WO2009127843A1

WO2009127843A1 - Terahertz signal generator

Info

Publication number: WO2009127843A1
Application number: PCT/GB2009/001004
Authority: WO
Inventors: Roger Pollard; Robert Miles; Mira Naftaly
Original assignee: University Of Leeds
Priority date: 2008-04-18
Filing date: 2009-04-17
Publication date: 2009-10-22
Also published as: GB0807176D0

Abstract

A signal generator (1) including an optical radiation means for generating a first optical radiation (4) having a first frequency, a second optical radiation means (5) having a second frequency and a third optical radiation means (12) having a third frequency; a difference- frequency means (3) for generating from a said first optical radiation and a said second optical radiation an electromagnetic local oscillator signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the second frequency, and for generating from a said first optical radiation and a said third optical radiation an electromagnetic probe signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the third frequency; output means for concurrently outputting from the signal generator a said local oscillator signal (33) and a said probe signal (34) for use.

Description

TERAHERTZ SIGNAL GENERATOR

The invention relates to methods and apparatus for generating electromagnetic signals e.g. for use in analysis of a sample or device such as, for example via a heterodyne or superheterodyne detection process.

Advances occur rapidly in electronic, electrical and microwave devices and circuits operating at terahertz (THz) speeds, with potential in applications such as high-speed wireless communications. Instrumentation for testing and characterising such new devices include vector network analysers (VNA) . VNAs are devices which analyse a device under test (DUT) by inputting to the DUT a predetermined "incident signal", such as an electrical signal of predetermined form, and then comparing the incident signal with either (or both) the parts of the incident signal transmitted through the DUT or the parts of the incident signal reflected from the DUT. Many circuit characteristics can be determined from such a comparison, and VNAs are typically arranged to determine such characteristics in this way.

However, a VNA is only able to perform such measurements as are possible or available according to the frequency and intensity of the incident signal against which the VNA is to make such comparisons. The ability to provide an incident signal of suitable frequency and strength is paramount. Existing signal generators for this purpose, able to generate such incident signals for use in VNA analysis of devices, are able to generate incident signal frequencies of up to 325 GHz. These instruments are very expensive and use multiple special, ad-hoc attachments to enable the signal generator to cover different frequency ranges . At present there are believed to be no such signal generators for use with VNAs whose range extends beyond 1 THz. Existing methods for producing incident signals for VNA use employ a process of up-converting lower-frequency electrical signals into higher-frequency electrical signals.

Accordingly, the present invention aims to address this shortcoming in the prior art and to provide a signal generator suitable for use with a VNA, or the like, for the analysis of devices or samples (e.g. materials) using incident signal frequencies within the THz range.

At its most general, the invention proposed is the use of three mixed optical radiations of differing frequencies in a frequency down-conversion operation to produce a local oscillator signal and a probe signal having frequencies of the order of terahertz. The invention may provide a signal generator for use with THz test instrumentation, such as a VNA or the like, which produces electromagnetic signal radiation in the range e.g. from 0.3 THz to 3 THz, or more. Pulsed fibre lasers may be employed to produce two 1 mW THz beams by difference frequency generation (DFG) , differing in frequency by a controllable amount (e.g. 10GHz or thereabouts) .

A laser means may serve as a probe signal (e.g. laser beam), the another laser means may act as a local oscillator signal (e.g. laser beam) . Heterodyne detection may be employed in conjunction with the signal generator for signal acquisition from a DUT. A 50 dB dynamic range may be provided. The signal generator may generate probe and local oscillator signals having frequencies in the frequency range 0.3-3 THz with a resolution of 1 GHz or less . The expected foot print of the signal generator may be defined by the space occupied by the laser components, which could fit a standard 19 inch (approx. 50cm) rack with a height of a few inches (approx. 5cm to 10cm) . Employing the signal generator, a terahertz (THz) frequency Vector Network Analyzer front-end may be provided which may be employed to feed an intermediate frequency signal, generated thereby, directly into an existing VNA signal processing chain. The THz VNA may then use heterodyne detection methods. The probe and local oscillator signals (e.g. beams) may be obtained by mixing three frequency components from a fiber Master Oscillator Power Amplifier (MOPA) system. Power amplification in the MOPA may be provided via pumped fibre amplifiers arranged to amplify a first combination of a tuneable first optical radiation combined with a fixed second optical radiation, and concurrently to separately amplify a second combination of the tuneable first optical radiation and a selected third optical radiation. Difference frequency generation applied to the first combination may provide a local oscillator signal and concurrent difference frequency generation applied to the second combination may provide a probe signal. Since both the first and the second combinations contain the tuneable first optical radiation, tuning thereof causes a concurrent tuning, in equal measure, of the local oscillator and probe signals. The frequency separation between may thus be held constant according to the selected value of the frequency of the third optical signal.

In a first of its aspects, the invention may provide a signal generator including: an optical radiation means for generating a first optical radiation having a first frequency, a second optical radiation having a second frequency and a third optical radiation having a third frequency; a difference-frequency means for generating from a said first optical radiation and a said second optical radiation an electromagnetic local oscillator signal having a frequency substantially equal in value -to the difference between the respective values of the first frequency and the second frequency, and for generating from a said first optical radiation and a said third optical radiation an electromagnetic probe signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the third frequency; output means for concurrently outputting from the signal generator a said local oscillator signal and a said probe signal for use.

Accordingly, a tuneable signal generator is provided in which the frequencies of both the local oscillator signal and the probe signal may be tuned in tandem and in equal measure by simply tuning the value of the first optical frequency. This is relatively simple to perform. Operation and stability of optical sources, such as lasers, is generally easier to achieve and maintain than it is in electrical signals and the means for generating electrical signals in existing signal generators. Since the frequency separation between the local oscillator and probe signals (the difference frequency) is preferably held constant according to the selected value of the frequency of the third optical radiation, a sweeping of the local oscillator and probe frequencies preferably does not change the frequency of the intermediate signal employed in heterodyne, or superheterodyne detection. This may ensure that such tuning does not require a corresponding re-tuning of a (super) heterodyne detector employed in conjunction with the signal generator being tuned/swept in frequency when testing a DUT.

The optical radiation means preferably comprises first laser means for generating said first optical radiation, second laser means for generating said second optical radiation, and modulator means for modulating the second optical radiation to generate the third optical radiation from the result. The first and second laser means may comprise continuous-wave (cw) lasers or pulsed lasers. The first laser means may be frequency tuneable to a selected first frequency and the second laser means may be of fixed second frequency. The optical radiation means may include amplitude modulator means arranged to modulate the amplitude if first and/or second optical radiation generated by the first and/or second laser means to produce pulsed first and/or second optical radiation. The pulses may be between 0.5mW and 5mW in average power. The first and/or second laser means may be lasers directly controlled to generate pulses of laser radiation to provide the first and/or second optical radiation as desired. The first frequency and/or second frequency may correspond to optical radiation having a wavelength in the range 500nm to 1500nm. Pulsed first and second optical radiation is preferable since it enables relatively high peak optical power to be conveyed thereby. This can assist in the efficient implementation of difference frequency generation.

The signal generator may include phase control means arranged to control the first laser means and the second laser means to maintain a constant phase difference (of any selectable value) between the first optical radiation and the second optical radiation such that the local oscillator signal and the probe signal are phase- locked. It is preferable to provide such phase-locking in applications such as vector network analysis, or the like, so that a phase difference between the local oscillator signal and the probe signal returned from a device under test can be attributed solely to the device under test.

Phase locking may be achieved automatically, as described above. That is to say the local oscillator radiation and the probe radiation may be produced by frequency difference generation based on the same first and second optical radiation. The local oscillator signal may be generated using the first and second optical radiation directly, while the probe signal may be generated using the first optical radiation and a side band radiation resulting from modulation of the second optical radiation. Thus, the phase relation between the first and second optical radiation is presented in each of the local oscillator and probe signals since the second optical radiation and its modulation side band radiation (third optical radiation) are automatically phased-locked .

The signal generator may include frequency control means arranged to control the first laser means and the second laser means to maintain a constant difference (of any selectable value) between the value of the first frequency and the value of the second frequency such that a constant corresponding difference is maintained between the value of the local oscillator signal frequency and the probe signal frequency.

For example, the first laser means and the second laser means may each comprise an optical grating placed within the respective laser cavity thereof, which optical grating possesses an optical grating spectrum which is controllably tuneable thereby to control the frequency at which the respective laser means lases . For example, the laser means may comprise fibre-based lasers and the optical grating may be an optical fibre grating. The optical grating may be a thermally tuneable optical grating responsive to controlled variations in temperature to change the spectral optical grating characteristics thereof. Other means and methods for tuning the grating, such as electro-optically (e.g. piezo electrically) may be employed to this end. it is to be noted that provision of such a grating also, at least to some extent, controls the line width of the laser radiation produced by a respective laser means. It is preferable that the line width of each of the first and second laser means is narrow (e.g. as narrow as possible) such that the frequency of the first and second optical radiations are not only stable but are well defined.

The modulator means may include phase modulator means arranged to receive second optical radiation from the second laser means, to modulate the phase of the received second optical radiation with a predetermined modulation frequency thereby to produce a side-band optical radiation and to output the result as said third optical radiation such that the value of the third frequency is equal to the sum of or the difference between the value of the modulation frequency and the value of the second frequency.

It will be appreciated that other forms of modulation may be implemented to produce such a side-band. Frequency modulation or amplitude modulation are possible alternatives to the phase modulation method described above. However, it has been found that phase modulation is typically most suitable for reasons not only concerning the expense of implementation, but also with regards to the relative difficulty in producing high-speed frequency modulation of sufficient precision, and bearing in mind that methods of amplitude modulation may reduce the average power conveyed by the side band radiation produced thereby.

The optical radiation means may include optical coupler means for combining a said first optical radiation and a said second optical radiation and co-propagating the combination to the difference-frequency means, and for combining a said first optical radiation and a said third optical radiation and co-propagating the combination to the difference-frequency means.

The optical coupler means may comprise a first optical input portion arranged for receiving a said first optical radiation, a second optical input portion arranged for receiving a said second optical radiation and a said third optical radiation concurrently, a first optical output portion arranged for receiving and combining said first optical radiation from the first optical input portion and said second optical radiation from the second optical input portion and for outputting the result to the difference- frequency means, and a second optical output portion arranged for receiving and combining said first optical radiation from the first optical input portion and said third optical radiation from the second optical input portion and for outputting the result to the difference- frequency means .

The optical coupler means may include first optical filter means placing the second optical input portion in optical communication with the first optical output portion and arranged to prevent transmission of a said third optical radiation therethrough and to permit transmission of a said second optical radiation therethrough, and second optical filter means placing the second optical input portion in optical communication with the second optical output portion and arranged to prevent transmission of a said second optical radiation therethrough and to permit transmission of a said third optical radiation therethrough.

The optical radiation means may include tuning means operable or arranged to controllably vary, as desired by a user, said first frequency with said second frequency maintained at a constant value thereby to controllably vary or tune the frequency of the local oscillator signal and the frequency of the probe signal concurrently and by equal measure such that the difference between the two remains unchanged. The tuning means may be arranged to maintain the second frequency constant, or the source of the second optical frequency may be arranged to inherently produce second optical radiation of a constant second frequency. This may enable a user to sweep the probe signal frequency across a desired frequency range, or simply re-tune the probe signal frequency to selected other values, as desired. This may be especially useful when the probe signal is used to irradiate a device or sample for testing the characteristics of the device or sample using the probe signal. For example, if the signal generator is used as part of a device analyser (e.g. with a vector network analyser) including means for analysing probe signal radiation returned from a device under test using heterodyne detection methods, then the ability to sweep/vary the value of the probe signal frequency with no change to the difference frequency (i.e. the difference between the local oscillator frequency and the probe signal frequency) means that the analyser (e.g. VNA) need not be itself retuned in response to variation of the probe frequency. This is because heterodyne detection methods employ an "intermediate frequency" (IF) generated by mixing a local oscillator signal radiation with a received probe signal radiation. The IF signal has a frequency equal to the difference frequency. Thus, maintaining the value of the difference frequency maintains the value if the frequency of the IF signal upon which the analyser (e.g. VNA) acts.

The difference-frequency means may include a first difference-frequency generator arranged in optical communication with the optical radiation means to permit concurrent receipt therefrom of a said first and a second optical radiations and to generate the local oscillator signal therefrom by a difference-frequency operation, and a second difference-frequency generator arranged in optical communication with the optical radiation means to permit concurrent receipt therefrom a said first and third optical radiations and to generate the probe signal therefrom by a difference-frequency operation. The first difference- frequency generator is preferably not in optical communication with the second difference-frequency generator such that each may perform a difference-frequency operation on separate optical radiation.

One or more of the first and second difference-frequency generators may comprise a nonlinear optical crystal having a nonzero second-order nonlinear susceptibility. Preferably, the value of the second-order nonlinear susceptibility of the nonlinear optical crystal exceeds a value necessary for production of local oscillator and probe signal intensities or powers suitable for practicable use in e.g. analysing a device or sample under test by measurement of the scattering of probe signals.

The lower the value of the second-order nonlinear susceptibility of the non-linear optical crystal, the lower the power or intensity of the THz local oscillator and probe radiation produced thereby. This may be compensated for with higher intensities of first, second and third optical radiation and/or by increasing the interaction length of first, second and third optical radiation within the nonlinear optical crystal. In general, it can be preferable to select the non-linear optical crystal with the highest value of second-order nonlinear susceptibility.

The optical crystal may be formed from Gallium Phosphide

(GaP) Gallium Selenide (GaSe), or Gallium Arsenide (GaAs) or may be a periodically-poled crystal such as a periodically poled Lithium Niobate (LiNb) crystal (PPLN) or an organic crystal such as an organic ionic salt crystal such as 4-N, N-dimethylamino-4' -N' -methyl-4-stibazolium tosylate ("DAST") .

The optical radiation means may be arranged to generate pulsed first optical radiation and pulsed second optical radiation, and synchrony means be provided therein and arranged or operable to synchronise the generation of said pulsed first and second optical radiations such that a local oscillator signal and a concurrent probe signal. generated by the signal generator comprise mutually synchronised electromagnetic pulses.

The optical radiation means may include optical amplifier means arranged to amplify first, second and third optical radiation prior to generation of the local oscillator signal and the probe signal by the difference-frequency means. The optical amplifier means may include a pulse regenerator means arranged to reshape pulses of first, second and third optical radiation received thereby, and to output the results to the difference-frequency means . The optical radiation means may comprise a Master Oscillator Power Amplifier (MOPA) comprising the first laser means, a first optical amplifier part and a second optical amplifier part not in optical communication with the first optical amplifier part, wherein each of the first and second optical amplifier parts is arranged to receive concurrently first optical radiation from the first laser means . The MOPA may include the second laser means, and the first optical amplifier part may be arranged to receive concurrently first optical radiation from the first laser means and second optical radiation from the second laser means. The second optical amplifier part may be arranged to receive concurrently first optical radiation from the first laser means and third optical radiation. Concurrently received first and second optical radiation may be co-amplified (and regenerated, when pulsed) by the first optical amplifier part for output to the difference-frequency means, and concurrently received first and third optical radiation may be co-amplified (and regenerated, when pulsed) by the second optical amplifier part for output to the difference- frequency means .

Certain advantages are inherent in employing such an arrangement. For example, the optical frequencies used to generate the local oscillator signal and the probe signal may be relatively close in value (e.g. first and third optical radiations) each being e.g of an infra frequency (e.g. wavelengths of the order of a micron), but of sufficient similarity that the frequency difference is of the order of THz. The double or dual-MOPA arrangement described above obviates the need to separate the local oscillator signal and the probe signal at the optical output of the device as would be required were a single MOPA design employed in which both a local oscillator and a probe signal were amplified on, and transmitted along, the same optical transmission line and optical output. Such a separation would be complicated and would cause very significant power losses in separating or disentangling the output local oscillator signal from the output probe signal. A further advantage of the double-MOPA arrangement is that it permits the generation of double the total available optical power in generating the first, second and third optical signals. In a second of its aspects, the invention may provide a probe apparatus for a sample or device analyser for producing an electromagnetic local oscillator signal and an electromagnetic probe signal with which to analyse a sample or device and including a signal generator therefor according to the invention in its first aspect, and mixer means for receiving a said local oscillator signal and a said probe signal output from the signal generator and for superimposing the received probe signal and received local oscillator signal to produce a mixer signal having a frequency substantially equal in value to the difference between the respective values of the second frequency and the third frequency.

The probe apparatus may comprise mixer means including a plurality of separate mixers each arranged for receiving local oscillator signal radiation and probe signal radiation, originating from the signal generator, separately from other mixers of the mixer means and arranged to produce a said mixer signal therefrom. Each mixer may comprise a metal feed horn arranged to receive local oscillator and probe radiation concurrently to superimpose such radiation and feed the result to an optoelectric sensor responsive thereto to generate said mixer signal

The probe apparatus may include input port means arranged to receive a said probe signal from the signal generator and operable to direct such probe signal to a sample or device under analysis, and output port means operable to receive a return probe signal originating from the signal generator and returned from the sample or device under analysis and to output the return probe signal for subsequent analysis.

Each of, or either of, the input port means and output port means may comprise a plurality of separate port elements each arranged to input/output probe signal radiation to/from a different part or port of a device/sample under test.

In another aspect, the invention may provide a device analyser including probe apparatus of the invention in its second aspect and including a heterodyne detector operably coupled to the mixer means to receive a mixer signal produced by the mixer means and therewith to perform a heterodyne operation on the mixer signal to produce an intermediate frequency signal therewith to determine the amplitude and/or the phase of a probe signal originating from the signal generator. The heterodyne detector may include a vector network analyser (VNA) . The probe apparatus may include a reference signal mixer arranged to receive from the signal generator local oscillator signal radiation and, concurrently, probe signal radiation which has not interacted with a device or sample under test. The heterodyne detector preferably possesses mixer signal input ports for receiving mixer signals from any or all mixers of the mixer means . A VNA measures the scattering or S-parameters of a Device Under Test (DUT) . That is to say, it records simultaneously the amplitude and phase of the transmitted and reflected beams and compares them with a reference signal. The S- parameters are the complex transmission and reflection ratios. For complete characterisation of a general DUT this ratio should be determined at all of the device ports. Such measurement according to the present invention in this aspect may be achieved by providing, via the probe apparatus, a reference signal (no interaction with the DUT), a transmitted probe signal (as transmitted through the DUT) and a reflected signal (as reflected from the DUT) from each port of the DUT. The usual arrangement for scattering parameter measurement requires a reference signal (i.e. a sample of the incident wave) and a test signal (a sample of the wave that has been either reflected by or transmitted through the device under test) . The resultant scattering coefficients are displayed as the complex ratio of the test signal to the reference signal. In this aspect the invention may be arranged to superheterodyne both signals to a frequency that can be received by a standard microwave instrument. Ensuring that the reference and test signals are derived from common coherent sources preferably preserves their magnitude and phase relationships. THz radiation may be coupled into and out of the DUT using metal horns, or in the case of material characterisation, by means of polymer collimating lenses and focusing solid immersion Si lenses. Directional coupling may be provided by a set of THz beam-splitters (e.g. 50:50 spliters) .

In another aspect the invention may provide a spectroscope or spectrometer comprising any one or more of the aspects described above.

It will be appreciated that the aspects of the invention described above realise a corresponding and equivalent method of signal generation, or device/sample analysis which the scope if the invention is intended to encompass.

In a third of its aspects, the invention may provide a method for signal generation including: generating a first optical radiation having a first frequency, a second optical radiation having a second frequency and a third optical radiation having a third frequency; generating from a said first optical radiation and a said second optical radiation an electromagnetic local oscillator signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the second frequency, and generating from a said first optical radiation and a said third optical radiation an electromagnetic probe signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the third frequency; concurrently outputting from the signal generator a said local oscillator signal and a said probe signal for use.

The method may include generating said first optical radiation as laser radiation, generating said second optical radiation as laser radiation, and modulating the second optical radiation to generate the third optical radiation from the result.

The method may include maintaining a constant phase difference between the first optical radiation and the second optical radiation such that the local oscillator signal and the probe signal are phase-locked.

The method may include maintaining a constant difference between the value of the first frequency and the value of the second frequency such that a constant corresponding difference is maintained between the value of the local oscillator signal frequency and the probe signal frequency.

The method may include modulating the phase of second optical radiation with a predetermined modulation frequency thereby to produce a side-band optical radiation and outputting the result as said third optical radiation such that the value of the third frequency is equal to the sum of or the difference between the value of the modulation frequency and the value of the second frequency. The method may include combining a said first optical radiation and a said second optical radiation applying a difference-frequency operation to the combination to generate the local oscillator signal, and combining a said first optical radiation and a said third optical radiation and applying a difference-frequency operation to the combination to generate the probe signal.

The method may include varying said first optical frequency and maintaining said second optical frequency at a constant value thereby to vary or tune the frequency of the local oscillator signal and the frequency of the probe signal concurrently and by equal measure such that the difference between the two remain unchanged.

The method may include providing a first difference- frequency generator and concurrently receiving thereat a said first and a second optical radiations and generating the local oscillator signal therefrom by a difference- frequency operation, and providing a second difference- frequency generator and concurrently receiving therefrom a said first and third optical radiations and generating the probe signal therefrom by a difference-frequency operation.

One or more of the first and second difference-frequency generators may comprise a nonlinear optical crystal having a nonzero second-order nonlinear susceptibility. The method may include generating pulsed first optical radiation and pulsed second optical radiation, and synchronising the generation of said pulsed first and second optical radiations such that a local oscillator signal and a concurrent probe signal comprise mutually synchronised electromagnetic pulses.

In a fourth of its aspects, the invention may provide a method for analysing a sample or device including producing an electromagnetic local oscillator signal and an electromagnetic probe signal with which to analyse a sample or device according to the invention in its third aspect, and receiving a said local oscillator signal and a said probe signal so generated and superimposing the received probe signal and received local oscillator signal to produce a mixed signal having a frequency substantially equal in value to the difference between the respective values of the second frequency and the third frequency.

The method may include receiving a said probe signal so generated directing such probe signal to a sample or device under analysis, receiving a return probe signal returned from the sample or device under analysis and outputting the return probe signal for subsequent analysis.

In a further aspect , the invention may provide a method for analysing a device analyser including generating a probe signal according to the invention in its fourth aspect , providing a heterodyne detector, receiving said mixed signal at the heterodyne detector and therewith performing a heterodyne operation on the mixed signal to produce an intermediate frequency signal therewith to determine the amplitude and/or the phase of a probe signal originating from the signal generator.

GaSe is a preferred choice of non-linear crystal for use in the difference-frequency generating means owing to its high second-order nonlinear coefficient, high transparency both at 1 μm and at THz frequencies, and its wide phase-matching range. The choice of crystal may also depend on the availability of crystal thicknesses and on cost. Exact phase matching may be achieved in GaSe by adjusting the orientation of the crystal. By use of very large mode area fiber amplifiers, (e.g. of or about 10 μπi², or 20 μm², or

30 μm², or a value therebetween) a crystal just a few mm thick (e.g. from 1 mm to 5 mm) and careful management of pulse shapes and nonlinear effects in the amplifiers, the signal generator may generate in excess of 1 mW peak or average THz power in 1 μs duration pulses with a bandwidth of 1 GHz at the lower end of the frequency range, rising by up to 2 orders of magnitude at about 2THz and then falling again as frequencies approach 3THz.

An advantage of employing relatively large mode area fibre amplifiers, and a motivation for managing optical pulse shapes, is to avoid producing high peak optical intensities in the fibre amplifier. Unsuitably high peak intensities may tend to induce non-linear effects in the fibre. Nonlinear effects should preferably be avoided where possible since they produce additional frequencies (which may appear at the output of the signal generator) . It is preferable that the shapes of the pulses are controlled to be broad in form (e.g. rectangular or square), and that the mode area is sufficiently large to keep the peak intensity in the fibre amplifier below the non-linear threshold of that fibre (i.e. the intensity threshold at which non-linear effects become observable and/or problematic) .

In use, three phase locked laser frequencies may be used to generate two THz beams, separated in frequency by 10 GHz. One of the THz beams may provide the local oscillator, while the other may provide a probe signal for use in probing devices or samples under test. The laser sources may be electronically synchronised and frequency-stabilised in order to maintain synchronicity and a constant frequency and phase difference between the local oscillator and probe beams. This may be possible because of the small and constant frequency difference between the two beams. During frequency tuning, both beams may be scanned simultaneously. Down-conversion may be achieved either by a Schottky diode or by a photoconductive detector. Following their interaction with a DUT, the probe and local oscillator beams may be combined optically, and down-converted to an IF frequency of 10 GHz with -1 GHz linewidth. The IF may then be fed into a commercial VNA via a notch filter to further reduce the linewidth. The laser beams may also be phase locked to the VNA. The THz pulse length may be ~1 μs, which will allow sufficient time for signal processing

The wavelengths required for THz generation by DFG may be generated by a fibre MOPA (Master Oscillator Power Amplifier) system based on amplification in Yb-doped fibres. Diode pumped Yb fibre amplifiers are highly efficient, with wall plug efficiencies of around 30%, and are compact and robust. Commercial narrow-linewidth fixed frequency and tunable lasers may be employed to produce output at around 1060nm, with a tuneable frequency difference of 0.3-3 THz , corresponding to δλ«l-12 nm. An electro-optic amplitude modulator (EOAM) may be employed to carve pulses from the CW sources. The phase of the IF signal obtained from a reference detector, having had no interaction with a DUT, may be employed to act as a comparison reference for the phases of the IF signals associated with the probe signals as transmitted and as reflected by a DUT. In order to obtain two lines separated by 10 GHz, an electro-optic phase modulator (EOPM) driven by an external RF signal may be employed to generate sidebands on the one of the beams conveying the second optical radiation: the fundamental and one sideband may be selected for amplification using narrow- bandwidth fiber gratings. This is a much simpler and more robust approach than attempting to phase lock two laser cavities. The stability of the difference frequency (δω) may be set by the stability of the electronic signal driving the EOPM, rather than by the stability of the laser frequencies.

Square pulses of optical radiation are generally most efficient for difference-frequency generation (DFG) , maintaining a high power for the duration of the pulse, whilst not exceeding peak power limits imposed by the onset of nonlinear effects in the amplifier, while also facilitating the THz signal processing. Transmission of optical radiation within the signal generator may be via polarisation maintaining optical fibres to maintain the (orthogonal) polarisations of the two wavelength components as is desirable for DFG phase matching. Peak powers of a few kW in such pulses may be generated preferably with duty cycles of ~ 1% thereby to give average powers of a few 10s of Watts. The amplified outputs of the optical radiation means may be made incident on two DFG crystals and may thus produce a pair of beams, tunable from 0.3-3 THz with a fixed frequency separation of 10 GHz.

Advantages of the signal generator, and device analyser include;

• Broad bandwidth of -0.3-3 THz.

• Ability to function as a front-end to a standard microwave VNA

• High THz power >1 mW peak power at any frequency, sufficient for heterodyne detection • Utilization of the high peak powers available from a pulsed fiber laser

• Use of heterodyne detection, providing a dynamic range of >50 dB and a low noise floor In order to tune the THz frequency generated by a non-linear crystal, the crystal may preferably be rotated to maintain optimal phase matching conditions . The generation of a photon at a third optical frequency, from photons at first and second optical frequencies incident upon the non-linear crystal, requires conservation of energy to be observed and, if the process is to occur with an appreciable efficiency, also requires phase-matching to occur, such that K₃=K₁ - K₂, while K₁, K₂ and K₃ are the wave vectors of photons of the first, second and third optical radiation respectively. A computer-controlled rotary turning stage may be employed to obtain, and maintain, crystal rotation angles necessary to achieve optimal phase matching. First, second and third optical radiation may be focused on to the difference frequency generating elements (e.g. crystals) in order to increase THz power generation. In difference-frequency generation, the generated difference-frequency power is proportional to the product of the power densities of the two input beams used to generated the difference frequency signals .

While the difference-frequency generating element comprises a GaSe crystal, phase matching, and optical THz radiation generation, may be tuned, as a function of crystal orientation, according to the phase-matching (PM) angles disclosed in: "Efficient, Tuneable and Coherent 0.18 - 5.27 THz source based on GaSe Crystal"; Wei Shi et al . , Optic Letters/Vol. 27, No. 16/August 15, 2002, pp 1454 - 1456.

There now follow non-limiting examples of embodiments of the invention made with reference to the accompanying drawings of which:

Figure 1 illustrates an optical signal generator; Figure 2 illustrates a device analyser in the form of a vector network analyser comprising a signal generator as illustrated in Figure 1;

Figure 3 illustrates an optical signal generator of Figure 1 adapted for use in the device analyser of Figure 2.

In the figures, like items are assigned like reference symbols. Figure 1 schematically illustrates a signal generator (1) constructed and arranged for producing an electromagnetic probe signal (34) and, concurrently, an electromagnetic local oscillator signal (33) . The signal generator includes an optical radiation generator (2) and a difference-frequency generator (3) , and collimating output apparatus (3B, 3C) .

The optical radiation generator (2) is arranged and operable to generate first optical radiation having a first frequency, a second optical radiation having a second frequency and a third optical radiation having a third frequency. A first output port (260) of the optical radiation generator is arranged to output first optical radiation and second optical radiation concurrently, and a second optical output (280) of the optical radiation generator is arranged to output first optical radiation and third optical radiation concurrently.

The difference-frequency generator (3) comprises a first optical input port arranged to place the first optical output port (260) of the optical radiation generator in optical communication with a first difference-frequency generating element (29) . The first difference-frequency generating element is adapted and arranged to generate from first optical radiation and second optical radiation concurrently received thereby from the first optical output port (260) of the optical radiation generator, an electromagnetic signal (30) having a frequency (G_h-Co₂ ) with a value equal to the difference between the respective values of the frequencies of the concurrently received first optical radiation (coi) and the second optical radiation (CO₂) . The first difference frequency generating element is arranged to output the result (30) to a first optical output element (3B) of the collimating output apparatus which comprises one or more collimating lenses or other suitable collimating optical apparatus adapted to produce a collimated beam (33) from the electromagnetic radiation (30) output from the first difference frequency generating element (29) . The output (30) of the first difference frequency generating element (29) defines a electromagnetic local oscillator signal made available for use in collimated form (33) .

Similarly, the difference-frequency generator (3) comprises a second difference-frequency generating element (31) arranged in optical communication with the second optical output port (280) of the optical radiation generator so as to enable the second difference frequency generating element to receive, concurrently, first optical radiation and third optical radiation generated by the optical radiation generator. The second difference frequency generating element (31) is constructed and arranged to generate from first optical radiation and third optical radiation concurrently received thereby, an electromagnetic probe signal (32) having a frequency (tθi-ω₂-δω) substantially equal in value to the difference between the respective values of the frequency of the first optical radiation (coi) and the third optical radiation (ω₂-δω) . A second optical output element (3C) of the collimating output apparatus is arranged to receive the electromagnetic probe signal (32) so produced, and to form a collimated electromagnetic probe signal (34) therefrom. The second output element (3C) comprises one or more lens elements, or the like, or other optical elements suitable for collimating electromagnetic radiation such as would be readily apparent to the skilled person.

The value of the frequency of the local oscillator signal (30) and the probe signal (32), is a respective value selected from the range: 0.3-3 terahertz.

Each of the first and second difference-frequency generating elements (29, 31) is a non-linear optical crystal having a non-zero second-order, non-linear susceptibility. Any such non-linear crystal, such as would be readily apparent to the skilled person, may be employed for this purpose. Gallium selenide crystal (GaSe) , or periodically-polled lithium niobate (PPLN) , or gallium arsenide (GaAs) or Gallium Phosphide (GaP) crystals may be employed. An organic crystal such as "DAST" may be used. Crystals of GaAs are suitable for difference-frequency generation using input radiation at wavelengths typically longer than those input wavelengths for which the other examples of non-linear crystals are optimal. As an alternative, either or each of the first and second difference-frequency generating elements (29, 31) may comprise a quantum-cascade-laser with an active region engineered to possess a suitably large second-order non-linear susceptibility arranged to be concurrently pumped by the optical radiation it receives from the optical output port of the optical radiation generator (1) in which it is in optical communication. The optical radiation generator (2) comprises a first tuneable continuous-wave laser (4) operable to generate the first optical radiation having a fixed intensity and a first frequency (CD₁) and to output the result to a first amplitude modulator unit (6) . The first amplitude modulator is arranged to receive first optical radiation from the tuneable laser (4) via a first segment of a first optical transmission line (10) placing the two in optical communication, and is arranged to modulate the amplitude of the received radiation according to amplitude modulation control signals (103) originating from a control unit (100) . The control unit (100) is arranged to control the first amplitude modulator (6) to amplitude-modulate the fixed- intensity first optical radiation received thereby so as to carve therefrom a sequence of square-wave optical pulses, and to output the result to a second segment of the first optical transmission line (10) for further processing.

A wave meter (9) is arranged in optical communication with the optical transmission line (10) via an optical coupler unit (8) arranged between the second segment of the first optical transmission line, and a subsequent third segment thereof. The optical coupler unit is arranged to extract from the first optical transmission line (10) a predetermined (small) portion of first optical radiation propagating there along from the first amplitude modulator unit (6) , and to pass the result to the wave meter for analysis. Typically, the optical coupler unit may extract about 1 to 2%, preferably less than 10%, of the first optical radiation.

The wave meter which is adapted to measure the frequency of the first optical radiation input thereto and to transmit a signal (106) conveying the value of that measured frequency to the control unit (100) with which the wave meter (9) is in communication for this purpose. The control unit is operable to control the first frequency (coi) of the first tuneable laser by issuing thereto a tuning instruction (101) to which the first tuneable laser (4) is responsive to vary the first frequency according to the instruction. The wave meter (9) and the measured frequency value (106) provide a feedback loop to the control unit (100) via which it is able to stabilise, monitor or adjust the tuning control signal (101) issued to the tuneable laser (4) in order to ensure that the tuneable laser is generating first optical radiation at the desired first frequency (CU₁) .

A second continuous-wave laser (5) is arranged to generate second optical radiation having a fixed intensity and a second frequency of a substantially fixed value (ω₂) and to output the result to an optical input of a second amplitude modulator unit (7) arranged in optical communication with the second laser via a first segment of a second optical transmission line (11) . The second amplitude modulator unit (7) is arranged to function in a similar manner to that in which the first amplitude modulator unit (6) is so arranged. In particular, the second amplitude modulator unit (6) arranged to receive second optical radiation from the second laser (5) via the first segment of the second optical transmission line, and to modulate the amplitude of the received radiation according to amplitude modulation control signals (104) originating from a control unit (100) (discussed below) . The control unit (100) is arranged to control the second amplitude modulator (7) in such a way as to generate from the continuous-wave, fixed-intensity second optical radiation received thereby, a sequence of square- wave optical pulses and to output the result to a second segment of the second optical transmission line (11) for further processing.

The control unit may comprise computer means suitably programmed or programmable to implement those control functions . The control unit may include sensors such as would be readily apparent to the skilled person, for collecting measurements (e.g. signal frequency, phase, amplitude) to which the computer means is responsive to implement control functions .

Frequency control and phase-locking control signals (101, 102) are issued to both of the first and second lasers (4, 5) from the control unit (100) to control and stabilise the frequency of optical radiation they generate and to maintain a desired value for the difference between the respective phases of the optical radiations they generate. As discussed above, the two difference frequencies (local oscillator signal and probe signal) are automatically phase- locked together by virtue of each being generated using the same first and second optical radiation from the first and second lasers (4, 5) with the third optical radiation (discussed below) being inherently in-phase with the second optical radiation.

A phase modulator unit 12 is arranged upon the second optical transmission line between a second segment and a third segment thereof, and has an optical input port in optical communication with the optical output port of the second amplitude modulator unit to receive square-wave optical pulses output by the second amplitude modulator. The phase modulator unit is arranged to modulate the phase of the electromagnetic waves comprising the second optical radiation by a variable amount determined according to a phase modulation control signal 105 originating from the control unit 100. The phase modulation control signal is effective to control the modulation frequency δω (typically radio frequencies, such as lOGhz) with which the phase modulator unit modulates received second optical radiation. The result of such phase modulation is output by the phase modulator unit onto a third segment of the second optical transmission line which optically couples an output port of the phase modulator unit to a second optical input port 13 of a coupler apparatus (collectively, items 13 to 22) . The phase modulator unit (12) may be phase locked to a vector network analyser (see Figure 2) in one embodiment, electronically such that the phase of the modulation implemented by the phase modulator, and the phase of an intermediate frequency ("IF") signal resulting from that, is phase-locked to an electronic signal within the network analyser.

The optical coupler apparatus is arranged to combine first optical radiation and second optical radiation and to co- propagate the combination to the first difference-frequency generating element (29) , and to combine first optical radiation and third optical radiation and co-propagate the combination to the second difference-frequency generating element (31) .

The optical coupler apparatus comprises a first optical coupler element (18) with an optical input port placed in optical communication with an end of the third segment of the first optical transmission line (10) for receiving pulses of first optical radiation output from the first amplitude modulator unit 6. The first optical coupler element is arranged to split received optical radiation into two parts of equal intensity and to output each part to a respective one of a first bifurcation optical transmission line (19) and a separate second bifurcation optical transmission line (20) thereof.

The optical coupler apparatus also includes a second optical coupler element (13) with an optical input port placed in optical communication with an end of the third segment of the second optical transmission line (11) for receiving concurrently pulses ^'of second and third optical radiation output from the phase modulator unit. The second optical coupler element is arranged to split received optical radiation into two parts of equal intensity and to output each part to a respective one of a third bifurcation optical transmission line (16) and a separate fourth bifurcation optical transmission line (17) thereof.

The third bifurcation optical transmission line includes a first optical waveguide transmission grating filter (14) portion, such as a long-period grating, structured and arranged to possess a band-pass grating transmission spectrum to prevent transmission therethrough of side-band radiation generated by the phase modulator unit (12) (including the third optical radiation) , and to permit transmission of second optical radiation therethrough. The fourth bifurcation optical transmission line includes a second optical waveguide transmission grating filter portion (15) , such as a long-period grating, structured and arranged to possess a high-pass or a band-pass grating transmission spectrum to prevent transmission therethrough of second optical radiation and of side-band radiation generated by the phase modulator unit (12) other than the third optical radiation, and to permit transmission of third optical radiation therethrough.

The optical coupler apparatus also includes a third optical coupler element (21) arranged for receiving and combining first optical radiation from the first optical coupler element and second optical radiation from the second optical coupler element and for outputting the result to the first difference-frequency generator element (29) . The coupler apparatus further includes a fourth optical coupler element (22) arranged for receiving and combining first optical radiation from the first optical coupler element and third optical radiation from the second optical coupler element and for outputting the result to the second difference- frequency generator element (31) .

The third optical coupler element (21) possesses a first optical input port placed in optical communication with the first optical coupler element (18) via an end of the first bifurcation transmission line (19) for receiving pulses of bifurcated first optical radiation output from the first optical coupler element. The third optical coupler element also includes a second optical input port placed in optical communication with the second optical coupler element (13) via an end of the third bifurcation transmission line (16) for receiving pulses of bifurcated second optical radiation output from the second optical coupler element via the first filter (14) thereof. The third optical coupler element is structured and arranged to combine pulses of bifurcated first and second optical radiation concurrently received thereby and to output the result to a third optical transmission line (23) for further processing.

Similarly, the fourth optical coupler element (22) possesses a first optical input port placed in optical communication with the first optical coupler element (18) via an end of the second bifurcation transmission line (20) for receiving pulses of bifurcated first optical radiation output from the first optical coupler element. The fourth optical coupler element also includes a second optical input port placed in optical communication with the second optical coupler element (13) via an end of the fourth bifurcation transmission line (17) for receiving pulses of bifurcated third optical radiation output from the second optical coupler element via the second filter (15) thereof. The fourth optical coupler element is structured and arranged to combine pulses of bifurcated first and third optical radiation concurrently received thereby and to output the result to a fourth optical transmission line (24) for further processing.

The optical output port of the third optical coupler element (21) is optically coupled via the third optical transmission line (23) to an optical input port of a first sequence of two concatenated optical fibre amplifiers (25, 26) arranged to amplify and re-shape concurrent and combined optical pulses of first and second optical radiation received thereby from the third optical coupler element. The terminal fibre amplifier (26) of the first sequence of amplifiers possesses an optical output port (260) which defines a first optical output port of the optical radiation apparatus (2) arranged, as described above, in optical communication with an optical input port of the first difference-frequency generator element (29) of the difference-frequency generator (3) .

The optical output port of the fourth optical coupler element (22) is optically coupled via the fourth optical transmission line (24) to an optical input port of a second sequence of two concatenated optical fibre amplifiers (27, 28) arranged to amplify and re-shape concurrent and combined optical pulses of first and third optical radiation received thereby from the fourth optical coupler element. The terminal fibre amplifier (27) of the second sequence of amplifiers possesses an optical output port (280) which defines a second optical output port of the optical radiation apparatus (2) arranged, as described above, in optical communication with an optical input port of the second difference-frequency generator element (31) of the difference-frequency generator (3) . During transmission between the first and second amplitude modulators (6, 7) and the third and fourth optical coupler elements, square pulses of first, second and third optical radiation suffer a degeneration of shape. The first and the second sequences of optical amplifiers are structured and arranged to re-shape and amplify such pulses into a square pulse shape of increases intensity prior to outputting their respective result (260, 280, respectively) to a respective one of the first and second difference-frequency generator elements (29, 31, respectively) as discussed above.

The control unit (100) is arranged to control the first and second amplitude modulators (6, 7) not only to control the amplitude and duty cycle of modulated pulses, but also to control relative timing of amplitude modulation as between the first and second amplitude modulator units. That is to say, the control unit 100 controls each of the first and second amplitude modulator units such that re-generated square pulses of first and second optical radiation concurrently output at the first optical output (260) of the optical radiation apparatus (2), are synchronised so as to be at least partly, and preferably wholly, temporally overlapping, and such that re-generated square pulses of first and third optical radiation concurrently output at the second optical output (261) of the optical radiation apparatus (2) , are synchronised so as to be at least partly, and preferably wholly, temporally overlapping. This ensures that first and second optical radiation are input together to the first difference-frequency generator element (29) to permit difference-frequency generation to occur to produce the local oscillator signal radiation (30, 33), and that first and third optical radiation are input together to the second difference-frequency generator element (29) to permit difference-frequency generation to occur to produce the probe signal radiation (32, 43) .

The stability of the difference frequency (δω) as between the local oscillator signal radiation and the probe signal radiation is determined by the stability of the electronic control signal (105) with which the phase modulator unit (12) is driven (i.e. phase modulation frequency (δω) e.g. 10GHz), and not by the stability of the first and second laser frequency. The control unit is operable to control the first laser and the second laser to maintain a constant difference between a selected value of the first frequency and the value of the second frequency such that a constant corresponding difference is maintained between the value of the local oscillator signal frequency and the probe signal frequency. The control unit is operable to vary the modulation frequency (δω) with which the phase modulator unit is driven thereby to vary, sweep or tune the value of the difference frequency as desired by the user .

The control unit is operable to vary the frequency of the first laser (4) as required by the user, to vary, sweep or tune that frequency as desired thereby to vary sweep or tune the frequencies of the local oscillator signal and the probe signal (33, 34) together, concurrently and in equal measure without changing the value of the difference frequency (δω) .

Figure 2 schematically illustrates a device analyser including a signal generator (1) , probe apparatus (35 to 58 collectively, excluding items 38 and 52) for directing probe radiation from the signal generator (1) of figure 1, to a device under test (DUT, 41) and therefrom, concurrently with local oscillator radiation from the signal generator, to each one of three signal input ports (38, 52, 60) of a superheterodyne detector comprising a vector network analyser (VNA) .

The three separate difference-frequency (local oscillator) signal generator elements (29 and 3B, 29' and 3B' , and 29' ' and 3B'', are each separately and concurrently connected in optical communication with the same single optical output port (260) of the optical signal generator conveying the first and second optical radiation as schematically illustrated in Figure 3. It is preferable to split the first and second optical radiation output via this output port (260) into three concurrent and phase-locked optical signals for separate frequency-difference generation into concurrent phase-locked local oscillator signals, since this merely requires use of optical splitters, wave guides or transmission lines etc. As all of the laser pulses produced to form local oscillator signals and probe signals are generated using the same two lasers, they are necessarily not only phase-locked but synchronised with each other.

Referring to Figure 3, three such concurrent local oscillator signals 33, 33' and 33'' are produced by difference-frequency generation from the same first and second optical radiation output at the same output port (260) of the optical radiation generator as follows. The first and second optical radiation is output to the first difference-frequency generating element 29 via a first optical splitter unit 100 arranged to split optical radiation received thereby into two separate paths (150, 160) bearing optical radiation intensities in any desired relative proportion. In this example, optical radiation directed via the first optical splitter towards the first frequency difference generating element 29 conveys one third of the optical energy incident upon the first optical splitter unit. A two-thirds proportion of the incident optical energy is directed by the first optical splitter along a second optical path 150 towards a third difference- frequency generating element 29' and a fourth difference- frequency generating element 29' ' , via a second optical splitter element 200. Once more, the ratio in which the second optical splitter element 200 is arranged to divide input optical energy as between its two optical output ports may be any desired or suitable value. In this example, the second optical splitter unit 200 is arranged to receive optical radiation via an optical transmission line 150 from the first optical splitter unit 100, and to split the received optical radiation into two concurrent optical signals (170, 180) of equal optical power. A first of the two optical signals (170) is conveyed from the second optical splitter unit to the third difference-frequency generating element 29' via suitable optical transmission means (e.g. optical wave guides) while the second optical signal 180 output by the second optical splitter unit 200 is conveyed (e.g. via a suitable wave guide) to the fourth difference-frequency generating element 29'' . In this way, first and second optical radiation output from a common source (260) is employed to generate three concurrent, synchronised and phase-locked local oscillator signals by difference-frequency generation at three different locations, concurrently with generation of a common probe signal. Of course, additional such concurrent local oscillator signals, or additional probe signals may be generated according to this technique.

Concurrent local oscillator signals are each separately collimated using respective dedicated collimating output apparatus, such as lenses, 3B, 3B' and 3B'', respectively.

The probe apparatus includes a first beam-splitting element 44 arranged to receive an electromagnetic probe signal (34) from the signal generator (1), to split the received beam in equal measure by reflecting a probe beam reflection portion 34A to a first microwave feed horn (35) via a second beam- splitting element (45) , and by transmitting a probe beam transmission portion (34B) to a second microwave feed horn (39) .

Feed horns are generally tapering or Y-shaped devices for guiding or concentrating radiation towards a specific location or region, and function in a manner analogous to loudspeaker operating in reverse. They are an efficient way to capture and focus THz radiation. As an alternative, lenses or mirrors may be employed, however, feed horns have been found to be more efficient in that they are able to couple free-space THz radiation into a device with relatively far fewer losses. Consequently, a greater proportion of THz radiation power is able to be delivered into and out of a device under test, and made available for the purposes of measurement.

The second microwave feed horn is arranged to direct received probe signal radiation into a DUT (41) via a suitable intermediate interface element (40) , such as a fiber-coupled probe, and to direct returned parts (53) of radiation of the probe beam transmission portion (reflected by the DUT) back to the first beam-splitting element for reflection thereat (54) to a third microwave feed horn (57) via a third beam-splitting element (55) identical in structure and characteristics to the first beam-splitting element. A fourth microwave feed horn (43) is arranged to direct probe signal radiation out from a DUT (41) , having been transmitted through the DUT from the second feed horn (39) , via a suitable intermediate interface element (42), such as a fiber-coupled probe, between the feed horn and the DUT, and to direct that radiation onwards to a fourth beam- splitting mirror (47) for transmission therethrough to a fifth microwave feed horn (49) .

Each, intermediate interface element (40, 42) may be any one of existing types of intermediate interfaces such as would be readily apparent to the skilled person in consideration of the nature of the DUT. The purpose of the intermediate interface is to couple an associated feed horn to a DUT to maximise the transmission of the probe signal into and/or out from the DUT via the feed horn to which it is coupled.

The intermediate interface may be a wave guide, or a strip line, or the like, suitable for transmitting THz radiation from the focus or output of a feed horn into a device under test. Other examples, such as would be readily apparent to the skilled person may be employed for this purpose.

The second, third and fourth beam-splitting elements (45, 55, 47) are each identical in structure and characteristics to the first beam-splitting element and are arranged to receive probe signal radiation to enable received probe radiation to pass therethrough, and to receive local oscillator radiation . (33, 33', 33''} originating from a signal generator (1) to reflect such received local oscillator radiation in a direction coincident with the direction in which the element in question is arranged to transmit probe signal radiation therethrough. The second beam-splitting element (45) is arranged to coincidently direct to the first feed horn (35) , probe signal and local oscillator signal radiation concurrently received thereby. The third beam-splitting element (55) is arranged to coincidently direct to the third feed horn (57), returned probe signal (reflected by the DUT) and local oscillator signal radiation concurrently received thereby. The fourth beam-splitting element (47) is arranged to coincidently direct to the fifth feed horn (49) , returned probe signal (transmitted by the DUT) and local oscillator signal radiation concurrently received thereby.

It is to be understood that the term "beam-splitting element" refers to a beam-splitting element being partially reflective and partially transmitted in such a way as to controllably and directionally split an incident THz radiation beam in a manner directly analogous to the splitting of an optical beam using a beam-splitting mirror of optical design. A wire-grid, of a design such as would be readily apparent to the skilled person, may be employed for this purpose. First focussing optics (560), second focussing optics (56), and third focussing optics (48) each comprising a suitable lens or lenses, are arranged to focus coincident local oscillator signal and probe signal radiation onto an input port of the first, third or fifth feed-horn, respectively. The output port of each of the first, third and fifth feed horns is coupled to an associated first, second or third mixing unit (36, 58, 50), respectively, comprising a Schottky diode in each case arranged to generate an electrical signal in proportion to the intensity of microwave (e.g. THz) radiation received thereby from the output port of the feed horn with which it is associated.

The structure of each of the first, third and fifth feed horns is such as to superimpose concurrently received local oscillator signal radiation and probe signal radiation, and to output the result at the output port thereof to which an associated mixing unit is coupled.

Thus, let the local oscillator signal received by either of the first, third and fifth feed horns be of the form:

E_w =E_oexv{iω_ωt} where ω_w=CO_ι—ω₂ is the local oscillator signal frequency, and E₀ is the amplitude of the local oscillator signal received at a given mixer unit. Let the concurrently received probe signal radiation is of the form:

where ω_Slg — CO₁- ω₂—δco is the probe signal frequency, and E₃ is the amplitude of that probe signal as received at the mixer unit (via the DUT or otherwise) . Then, the form of the superimposed signal output by the respective feed horn to an associated mixing unit, is:

Esφ∞mpovl = ^ES ∞PiiiPsv* + Φ)) + ^E0 «φ{»^flW}

where φ is a phase difference. The form of the electrical mixing signal is:

¹Mixer E_s exp{i(ω_Sιgt + φ)} + E₀

= E* +Eo + 2E₅E₀ cos(δωt + φ)

This form of mixer signal (37, 59, 51) is input to each of a first (38), second (60) and third (52) signal input ports of a common vector network analyser (VNA) . Each of the first, second and third signal input ports of the VNA is operably coupled to a respective first (36), second (58) or third (50) mixer unit, respectively. The common VNA is arranged to perform a superheterodyne detection operation, separately, upon each of the first (37) , second (59) and third (51) mixer signals to extract from the received mixer signal both the amplitude E_s and the phase φ of received probe signal radiation.

The first mixer signal (37) comprises a superposition of merely an "unused" probe signal radiation (probe beam reflection portion 34A) which has not interacted with the DUT, and a local oscillator signal (33) . The VNA is arranged to identify as deriving from the "incident signal" the amplitude B_s and the phase φ contained/mixed within the first mixer signal it is arranged to receive via its first signal input port (38) . Accordingly, the VNA is arranged to calculate characteristics and parameters (e.g. scattering matrix parameters, or the like) of the DUT by comparing the amplitude and phase of the "incident signal" with those of the retuned probe signal (i.e. reflected or transmitted by the DUT) as received at either or each of the second VNA signal input port (60) and the third VNA signal input port (52) . In this way the VNA is operable to determine or analyse properties of the transmission path along which the signal probe radiation has propagated before reaching the mixer unit in question - i.e. via the DUT. Each of the two constant terms E\ and E₀ ² may be removed, by a suitable filtering process, in the respective VNA leaving only the intermediate frequency term containing values of the received probe signal amplitude E_s and phase φ thereof, as unconstrained quantities determined by the properties of the DUT. The values of the difference frequency Sύ) and the local oscillator signal amplitude E₁₀ are predetermined by the signal generator (1) , and by knowledge of the optical properties of the first, second, third and fourth (44, 45, 55, 47) beam-splitting elements.

The purpose of the VNA port (38), the optical path which feeds it the "un-used" probe signal (34A) and local oscillator signal (32), is to provide a reference signal. The signals give a reference value which allows calculation of scattering parameters associated with scattered probe signal radiation returned from a device under test. Scattering parameters are to be calculated as the ratios of the power of returned (scattered) probe signal radiation to the power of the probe signal radiation input to the device under test. Reference signals as described above provide a value for the input power used in this ratio measure. Such basic methodology would be readily apparent to the skilled person and details will not be entered into here for the sake of conciseness. Accordingly, the signal generator (1) of figure 1 may provide a signal source suitable for vector network analysis by superheterodyne detection using local oscillator signals and probe signals of frequencies ranging from about 0.3 THz to about 3THz or more. The DUT may be a circuit component, such as a microwave circuit component, or may be any other "device" such as a sample of material suitable for analysis by THz microwaves . The device analyser of figure 2 may function as a sample analyser or spectrometer. The device analyser may output directly into a commercial VNA signal processing unit e.g. as currently available.

The invention may provide a Vector Network Analyser operating in the frequency range 0.3 - 3 THz, with < 1 GHz resolution and >50 dB dynamic range. This is a breakthrough in terms of frequency range. The invention may provide a

"work-horse" instrument with value for a wide variety of THz spectroscopy and ultrafast electronics applications. It may extend the range of impedance measuring up to 3 THz, and hence enable the characterisation of high-speed devices, circuits and specialised materials. Use is made of a novel scheme to generate and detect THz radiation for such purposes. The above examples are intended to be non-limiting and modifications or variants thereto, such as would be apparent to the skilled person are encompassed within the scope of the invention.

Claims

CLAIMS :

1. A signal generator including: an optical radiation means for generating a first optical radiation having a first frequency, a second optical radiation having a second frequency and a third optical radiation having a third frequency; a difference-frequency means for generating from a said first optical radiation and a said second optical radiation an electromagnetic local oscillator signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the second frequency, and for generating from a said first optical radiation and a said third optical radiation an electromagnetic probe signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the third frequency; output means for concurrently outputting from the signal generator a said local oscillator signal and a said probe signal for use.

2. A signal generator according to any preceding claim wherein the optical radiation means comprises first laser means for generating said first optical radiation, second laser means for generating said second optical radiation, and modulator means for modulating the second optical radiation to generate the third optical radiation from the result.

3. A signal generator according to claim 2 including phase control means arranged to control the first laser means and the second laser means to maintain a constant phase difference between the first optical radiation and the second optical radiation such that the local oscillator signal and the probe signal are phase-locked.

4. A signal generator according to claim 2 or claim 3 including frequency control means arranged to control the first laser means and the second laser means to maintain a constant difference between the value of the first frequency and the value of the second frequency such that a constant corresponding difference is maintained between the value of the local oscillator signal frequency and the probe signal frequency.

5. A signal generator according to claim 2 in which the modulator means includes phase modulator means arranged to receive second optical radiation from the second laser means, to modulate the phase of the received second optical radiation with a predetermined modulation frequency thereby to produce a side-band optical radiation and to output the result as said third optical radiation such that the value of the third frequency is equal to the sum of or the difference between the value of the modulation frequency and the value of the second frequency.

6. A signal generator according to any preceding claim in which the optical radiation means includes optical coupler means for combining a said first optical radiation and a said second optical radiation and co- propagating the combination to the difference- frequency means, and for combining a said first optical radiation and a said third optical radiation and co-propagating the combination to the difference- frequency means .

7. A signal generator according to claim 6 wherein the optical coupler means comprises a first optical input portion arranged for receiving a said first optical radiation, a second optical input portion arranged for receiving a said second optical radiation and a said third optical radiation concurrently, a first optical output portion arranged for receiving and combining said first optical radiation from the first optical input portion and said second optical radiation from the second optical input portion and for outputting the result to the difference-frequency means, and a second optical output portion arranged for receiving and combining said first optical radiation from the first optical input portion and said third optical radiation from the second optical input portion and for outputting the result to the difference-frequency means .

8. A signal generator according to claim 7 in which the optical coupler means includes first optical filter means placing the second optical input portion in optical communication with the first optical output port and arranged to prevent transmission of a said third optical radiation therethrough and to permit transmission of a said second optical radiation therethrough, and second optical filter means placing the second optical input portion in optical communication with the second optical output port and arranged to prevent transmission of a said second optical radiation therethrough and to permit transmission of a said third optical radiation therethrough.

9. A signal generator according to any preceding claim wherein the optical radiation means includes tuning means operable or arranged to controllably vary said first optical frequency and to maintain said second optical frequency at a constant value thereby to controllably vary or tune the frequency of the local oscillator signal and the frequency of the probe signal concurrently and by equal measure such that the difference between the two remain unchanged.

10. A signal generator according to any preceding claim in which the difference-frequency means includes a first difference-frequency generator arranged in optical communication with the optical radiation means to permit concurrent receipt therefrom of a said first and a second optical radiations and to generate the local oscillator signal therefrom by a difference- frequency operation, and a second difference-frequency generator arranged in optical communication with the optical radiation means to permit concurrent receipt therefrom a said first and third optical radiations and to generate the probe signal therefrom by a difference-frequency operation.

11. A signal generator according to claim 10 in which one or more of the first and second difference- frequency generators comprises a nonlinear optical crystal having a nonzero second-order nonlinear susceptibility.

12. A signal generator according to any preceding claim in which the optical radiation means is arranged to generate pulsed first optical radiation and pulsed second optical radiation, and synchrony means arranged or operable to synchronise the generation of said pulsed first and second optical radiations such that a local oscillator signal and a concurrent probe signal generated by the signal generator comprise mutually synchronised electromagnetic pulses.

13. A probe apparatus for a sample or device analyser for producing an electromagnetic local oscillator signal and an electromagnetic probe signal with which to analyse a sample or device and including a signal generator therefor according to any preceding claim, and mixer means for receiving a said local oscillator signal and a said probe signal output from the signal generator and for superimposing the received probe signal and received local oscillator signal to produce a mixer signal having a frequency substantially equal in value to the difference between the respective values of the second frequency and the third frequency.

14. A probe apparatus according to claim 13 including an input port means arranged to receive a said probe signal from the signal generator and operable to direct such probe signal to a sample or device under analysis, and an output port means operable to receive a return probe signal originating from the signal generator and returned from the sample or device under analysis and to output the return probe signal for subsequent analysis.

15. A device analyser including probe apparatus of any of claims 13 or 14 and including a heterodyne detector operably coupled to the mixer means to receive a mixer signal produced by the mixer means and therewith to perform a heterodyne operation on the mixer signal to produce an intermediate frequency signal therewith to determine the amplitude and/or the phase of a probe signal originating from the signal generator.

16. A method for signal generation including: generating a first optical radiation having a first frequency, a second optical radiation having a second frequency and a third optical radiation having a third frequency; generating from a said first optical radiation and a said second optical radiation an electromagnetic local oscillator signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the second frequency, and generating from a said first optical radiation and a said third optical radiation an electromagnetic probe signal having a frequency substantially equal in value to the difference between the respective values of the first frequency and the third frequency; concurrently outputting from the signal generator a said local oscillator signal and a said probe signal for use.

17. A method according to claim 16 including generating said first optical radiation as laser radiation, generating said second optical radiation as laser radiation, and modulating the second optical radiation to generate the third optical radiation from the result.

18. A method according to claim 17 including maintaining a constant phase difference between the first optical radiation and the second optical radiation such that the local oscillator signal and the probe signal are phase-locked.

19. A method according to claim 17 or claim 18 including maintaining a constant difference between the value of the first frequency and the value of the second frequency such that a constant corresponding difference is maintained between the value of the local oscillator signal frequency and the probe signal frequency.

20. A method according to claim 17 including modulating the phase of second optical radiation with a predetermined modulation frequency thereby to produce a side-band optical radiation and outputting the result as said third optical radiation such that the value of the third frequency is equal to the sum of or the difference between the value of the modulation frequency and the value of the second frequency.

21. A method according to any of claims 16 to 20 including combining a said first optical radiation and a said second optical radiation applying a difference- frequency operation to the combination to generate the local oscillator signal, and combining a said first optical radiation and a said third optical radiation and applying a difference-frequency operation to the ■ combination to generate the probe signal.

22. A method according to any of claims 16 to 21 including varying said first optical frequency and maintaining said second optical frequency at a constant value thereby to vary or tune the frequency of the local oscillator signal and the frequency of the probe signal concurrently and by equal measure such that the difference between the two remain unchanged.

23. A method according to any of claims 16 to 22 including providing a first difference-frequency generator and concurrently receiving thereat a said first and a second optical radiations and generating the local oscillator signal therefrom by a difference- frequency operation, and providing a second difference-frequency generator and concurrently receiving therefrom a said first and third optical radiations and generating the probe signal therefrom by a difference-frequency operation.

24. A method according to claim 22 in which one or more of the first and second difference-frequency generators comprises a nonlinear optical crystal having a nonzero second-order nonlinear susceptibility.

25. A method according to any of claims 16 to 24 including generating pulsed first optical radiation and pulsed second optical radiation, and synchronising the generation of said pulsed first and second optical radiations such that a local oscillator signal and a concurrent probe signal comprise mutually synchronised electromagnetic pulses.

26. A method for analysing a sample or device including producing an electromagnetic local oscillator signal and an electromagnetic probe signal with which to analyse a sample or device according to any of claims 16 to 25, and receiving a said local oscillator signal and a said probe signal so generated and superimposing the received probe signal and received local oscillator signal to produce a mixed signal having a frequency substantially equal in value to the difference between the respective values of the second frequency and the third frequency.

27. A method according to claim 26 including receiving a said probe signal so generated directing such probe signal to a sample or device under analysis, receiving a return probe signal returned from the sample or device under analysis and outputting the return probe signal for subsequent analysis .

28. A method for analysing a device analyser including generating a probe signal according to any of claims 26 or 27, providing a heterodyne detector, receiving said mixed signal at the heterodyne detector and therewith performing a heterodyne operation on the mixed signal to produce an intermediate frequency signal therewith to determine the amplitude and/or the phase of a probe signal originating from the signal generator.

29. A method substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings .

30. Α signal generator substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings .

31. A probe apparatus substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings .

32. A device analyser substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings .