US 20060238866 A1
A method and an optical device in all-optical signal processing. It provides a novel way of taking into use the potential bandpass filtering capabilities of an optical complex one-pole resonator by, 1) excitating with at least part of the input signal an optical resonator arrangement that comprises two substantially parallel, independent, complex one-pole resonators arranged in a manner that one of the resonators is matched, and the other one is non-matched with the input signal, and 2) based on polarization separating further from the output of the optical resonator arrangement at least one optical output signal so that both the matched and non-matched resonators contribute to the formation of the output signal. The method and device can be utilized, for example, for optical signal analysis, optical clock recovery, or for producing high-frequency outputs from lower frequency input signals, like e.g. optical microwave generation.
1. A method in optical signal processing of an optical input signal, wherein the processing of said input signal comprises:
excitating with at least part of said input signal an optical resonator arrangement that comprises at least two substantially parallel, independent, complex one-pole resonators arranged in a manner that the first one of said resonators is matched with the wavelength of the input signal and the second one is non-matched with the wavelength of the input signal, and
forming at least one optical output signal so that both said first and second resonators of said optical resonator arrangement contribute to the formation of said output signal.
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10. An optical device for optical signal processing of an optical input signal, wherein for the processing of said input signal the device comprises:
an optical resonator arrangement excitated with at least part of said input signal and comprising at least two, substantially parallel, independent, complex one-pole resonators arranged in a manner that the first one of said resonators is matched with the wavelength of the input signal and the second one is non-matched with the wavelength of the input signal, and
combining means to combine the outputs.
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The present invention relates to a method in optical signal processing and to an optical device for carrying out said method. More particularly, the invention relates to all-optical filters based on an optical resonator arrangement, which has a capacity to store temporally the electromagnetic energy of the incoming light and therefore has some memory about its past so that the filter can stay on a desired state for some time regardless of fast perturbations on the optical input signal. Such a filter may be referred to as a “slow” all-optical filter. The invention can be utilized, for example, for optical signal analysis, optical clock recovery, or for producing high-frequency outputs from lower frequency input signals, like e.g. optical microwave generation.
The frequency of light is typically understood as the inverse of the wavelength of the electromagnetic field, i.e. ν=c/(nλ), where ν is the frequency, c is the speed of light in vacuum, n is the index of refraction of the medium and λ is the wavelength. In the context of this application, the word frequency, however, addresses to the rate of temporal, periodic intensity change of the light (e.g. a pulse frequency), and is therefore not connected to the wavelength. To highlight this conceptual difference, this rate of intensity change is called hereafter simply the frequency f, and the frequency ν as the fundamental physical property of the electromagnetic field is discussed in terms of wavelength λ. In other words, frequency f relates hereafter to temporal pulses of light, which pulses are formed from light having wavelength λ, and frequency ν corresponding to said wavelength.
Due to historical reasons optical resonators are called sometimes as optical cavities. These two expressions are used interchangeably throughout the document.
Electrical Signal Processing
Bandpass filtering is traditionally made with electrical means and optical signals processed by conventional methods require therefore optical-to-electrical conversion before the filtering can be performed. If the signal is transmitted further by optical means, one has to convert the filtered signal back to optical domain. Conversion to and from electrical domain is non-trivial with current state-of-the-art whenever signal frequencies go above 40 GHz. The conversions forth and back generate also additional cost and possibly degrade optical signal quality. For example patent U.S. Pat. No. 4,737,970 discloses a clock recovery device utilizing a cavity resonator in electrical domain. The present invention aims to all-optical conversion requiring thus no conversions to and from electrical domain and eliminating therefore any problems related to said conversions.
All-Optical Signal Processing
All-optical signal processing traditionally deals with different wavelengths of light, where e.g. different channels in WDM (Wavelength Division Multiplexing) system are separated from each other using optical filters having different wavelength bands.
Optical processing based on temporal input signal frequencies, not wavelengths of light, has been done far less, but is also known from prior-art. Ring-resonators have been used for construction of all-pass optical filters (Azana and Chen, IEEE Photonic Technology Letters, 14, 2002) and together with Mach-Zehnder interferometers they typically are considered as basic building blocks of all-optical filters. Cascaded ring-resonators and Mach-Zehnder interferometers can be used to perform a Fourier-transform and other optical functions. Optical resonators have been used for all-optical signal processing (Lenz et al, IEEE J. Quantum Electron., 34, August 1998) and clock recovery (Jinno and Matsumoto, IEEE J. Quantum Electron., 4, April 1992). The clock recovery made by Jinno was based on simple optical resonator, whose optical length was carefully matched with: 1) wavelength of the incoming light and 2) data frequency. The device is, however, very limited in its capabilities to process more than one wavelength (or WDM channel) at time.
Some other all-optical filtering techniques are presented in the following patent publications. U.S. Pat. No. 5,446,573 discloses an all-optical regenerator that is based on the use of a non-linear ring resonator. U.S. Pat. No. 6,028,687 discloses a laser arrangement between two resonator mirrors for recovering a clock from a modulated optical input signal. Here, bi-directional mux/demux components have been arranged within the resonator. U.S. Pat. No. 6,388,753 discloses an all-optical bit phase sensor utilizing non-linear interferometers, where the refractive index of the non-linear material is varied.
Basically and generally expressed, the present invention relies on a method, where the processing of an optical input signal includes at least the steps of: 1) excitating with at least part of said input signal an optical resonator arrangement that comprises two substantially parallel, independent, complex one-pole resonators arranged in a manner that that one of said resonators is matched with the input signal and the other one is non-matched with the input signal, and 2) based on polarization separating further from the output of said optical resonator arrangement at least one optical output signal so that both said matched and non-matched resonators of said optical resonator arrangement contribute to the formation of said linearly polarized output signal.
The method may be carried out in practise, for example, by using a single optically birefringent resonator, whose output is further directed to a suitably located polarization selective device, that selects at least one direction of polarization from the output said birefringent resonator. In such an embodiment, the matched and non-matched resonators become formed inside said resonator arrangement due to the birefringency as two virtually independent resonators.
As will be shown later, the matched and non-matched resonators together with the polarization separating or selective device or means may be realized in several ways and using various types of optical elements. All the optical devices carrying out the method generate different types of bandpass filtering of the incoming optical signal and provide thus useful tools for various all-optical signal-processing applications.
The significant benefits of the invention include, for example, the fact that the present invention can be used to realize optical devices that process multiple wavelength channels simultaneously, that is to say in parallel. The invention requires that the optical length of one polarization mode of the resonator arrangement is matched with the wavelength of the incoming light, but it has no temporal signal length requirements, i.e. requirements for the duration regarding the rate of the incoming optical data. The data rate is matched by selecting, for example, suitable amount of birefringence into the resonator. The method of the invention thus enables parallel processing of multiple wavelengths simultaneously, provided that the resonator length is matched to all used wavelengths and the birefringence is suitable for the data rate.
One promising application of the invention can be found in all-optical clock recovery. However, the parallelism mentioned above holds true for all applications, provided that different wavelengths can later be separated from each other.
The most profound benefits of the invention vary according to the specific applications. When used for all-optical clock recovery the method liberates from the dependency between the resonator length (WDM channel separation) and the data rate. This freedom opens opportunity to build truly parallel systems in practise. When compared to U.S. Pat. No. 5,446,573, U.S. Pat. No. 6,028,687, or U.S. Pat. No. 6,338,753, the present invention requires smaller number of optical elements and gives thus simplicity of construction and savings in cost. In particular, the present invention does not necessarily require the use of any optically active medium (semiconductor-optical amplifier or laser source) to provide adequate signal levels and operation. In its simplest form, the present invention requires only the use of one birefringent resonator arrangement together with one polarization selective element. For example, U.S. Pat. No. 5,446,573 requires a non-linear ring, which consists of two laser sources, and four phase-modulators. U.S. Pat. No. 6,028,687 requires an optical mux/demux element and at least two semiconductor optical amplifiers within an optical resonator.
U.S. Pat. No. 6,338,753 requires non-linear interferometer, whose refractive index is varied with external optical pulses, which in practice means a sufficient high-energy laser build into a working system. More details of the present invention are set forth in the foregoing description and in the accompanying drawings describing selected embodiments of the invention. The preferred embodiments and possible variants of the invention will become more apparent to a person skilled in the art also through the appended claims.
It should be understood that in the following the purpose of the theoretical and mathematical descriptions is merely to function as tools for describing the invention better for a person skilled in the art. Therefore, even though the theory might not be completely immaculate in all respects, it nevertheless represents the Applicant's best understanding of the phenomena underlying the invention.
In the accompanying drawings:
In the following the invention is mainly described referring to a birefringent resonator arrangement, where the matched and non-matched resonators are formed within a single physical cavity due to the birefringency. However, the invention is not limited to such embodiments, but the invention may also be realized using separate matched and non-matched resonators. One example of such an embodiment is later given in
In the FIGS. 1,16,19 and 20 the polarization axes are depicted with “crossed” markings using both solid and dashed lines.
It is known from prior-art, as such, that an optical resonator behaves as an one-pole complex filter. Complex one-pole resonators generally, working either in optical, digital-electronic etc. domain, are able to transform purely real input signal to a complex one. Only when the wavelength of excitation is matched with the resonator, it has lowpass filtering function with no imaginary parts and therefore, it can directly be used for signal processing purposes. Alternatively, if the wavelength and rate of data both are matched, then it operates as a bandpass filter. However, when the wavelength of excitation does not match with the resonator, it always behaves as a complex bandpass filter. Although real and imaginary parts both pass only a narrow band of signal spectrum with equal gain, they differ π/2 in phase. While the optical output is always measured as the square of amplitude (i.e. intensity) and because of the quadratic phase-shift between real and imaginary parts of the signal, the output is flattened out and the resonator cannot be used directly as a bandpass filter (later this will be shown graphically).
If one could separate real and imaginary parts from each other, or suppress one of them, the resonator bandpass characteristics could effectively be utilized also in case of wavelength mismatch. The current invention provides a novel and inventive way of doing this.
The key of the invention lies in the novel way of taking into use the potential bandpass filtering capabilities of an optical complex one-pole resonator by, 1) excitating with at least part of the input signal an optical resonator arrangement that comprises two substantially parallel, independent, complex one-pole resonators arranged in a manner that one of said resonators is matched with the input signal and the other one is non-matched with the input signal, and 2) based on polarization separating further from the output of said optical resonator arrangement at least one optical output signal so that both said matched and non-matched resonators of said optical resonator arrangement contribute to the formation of said output signal.
As will be shown later, such an arrangement, that may be realized for example by using a birefringent resonator combined with a polarization beam splitter, will be able to amplify the real part of the mismatched optical field and suppress the imaginary part. Therefore, the output of such resonator-polarizer setup can be made highly dependent on the filtering properties of the complex one-pole resonator.
In the following some basic principles of optical resonators are first discussed. Real and imaginary part transfer functions of a complex one-pole resonator are presented and an intensity output of the resonator is presented. Especially, the consequence of the birefringence of the resonator OR and the effect of the inclusion of a polarization beam splitter PBS at the resonator output are explained. Finally, selected examples of systems according to the invention, and using various optical excitation forms are presented together with some applications.
Optical Resonator and Phasors
Amplitude and phase of an electric field inside an optical resonator can be presented with phasors as depicted in
When phase is matched (φ=0), the energy starts to build up within the resonator and the net electric field has no imaginary part. Now the resonator behaves as a low-pass filter and has time constant τ=l(−cln r) for light intensity, where c is the speed of light within the medium, l is the one-pass length of a Fabry-Perot resonator and r is one-pass loss factor of light intensity within the resonator.
When phase is not matched, the electric field becomes complex and the phasor draws a converging spiral loop. As visible in
Impulse response of the resonator is
Filtering property of the resonator is now shown in time domain for a periodic input. Let us assume a train of pulses with period T=188.5 as shown in
Optical Resonator Birefringence and Polarization Beam Splitter
When an optical resonator OR is birefringent, the path lengths inside the resonator are uneven for orthogonal polarization components. When this type of resonator is combined with a polarization beam splitter PBS and circulating electric field has phase-matched and non-phase-matched components, the imaginary part of the electric field component can be suppressed. In other words, according to the present invention the system output can be made strongly dependent on the real part electric field of the mismatched resonator and such system behaves as an optical signal bandpass filter.
Let's define x-axis of the resonator phase-matched and y-axis non-phase-matched, the amount of mismatch symbolized by φ. Intracavity electric fields are
We assume that the incoming field has angle α with respect to the axis of birefringence of the resonator OR and the polarization beam splitter PBS divides the outcoming electric field according to angle θ, as shown in
In order to show the effect, the same pulse excitation than in previous case is used with a resonator, where x-axis is matched (φ=0), y-axis is non-phase-matched (φ=0.1), reflectivity is r=0.999, angle of input polarization is α=45° and angle of polarization beam splitter PBS is θ=45°. In the upper part of
The operation can be proved also for other type of signals. A train of Gaussian pulses with period T=125.7, as depicted in
Applications, All-Optical Multiwavelength Clock Recovery
One interesting application of the invention can be found in all-optical clock recovery. When compared to prior art solutions to make all-optical clock recovery by use of Fabry-Perot resonator, the arrangement of the invention is superior by its ability to operate simultaneously at multiple wavelengths. A burst of data (half of bits randomly shut off) with clock period T=62.8 is fed into the resonator OR as shown in
In the following an example calculation of making filtering at several wavelengths simultaneously is given.
We shall assume a set DWDM channels (Δν=100 GHz) operating at λ=1550 nm wavelength region. Data rate at each channel is f=40 GHz. A resonator medium has refractive index n=1.5. In following we will determine the required Fabry-Perot resonator length l, degree of birefringence Δn, and one-pass intensity loss factor r for simultaneous operation of the system at multiple wavelengths.
Resonator x-modes are set to coincide with channel separation of the DWDM system, i.e., Δν=c/(2nl), which gives the resonator length l=1 mm. The DWDM channels would also be matched with resonator modes when the length is integer multiple of l, as clear for the persion skilled in the art.
The filtered frequency is dependent on the difference of x- and y-mode refractive indices
As present from Eq. 8 the filtered frequency is dependent on the wavelength of light. While different channels have slightly different wavelength, the filtered frequency is also changed. The difference in filtered signal frequency can be expressed in function of the wavelength change
The channel separation can be expressed in terms of wavelength dλ=−λ2dν/c=−0.8 nm, which translates to shift of filtering frequency according to following Table 1.
Passband of a Fabry-Perot resonator is defined as
The filter operates for those DWDM channels where the filtering frequency f shifts stays within the passband of the resonator. This happens when df<0.5 δν.
The passbands and the number of possible simultaneously processed channels for various loss factors r are presented below in Table 2.
As can be seen, the resonator is capable of processing smaller number of channels the lower the loss factor (higher the mirror reflectivity) is. High loss factor (low mirror reflectivity) gives larger number of channels to be processed, but with lower capability for recovery (the energy storage of the resonator is lossy). One has to make a tradeoff between the number of processed channels and the capability to recover the clock signal.
The situation can be compensated by suitable control of the wavelength dependence of the birefringence Δn(λ).
One possible setup contains, as depicted schematically in
The first polarization controller PC1 sets the state-of-polarization of incoming light such that the angle of polarization (azimuth) differs from the axis of birefringence of the resonator OR. Should they completely coincide, no oscillation may be observed at the output while all energy is directed either on matched or non-matched resonator mode.
In this embodiment, the fiber optical resonator OR is constructed by coating cleaved or polished fiber end with high reflectivity dielectric coating. The fiber length is chosen such that free spectral range (FSR) is matched with the channel spacing of the incoming light. The optical length of the resonator is regulated in order to keep the x-mode optical length matched with channels of the incoming light. In the context of this invention the aforementioned matching is shortly referred to as mode-locking and in addition to a fiber resonator it applies also to other type of resonators. Instead, or in addition to adjusting the length of the resonator, mode-locking may also be achieved by tuning the wavelength of the incoming light. The resonator has preferably low internal loss and has high reflectivity dielectric mirrors. The degree of birefringence, and thus the angular frequency of the mismatch, is adjusted by twisting the optical fiber constituting the resonator. The amount of angular frequency mismatch should be equal with the angular frequency of the incoming data.
Second polarization controller PC2 sets the state-of-polarization of the resonator OR output such that the polarization beam splitter PBS divides signals suitably between the two output arms Bout1, Bout2.
If the processed signal has multiple wavelengths (channels), each channel should have data frequency matched with the degree of angular mismatch from channel to channel.
The phase may differ. Later one may want to separate channels from each other, but this speculation and related techniques are beyond the scope of the present invention.
In the following certain variations and alternatives for the different parts of the setup carrying out the invention have been listed. These apply to the clock recovery application already explained, but are also applicable, mutatis mutandis, to other applications and optical setups described in this text. Further, one should not consider the invention to be limited even by these variations and alternatives, but the invention should be restricted only in the manner indicated by the scope of the claims appended hereto.
The resonator OR does not need to be of a Fabry-Perot type, but it could also be, for example, a fiber loop resonator (preferably with weak input and output couplings). It may also be a micro-ring, -sphere, -toroid or a photonic bandgap resonator. In such “non-Fabry-Perot” loop or ring type resonators the optical medium is arranged to form at least partly closed optical circle where the light travels substantially only one common direction without multiple back-and-forward reflections.
The cavity medium of the resonator OR may constitute of a normal single mode fiber or polarization maintaining fiber. However, the setup may also be build completely without any fiber optic components. The required optical functions can be achieved with free-space optics or integrated optics. Therefore, the resonator OR could be, for example, a coated dielectric rod, a pair of free-space dielectric mirrors or even a semiconductor device. The cavity medium may be simply air or other transparent gas or liquid instead other optically transparent, dielectric or semiconductor materials. It should be noted, that the invention is not limited only to visible wavelengths, but it can be used broadly with any optical wavelengths that may be shorter or longer than visible wavelengths. It is also obvious for a person skilled in the art that certain coherence of the inputted light is required.
The “mirrors” of the resonator OR may be, for example, dielectric stack mirrors, metallic mirrors, fiber loop mirrors, fiber couplers (not really a mirror, but input port with weak efficiency) or different type of waveguide couplers. Further, said mirrors may have different reflectivities for x- and y-directions. High reflectivity mirrors are typically desired.
Light is necessarily not coupled into the resonator OR through the aforementioned mirrors, but evanescent coupling, or any other optical coupling methods known in the art may also be used. Input coupling may be realized, for example, with prisms arranged in planar waveguides.
The degree of birefringence in the resonator OR can be adjusted with not only twist, but also by stress, tension, bend, or other structural deformations of the cavity medium.
Temperature and electro-optical effects may also be used to affect the birefringence of the resonator depending on the cavity medium/material. For example, if the cavity medium is gas or vacuum, then the birefringency of the resonator may be altered by deformation of the end mirrors or corresponding cavity forming reflectors.
As already mentioned, the resonator arrangement does not need to based on a birefringent cavity, but instead it may also be formed from two physically separate resonators one matched with the incoming light and the other one non-matched with the incoming light. The polarization state of the incoming light is required to be such that both of these resonators participate in the formation of the final output signal available after the polarization separating means.
Polarization controllers PC1,PC2 before and after a resonator OR, as shown schematically in
Polarization separating means may be a beam splitter PBS or any optical component or set of components known in the art, which selects or separates at least one polarization component from the inputted light. It is not even necessary to divide the polarization components of the light into different optical arms, but one of these components may be absorbed or otherwise dumped during the separation process. Possible devices include, but are not limited to the following: Wollaston prism, Glan-Foucault polarizer, Nicol prism, Rochon prism, dielectric coating polarizer, wire grid polarizer, polymer based film polarizer, single polarization mode transmitting fiber, photonic crystal polarization separator. The polarization separation means do not need to be comprised in a separate optical component, but it/they can be, for example, integrated directly on the output side of the resonator arrangement.
In case when the birefringence of the resonator OR is wavelength dependent, then the extracted frequency may vary from inputted channel to channel, i.e. data rate is xx GHz for inputted channel one, yy GHz for channel two etc.
The mode locking may be active or passive. For example, if the light source is in vicinity of the resonator OR, the out-leaking light from resonator OR may be used to feed some light back to the coherent light source (typically laser). For the correct operation of the setup, however, the resonator OR is typically required to be substantially mode-locked with the incoming light/light source.
The excitation of the system according to the invention may be due to change of the incoming light intensity, change of its polarization, change of the resonator length (so that x-axis also becomes non-phase-matched), or change of the resonator medium refraction index. The excitation may be generated by the light source, an external modulator (modulating light intensity or polarization), or coupling strength of the input port of the resonator. Also the input port coupling strength may be modulated. Excitation forms may be periodic: train of square pulses, Gaussian pulses, solitons (sech), or non-periodic: stream of data . Other type of input signals may also be used. Therefore, the excitation may be due to the properties of the incoming light itself, or the excitation may also be accomplished by altering the properties of the resonator arrangement.
Applications, Signal Frequency Component Analysis The setup described schematically in
Applications, Higher Harmonic Signal Generation
The setup of the previous cases explained abobe (
Applications, Measurement of Birefringence
The filtering property of a resonator system according to the invention is dependent on the degree of birefringence in the optical resonator OR . This property can be used for the measurement of minute birefringence in the following way.
When the system is excited with an optical signal Bin, which includes multiple ranges of frequencies, or continuum of frequencies, the change of filtering frequency can be determined from frequency output oscillations. An example of a signal with continuum of frequencies is a step-function, and a signal with multiple frequencies is, for example, a periodic sinc signal.
A system with parameter values: r=0.99 and θ=75°, is subject to the change of birefringence. This system is excited with an inputted step-function. The system intensity output Bout1 from arm 1 is depicted in
If we know the initial state of the studied optical system, i.e., its birefringence, the change of oscillation frequency is an indication of the change of birefringence. While the oscillation will dampen out on course of time, the excitation should be repeated at some time interval.
An alternative way to measure birefringence is to excite the system with a periodic signal, whose fundamental frequency, or some of its harmonics, coinside with the filtering frequency of the resonator system. The excitation frequency is actively tracking the filter transmission maximum by, e.g., using a voltage controlled oscillator. If the filter output amplitude decreases, the excitation frequency is adjusted such that the output amplitude reaches again its maximum. Thus, the frequency of excitation is an indication of the birefringence of the optical system.
Applications, Miniaturized Embodiment
The invention may also be realized as a miniaturized optical arrangement described schematically in
Other Applications and Further Variations
The invention can be used for wide variety of applications related to all-optical filtering. Because of the auto-regression nature , i.e. “memory” of the filter, filtering does not however apply for very fast changes. For certain applications, this may be a desired feature instead of a limitation.
Again, it needs to be emphasized, that while the invention has been shown and described here with respect to a few selected embodiments, it should be understood that these embodiments are only examples, and that a person skilled in the art could construct other embodiments utilizing technical details other than those specifically disclosed herein while still remaining within the spirit and scope of the present invention. It should therefore be understood that various omissions and substitutions and changes in the optical design of the resonator arrangement, polarization selective element and related optical components coupling light in and out from said components, as well as in the mutual organisation and operation of the same, may be made by those skilled in the art without departing from the spirit of the invention.
For example, according to the invention it is also possible to arrange a first polarization beam splitter PBS1 in the beginning of the setup, then two resonators OR1 and OR2 (one matched and other non-matched), whose outputs are further combined with another polarization beam splitter PBS2. This embodiment is schematically shown in
The setup according to the invention has been shown in the above examples to operate in transmission. However, it should be obvious for a person skilled in the art that embodiments also operating in reflection are possible. The resonator arrangement OR may thus be realized in such a way that the light inputted in the resonator through a port (typically a mirror) outputs from the same port. In the above given examples the resonator has been arranged with separate ports for input and output.
As a general way of optimizing the performance of the optical setup according to the invention, the angle of the linear polarization of the incoming light may be adjusted respect to the polarization axes of the resonator arrangement so that the oscillations at the output are maximized. Similarly, the angle of the polarization separating element after the resonator arrangement may be adjusted for the same purpose.
A light amplifying optical element or light amplifying means may be arranged in front of the resonator arrangement, after it or within it. Preferably, such an element is arranged between the resonator and the polarization separating means. Suitable light amplifying elements include, for example, active optical fibers and other active waveguides, which are typically based on the use of rare-earth element doped materials. Another possibility is the use of various semiconductor optical amplifier (SOA) devices as shown in