|Publication number||US20050047702 A1|
|Application number||US 10/928,428|
|Publication date||Mar 3, 2005|
|Filing date||Aug 27, 2004|
|Priority date||Aug 27, 2003|
|Also published as||EP1517173A1, EP1517173B1, US6856737, US20050047739|
|Publication number||10928428, 928428, US 2005/0047702 A1, US 2005/047702 A1, US 20050047702 A1, US 20050047702A1, US 2005047702 A1, US 2005047702A1, US-A1-20050047702, US-A1-2005047702, US2005/0047702A1, US2005/047702A1, US20050047702 A1, US20050047702A1, US2005047702 A1, US2005047702A1|
|Inventors||Greg Parker, Jeremy Baumberg, James Wilkinson, Martin Charlton, Majd Zoorob, Maria Netti, Nicolas Perney, John Lincoln|
|Original Assignee||Mesophotonics Limited|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Referenced by (14), Classifications (9), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to nonlinear optical devices and in particular to a planar waveguide device for nonlinear optical signal generation with large accessible bandwidth, including optical continuum generation.
Optical sources which can generate radiation over a wide wavelength range currently have many applications in scientific research, engineering and medicine. Often the application requires the coherence properties associated with laser radiation and so various laser materials have been developed which exhibit a broad gain bandwidth when suitably pumped. A good example is titanium doped sapphire, which is characterized by a gain bandwidth covering the range 650-1100 nm. The coherence properties of lasers based on broadband material systems may be further enhanced in a number of ways. The technique of modelocking permits the utilization of much of the available laser bandwidth to obtain a repetitive train of short (or ultra short) pulses. Alternatively, the laser cavity may contain frequency selective elements to ensure that the laser only emits radiation with a relatively narrow spectrum centered around a particular wavelength. By adjusting a frequency selective element this wavelength may be tuned across the available gain bandwidth.
However, as many laser sources only operate over narrower, well-defined wavelength ranges, nonlinear optical processes have been employed to generate other wavelengths using the output from available laser sources. A wide range of nonlinear optical processes are known, with the common factor being a nonlinear dependence of the electric polarization that is induced in the nonlinear material on the electric field (or intensity) of an optical input, resulting in the generated optical output. The order of nonlinearity relates to the specific integer power of the electric field on which the induced polarization depends. Second order effects include second harmonic generation, which is commonly used to “frequency double” the output from a laser source, and also two-wave mixing. Third order effects are more numerous and include third harmonic generation, four-wave mixing, self-phase modulation, self-focussing and Raman scattering.
As indicated, non-linear optical devices have the advantage that they can be “bolted on” to the output of existing laser sources in order to extend the available wavelength range or simply to generate nonlinearly another well-defined wavelength from the laser radiation. The strength of the nonlinear effect is generally determined by the relevant non-linear coefficient of the material and the peak intensity of the input (pump) beam inside the material. However, other factors such as interaction length and accurate phase-matching can be very important in maximizing the efficiency of conversion in the non-linear interaction. Nonlinear processes and materials capable of generating radiation over a wide wavelength range from a relatively narrow band optical input are of particular interest.
One example is optical continuum generation (CG), whereby a cascade of (generally) third order processes enables the generation of an optical signal with a continuous, or near continuous, spectrum over a very broad bandwidth. The continuum may share a number of properties with laser light, including spatial coherence. However, the broad bandwidth means the continuum is of lower temporal coherence, which make it an attractive source for some applications, such as low coherence interferometry. The continuum generated can be used in its entirety or optically filtered or sliced as the application requires.
Another example are the optical parametric processes, where one or more optical signals that are tunable over a wide bandwidth are generated from an input pump at a fixed wavelength. In particular, the optical parametric amplifier (OPA) provides parametric amplification at two tunable wavelengths (the signal and idler), whereas the optical parametric oscillator (OPO) employs (tunable) optical feedback at one, or both, of these wavelengths to achieve self-oscillation.
A variety of techniques and materials have been investigated for enhancing the bandwidth that can be accessed by the nonlinear processes described above, including CG and optical parametric devices. However, as will now be described, all of these have their attendant drawbacks.
Optical CG has typically been performed in bulk materials, both liquid and gas, due to the simplicity of implementation and the relatively small sample size required. However, due the low nonlinear coefficient associated with many materials, the characteristic threshold intensity is high and so a high peak intensity laser source is required. This usually takes the form of a modelocked laser system generating very short (or ultrashort) pulses which are then amplified and the radiation focussed tightly onto the target material. Consequently, there is a high attendant risk of surface or bulk damage to the sample unless it is a material exhibiting a high damage threshold such as sapphire, which also exhibits good stability of CG.
One approach to reducing the threshold pulse energy required has been the utilization of optical fibers for CG. Despite a relatively low nonlinear coefficient for the fiber material, the lateral optical confinement ensures that an adequate optical intensity can be maintained throughout a long interaction length of fiber for efficient CG. Nevertheless, the associated pulse energy damage threshold is also reduced and so end facet damage may occur, requiring the cleaving of a new facet or provision of a new fiber entirely. In addition, the stability of CG in optical fibers is typically low, the overall size may limit the compactness of the source and the optical mode properties are not easily compatible with the planar waveguide devices used in photonic integrated circuits. A further problem is maintaining the polarization (electric field orientation) of the light generated, which affects its usefulness in applications.
Another issue associated with CG is the characteristic optical dispersion (variation of refractive index with wavelength) of the device in which the continuum is generated. It is known that the threshold for CG is lowered when the pump is at a wavelength where the dispersion of the device is near zero. Furthermore, due to the proximity of the anomalous dispersion region, more nonlinear processes may be accessed and bandwidth may more easily be generated beyond the zero dispersion wavelength, extending the spectrum further into the infrared. In the case of bulk materials, the characteristic dispersion is simply the material dispersion, which usually lies within the normal dispersion region at the optical wavelengths of interest. But for waveguides, the situation is more complex, with the total dispersion also depending on waveguide and modal dispersion. This provides scope for controlling the total dispersion via the waveguide design parameters. To this end, zero dispersion fibers and tapered fibers have been manufactured.
A further development in the control of fiber dispersion has been the fabrication of the so-called microstructured fiber (MF) or photonic crystal fiber (PCF). These fibers consist of a solid silica core surrounded by an array of air holes running along the fiber, which provides a wavelength-dependent effective index for the cladding and can allow single-mode guidance throughout the visible and near infra-red. By suitable choice of arrangement and size of holes, the dispersion properties of the fiber can be tailored, as can the effective area of the propagating mode. Such dispersion engineered fibers have been used to enhance the continuum bandwidth that can be generated from a short pulse pump, resulting in so-called supercontinuum generation.
Here, bandwidth in excess of 800 nm has been generated as a result of a cascade of processes, including self-phase modulation (SPM), four-wave mixing (FWM), Raman scattering (RS), soliton formation and decay, soliton self-frequency shifting (SSFS) and self-steepening (SS). However, despite the improvement in CG bandwidth, the microstructured fibers still suffer from the drawbacks associated with more conventional fibers, as outlined above. In addition, there are many materials that can not be fabricated in bulk or fiber form and are therefore unavailable for CG in these configurations.
Another approach for enhancing a nonlinear phenomenon has been the employment of a planar chalcogenide glass (ChG) waveguide. Here, the intention was to enhance the level of nonlinear phase shift that could be obtained via SPM for a given pump pulse energy, a key application being optical switching for optical communication systems. A thin film of high refractive index GeSe-based glass material formed the core of a planar waveguide that was subjected to pump pulses from an amplified mode-locked fiber ring laser. A maximum peak phase shift of 1.6π was recorded for an input pulse energy of 461 pJ. However, although the process was accompanied by nonlinear spectral broadening, which resulted in an optical output having a bandwidth broader than that of the input pulse, the degree of spectral broadening was not sufficient to generate an optical continuum.
Planar waveguides have also been employed for enhanced performance in parametric devices, such as the optical parametric oscillator (OPO) and optical parametric amplifier (OPA). Typical devices comprise a layer of periodically poled material such as Lithium Niobate (Li2O3), also known as PPLN. Although improved performance is obtained in terms of threshold power and conversion efficiency, the available tuning range is still limited.
According to a first aspect of the present invention there is provided a non-linear optical device comprising a planar optical waveguide, at least a section of the planar optical waveguide being operative to generate an optical output from at least a portion of an optical input having an input bandwidth by means of a non-linear optical process, the optical output having a wavelength within an accessible bandwidth, wherein the planar optical waveguide is operative to enhance the accessible bandwidth such that the ratio of the accessible bandwidth to the input bandwidth is at least 4.
However, it is preferred that the ratio of the accessible bandwidth to the input bandwidth is at least 10.
Here, the term “bandwidth” is defined as the wavelength interval beyond which the spectral radiant intensity remains below a level of −30 decibels (0.001) of the maximum value. The logarithmic definition is appropriate due to the widely differing spectral intensity of the different wavelengths that may be generated by the nonlinear optical process.
Strong optical confinement in the planar waveguide is obtained when the refractive index of the core layer of the waveguide is high. Preferably, the planar waveguide has a core layer with a refractive index of at least 1.7.
A variety of materials exhibit both the high linear and nonlinear refractive index preferred for the core layer in the present invention.
Preferably, the planar waveguide has a core layer comprising a metal oxide material. More preferably, the planar waveguide has a core layer comprising a material selected from a group including the oxides of tantalum, hafnium, zirconium, titanium and aluminium. Alternatively, the planar optical waveguide may have a core layer which comprises silicon nitride (SiN).
The performance and wavelength range of the nonlinear device can be extended by suitable doping of the core material. Preferably, the planar optical waveguide has a core layer which comprises a material doped with a rare earth element. An example is Neodymium (Nd).
Although the performance of the nonlinear device may be characterized in terms of the enhancement of the accessible bandwidth relative to the bandwidth of the optical input, it is also desirable that the device is characterized by a large absolute accessible bandwidth. Preferably, the accessible bandwidth is at least 200 nm. More preferably, the accessible bandwidth is at least 500 nm.
Preferably, the ratio of the accessible bandwidth to the input bandwidth is non-linearly dependent on the peak intensity of the optical input. Alternatively, it may be linearly dependent intensity.
A non-linear optical device according to the present invention will typically operate by means of a nonlinear interaction, which comprises one or more third order nonlinear optical processes. Preferably, the non-linear optical process comprises one or more processes selected from a group which includes self-phase modulation, self-focussing, four-wave mixing, Raman scattering and soliton formation.
In addition to the simple broad area broad area waveguide configuration, a nonlinear device according to the present invention may comprise other forms of planar waveguide structure, which may provide enhanced optical confinement. Preferably, the planar waveguide comprises a ridge waveguide. Alternatively, the planar waveguide may comprise a rib waveguide.
One of the problems associated with planar structures, is the coupling in of light from other devices or sources having a different geometry, such as optical fibre. This problem can be mitigated by employing beam shaping or spot-size converting structures, which can be integrated on the same chip.
Preferably, a portion of the planar waveguide is tapered. Preferably, the tapered region is proximate the input of the planar waveguide. It is preferred that the taper is characterized by a gradually increasing waveguide (core) width. However, the taper may be characterized by a gradually decreasing waveguide (core) width Preferably, the taper is symmetrical.
The non-linear optical device may also comprise other structures for pre-processing of the optical input or post-processing of the optical output. Preferably, a portion of the planar waveguide includes a structure, the structure being operative to modify the optical input and/or optical output.
Preferably, the structure comprises a photonic structure. Such structures may perform many functions and can be tailored by appropriate design. Many examples of the photonic structure (crystal) and applications of photonic structures (crystals) can be found in the Applicant's co-pending U.S. patent application Ser. Nos. 09/910,014, 10/147,328, 10/185,727, 10/196,727, 10/240,928, 10/287,792, 10/287,825 and 10/421,949, the discussion of which is included herein by reference.
Preferably, the structure is operative to filter the optical input and/or optical output. The optical transfer function of the filter may result in changes to both the phase and amplitude of the different spectral components of the optical signal spectrum. For example, particular wavelengths or ranges of wavelengths may be transmitted, whilst others are blocked or reflected.
Preferably, the non-linear optical device has a structure which is operative to compress temporally the optical input and/or optical output. Pulse compression can serve to increase pulse peak power, leading to a stronger induced nonlinear effect, and can also pre-compensate for pulse broadening during subsequent propagation due to refractive index dispersion.
Preferably, the non-linear optical device has a structure which is operative to modify the optical dispersion characteristics of the planar optical waveguide. The structure will typically be disposed either proximate or in the region of nonlinear signal generation, and can serve to tailor the waveguide dispersion characteristics to optimize device performance and accessible bandwidth enhancement.
Preferably, the planar optical waveguide comprises a photonic structure which is operative to modify the optical dispersion characteristics of the planar optical waveguide. Alternatively, the planar optical waveguide comprises at least a further planar layer which is operative to modify the optical dispersion characteristics of the planar optical waveguide.
Of course, there are many other applications of the present invention, including integration into a more complex optical system.
Preferably, an optical system includes a non-linear optical device according to the first aspect of the present invention.
A particular application of the present invention is in optical continuum generation. The nonlinear device can give rise to optical continua and supercontinua characterized by particularly large bandwidth.
According to a second aspect of the present invention, an optical continuum source comprises a planar optical waveguide, at least a section of the planar optical waveguide being operative to generate an optical output having an output bandwidth from at least a portion of an optical input having an input bandwidth by means of a non-linear optical process, wherein the optical output has an optical spectrum comprising an optical continuum as a result of non-linear broadening of the optical input, the planar optical waveguide being operative to enhance the ratio of the output bandwidth to the input bandwidth to at least 4, the term “bandwidth” being defined here as the wavelength interval beyond which the spectral radiant intensity remains below a level of −30 decibels (0.001) of the maximum value.
Preferably, the degree of non-linear broadening is by at least a factor of 4.
Preferably, the output bandwidth of the optical continuum is at least 200 nm.
Preferably, the degree of broadening is non-linearly dependent on the peak intensity of the optical input. Alternatively, the degree of broadening may be linearly dependent on intensity.
In order to enhance the continuum generation process it is preferred that the non-linear optical process is seeded with an optical seed input.
Preferably, a portion of the planar waveguide in the optical continuum source includes a structure, the structure being operative to modify the optical dispersion characteristics of the planar optical waveguide.
Preferably, the structure comprises a photonic structure.
Preferably, the optical dispersion characteristics of the planar optical waveguide are modified by the structure to achieve zero dispersion at points along the waveguide. Alternatively, the optical dispersion characteristics of the planar optical waveguide are modified to achieve normal dispersion at a predetermined wavelength.
The waveguide devices according to the first and second aspects may be incorporated into more complex optical or photonic devices.
According to a third aspect of the present invention, an interferometer comprises:
Preferably, the interferometer comprises a photonic integrated circuit, allowing easy integration of all the components on a single chip.
Various arrangements of interferometer are possible. However, it is preferred that a single optical component acts as the beam splitter and the beam combiner, in a Michelson type arrangement.
Preferably, an arm of the interferometer includes a photonic crystal structure. Such a structure may be used to introduce dispersion, dispersion compensation or time delay into the interferometer arm.
This type of interferometer with broad band source is particularly useful for applications involving low coherence interferometry, such as optical coherence tomography.
Preferably, a system for performing optical coherence tomography on a sample comprises:
A further application of the present invention is in optical parametric devices for the tunable generation and amplification of an optical output over a wide wavelength range.
According to a fourth aspect of the present invention, an optical parametric oscillator comprises:
Preferably, the accessible bandwidth of the optical parametric oscillator is at least 200 nm.
Preferably, the optical feedback means is provided at least in part by a photonic structure.
Preferably, a portion of the planar waveguide in the optical parametric oscillator includes a structure, the structure being operative to modify the optical dispersion characteristics of the planar optical waveguide.
Preferably, the structure comprises a photonic structure.
Preferably, the optical dispersion characteristics of the planar optical waveguide are modified to achieve negative (anomalous) dispersion at a predetermined wavelength.
According to a fifth aspect of the present invention, an optical parametric amplifier comprises a planar optical waveguide for receiving a first optical input having a first input bandwidth and a second optical input having a second input bandwidth, at least a section of the planar optical waveguide being operative to amplify the second optical input by generating an optical output from at least a portion of the first optical input by means of a non-linear optical process, the optical output and the second optical input having a wavelength within an accessible bandwidth, wherein the planar optical waveguide is operative to enhance the accessible bandwidth such that the ratio of the accessible bandwidth to the first input bandwidth is at least 4, the term “bandwidth” being defined here as the wavelength interval beyond which the spectral radiant intensity remains below a level of −30 decibels (0.001) of the maximum value.
Preferably, the accessible bandwidth of the optical parametric amplifier is at least 200 nm.
Preferably, a portion of the planar waveguide in the optical parametric amplifier includes a structure, the structure being operative to modify the optical dispersion characteristics of the planar optical waveguide.
Preferably, the structure comprises a photonic structure
Preferably, the optical dispersion characteristics of the planar optical waveguide are modified to achieve negative (anomalous) dispersion at a predetermined wavelength.
According to a sixth aspect of the present invention, a method for enhancing the bandwidth accessible in the generation of an optical output, comprises the step of providing a planar optical waveguide for receiving an optical input having an input bandwidth, wherein at least a section of the planar optical waveguide is operative to generate an optical output from at least a portion of the optical input by means of a non-linear optical process, the optical output having a wavelength within an accessible bandwidth, wherein the planar optical waveguide is operative to enhance the accessible bandwidth such that the ratio of the accessible bandwidth to the input bandwidth is at least 4, the term “bandwidth” being defined here as the wavelength interval beyond which the spectral radiant intensity remains below a level of −30 decibels (0.001) of the maximum value.
According to a seventh aspect of the present invention, a method for generating an optical signal comprises the steps of:
Thus the present invention provides an extremely flexible nonlinear device, which substantially enhances the bandwidth accessible in a nonlinear optical interaction. The key element of the device is a planar waveguide formed from material having both high linear and nonlinear refractive index, which combines the advantages of strong optical confinement and high intensity over an extended interaction region with those of a highly nonlinear material. The net result is an extremely efficient nonlinear interaction with a considerably enhanced accessible bandwidth, as compared to that achievable in prior art planar devices. The device has particular application in optical continuum and supercontinuum generation and also in broadly tunable parametric devices. The geometry of the planar device makes it particularly amenable to the integration of other functionality on the same chip and also compatible with modern photonic integrated circuits. By using tapers, ridge and rib waveguides, and also pulse compression, dispersion modifying and filtering structures (particularly photonic crystal structures) the performance and range of applications of the device can be greatly improved. The planar waveguide device may be incorporated in more complex photonic integrated circuits, such as a Michelson interferometer for low coherence interferometry based optical coherence tomography.
Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:
The present invention is directed to a device for non-linear bandwidth generation.
The propagation of ultra-short, intense pulses in a high index planar waveguide according to the present invention is accompanied by an extremely large nonlinear spectral broadening, which may be exploited in a number of ways. A particularly useful application is in the generation of optical continua. Although the phenomenon of continuum generation is well known in bulk material and optical fiber, the spectral broadening achieved in planar waveguides according to the present invention exhibits unique characteristics. Unlike optical fiber, the high index planar waveguide enables the generation of a broad continuum of wavelengths by exploiting only a smaller number of nonlinear effects.
Planar optical waveguides are key devices in the construction of integrated optical circuits and lasers. The potential of those waveguides lies in the way the electric fields can distribute and propagate in the planar platform, providing a unique way to implement functionality on chip.
The key material property is the refractive index n, with the condition nc>ns required for optical confinement within the waveguide, where nc is the refractive index of the core layer and ns is the refractive index of the neighboring buffer and cladding layers. The refractive index is generally a wavelength dependent function, n(λ), a property that is known in optics as dispersion.
The wavelength dependence of the linear refractive index, n0(λ), of a material is the property that accounts for the difference in propagation speed (phase velocity) experienced by different wavelengths (colours) of light, when travelling through the material. The graph of
However, dispersion in optical waveguides is more complex phenomena, being given by the combined effect of two contributions: material dispersion and waveguide dispersion. Waveguide dispersion may also be characterised in terms of index by introducing an effective refractive index ne(λ), which corresponds to the guiding condition. This effective index takes into account the speed at which a particular optical mode (described by both polarization state and order) propagates in the waveguide. The index is proportional to the propagation constant for the specific mode and its value is always in the range ns<ne<nc. Thus, for planar waveguides, ne(λ) depends on both the geometry of the waveguide (core and cladding thickness) and on the value of the refractive indices of the constituent materials.
Another useful way to express dispersion is to look at its temporal effects on light propagation within a material. The phase velocity, V100 of a wave is inversely proportional (V100=c/n) to the linear refractive index of the material, n. The phase velocity is the velocity at which the phase of any one frequency component of the wave will propagate. This is not the same as the group velocity of the wave, which is the rate at which changes in amplitude (known as the envelope of the wave) will propagate. The group velocity, Vg, is often thought of as the velocity at which energy or information is conveyed along the wave and is given by
The group velocity is also generally a function of wavelength. This dependence results in group velocity dispersion (GVD), which causes a short pulse of light to spread in time as a result of different frequency components of the pulse travelling at different velocities. This effect provides one of the limitations on achieving and maintaining short pulse duration and high data rate in optical communication systems. GVD is often quantified by the group delay dispersion parameter:
The D parameter accounts for the propagation delay per unit wavelength introduced by a unit length of material, and is usual quoted in units of ps/nm/Km). If D is less than zero, the medium is said to have normal, or positive dispersion. If D is greater than zero, the medium has anomalous, or negative dispersion. When a light pulse is propagating through a normally dispersive medium, the higher frequency components travel slower than the lower frequency components. Conversely, when a light pulse travels through an anomalously dispersive medium, high frequency components travel faster than the lower frequency components.
From the above discussion, it is clear that tailoring the dispersion is a fundamental ingredient for controlling or engineering the propagation of light and, as such, is currently exploited in optical fibre communication. It is noted though that, thus far, only linear processes have been considered. This, however, is not a good approximation when dealing with the propagation of intense, ultra-short laser pulses. High index materials of the type used in the present invention can exhibit nonlinear optical effects, which can be particularly strong when induced by intense, ultra-short laser pulses. Although the presence of these nonlinearities in bulk material has been known since the introduction of ultrafast lasers, the more recent diffusion of planar waveguides and use of optical fibres in optical circuits has boosted investigation of nonlinear optical effects in such systems.
One of the most important characteristics of ultra-short laser pulse interaction with matter is the delivery of high energy in a very short time (˜100 femtoseconds=10−13 s) without permanently damaging the material. The light pulse itself can induce a whole range of physical phenomena as it propagates through the material. When the electric field, Elight, of a laser pulse is comparable with the internal field, Eat, of the atoms in the material, the laser light can “drive” the atoms and, in turn, be modified by this interaction. A simple representation of such an interaction is illustrated in
A key parameter in quantifying these particular phenomena is the strength of the nonlinearity which can be characterised by an intensity-dependent higher order contribution to the index of refraction, n, as follows
n(λ,r,t)=n 0(λ)+n 2(λ)I(r,t)
where n0(λ) is the ordinary linear index, n2(λ) is the nonlinear refractive index (in units of m2W−1) and I(r,t) is the temporally and spatially varying intensity of the laser pulse. The nature of the linear term has been described previously and gives rise to optical phenomena such as refraction and reflection, in which light is merely deflected or delayed but remains unchanged in terms of its frequency (wavelength). The nonlinear term is rather different and depends on both the characteristic nonlinear coefficient of the material at the laser wavelength and on the spatial-temporal characteristics of the laser pulse. The higher the nonlinear index of refraction n2 and/or the higher the intensity of the laser pulse, the stronger the nonlinear effect and the greater the nonlinear contribution to the total refractive index. This third order nonlinear effect is commonly known as the Optical Kerr effect.
One practical use of the Optical Kerr effect is the generation of a range of new wavelengths around the input wavelength. If the spread of wavelengths in broad and continuous, the result is termed an Optical Continuum. When there is both a spatial and temporal variation in the local intensity of the optical input field, the Optical Kerr effect can be resolved into two contributions, known as Self-Focussing (SF) and Self-Phase Modulation (SPM). Both effects have their origin in the spatio-temporal dependence of the refractive index n(I(r,t)), with the spatially-varying contribution, I(r), giving rise to self-focussing and the temporally-varying contribution, I(t), giving rise to self-phase modulation.
The intensity of an ultrashort pulse also changes rapidly with time and so, due to the near-instantaneous response of the material, different parts of the pulse will induce different magnitudes of nonlinear refractive index. This time-varying refractive index leads to a phase change, Δφ(t), across the temporal profile of the pulse, which is dependent on the instantaneous intensity in the following manner
where L is the length of the material. Since a time-varying phase corresponds to frequency (frequency is the time derivative of phase, ω=−dφ/dt), the phase delay, Δφ(t), results in a frequency chirp, Δω, across the pulse, given by
As a result, the time-varying nonlinear refractive index of the material leads to what is termed self-phase modulation of the pulse. The frequency shift introduces new spectral components to the pulse, leading to a broadening of the spectral bandwidth. The phase shift and corresponding frequency shift induced across a Gaussian pulse by self-phase modulation is illustrated in
Thus, a planar waveguide with a high associated nonlinear index of refraction has great potential, as it combines the properties of light confinement and guiding with those of a highly nonlinear material. Furthermore, by seeding the interaction with a pulse having a wavelength in the vicinity of zero group velocity dispersion or in the anomalous dispersion region, a wide range of nonlinear processes can occur in the medium. Self-phase modulation, self-focussing, four-wave mixing, Raman scattering, harmonic generation, soliton formation are among the nonlinear effects that may be initiated.
On the logarithmic decibel scale, the half-maximum intensity point, at which FWHM is conventionally measured, corresponds to the −3 dB, However, here the point at which the spectral (radiant) intensity, I(λ), falls permanently below 10−3 (0.001) of the peak (maximum) value, Imax is chosen for the measure of bandwidth. This equates to a relative intensity of −30 dB on the decibel scale. Thus, the working definition of bandwidth is the wavelength interval (Δλ) between the maximum (λmax) and minimum (λmin) wavelengths beyond which the relative intensity is less than −30 dB, as follows:
Δλ=λ max(−30 dB)−λ min(−30 dB)
As can be seen from
At λp=800 nm, the pump is tuned to a wavelength within the normal dispersion regime of the planar waveguide. Furthermore, the dispersion present in the planar waveguide is less structured than that in present in the microstructured fibres used in prior art continuum generation. As a consequence the nonlinear interaction is dominated by the twin Optical Kerr effects of self-focussing and self-phase modulation. Nevertheless, it is clear that unusually broad continuum generation is occurring, with the spectrum extending into the anomalous dispersion region.
Using a planar waveguide with a 500 nm Ta2O5 core layer, continuum generation is achieved for an average pump power as low as 10 mW (pulse energy 40 nJ, peak power 0.25 mW). Alternative dimensions and materials should reduce the threshold sufficiently for continuum generation with unamplified pump pulses from the TiS laser oscillator alone. Suitable core materials include oxides such as hafnium oxide, zirconium oxide, titania, aluminium oxide and also silicon nitride. However, there are many other possible high index candidate materials, some of which can not be fabricated in the bulk but which can be deposited in a thin film to form the core of a planar waveguide. This is another advantageous feature of the planar waveguide device according to the present invention. Doping the core material with rare earth metals will modify the properties still further. Tantalum pentoxide films doped with Neodymium (Nd) have been investigated and demonstrate that such doping permits continuum generation to be extended into the ultraviolet (UV) spectral region. Further advantages of the planar waveguide geometry for continuum generation are preservation of polarization state and very low noise. Other techniques for CG do not preserve the polarization state of the continuum generated, leading to an optical signal with a polarization state that is spatially-varying or possibly depolarized. Such beams have a reduced range of applications. Similarly, a low level of unwanted background noise is also desirable.
The basic waveguide structure for a planar device 100 according to the present invention is reproduced in
As indicated in
Efficient coupling of the pump beam into the waveguide core is one of the most important issues for generating continua, as power density is critical for strong non-linearity. Thus, by launching most of the power into a high index thin film waveguide for optical confinement, high power density can be maintained over an interaction length. In the basic arrangement 10 of
Adding on-chip functionality is one of the great advantages of planar waveguide devices. On-chip structures for spatial profiling and beam shaping have already been described in the context of waveguide tapers. However, other types of functionality can be included that modify the phase or amplitude of a beam propagating in the device. In particular, structures for filtering, shaping or slicing the spectra of the optical input or output may be integrated. Such filtering may be performed by conventional structures with wavelength-dependent absorption, transmission or diffraction properties. However, all the above operations can be performed by photonic crystal structures, which can control propagation of light, both spatially and spectrally, by virtue of their detailed structure which results in forbidden bands of optical propagation constant.
The on-chip processing functions described so far can, of course, be combined.
Thus far, only conventional broad area planar waveguides have been considered, which predominantly provide optical confinement in only one dimension, the vertical. Lateral confinement may be provided by employing ridge or rib type planar waveguide structures, as illustrated in
Such structures give rise to higher peak intensity within the guided mode and a correspondingly larger degree of nonlinearity induced in the nonlinear material. This in turn leads to a lower threshold power (pulse energy) requirement for initiating the nonlinear process. Furthermore, a more symmetrical optical mode may be promoted within the waveguide, which is more easily mode matched to other devices, such as optical fiber, when coupling light into and out of the waveguide. The precise dimensions of the structures will determine whether the waveguide is single-mode or multi-mode.
Of course, many of the pre- and post-processing structures applied to the basic broad area planar waveguide structure may also be applied the ridge and rib waveguide embodiments.
In a manner analogous to that shown in
The geometry of the ridge and rib waveguide structures naturally lend themselves to the construction of a single chip device with multiple waveguide channels.
A further embodiment of the present invention incorporates a structure within the region of the planar waveguide where the signal is nonlinearly generated, the structure being designed to modify the local dispersion characteristics of the waveguide in a predetermined way.
Furthermore, operating near the zero dispersion point can lead to broader continuum generation.
An alternative device structure for modifying the dispersion characteristics of the waveguide is a multilayered structure 290, as illustrated in
As has been described previously, in relation to tapers, the planar waveguide device may comprise integrated structures for the pre-processing of the optical input signal prior to nonlinear generation of the output signal. In the context of a short pump pulse input, one particularly useful function provided by such structure is on-chip pulse compression.
The pulse compression described above may simply be for the purposes of increasing peak power in the pulse in order to induce a stronger nonlinear effect in the continuum generation section of the core. Alternatively, or in addition, the pulse compression may be a pre-processing of the pulse to compensate for dispersion effects on propagation through the remainder of the waveguide device. Of course, this pre-processing function may be applied to any of the embodiments described previously.
We now turn our attention to an application of the optical continuum generated in a planar waveguide according to the present invention. In particular the integration of a CG waveguide in a device for application to Optical Coherence Tomography (OCT). OCT is a relatively recent technique for performing optical ranging in biological tissue to determine local variations in the structure and is similar to ultrasound imaging, but uses near-infrared optical radiation. Optical ranging has been widely used in telecommunications for locating faults or defects in optical fibers by sending optical pulses through the fiber and measuring the time delay between the original pulse and a reflected pulse. However, due to the velocity of light, the time delay between original and reflected pulses cannot be measured directly, and so an interferometric technique is used.
One suitable method is low coherence interferometry, or coherence domain reflectometry, performed using a Michelson type interferometer.
Axial scanning of the reference arm optical length permits measurement of interference fringes, and associated coherence envelope, formed by interference between the reference and signal beams. From this the time-of-flight and optical ranging information can be deduced. By scanning the beam in the transverse directions, horizontal and vertical, a two dimensional image may be built up. Axial scanning gives depth information and the detected beam intensity gives further information about the composition of the sample. In this way, a data array of up to four-dimensions may be built that represents the optical backscattering from the specimen sample.
A key feature of this technique is that the axial (depth) and transverse (lateral) resolution are independent. The axial resolution is determined by the coherence length of the source of the source and is inversely proportional to the spectral bandwidth. For an optical source with a Gaussian-shaped spectral distribution, the axial resolution is given by
where λ is the centre wavelength and Δλ is the spectral bandwidth. Thus, for high axial resolution, a wide bandwidth and hence low coherence source is required, which makes optical continua an attractive option. For application to biological samples another consideration is the degree of scatter, which is stronger at shorter wavelengths, and absorption, which is stronger at longer wavelengths. Thus, an optimum “biological window” exists in the near infrared between 800 nm and 1500 nm which, as shown in
where d is the diameter of the beam size incident on a lens of focal length f. As λ is largely fixed, Δx can be varied by choosing suitable values of f and d, although a trade-off exists between transverse resolution and depth of focus (∝Δx2/λ).
As described previously with reference to
Such data analysis can be simplified by including further functionality in the detection arm, such as a dispersive element.
A final addition to the MI-based OCT systems described above is the use of modulation, which may be implemented by an integrated on-chip modulator.
The majority of the foregoing discussion has centered on the ability of the present invention to generate large bandwidth, particularly in the context of optical continuum generation, and its manipulation and application, such as in OCT. However, another application of the planar waveguide according to the present invention is in parametric devices such as the optical parametric oscillator (OPO) and optical parametric amplifier (OPA). Such devices are used to generate or amplify one or more signals at a discrete wavelength from a input pump beam at a different wavelength. Typically, in a third order parametric process, two output beams are generated, the signal (s) and idler (i), from the single input pump (p) beam. The total photon energy is conserved in the process such that the pump, ωp, signal, ωs, and idler, ωi, frequencies are related by
ω p=ωs +ω i
Each signal typically has a relatively narrow bandwidth centered around these frequencies. However, the wavelengths of the signal and idler can be tuned over a broad range by satisfying the appropriate phase matching condition. In the case of bulk nonlinear crystals, this is often achieved by rotation of the crystal. In the case of modern planar devices, quasi phase-matching can be achieved by periodic poling of the material.
Another common parametric device is the OPO, which comprises a suitable nonlinear material (parametric generator) with optical feedback provided at either the signal or idler wavelength, or both. A certain degree of feedback may also be provided at the pump wavelength. In an OPO, the presence of optical feedback, together with the parametric gain provided by the pump beam, gives rise to a laser-like growth in the signal being fed back. The feedback may be in the form of a linear cavity or a ring (loop) cavity. As with a laser, by making one of the feedback elements partially transmitting at the resonating wavelength, an optical output can be obtained, which can be tuned by employing tunable wavelength selective feedback element.
Optical parametric amplification of a second input beam at the signal or idler wavelength is also possible using a planar waveguide according to the present invention. The arrangement is much the same as the OPO above but without any feedback mechanism. In all parametric device embodiments it is preferred that the nonlinearly generating region comprises a structure to modify the dispersion of the planar waveguide. This may be provided by photonic crystal structures or multilayer structures as described above, or any other suitable structure. In order for accurate phase-matching and efficient operation to be achieved, it is preferred that the total dispersion is in the anomalous regime for the wavelengths of interest.
In summary, the present invention provides an extremely flexible nonlinear device, which substantially enhances the bandwidth accessible in the nonlinear optical interaction. The use of a planar waveguide formed from material having a high linear and nonlinear refractive index combines the benefit of strong optical confinement and high intensity with high material nonlinearity. The device may be pumped by a variety of standalone sources. Typically these are ultrashort pulse laser sources. The net result is an extremely efficient nonlinear interaction with a considerably enhanced accessible bandwidth. The device has particular application in optical continuum and supercontinuum generation, but also in broadly tunable parametric devices. The geometry of the planar device makes it particularly amenable to the integration of other functionality on the same chip and also compatible with modern photonic integrated circuits. As has been described, there are very many embellishments that can be made to the basic device to incorporate added functionality. In particular, the use of tapers, ridge and rib type waveguides, and other modifying structures for pulse compression, dispersion control and filtering (particularly photonic crystal structures) has been shown to improve greatly the performance and range of applications of the device. Furthermore, the planar waveguide, as optical continuum generator, may be integrated in a variety of larger photonic devices. A particular example that has been described is a PIC implemented Michelson interferometer with a continuum broadband source, which finds great application in optical coherence tomography by low coherence interferometry.
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|U.S. Classification||385/1, 385/129|
|International Classification||G02F1/35, G02F1/365|
|Cooperative Classification||G02F2001/3528, G02F1/365, G02F1/353|
|European Classification||G02F1/365, G02F1/35W|
|Nov 9, 2004||AS||Assignment|
Owner name: MESOPHOTONICS LIMITED, UNITED KINGDOM
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:PARKER, GREG;WILKINSON, JAMES;ZOOROB, MAJD;AND OTHERS;REEL/FRAME:015360/0548;SIGNING DATES FROM 20040929 TO 20041005