Patent Publication Number: US-9841658-B1

Title: Controllable opto-electronic time stretcher, an electro-optical analog to digital converter having non-uniform sampling using the same, and related methods of operation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and is a divisional of U.S. patent application Ser. No. 14/702,195 filed on May 1, 2015, the disclosure of which is incorporated herein by reference. 
     This application claims the benefit of provisional application Ser. No. 61/988,133, entitled “A CONTROLLABLE OPTO-ELECTRONIC TIME STRETCHER, AN ELECTRO-OPTICAL ANALOG TO DIGITAL CONVERTER HAVING NON-UNIFORM SAMPLING USING THE SAME, AND RELATED METHODS OF OPERATION” and filed on May 2, 2014, which is hereby incorporated by reference. 
     This application claims the benefit of provisional application Ser. No. 62/147,493, entitled “A CONTROLLABLE OPTO-ELECTRONIC TIME STRETCHER, AN ELECTRO-OPTICAL ANALOG TO DIGITAL CONVERTER HAVING NON-UNIFORM SAMPLING USING THE SAME, AND RELATED METHODS OF OPERATION” and filed on Apr. 14, 2015, which is hereby incorporated by reference. 
     This application claims the benefit of provisional application Ser. No. 62/147,473, entitled “HARDWARE BASED COMPRESSIVE SAMPLING ADC ARCHITECTURE FOR NON-UNIFORM SAMPLED SIGNAL RECOVERY” and filed on Apr. 14, 2015, which is hereby incorporated by reference. 
    
    
     REFERENCE TO A CONTRACT 
     This disclosure does not relate to work performed under a specific Government contract. 
    
    
     INCORPORATION BY REFERENCE 
     The present disclosure relates to U.S. Pat. No. 8,334,797, issued on Dec. 18, 2012 and entitled: “Wideband High Resolution Time-Stretched Photonic Analog-to-Digital Converter”, which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to electro-optic elements; in particular to an opto-electronic time stretcher for controllably stretching in time an optical pulse. The present disclosure also relates to an analog to digital converter using said opto-electronic time stretcher, as well as related methods of sampling an analog signal. 
     BACKGROUND 
     The resolution of Analog To Digital Converters, or ADCs, as measured by their effective number of bits (ENOB) is limited by their “aperture-jitter” at high sampling rates. Photonic time-stretching allows effective compression of the analog-input bandwidth, so that quantizers that sample at much lower rates fi can be used to accomplish A/D conversion with high resolution. 
     As documented in W. Ng, T. Rockwood, G. Sefler, G. Valley: “Demonstration of a Large Stretch-Ratio (M=41) Photonic Analog-to-Digital Converter with 8 ENOB for an Input Signal Bandwidth of 10 GHz”, IEEE Photonic Technology Letters, Vol. 24 (14), 1185-1187 (2012), an ENOB &gt;8 could be attained for f sig =10 GHz with a Photonic Time Stretch ADC. 
     As illustrated in  FIG. 1 , a photonic TS ADC  10  comprises a source of light  12 , such as a mode-locked laser, for generating a series of pulses having each a broad (˜30-40 nm) supercontinuum (SC) spectrum. 
       FIG. 2  is a time-domain illustration of a series of pulses  13  comprising each a plurality of wavelengths. In  FIG. 2 , each exemplary pulse of light comprises four wavelengths of light represented on the figure by the symbols “Δ”, “□”, “◯”, and “*”. 
     Returning to  FIG. 1 , ADC  10  comprises a first dispersion element  14  that chirps each pulse  13  by spreading in time the wavelengths comprised in the pulse. Dispersion element  14  can be made of a material the refractive index of which varies with wavelength, such that the wavelengths of the pulse traverse the material at different speeds and exit the material at different times. Dispersion element  14  is arranged so that the time-stretched optical pulses overlap with each other and dispersion element  14  outputs a continuous optical signal  16 , comprised of wavelengths that periodically vary with time. 
       FIG. 3  is a time-domain illustration of the continuous optical signal  16  comprised of the overlapping juxtaposition of the time-spread optical pulses  13 . Continuous optical signal  16  is comprised of various wavelengths, represented on the figure by the symbols “Δ”, “□”, “◯”, and “*”, that periodically vary with time. The wavelength of a pulse  13  that gets out of dispersion element  14  the faster (represented by symbol “Δ” in  FIG. 3 ) overlaps with the wavelength of the next pulse  13  that gets out of dispersion element  14  the slower (represented by symbol “*” in  FIG. 3 ). 
     Returning to  FIG. 1 , ADC  10  comprises an electro-optic modulator  18  arranged for modulating continuous optical signal  16  with an input analog signal  20  into a modulated optical signal  22 . 
       FIGS. 4 and 5  are time-domain illustrations of input analog signal  20  and modulated optical signal  22 . 
     Returning to  FIG. 1 , ADC  10  comprises a time-controlled demultiplexer  24  arranged for separating the modulated optical signal  22  into a plurality of modulated optical signal segments  26 . ADC  10  further comprises a plurality of second dispersion elements  28 , coupled each to an output of demultiplexer  24 , for spreading in time the wavelengths comprised in each modulated optical signal segment  26 . 
       FIG. 6  is a time-domain illustration of a time-spread modulated optical signal segment  30  as output by a second dispersion element  28 . 
     Returning to  FIG. 1 , ADC  10  comprises, coupled to the output of each second dispersion element  28 , a sampler  32  arranged for sampling the time-spread modulated optical signal segments  30 . 
       FIG. 7  is a time-domain illustration of a series of samples  34  obtained by sampling the time-spread modulated optical signal segment  30  as output by a second dispersion element  28 . 
     Returning to  FIG. 1 , ADC  10  comprises a calculator  36  arranged for receiving the samples  34  output by each sampler  32 , and for constructing a digitized image of the input analog signal  20  based on the samples  34 . 
     If D 1  is the dispersion coefficient (given in psec/nm) of first dispersion element  14  and D 2  the dispersion coefficient of each dispersion element  28 , the stretch ratio M of ADC  10  is given by: M=1+D 2 /D 1 . However, the number of channels in output of demultiplexer  24 , which is needed to de-serialize an input-signal of continuous time-duration (CT) in ADC  10 , is directly related to M. It follows that a stretch-factor M of 20 typically requires 20 or more parallel channels to de-serialize the signal. This, in turn, increases the size, weight and power consumption (SWAP) of the ADC. 
     There exists a need for a high resolution ADC having reduced size, weight and power consumption with respect to ADC  10 . 
     SUMMARY OF THE INVENTION 
     The present disclosure relates to a controllable opto-electronic time stretcher, or dispersion element, that has a controllable dispersion. The present disclosure further relates to an electro-optical ADC that uses such controllable opto-electronic time stretcher to change the stretch ratio M of the ADC according to a predetermined non-uniform pattern, thus outputting a pseudo-randomly time stretched modulated optical signal; the ADC comprising a calculator capable of finding back the input signal of the ADC using said predetermined non-uniform pattern and periodic samples of the pseudo-randomly time stretched modulated optical signal. 
     An embodiment of the present disclosure comprises a controllable opto-electronic time stretcher having: a first wave guide and a second waveguide coupled to the first waveguide along a coupling portion; wherein at least one of the first and second waveguides in the coupling portion has a controllable refractive index. 
     According to an embodiment of the present disclosure, said at least one of the first and second waveguides in the coupling portion having controllable refractive index comprises an electro-optic portion and an electrode capable of submitting the electro-optic portion to a controllable electric field. 
     According to an embodiment of the present disclosure, said electro-optic portion comprises an electro-optic polymer or material. 
     According to an embodiment of the present disclosure, the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     According to an embodiment of the present disclosure, the first waveguide comprises a Si core and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     According to an embodiment of the present disclosure, a layer of electro-optic material is arranged between the cores of the first and second waveguides in the coupling portion. 
     According to an embodiment of the present disclosure, said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     An embodiment of the present disclosure comprises an electro-optical analog to digital converter having non-uniform sampling, having: a source of light arranged for generating at least one light pulse having a first spectral width; a chirp element arranged for spreading in time the wavelengths of said at least one light pulse; an electro-optic modulator arranged for modulating the time-spread light pulse with an input analog signal into a modulated optical signal; at least one controllable opto-electronic time stretcher according to concept 1 as detailed hereafter, arranged for receiving the modulated optical signal in the first wave guide and for controlling the refractive index according to a predetermined non-uniform pattern; at least one sampler arranged for sampling at a predetermined rate the output of the at least one controllable opto-electronic time stretcher; and a calculator arranged for constructing a digitized image of the input analog signal based on the samples generated by the at least one sampler and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, said source of light is arranged for generating said at least one light pulse as one light pulse of a train of identical light pulses; the chirp element being arranged to generate overlapping time-spread light pulses, and the electro-optic modulator being arranged to modulate the overlapping time-spread light pulses with an input analog signal into said modulated optical signal; the electro-optical analog to digital converter further comprises a time-controlled demultiplexer for separating said modulated optical signal into a plurality of modulated optical signal segments; said at least one controllable opto-electronic time stretcher comprises one controllable opto-electronic time stretcher for receiving each modulated optical signal segment in its first wave guide; said at least one sampler comprises one sampler for sampling the output of each controllable opto-electronic time stretcher; and said calculator is arranged for constructing a digitized image of the input analog signal based on the samples generated by each sampler, based on the order in which each modulated optical signal segment is generated by the demultiplexer, and based on said predetermined non-uniform pattern. 
     An embodiment of the present disclosure comprises a method for controllably time stretching an input optical signal; the method comprising: providing a first wave guide having a first refraction index characteristic, said first waveguide receiving said input optical signal; providing a second waveguide having a second refraction index characteristic; and coupling the second waveguide to the first waveguide with a controllable degree of coupling. 
     According to an embodiment of the present disclosure, coupling the second waveguide to the first waveguide with a controllable degree of coupling comprises controllably changing the refraction index of one of the first and second waveguide at a point of coupling between the first and second waveguides. 
     According to an embodiment of the present disclosure, said controllably changing the refraction index of one of the first and second waveguide comprises providing at least one of the first and second waveguides at said point of coupling with an electro-optic portion and submitting the electro-optic portion to a controllable electric field. 
     According to an embodiment of the present disclosure, said electro-optic portion comprises an electro-optic polymer. 
     According to an embodiment of the present disclosure, the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     According to an embodiment of the present disclosure, the first waveguide comprises a Si core, with a SiO2 inner core sandwiched by Si slabs, and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     According to an embodiment of the present disclosure, said layer of electro-optic material is arranged between the cores of the first and second waveguides at the point of coupling. 
     According to an embodiment of the present disclosure, said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     An embodiment of the present disclosure comprises a method of converting an analog input signal into a digital output signal, the method, comprising: generating at least one light pulse having a first spectral width; spreading in time the wavelengths of said at least one light pulse; modulating the time-spread light pulse with said input analog signal into a modulated optical signal; controllably time stretching the modulated optical signal according to the method of the previous concepts, said first waveguide receiving the modulated optical signal and said coupling with a controllable degree of coupling following a predetermined non-uniform pattern; sampling at a predetermined rate the output of the first and second waveguide; and constructing a digital output signal corresponding to a digitized image of the analog input signal, based on the samples generated by the sampling and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, said generating at least one light pulse comprises generating said at least one light pulse as one light pulse of a train of identical light pulses; said spreading in time the wavelengths of said at least one light pulse comprises generating overlapping time-spread light pulses; said modulating the time-spread light pulse comprises modulating the overlapping time-spread light pulses with said input analog signal into said modulated optical signal; the method further comprising separating said modulated optical signal into a plurality of successive modulated optical signal segments; wherein said controllably time stretching the modulated optical signal according to the method of the previous concepts comprises controllably time stretching separately each of the successive modulated optical signal segments by varying, according to said predetermined non-uniform pattern, a coupling between a first waveguide receiving each modulated optical signal segment and a second waveguide; wherein said sampling at a predetermined rate the output of the first and second waveguides comprises sampling each time-stretched modulated optical signal segment at said predetermined rate; and wherein said constructing a digital output signal comprises constructing a digitized image of the input analog signal based on the samples generated for each time-stretched modulated optical signal segment, based on the order in which each time-stretched modulated optical signal segment is generated, and based on said predetermined non-uniform pattern. 
     An embodiment of the present disclosure comprises a method of sampling an analog signal having a predetermined spectrum, the method comprising: assuming that said analog signal corresponds to a K sparse vector of N coefficients, with K&lt;&lt;N; spreading in time said analog signal according to a predetermined pseudo-random pattern into a pseudo-randomly time-spread signal; on a predetermined time period, taking M samples by sampling the pseudo-randomly time-spread signal at a predetermined rate, said predetermined rate being inferior to the Nyquist rate, where M≧K·log(N/K); associating the M samples to the time at which they would have been taken if said analog signal had been spread uniformly during said time period by a spread factor equal to the mean of the pseudo-randomly spread factor on said time period; and determining iteratively the closest output signal that would have allowed extracting the M samples at their associated times. 
     According to an embodiment of the present disclosure, the method comprises: assuming that the analog signal can be expressed as a K sparse vector comprising a number N of DFT coefficients, with K&lt;&lt;N; assuming that the set Y of M samples is such that Y=Φ·X (1), where Φ is a M by N matrix; assuming that X can be written as X=Ψ·S (2), where W is a N×N matrix and S is a N-coefficients vector having only K non-zero coefficients, whereby Y=θ·S, with θ=Φ·Ψ; solving the linear program: Ŝ=argmin∥S∥ 1 , subject to Y=θ·S, where 
                      S        1     =       ∑     k   =   1     N     ⁢          S   ⁡     (   k   )                ;         
and finding X using X=Ψ·S.
 
     An embodiment of the present disclosure comprises an electro-optical analog to digital converter having non-uniform sampling, having: a source of light arranged for generating at least one light pulse having a first spectral width; a chirp element arranged for spreading in time the wavelengths of said at least one light pulse; an electro-optic modulator arranged for modulating the time-spread light pulse with an input analog signal into a modulated optical signal; at least one controllable opto-electronic time stretcher arranged for variably spreading in time the modulated optical signal according to a predetermined non-uniform pattern; at least one sampler arranged for sampling at a predetermined rate the output of the at least one controllable opto-electronic time stretcher; and a calculator arranged for constructing a digitized image of the input analog signal based on the samples generated by the at least one sampler and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, said source of light is arranged for generating said at least one light pulse as one light pulse of a train of identical light pulses; the chirp element being arranged to generate overlapping time-spread light pulses, and the electro-optic modulator being arranged to modulate the overlapping time-spread light pulses with an input analog signal into said modulated optical signal; the electro-optical analog to digital converter further comprises a time-controlled demultiplexer for separating said modulated optical signal into a plurality of modulated optical signal segments; said at least one controllable opto-electronic time stretcher comprises one controllable opto-electronic time stretcher for variably spreading in time according to a predetermined non-uniform pattern each modulated optical signal segment; said at least one sampler comprises one sampler for sampling the output of each controllable opto-electronic time stretcher; and said calculator is arranged for constructing a digitized image of the input analog signal based on the samples generated by each sampler, based on the order in which each modulated optical signal segment is generated by the demultiplexer, and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, the controllable opto-electronic time stretcher comprises: a first wave guide; and a second waveguide coupled to the first waveguide along a coupling portion; wherein at least one of the first and second waveguides in the coupling portion has a controllable refractive index. 
     According to an embodiment of the present disclosure, said at least one of the first and second waveguides in the coupling portion having controllable refractive index comprises an electro-optic portion and an electrode capable of submitting the electro-optic portion to a controllable electric field. 
     According to an embodiment of the present disclosure, said electro-optic portion comprises an electro-optic polymer. 
     According to an embodiment of the present disclosure, the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     According to an embodiment of the present disclosure, the first waveguide comprises a Si core and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     According to an embodiment of the present disclosure, said electro-optic portion is arranged between the cores of the first and second waveguides in the coupling portion. 
     According to an embodiment of the present disclosure, said electro-optic portion is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     An embodiment of the present disclosure comprises a method of converting an analog input signal into a digital output signal, the method, comprising: generating at least one light pulse having a first spectral width; spreading in time the wavelengths of said at least one light pulse; modulating the time-spread light pulse with said input analog signal into a modulated optical signal; controllably time stretching the modulated optical signal according to a predetermined non-uniform pattern; sampling at a predetermined rate the output of the first and second waveguide; and constructing a digital output signal corresponding to a digitized image of the analog input signal, based on the samples generated by the sampling and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, said generating at least one light pulse comprises generating said at least one light pulse as one light pulse of a train of identical light pulses; said spreading in time the wavelengths of said at least one light pulse comprises generating overlapping time-spread light pulses; said modulating the time-spread light pulse comprises modulating the overlapping time-spread light pulses with said input analog signal into said modulated optical signal; the method further comprising separating said modulated optical signal into a plurality of successive modulated optical signal segments; wherein said controllably time stretching the modulated optical signal comprises controllably time stretching separately each of the successive modulated optical signal segments according to said predetermined non-uniform pattern; wherein said sampling at a predetermined rate the output of the first and second waveguides comprises sampling each time-stretched modulated optical signal segment at said predetermined rate; and wherein said constructing a digital output signal comprises constructing a digitized image of the input analog signal based on the samples generated for each time-stretched modulated optical signal segment, based on the order in which each time-stretched modulated optical signal segment is generated, and based on said predetermined non-uniform pattern. 
     According to an embodiment of the present disclosure, said controllably time stretching the modulated optical signal comprises: providing a first wave guide having a first refraction index characteristic, said first waveguide receiving said input optical signal; providing a second waveguide having a second refraction index characteristic; and coupling the second waveguide to the first waveguide with a controllable degree of coupling. 
     According to an embodiment of the present disclosure, coupling the second waveguide to the first waveguide with a controllable degree of coupling comprises controllably changing the refraction index of one of the first and second waveguide at a point of coupling between the first and second waveguides. 
     According to an embodiment of the present disclosure, said controllably changing the refraction index of one of the first and second waveguide comprises providing at least one of the first and second waveguides at said point of coupling with an electro-optic portion and submitting the electro-optic portion to a controllable electric field. 
     According to an embodiment of the present disclosure, said electro-optic portion comprises an electro-optic polymer. 
     According to an embodiment of the present disclosure, the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     According to an embodiment of the present disclosure, the first waveguide comprises a Si core and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     According to an embodiment of the present disclosure, said layer of electro-optic material is arranged between the first and second waveguides cores at the point of coupling. 
     According to an embodiment of the present disclosure, said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a known Photonic Analog-to-Digital Converter. 
         FIG. 2  illustrates a series of pulses output by the source of light of  FIG. 1 . 
         FIG. 3  illustrates a continuous optical signal provided to the optical input of the modulator of  FIG. 1 . 
         FIG. 4  illustrates an input analog signal provided to the electrical input of the modulator of  FIG. 1 . 
         FIG. 5  illustrates the signal output by the modulator of  FIG. 1 . 
         FIG. 6  illustrates the signal provided to a sampler of  FIG. 1 . 
         FIG. 7  illustrates the signal output by a sampler of  FIG. 1 . 
         FIG. 8  illustrates a Photonic Analog-to-Digital Converter according to an embodiment of the present disclosure. 
         FIG. 9  illustrates the signal provided to a sampler of  FIG. 8 . 
         FIG. 10  illustrates the signal output by a sampler of  FIG. 8 . 
         FIG. 11  illustrates the effect of controllably varying the stretch ratio M of the ADC of  FIG. 8  on the sampling of the time-stretched optical signal. 
         FIG. 12A  illustrates signal aliasing when using uniform sampling. 
         FIG. 12B  illustrates signal aliasing reduction when using non-uniform sampling. 
         FIG. 13  illustrates the elimination of sub-sampled aliases via non-uniform sampling. 
         FIG. 14  illustrates a method followed for reconstructing the input signal from the samples taken according to an embodiment of the present disclosure. 
         FIG. 15  illustrates the refractive index of the coupling portion of a controllable opto-electronic time stretcher according to an embodiment of the present disclosure. 
         FIG. 16  illustrates the dispersion enhancement over the resonance width of the time stretcher of  FIG. 16 . 
         FIG. 17  illustrates a cross-section of an embodiment of the time stretcher of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
       FIG. 8  illustrates a Photonic Analog-to-Digital Converter  40  according to an embodiment of the present disclosure. According to an embodiment of the present disclosure, ADC  40  comprises a source of light  12 , a first dispersion element  14 , an electro-optic modulator  18  and a demultiplexer  24  arranged and operating as in the ADC  10  of  FIG. 1 .  FIGS. 2, 3 and 5  can respectively be used to illustrate the outputs of source of light  12 , first dispersion element  14 , and electro-optic modulator  18  of ADC  40  as detailed above with respect to ADC  10 . 
     According to an embodiment of the present disclosure, in ADC  40 , each output of demultiplexer  24  is coupled to a controllable time stretcher  42  that has a controllable dispersion. The structure of an exemplary time stretcher is detailed hereafter. Demultiplexer  24  is time controlled for slicing the modulated optical signal  22  into a plurality of modulated optical signal segments  26 , such that each controllable time stretcher  42  receives in input a different one of the plurality of modulated optical signal segments  26 . According to an embodiment of the present disclosure, each controllable time stretcher  42  is controlled by a source  44  of a predetermined non-uniform or pseudo-random pattern, such that the dispersion of each controllable time stretcher  42  is changed in time according to said pseudo-random pattern as the modulated optical signal segment  26  received by the controllable time stretcher  42  passes through the controllable time stretcher  42 . This results in spreading in time modulated optical signal segment  26  with a pseudo-random time-stretching factor controlled by source  44 . 
       FIG. 9  is a time-domain illustration of a pseudo-randomly time-spread modulated optical signal segment  50  as output by a controllable time stretcher  42 . Pseudo-randomly time-spread modulated optical signal segment  50  is to be compared with a regularly time-spread modulated optical signal segment  30  such as shown in  FIG. 6 . In the exemplary pseudo-randomly time-spread modulated optical signal segment  50  of  FIG. 9 , the first and last portions of signal segment  26 , having the wavelengths represented by “Δ”, “*” and “◯”, were time-spread less than the same portion of signal in  FIG. 6 , and the middle portion of signal segment  26 , having the wavelengths represented by “□”, was time-spread more than the same portion of signal in  FIG. 6 . According to an embodiment of the present disclosure, signal segment  50  is presented to a sampler  52  having a fixed predetermined sampling rate. 
       FIG. 10  is a time-domain illustration of a series of samples  54  obtained by sampling the pseudo-randomly time-spread modulated optical signal segment  50  of  FIG. 9 , as output by a sampler  52 . According to an embodiment of the present disclosure, because the samples  54  are taken at a fixed sampling rate from a pseudo-randomly time-stretched signal, samples  54  are identical to samples that would be taken at a pseudo-random sampling rate from a signal time-stretched with a fixed time-stretch factor. Sampler  52  can be a photodetector/photoreceiver. 
       FIG. 11  illustrates the position in time of samples  54  of  FIG. 10  with respect to a time-spread modulated optical signal segment  30  as output by a second dispersion element  28  having a fixed time-stretch factor.  FIG. 11  is to be compared to  FIG. 7 . Fewer samples were taken from the portions of signal segment  26  having the wavelengths represented by “Δ”, “*” and “◯” in  FIG. 11  than in  FIG. 7 . On another hand, much more samples were taken from the middle portion of signal segment  26 , having the wavelengths represented by “□”, in  FIG. 11  than in  FIG. 7 . The inventors have noted that, even for a total number of samples of a signal that would lead to sub-sampling and aliasing if the samples were taken at a fixed rate, taking the same number of samples of a signal at a pseudo-random sampling rate allows eliminating aliasing. 
       FIG. 12A  is a time-domain illustration of a plurality of exemplary sinusoidal signals  56 ,  58 ,  60  having different frequencies, which cannot be differentiated by a series of samples  62  taken at a fixed, sub-sampling, rate from signal  58 . 
       FIG. 12B  is a time-domain illustration of the signals  56 ,  58 ,  60  of  FIG. 12A  having different frequencies, which can be differentiated by a series of samples  64  of signal  58 , comprising as many samples as samples  62  but taken at a pseudo-random sampling rate. 
       FIG. 13  illustrates the measured/recovered frequency Fr for an input frequency Fi as recovered  66  using non-uniformly (pseudo-randomly) sub-sampled signals, as well as the measured/recovered frequency Fr for an input frequency Fi as recovered  68  using uniformly sampled signals. As shown in  FIG. 13 , frequency Fr recovered  68  from a uniform sampling is folded back into the first Nyquist zone capped at half the frequency of the uniform sampling. 
     According to an embodiment of the present disclosure, non-uniform/pseudo-random sampling of signals  26  allows recovering signal  26  with a smaller number of samples than with a regular sampling. Alternatively, with a number of samples unchanged non-uniform/pseudo-random sampling of signals  26  according to an embodiment of the present disclosure allows efficiently sampling signals  26  that are less time-stretched than if signals  26  were to be uniformly sampled, which in turn reduces the number of signals  26  that need be formed out of modulated signal  22 . This in turn reduces the size of multiplexer  24  as well as the number of controllable time stretchers  42  and samplers  52  in Photonic Analog-to-Digital Converter  40 . 
     According to an embodiment of the present disclosure, the input signal  26  from which non-uniform/pseudo-random samples  54  are taken can be reconstructed by a calculator  70 , arranged for constructing a digitized image of the input analog signal  20  based on the samples  54  as well as the output of pseudo-random pattern source  44 . 
     According to an embodiment of the present disclosure, the digitized image X of input analog signal  20 , for example the Discrete Fourier Transform of signal  20 , is reconstructed by considering the set Y of samples  54  as a compressive-sensing measurement of X. Considering that Y comprises M samples, and X comprises N coefficients, with M&lt;&lt;N, it can be written that
 
 Y=Φ·X   (1),
 
where Φ is a M by N matrix. Compressive sensing is for example described in: R. Baraniuk, “Compressive Sensing”, IEEE Signal Processing Magazine, p. 118-124, July, 2007.
 
     According to an embodiment of the present disclosure it is considered that X is K-sparse, whereby X can be written as
 
 X=Ψ·S   (2),
 
where Ψ is a N×N matrix and S is a N-coefficients vector having only K non-zero coefficients.
 
     The above equations (1) and (2) lead to the hypothesis that
 
 Y=θ·S   (3),
 
with θ=Φ·Ψ.
 
 FIG. 14A  illustrates equation (3) above in an exemplary case where S comprises K=4 non-zero coefficients.
 
     It has been shown that, if M≧K·log(N/K), and matrix Φ is a “random” measurement-matrix, the s vector of equation (3) above can be found by solving the linear program:
 
{circumflex over ( S )}=argmin∥ S∥   1 , subject to  Y=θ·S   (4)
 
     
       
         
           
             
               where 
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                    
                   S 
                    
                 
                 1 
               
             
             = 
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 N 
               
               ⁢ 
               
                  
                 
                   S 
                   ⁡ 
                   
                     ( 
                     k 
                     ) 
                   
                 
                  
               
             
           
         
       
     
     Such linear program is for example disclosed in: J. Tropp and S. J. Wright, “Computational methods for sparse solution of linear inverse problems,” Proc. IEEE, 98(6): 948-958, 2010. According to an embodiment of the present disclosure, X is then found back using X=Ψ·S. 
     According to embodiments of the present disclosure, other signal reconstruction algorithms such as orthogonal matching pursuit that can also be used to find the sparse vector s and the input signal X. 
     According to an embodiment of the present disclosure, knowledge of the pseudo-random pattern generated by source  44  allows associating samples  54  each to the pseudo-random position in time they would have in a time-spread modulated optical signal segment  30  as shown in  FIG. 11 , whereby the set Y of samples  54  can be considered as a non-uniform sampling of segment  30 . An embodiment of the present disclosure provides for finding the combination of DFT coefficients that allow reconstructing a signal X as close as possible to segment  30 . According to an embodiment of the present disclosure, because the set Y of samples  54  having pseudo-random positions in time can be considered as a non-uniform sampling of segment  30 , the number of samples  54  can be smaller than the number of samples that would be required by Nyquist-Shannon, and still allow reconstruction of signal X. Calculator  70  can be implemented as a processor programmed to retrieve signal X from samples Y, or as a FPGA or a custom-made integrated circuit having calculation modules for retrieving signal X from samples Y. 
       FIG. 15  shows a cross section in a plane perpendicular to the light propagation axis of controllable time stretcher  42  according to an embodiment of the present disclosure. Time stretcher  42  comprises a first wave guide  72 ; a second waveguide  74  coupled to the first waveguide along a coupling portion cut by the plane of the cross-section. According to an embodiment of the present disclosure, at least one of the first and second waveguides in the coupling portion has a controllable refractive index. In the exemplary embodiment of  FIG. 15 , first wave guide  72  in the coupling portion has a controllable refractive index. 
     According to an embodiment of the present disclosure, first wave guide  72  comprises an electro-optic portion  76 , for example comprising an electro-optic polymer, and an electrode  78  capable of submitting the electro-optic portion  76  to a controllable electric field, to controllably change the refractive index of first wave guide  72 . According to an embodiment of the present disclosure, first waveguide  72  is a slot waveguide, provided for guiding strongly confined light in a subwavelength-scale low refractive index region  80  by total internal reflection. According to an embodiment of the present disclosure, first waveguide  72  comprises a core having two strips or slabs  82 ,  84  of high-refractive-index materials separated by subwavelength-scale low-refractive-index slot inner core region  80 . A portion of core region  82  is covered by electro-optic portion  76 . The remaining portions of the core regions  80 ,  82 ,  84  are covered by low-refractive-index cladding materials  86  having a refractive index comparable to the refractive index of portion  76 . 
     According to an embodiment of the present disclosure, second waveguide  74  is a buried strip waveguide comprising a core slab  88 . According to an embodiment of the present disclosure, core slab  88  can be separated from electro-optic portion by a thin slab  90  of cladding material, followed by a thin tunnel layer slab  92 . According to an embodiment of the present disclosure, the remaining portions of core slab  88  can be covered by a thick layer of cladding material  86 . 
     According to an embodiment of the present disclosure, the first waveguide  72  comprises a SiO2 core  80  sandwiched by Si slabs  82 ,  84 ; the total height of the core regions  84 ,  80 ,  82  being 0.245 μm and their width being 0.5 μm. Cladding 86 can be SiO2. Tunnel layer slab  92  can be Oxy-Si x N y  and core  88  can be a Si x N y  core having a height of 1 μm. The height of the layer  76  of the electro optic polymer can be equal to the height of the tunnel layer slab  92  and be 0.245 μm. The height of the thin cladding material slab  90  and be 0.4 μm. The width of all the slabs can be 0.5 μm. 
     It is noted that the exemplary embodiment illustrated in  FIG. 15  shows a first waveguide  72  having an electro-optic portion  76  for controlling the refractive index of the first waveguide; but according to an embodiment of the present disclosure, waveguide  72  can also comprise a thermo-optic portion  76  for controlling the refractive index of the first waveguide. In such embodiment, electrode  78  would end with resistors for increasing the temperature of thermo-optic portion  76 , and could alternatively be used as heat drains for decreasing the temperature of thermo-optic portion  76 . A metal plate can alternatively surround the coupling portion of the waveguides to evacuate the heat. 
     The exemplary embodiment illustrated in  FIG. 15  shows a first waveguide  72  having a electro-optic portion  76  for controlling the refractive index of the first waveguide; but according to an embodiment of the present disclosure, the second waveguide can alternatively comprise a thermo-optic portion for controlling its refractive index. According to an embodiment of the present disclosure, the light can be input into controllable time stretcher  42  through only one of waveguides  72 ,  74 , or both of them. 
     According to an embodiment of the present disclosure, first waveguide  72  has a first refractive index n 1 , which changes with the wavelength A of the light propagating through first waveguide  72 , and second waveguide  74  has a second refractive index n 2 , which changes with the wavelength A of the light propagating through first waveguide  74 . 
       FIG. 16  is a diagram showing how refractive indexes n 1  and n 2  change, respectively along a line  94  having a strong decreasing slope and a line  96  having a weaker decreasing slope. Lines  94  and  96  cross at a resonance wavelength λr. According to an embodiment of the present disclosure, the refractive index n 3  of the coupling region of first and second waveguides  72 ,  74  follows a curve  98  essentially above and tangent to the highest of lines  94 - 96 . According to an embodiment of the present disclosure, at the coupling region of first and second waveguides  72 ,  74 , the light having a wavelength below λr propagates in first waveguide  72  and the light having a wavelength above λr propagates in second waveguide  74 . According to an embodiment of the present disclosure, the light having a wavelength comprised within a range Δλr propagates in both the first and the second waveguides. According to an embodiment of the present disclosure, Δλr can be 21 nm wide. 
     According to an embodiment of the present invention, first and second waveguides  72 ,  74  are provided for having each a reduced dispersion. The inventors have noted that the dispersion at the coupling region of first and second waveguides  72 ,  74  is very high for light having a wavelength comprised within the range Δλr. 
       FIG. 17  illustrates the dispersion at the coupling region of first and second waveguides  72 ,  74 : generally small but very high for light having a wavelength comprised within the range Δλr. 
     According to an embodiment of the present disclosure, changing the degree of coupling between waveguides  72  and  74  changes the values of λr and Δλr. An embodiment of the present disclosure provides for changing the degree of coupling, by changing the refractive index of waveguide  72 , for example by controllably changing the refractive index of region  76 , which changes the slope or position of line  94  of  FIG. 16 , thus changing the values of λr and Δλr. 
     Referring back to  FIG. 5 , it is noted that each signal segment  26  is comprised of a plurality of wavelengths represented by “Δ”, “□”, “◯”, “*”. According to an embodiment of the present invention, controllable time stretcher  42  is arranged such that controllably changing the refractive index of region  76  causes Δλr to controllably sweep the wavelengths represented by “Δ”, “□”, “◯”, “*” of each signal segment  26 . This can cause the dispersion/time stretch in controllable time stretcher  42  to increase strongly for a controlled portion of signal segment  26  if the wavelength (“Δ”, “□”, “◯”, “*”) of said portion is comprised in the range Δλr at the time said portion passes through time stretcher  42 . 
     According to an embodiment of the present disclosure, the control electrodes  78  of controllable time stretcher  42  receive a control voltage that follows a predetermined pseudo-random pattern from source  44 , such that the dispersion of each controllable time stretcher  42  is changed in time according to said pseudo-random pattern as the modulated optical signal segment  26  received by the controllable time stretcher  42  passes through the controllable time stretcher  42 . According to an embodiment of the present disclosure, this allows spreading in time the modulated optical signal segment  26  with a pseudo-random time-stretching factor controlled by source  44 . 
     According to an embodiment of the present invention, the average stretch factor introduced by controllable time stretcher  42  must not be larger than the number of modulated optical signal segments  26  produced by demultiplexer  24 . 
     Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . ” 
     The present disclosure discloses the following concepts: 
     1. A controllable opto-electronic time stretcher comprising: 
     a first wave guide; 
     a second waveguide coupled to the first waveguide along a coupling portion; 
     wherein at least one of the first and second waveguides in the coupling portion has a controllable refractive index. 
     2. The controllable opto-electronic time stretcher of concept 1, wherein said at least one of the first and second waveguides in the coupling portion having a controllable refractive index comprises an electro-optic portion and an electrode capable of submitting the electro-optic portion to a controllable electric field. 
     3. The controllable opto-electronic time stretcher of concept 2, wherein said electro-optic portion comprises an electro-optic polymer. 
     4. The controllable opto-electronic time stretcher of concept 2, wherein the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     5. The controllable opto-electronic time stretcher of concept 4, wherein the core of the first waveguide comprises a SiO2 inner core sandwiched by Si slabs and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     6. The controllable opto-electronic time stretcher of concept 5, wherein said layer of electro-optic material is arranged between the first and second waveguides cores in the coupling portion. 
     7. The controllable opto-electronic time stretcher of concept 6, wherein said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     8. An electro-optical analog to digital converter having non-uniform sampling, comprising: 
     a source of light arranged for generating at least one light pulse having a first spectral width; 
     a chirp element arranged for spreading in time the wavelengths of said at least one light pulse; 
     an electro-optic modulator arranged for modulating the time-spread light pulse with an input analog signal into a modulated optical signal; 
     at least one controllable opto-electronic time stretcher according to concept 1, arranged for receiving the modulated optical signal in the first wave guide and for controlling the refractive index according to a predetermined non-uniform pattern; 
     at least one sampler arranged for sampling at a predetermined rate the output of the at least one controllable opto-electronic time stretcher; and 
     a calculator arranged for constructing a digitized image of the input analog signal based on the samples generated by the at least one sampler and based on said predetermined non-uniform pattern. 
     9. The electro-optical analog to digital converter having non-uniform sampling of concept 8: 
     wherein said source of light is arranged for generating said at least one light pulse as one light pulse of a train of identical light pulses; the chirp element being arranged to generate overlapping time-spread light pulses, and the electro-optic modulator being arranged to modulate the overlapping time-spread light pulses with an input analog signal into said modulated optical signal; 
     wherein the electro-optical analog to digital converter further comprises a time-controlled demultiplexer for separating said modulated optical signal into a plurality of modulated optical signal segments; 
     wherein said at least one controllable opto-electronic time stretcher comprises one controllable opto-electronic time stretcher for receiving each modulated optical signal segment in its first wave guide 
     wherein said at least one sampler comprises one sampler for sampling the output of each controllable opto-electronic time stretcher; and 
     wherein said calculator is arranged for constructing a digitized image of the input analog signal based on the samples generated by each sampler, based on the order in which each modulated optical signal segment is generated by the demultiplexer, and based on said predetermined non-uniform pattern. 
     10. A method for controllably time stretching an input optical signal; the method comprising: 
     providing a first wave guide having a first refraction index characteristic, said first waveguide arranged for receiving said input optical signal; 
     providing a second waveguide having a second refraction index characteristic; and 
     coupling the second waveguide to the first waveguide with a controllable degree of coupling. 
     11. The method of concept 10, wherein coupling the second waveguide to the first waveguide with a controllable degree of coupling comprises controllably changing the refraction index of one of the first and second waveguide at a point of coupling between the first and second waveguides. 
     12. The method of concept 11, wherein said controllably changing the refraction index of one of the first and second waveguide comprises providing at least one of the first and second waveguides at said point of coupling with an electro-optic portion and submitting the electro-optic portion to a controllable electric field. 
     13. The method of concept 12, wherein said electro-optic portion comprises an electro-optic polymer. 
     14. The method of concept 12, wherein the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     15. The method of concept 14, wherein the core of the first waveguide comprises a SiO2 inner core sandwiched by Si slabs and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     16. The method of concept 15, wherein said layer of electro-optic material is arranged between the cores of the first and second waveguides at the point of coupling. 
     17. The method of concept 16, wherein said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     18. A method of converting an analog input signal into a digital output signal, the method, comprising: 
     generating at least one light pulse having a first spectral width; 
     spreading in time the wavelengths of said at least one light pulse; 
     modulating the time-spread light pulse with said input analog signal into a modulated optical signal; 
     controllably time stretching the modulated optical signal according to the method of concept 10, said first waveguide receiving the modulated optical signal and said coupling with a controllable degree of coupling following a predetermined non-uniform pattern; 
     sampling at a predetermined rate the output of the first and second waveguide; and 
     constructing a digital output signal corresponding to a digitized image of the analog input signal, based on the samples generated by the sampling and based on said predetermined non-uniform pattern. 
     19. The method of concept 18, wherein: 
     said generating at least one light pulse comprises generating said at least one light pulse as one light pulse of a train of identical light pulses; 
     said spreading in time the wavelengths of said at least one light pulse comprises generating overlapping time-spread light pulses; 
     said modulating the time-spread light pulse comprises modulating the overlapping time-spread light pulses with said input analog signal into said modulated optical signal; 
     the method further comprising separating said modulated optical signal into a plurality of successive modulated optical signal segments; 
     wherein said controllably time stretching the modulated optical signal according to the method of concept 10 comprises controllably time stretching separately each of the successive modulated optical signal segments by varying, according to said predetermined non-uniform pattern, a coupling between a first waveguide receiving each modulated optical signal segment and a second waveguide; 
     wherein said sampling at a predetermined rate the output of the first and second waveguides comprises sampling each time-stretched modulated optical signal segment at said predetermined rate; and 
     wherein said constructing a digital output signal comprises constructing a digitized image of the input analog signal based on the samples generated for each time-stretched modulated optical signal segment, based on the order in which each time-stretched modulated optical signal segment is generated, and based on said predetermined non-uniform pattern. 
     20. A method of sampling an analog signal having a predetermined spectrum, the method comprising: 
     assuming that said analog signal corresponds to a K sparse vector of N coefficients, with K&lt;&lt;N; 
     spreading in time said analog signal according to a predetermined pseudo-random pattern into a pseudo-randomly time-spread signal; 
     on a predetermined time period, taking M samples by sampling the pseudo-randomly time-spread signal at a predetermined rate, said predetermined rate being inferior to the Nyquist rate, where M≧K·log(N/K); 
     associating the M samples to the time at which they would have been taken if said analog signal had been spread uniformly during said time period by a spread factor equal to the mean of the pseudo-randomly spread factor on said time period; and 
     determining iteratively the closest output signal that would have allowed extracting the M samples at their associated times. 
     21. The method of concept 20, comprising: 
     assuming that the analog signal can be expressed as a K sparse vector comprising a number N of DFT coefficients, with K&lt;&lt;N; 
     assuming that the set Y of M samples is such that Y=Φ·X, where Φ is a M by N matrix; 
     assuming that X can be written as X=Ψ·S, where Ψ is a N×N matrix and S is a N-coefficients vector having only K non-zero coefficients, whereby Y=θ·S, with θ=Φ·Ψ; 
     solving the linear program: 
     Ŝ=argmin∥S∥ 1 , subject to Y=θ·S 
     
       
         
           
             
               where 
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                    
                   S 
                    
                 
                 1 
               
             
             = 
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 N 
               
               ⁢ 
               
                  
                 
                   S 
                   ⁡ 
                   
                     ( 
                     k 
                     ) 
                   
                 
                  
               
             
           
         
       
     
     and finding X using X=Ψ·S. 
     22. An electro-optical analog to digital converter having non-uniform sampling, comprising: 
     a source of light arranged for generating at least one light pulse having a first spectral width; 
     a chirp element arranged for spreading in time the wavelengths of said at least one light pulse; 
     an electro-optic modulator arranged for modulating the time-spread light pulse with an input analog signal into a modulated optical signal; 
     at least one controllable opto-electronic time stretcher arranged for variably spreading in time the modulated optical signal according to a predetermined non-uniform pattern; 
     at least one sampler arranged for sampling at a predetermined rate the output of the at least one controllable opto-electronic time stretcher; and 
     a calculator arranged for constructing a digitized image of the input analog signal based on the samples generated by the at least one sampler and based on said predetermined non-uniform pattern. 
     23. The electro-optical analog to digital converter having non-uniform sampling of concept 22: 
     wherein said source of light is arranged for generating said at least one light pulse as one light pulse of a train of identical light pulses; the chirp element being arranged to generate overlapping time-spread light pulses, and the electro-optic modulator being arranged to modulate the overlapping time-spread light pulses with an input analog signal into said modulated optical signal; 
     wherein the electro-optical analog to digital converter further comprises a time-controlled demultiplexer for separating said modulated optical signal into a plurality of modulated optical signal segments; 
     wherein said at least one controllable opto-electronic time stretcher comprises one controllable opto-electronic time stretcher for variably spreading in time according to a predetermined non-uniform pattern each modulated optical signal segment; 
     wherein said at least one sampler comprises one sampler for sampling the output of each controllable opto-electronic time stretcher; and 
     wherein said calculator is arranged for constructing a digitized image of the input analog signal based on the samples generated by each sampler, based on the order in which each modulated optical signal segment is generated by the demultiplexer, and based on said predetermined non-uniform pattern. 
     24. The electro-optical analog to digital converter having non-uniform sampling of concept 22, wherein the controllable opto-electronic time stretcher comprises: 
     a first wave guide; 
     a second waveguide coupled to the first waveguide along a coupling portion; 
     wherein at least one of the first and second waveguides in the coupling portion has a controllable refractive index. 
     25. The electro-optical analog to digital converter having non-uniform sampling of concept 24, wherein said at least one of the first and second waveguides in the coupling portion having controllable refractive index comprises an electro-optic portion and an electrode capable of submitting the electro-optic portion to a controllable electric field. 
     26. The electro-optical analog to digital converter having non-uniform sampling of concept 25, wherein said electro-optic portion comprises an electro-optic polymer. 
     27. The electro-optical analog to digital converter having non-uniform sampling of concept 24, wherein the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     28. The electro-optical analog to digital converter having non-uniform sampling of concept 27, wherein the core of the first waveguide comprises a SiO2 inner core sandwiched by Si slabs and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     29. The electro-optical analog to digital converter having non-uniform sampling of concept 25, wherein said electro-optic portion is arranged between the cores of the first and second waveguides in the coupling portion. 
     30. The electro-optical analog to digital converter having non-uniform sampling of concept 29, wherein said electro-optic portion is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide. 
     31. A method of converting an analog input signal into a digital output signal, the method, comprising: 
     generating at least one light pulse having a first spectral width; 
     spreading in time the wavelengths of said at least one light pulse; 
     modulating the time-spread light pulse with said input analog signal into a modulated optical signal; 
     controllably time stretching the modulated optical signal according to a predetermined non-uniform pattern; 
     sampling at a predetermined rate the output of the first and second waveguide; and 
     constructing a digital output signal corresponding to a digitized image of the analog input signal, based on the samples generated by the sampling and based on said predetermined non-uniform pattern. 
     32. The method of concept 31, wherein: 
     said generating at least one light pulse comprises generating said at least one light pulse as one light pulse of a train of identical light pulses; 
     said spreading in time the wavelengths of said at least one light pulse comprises generating overlapping time-spread light pulses; 
     said modulating the time-spread light pulse comprises modulating the overlapping time-spread light pulses with said input analog signal into said modulated optical signal; 
     the method further comprising separating said modulated optical signal into a plurality of successive modulated optical signal segments; 
     wherein said controllably time stretching the modulated optical signal comprises controllably time stretching separately each of the successive modulated optical signal segments according to said predetermined non-uniform pattern; 
     wherein said sampling at a predetermined rate the output of the first and second waveguides comprises sampling each time-stretched modulated optical signal segment at said predetermined rate; and 
     wherein said constructing a digital output signal comprises constructing a digitized image of the input analog signal based on the samples generated for each time-stretched modulated optical signal segment, based on the order in which each time-stretched modulated optical signal segment is generated, and based on said predetermined non-uniform pattern. 
     33. The method of concept 31, wherein said controllably time stretching the modulated optical signal comprises: 
     providing a first wave guide having a first refraction index characteristic, said first waveguide receiving said input optical signal; 
     providing a second waveguide having a second refraction index characteristic; and 
     coupling the second waveguide to the first waveguide with a controllable degree of coupling. 
     34. The method of concept 33, wherein coupling the second waveguide to the first waveguide with a controllable degree of coupling comprises controllably changing the refraction index of one of the first and second waveguide at a point of coupling between the first and second waveguides. 
     35. The method of concept 34, wherein said controllably changing the refraction index of one of the first and second waveguide comprises providing at least one of the first and second waveguides at said point of coupling with an electro-optic portion and submitting the electro-optic portion to a controllable electric field. 
     36. The method of concept 34, wherein said electro-optic portion comprises an electro-optic polymer. 
     37. The method of concept 36, wherein the first waveguide comprises a slot waveguide and the second waveguide comprises a strip waveguide. 
     38. The method of concept 37, wherein the core of the first waveguide comprises a SiO2 inner core sandwiched by Si slabs and the second waveguide comprises a Si x N y  core; the first and second waveguides having a SiO 2  cladding. 
     39. The method of concept 38, wherein said layer of electro-optic material is arranged between the cores of the first and second waveguides at the point of coupling. 
     40. The method of concept 39, wherein said layer of electro-optic material is arranged in contact with the core of the first waveguide; the controllable opto-electronic time stretcher further comprising a tunnel layer of oxidized Si x N y  arranged in contact with the electro-optic material and a SiO 2  layer arranged between the tunnel layer and the core of the second waveguide.