Abstract:
Apparatus and method for high-speed analog-to-digital conversion employs photonic circuits with variable light-absorption. A coherent light source is clocked to produce a stream of coherent light pulses onto which is modulated an analog signal to be converted to digital form. An optical splitter divides the stream of pulses into a number of streams that is proportional to the resolution sought. A passive photonic quantizer absorbs, to a predetermined degree, the light energy of the incident pulse streams. The output of a fully absorbed pulse does not trigger a comparator. Pulse streams of sufficient intensity pass through the light absorbers, and their output does trigger the comparator. The output state of the comparators may be read into a digital memory to form a digital word that represents the analog signal during any discrete clock interval. Most of the elements of the apparatus can be fabricated on a substrate of photonic semiconducting material.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to high speed electro-optic analog-to-digital converters, and, more specifically, to such analog-to-digital converters in which the quantization of the analog signal is performed entirely by passive photonic elements. This invention also relates to the field of integrated photonic structures in which such electro-optical analog-to-digital converters may be monolithically fabricated. 
     Analog-to-digital converters (“ADCs”) have traditionally been fabricated using microelectronics. For low-speed (i.e., low-bandwidth) analog signals, microelectronic based ADCs are adequate. A problem arises where the analog signal to be digitized is broadband or where it must be sampled at the carrier&#39;s frequency. Under these circumstances, microelectronic ADCs quickly approach the limits of their performance. Furthermore, while the speed and resolution of microelectronic ADCs have evolved over the years, this evolution is slow—e.g., six years of development may yield only one bit of improvement in the resolution of a microelectronic ADC. (Robert H. Walden, “Analog-to-Digital Converter Survey and Analysis”, 17  IEEE Journal on Selected Areas in Communications,  539-550 (April 1999)). Unfortunately, military radar and electronic warfare systems require more than what the state of the art can provide in microelectronic ADCs. 
     It is thus desirable to provide faster analog-to-digital conversion at higher resolution. Optical sampling provides a significant improvement in ADC performance. In conventional microelectronic ADCs, conversion speed (interchangeable with bandwidth) must be traded for resolution (the length in bits of the representative digital word). With microelectronics, it is simply not possible to achieve both wide bandwidth and high resolution. Optical ADCs, however, overcome this constraint by rapidly converting broadband analog signals to highly resolved digital representation (i.e., words with many bits). 
     In an optimal analog-to-digital conversion, i.e., with wide bandwidth and high resolution, most of the conversion takes place on a modulated light signal with a maximum of optical processing. Though the prior art tried this approach, it has failed to make use of the full extent of optical processing. Specifically, the prior art still relegates to microelectronics the quantization step of analog-to-digital conversion. Thus the prior-art optical ADCs are suboptimal. 
     Both time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) optical ADCs exist in the prior art. They are physically large, complicated branched structures that are very sensitive to optical path length and temperature effects. They require precise timing to reconstitute signals. Most importantly, they require microelectronic ADCs to quantize and digitize the sampled analog signals that emanate from each optical branch. Though they do sample a wide band analog signal by dividing it into a set of discrete optical signals, both TDM and WDM optical ADCs require microelectronic ADCs to complete the analog-to-digital conversion. 
     U.S. Pat. No. 5,010,346 to Hamilton et al. discloses a mode-locked laser source divided into a plurality of optical signals, each of which is modulated by an analog signal. This plurality of modulated optical signals is detected optically and amplified. Hamilton et al. requires that the modulated optical signal be split as many times as is necessary to ensure that the resultant bandwidth of the “split” signal falls within the speed and resolution capability of the microelectronic quantization portion of the circuit. Accordingly, Hamilton, et al. teaches a microelectronic ADC to quantize and digitize each and every one of the plurality of detected outputs. Thus the apparatus of Hamilton et al. ceases to be optical downstream of the detectors. Therefore the resolution and conversion speed limitations typical of microelectronic ADCs limits the utility of Hamilton et al.&#39;s apparatus for broadband signal applications. 
     U.S. Statutory Invention Registration USH0000353 to Taylor teaches pairs of optical waveguides on a substrate. A modulator induces either of two possible orthogonal phases that represent the amplitude of the sampled analog signal. Optical phase detectors produce one of two possible least significant bit (“LSB”) states (i.e., “1” or “0”) that depends on the phase state detected. Taylor teaches how to process LSB representations to form a complete digital word that represents the sampled analog signal. Taylor neither teaches nor suggests a means by which a quantized digital word can be formed without digital processing, and therefore, without slowing the analog-to-digital conversion by the time required for digital processing. Therefore the performance of Taylor&#39;s apparatus on broadband signals is inherently extremely limited. 
     U.S. Pat. No. 4,325,603 to Marom also teaches parallel optical waveguides disposed on a substrate wherein laser light is coupled between optical waveguide pairs. The amount of coupling is proportional to the analog signal applied. Marom teaches further that the length of the coupling region between waveguide pairs can be predetermined. Thus Marom eliminates the need for phase shifting the laser light (and the need for light polarizers to obtain bi-phase light). Marom requires, however, two optical waveguide channels for each single bit state determination (i.e., “1” or “0”) and two comparators to determine the relative intensity of the waveguides. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus that overcomes the prior art&#39;s dependency on conventional microelectronic circuits to perform optical analog-to-digital conversion. 
     It is a further object of the present invention to provide passive quantization with photonic materials that can be selectively altered to absorb different amounts of light. 
     It is still a further object of the present invention to provide an apparatus where a converted analog signal is digitally represented at the output of a passive photonic quantizer and stored in a digital memory. 
     It is yet a further object of the present invention to provide an apparatus that determines a digital bit state of “1” or “0” as the result of a single threshold detection of a single pulse of modulated light. 
     An additional object of the present invention is to provide a light source, an electro-optical modulator, an optical splitter, and the elements of a passive photonic quantizer on a common substrate. 
     Yet another additional object of the present invention is to provide an apparatus passive photonic quantizer substrates. 
     Briefly stated, apparatus and method for high-speed analog-to-digital conversion employs photonic circuits with variable light-absorption. A coherent light source is clocked to produce a stream of coherent light pulses onto which is modulated an analog signal to be converted to digital form. An optical splitter divides the stream of pulses into a number of streams that is proportional to the resolution sought. A passive photonic quantizer absorbs, to a predetermined degree, the light energy of the incident pulse streams. The output of a fully absorbed pulse does not trigger a comparator. Pulse streams of sufficient intensity pass through the light absorbers, and their output does trigger the comparator. The output state of the comparators may be read into a digital memory to form a digital word that represents the analog signal during any discrete clock interval. Most of the elements of the apparatus can be fabricated on a substrate of photonic semiconducting material. 
     A passive photonic quantizer receives the output of each branch of the optical power divided into an equal number of light absorbers. Each absorber exhibits a predetermined absorption, i.e., some light absorbers allow a given light pulse to pass through to a detector, while other light absorbers absorb the pulse so that it is not detected. The present invention thus achieves the requisite quantization for analog-to-digital conversion. Optical delay lines delay pulses that pass through channels with less absorbency compared to pulses that pass through channels with greater absorbency. This optical delay insures that all light pulses represent the same sample of the analog input signal at a particular instant. The output voltage of the optical detectors varies with the intensity of the incident light minus the effect of the absorption channel. The light pulses are effectively quantized at the output of the optical detectors, but they require comparison to a reference voltage in comparators. The output of the comparators forms a digital word that is read directly into a digital memory. The delay lines insure that all comparator outputs represent the same sample point on the input analog signal when they are read into the digital memory. 
     A greater or lesser degree of quantization can be achieved by increasing or decreasing the number of optical splitter branches and accordingly the number of light absorbers, optical delay lines, threshold detectors, and comparators. A longer or shorter digital word of greater or lesser resolution is thereby formed. 
     As to the photonic quantizer and the light absorbers, only one light absorber is required to quantize a single bit of the digital word, in contrast to prior-art devices, which require a minimum of two optical waveguides: one for the light pulse, and one to serve as the coupled light path. Alternatively, the prior art uses a pair of optical waveguides and detects the relative phase of the light pulse travelling down each of them. In either prior-art device, only one bit state is determined. In contrast, the present invention determines a bit state from only one light absorber. Nothing in the prior art teaches or suggests this feature of the present invention. 
     Thus the present invention (1) quantizes an analog signal with half the number of optical structures (channels or waveguides) that the prior art requires; and (2) directly forms a digital representation of the analog signal without having to compare the relative phase or intensity of light between each of a pair of waveguides. Therefore the present invention represents a significant improvement on the prior art. 
     According to an embodiment of the invention, a photonic analog-to-digital converter, comprises: a source of coherent light; a sampling clock, effective for converting the coherent light into a first stream of coherent light pulses; an electro-optic modulator, effective for receiving as inputs the first stream and an analog signal, the output of the modulator being a second stream of modulated coherent light pulses; an optical splitter, effective for splitting the second stream into a plurality of streams, each of the streams comprising modulated coherent light pulses; a passive photonic quantizer, effective for quantizing the plurality of streams, the quantizer further comprising a plurality of light absorbers; a plurality of optical delay lines, effective for aligning a quantized plurality of streams to a discrete instant at which each member of the plurality of streams was sampled from the analog signal; a plurality of optical detectors, effective for converting the quantized plurality of streams into a plurality of detected voltages; a plurality of comparators for comparing the plurality of detected voltages to a reference voltage, whereby a logical “1” is produced if the detected voltage is at least equal to a reference voltage and a logical “0” is produced if the detected voltage is less than the reference voltage; and a digital device, effective for storing a plurality of logical “1” and logical “0” as a digital word synchronous with the sampling clock. 
     According to a feature of the invention, a method for digitizing an analog signal, comprises the steps of: generating coherent light; generating a clock waveform; sampling the coherent light with the clock waveform to produce a first stream of coherent light pulses; modulating an input analog signal onto the first stream to produce a second stream of modulated coherent light pulses; splitting the second stream into a plurality of streams, each comprising modulated coherent light pulses; passively quantizing the plurality of streams by absorbing the pulses in a like plurality of light absorbers; adjusting in a predetermined fashion an absorption characteristic of each of the plurality of light absorbers; absorbing the plurality of streams in varying degree, whereby some pulses are passed and other pulses are fully absorbed; aligning a quantized plurality of streams to the same instant relative to the time at which they were sampled from the analog signal; detecting each of the quantized plurality of streams, thereby producing a proportional voltage; comparing, within the period of the clock waveform, the proportional voltage to a reference voltage, thereby producing a logical “1” if the proportional voltage is at least equal to the reference voltage and producing a logical “0” if the proportional voltage is less than the reference voltage; and storing the logical “1” and the logical “0” in a digital device. 
     According to another feature of the invention, a photonic analog-to-digital converter, comprises: means for generating coherent light; means for generating a clock waveform; means for sampling the coherent light with the clock waveform to produce a first stream of coherent light pulses; means for modulating an input analog signal onto the first stream to produce a second stream of modulated coherent light pulses; means for splitting the second stream into a plurality of streams, each comprising modulated coherent light pulses; means for passively quantizing the plurality of streams by absorbing the pulses in a like plurality of light absorbers; means for adjusting in a predetermined fashion an absorption characteristic of each of the plurality of light absorbers; means for absorbing the plurality of streams in varying degree, whereby some pulses are passed and other pulses are fully absorbed; means for aligning a quantized plurality of streams to the same instant relative to the time at which they were sampled from the analog signal; means for detecting each of the quantized plurality of streams, thereby producing a proportional voltage; means for comparing, within the period of the clock waveform, the proportional voltage to a reference voltage, thereby producing a logical “1” if the proportional voltage is at least equal to the reference voltage and producing a logical “0” if the proportional voltage is less than the reference voltage; and means for storing the logical “1” and the logical “0” in a digital device. 
     The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram representation of a 2-bit form of the present invention. 
     FIG. 2 plots the absorbency of the channel in the passive photonic quantizer of the present invention against the wavelength of incident light for both 250 nanometers (“nm”) and 750 nm InGaAs layers grown on InP substrates. 
     FIG. 3 plots the transmittance of the absorber in the passive photonic quantizer of the present invention against the intensity of incident light for both 250 nm and 750 nm InGaAs layers grown on InP substrates. 
     FIG. 4 is a perspective view of a 2-bit integrated passive photonic quantizer of the present invention showing a three-way optical power divider, three light absorbers, two optical delay lines, three optical detectors, three comparators and a digital memory integrated onto a single substrate. 
     FIG. 5 is a perspective view of an array of passive photonic quantizers formed through vertical stacking. 
     FIG. 6 depicts a 4-bit integrated passive photonic quantizer fabricated on a single substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a 2-bit photonic ADC of the present invention includes a coherent light source  110  (in the preferred embodiment, a mode-locked laser), an electro-optic modulator  100 , a sampling clock  120 , an optical splitter  130 , a passive photonic quantizer  140  containing light absorbers  151 ,  152 , and  153 ; optical delay lines  181  and  182 ; optical detectors  161 ,  162 , and  163 ; comparators  191 ,  192 , and  193 ; and a digital storage device  170  (in the preferred embodiment, a digital memory). 
     Coherent light source  110  samples the analog signal input at a clock rate provided by sampling clock  120  using electro-optic modulator  100 . The output of electro-optic modulator  100  is fed into optical splitter  130 . FIG. 1 shows, by a series of three dots, that a plurality of output branches from optical splitter  130  is possible. The number of outputs from optical splitter  130  in the preferred embodiment depends upon the bandwidth of the analog signal to be converted and the desired resolution (i.e., number of bits) of the digital representation. The number of light absorbers  151 ,  152 , and  153 ; optical delay lines  181  and  182 ; optical detectors  161 ,  162 , and  163 ; and comparators  191 ,  192 , and  193  scale in number equally to the number of output branches in optical splitter  130 . In turn, the number of branches in optical splitter  130  will equal to 2 N −1, where N is the number of bits in the digital representation. In FIG. 1 the bit length N is two; it follows that there must be three output branches from optical splitter  130 . 
     The sampled optical signal is a series of light pulses generated at the clock rate. Each pulse is modulated by the analog signal at successively different clock intervals, passes out of the optical splitter branches, and is passed on to passive photonic quantizer  140 . Passive photonic quantizer  140  converts these light pulses into a digital format. Within passive photonic quantizer  140  are a number of light absorbers  151 ,  152 , and  153 ; optical delay lines  181  and  182 ; optical detectors  161 ,  162 , and  163 ; and comparators  191 ,  192 , and  193 . The number of light absorbers  151 ,  152 , and  153 ; optical delay lines  181  and  182 ; optical detectors  161 ,  162 , and  163 ; and comparators  191 ,  192 , and  193  depends on the bandwidth of the analog signal and the digital resolution sought. 
     The design of passive photonic quantizer  140  is based on the absorbency of light absorbers  151 ,  152 , and  153 . The sampled and modulated optical signal travels in parallel to an array of 2 N −1 light absorbers  151 ,  152 , and  153  of varying absorbency. The optical properties of light absorbers  151 ,  152 , and  153  are tailored so that their linear absorbency scales monotonically over the range of channels. Non-linear absorbency can be added by varying the material properties of the absorber to yield an absorbency that is a function of incident light intensity, i.e., a non-linear function. The absorbency is scaled from a very low value for first light absorber  153  in the array to a very high value for the (2 N −1) th  light absorber  151  in the array. 
     Suppose a sampled light pulse of very low intensity, corresponding to a small voltage, is incident upon each of light absorbers  151 ,  152 , and  153 . The pulse will be absorbed by the (2 N −1) th  light absorber  151  and by light absorbers  152  and  153  in the array. Likewise, all corresponding optical detectors  161 ,  162 , and  163  remain “OFF”. Suppose now a sampled light pulse of slightly greater intensity is incident upon light absorbers  151 ,  152 , and  153 . Since light absorber  153  has the lowest absorbency, it may pass a very weak signal. If absorber  153  does indeed pass the sampled light pulse, optical detector  163  is turned “ON”. Optical detectors  161  and  162  remain “OFF”, because the optical signal did not pass through corresponding light absorbers  151  and  152 . The digital representation of the detected analog signal then corresponds to a bit pattern output from comparators  191 ,  192 , and  193  that follow optical detectors  161 ,  162 , and  163 . We consider the “OFF” comparators to have a value ‘0’ and the “ON” comparators to have a value ‘1’. 
     Similarly, suppose a signal with a slightly larger intensity, corresponding to a larger voltage, is incident upon light absorbers  151 ,  152 , and  153 . The signal is absorbed by the upper light absorbers but passed by the remaining light absorbers, turning on the corresponding comparators. A corresponding bit pattern is once again formed. Optical delay lines  181  and  182  are required to match the optical time delay through all 2 N −1  light paths of passive photonic quantizer  140 . Optical time delay through each light absorber is unique, related to the absorption property of that particular light absorber. Because the analog signal must be quantized at the same point on its waveform, optical delay must be employed to align all the sampled light signals to represent concurrent time samples of the analog signal at the moment they are clocked at the output of comparators  191 ,  192 , and  193  and into digital memory  170 . Optical delay lines  181  and  182  perform this function by providing maximum optical time delay to the light absorbers with the least absorbency and minimum optical time delay to the light absorbers with the greatest absorbency. Thus, no optical delay line need follow light absorber  151 , which has the maximum absorption. Since light absorber  153  has less absorption, optical delay line  182  follows it, giving it a greater time delay. 
     Referring to FIG. 2, the optical properties of semiconductor InGaAs are essential to the light absorbers of passive photonic quantizer  140  of the present invention. Of particular importance are the linear absorption coefficient and the saturation intensity. InGaAs was chosen for its optical properties around 1550 nanometers (nm) wavelength. This wavelength coincides with the minimum loss transmission window of optical fiber. The bulk samples tested consisted of a 250 nm In 0.53 Ga 0.47 As layer, on a semi-insulating InP substrate and a 750 nm In 0.53 Ga 0.47 As layer, also on a semi-insulating InP substrate. The absorbency, αL, of the quantum wells and is given (from Beer&#39;s law) by: 
     
       
         α L=− ln( T/T   o ), 
       
     
     where T is the transmission through the multiple quantum well (MQW) sample and T o  is the source transmission. The absorption coefficient α is found by dividing the absorbency by the total thickness L of the InGaAs layer. The absorbency spectra of the two light absorber samples, absorbency versus wavelength, are plotted in FIG.  2 . 
     The linear absorbency spectra of the 250 nm and the 750 nm samples are similar in shape. From the graphs, the absorption coefficient of the 250 nm sample is 5880/cm; of the 750 nm sample, 5506/cm. These values compare well with previously published values of 6800/cm and 6000/cm. The absorption spectra were taken at multiple spots on each sample to verify the results. 
     Referring to FIG. 3, the nonlinear absorbency (or saturation intensity) of the two InGaAs samples is critical. The architecture of the present invention operates in both the linear and non-linear absorbency regions of InGaAs. 
     FIG. 3 shows transmission through the samples as a function of incident intensity. The saturation intensity, I sat  is related to intensity dependent absorption as:            α        (     I   in     )       =       α   o       1   +       I   in       I   sat             ,                          
     where α o  is the small signal or linear absorption coefficient and I in  is the incident intensity. When I in =I sat , the absorption has decreased by half its linear value. However, I in  decreases along the length of the sample because the material absorbs light. The change in intensity per unit length is a function of position within the sample, z, as: 
     
       
           I ( z+∂z )= I ( z ) e   −α(z)∂)z   
       
     
     Integrating over the unit length yields:            ln        (     I   out     )       -     ln        (     I   in     )       +         I   out     -     I   in         I   sat         =       -     α   o          L                            
     We may solve for I sat  from the experimental data. The saturation intensity of the 250 nm sample is approximately 100 KW/cm 2 ; of the 750 nm sample, 30 KW/cm 2 . One might expect the saturation intensity to be much higher in the 750 nm sample, as it is three times as thick as the 250 nm sample. That the saturation intensity is lower may be attributed to strain in the 250 nm sample. 
     These measurements show the entire linear and non-linear regions over which the absorbers in the photonic analog-to-digital (ADC) architecture of the present invention can operate. 
     Referring to FIG. 4, the components of the present invention may be integrated on a single substrate  190  by integrated electro-optic techniques. These components include coherent light source  110  (a mode-locked laser, not shown); electro-optic modulator  100  (not shown); optical splitter  130 ; passive photonic quantizer  140 ; and digital memory  170 . Passive photonic quantizer  140  is in turn comprised of light absorbers  151 ,  152 , and  153 ; optical delay lines  181  and  182 ; optical detectors  161 ,  162 , and  163 ; and comparators  191 ,  192 , and  193 . The passive nature of this architecture makes it desirable for applications that require low power. As in electronic flash ADC systems, the total number of comparators (i.e., light absorption channels in the present invention) is 2 N −1, where N is the number of bits of resolution. For example, a system with two bits of resolution, as depicted here, would require three light  1 o absorbers  151 ,  152 ,  153 . 
     Referring to FIG. 5, the present invention may be configured in a stack to form an array of passive photonic quantizers  140 . A plurality of substrates  190  can be monolithically fabricated in layers, each layer possessing all the elements of passive photonic quantizer  140 . Each layer can independently convert a stream of modulated coherent light pulses. An independent digital memory  170  is associated with each layer, clocking in a discrete digital word that corresponds to each sample of a modulated, coherent stream of light pulses. Simultaneous parallel quantization, as shown in FIG. 5, provides real-time analog-to-digital conversion of a broadband signal, which is divided into a number of signals each of narrower bandwidth and input into such an array. Alternatively, the stacked configuration shown in FIG. 5 can digitize several independent coherent streams of light pulses emanating from multiple sources. 
     Referring to FIG. 6, large-scale planar quantization demands a high order of optical splitting, quantizing, detecting, comparing, forming digital words, and storing on a singular planar substrate  190 . Such quantization is limited by the minimum detectable quantization voltage on the one hand, and the fidelity with which the optical splitter, light absorbers, and optical delay lines can be fabricated on the other. When these limitations are reached, the same resolution can be achieved by employing a stacked array of passive photonic quantizers, thereby overcoming the limits of minimum detectable voltage and fabrication. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.