Optical conversion system and method with multiple amplitude processing

This application is related to U.S. patent application Ser. No. 13/963,899, titled “OPTICAL CONVERSION SYSTEM AND METHOD WITH MULTIPLE PHASE PROCESSING” filed on an even date herewith by Zanoni, U.S. patent application Ser. No. 13/787,202 titled “OPTICAL SAMPLE AND HOLD SYSTEM AND METHOD” filed Mar. 6, 2013, by Zanoni et al., U.S. patent application Ser. No. 13/240,226, titled “OPTICAL SAMPLE AND HOLD”, filed on Sep. 22, 2011, by Zanoni et al., now issued U.S. Pat. No. 8,456,336 U.S. patent application Ser. No. 13/204,158 entitled “WIDE BAND DIGITAL RECEIVER: SYSTEM AND METHOD”, filed on Aug. 5, 2011, by Zanoni et al., U.S. patent application Ser. No. 13/626,642 entitled “OPTICAL ANALOG-TO-DIGITAL CONVERSION SYSTEM AND METHOD WITH ENHANCED QUANTIZATION”, filed on Sep. 25, 2012, by Zanoni et al., and U.S. patent application Ser. No. 13/536,929 entitled “PIPELINED RECEIVER SYSTEM AND METHOD”, filed on Jun. 28, 2012, by Zanoni et al., all assigned to the Assignee of this patent application and incorporated in their entireties herein by reference.

The present specification relates to optical or photonic processors, including but not limited to a processor for a photonic analog-to-digital converter (pADC) in high frequency (e.g., 100+ megahertz (MHz) and multi-gigahertz (GHz) radio frequency (RF)) systems. More particularly, the present specification relates to a multi-amplitude photonic processor.

Photonic processing techniques are used in a wide variety of applications. Communication and sensing devices frequently use photonic processing based analog-to-digital conversion to convert an analog signal to a digital signal. High speed analog-to-digital conversion is utilized in high capacity optical communications, radio frequency (RF) receivers used in military, commercial and consumer applications. Products that utilize analog-to-digital signal conversion include, but are not limited to: electronic intelligence (ELINT) receivers, cell phones, wireless local area network components, radars (e.g., synthetic aperture radar (SAR)), sensors, high frequency (e.g., 100 megahertz (MHz) and multi-Giga Hertz (GHZ)) systems, coherent optical communications systems, etc.

Optical or photonic analog-to-digital converters (pADC) can reduce the size, weight, and power requirements of the systems in which they are employed. The pADC generally includes, temporal and or wavelength multiplexing/demultiplexing components, and at least one electronic analog-to-digital converter (eADC). The optical components can add to the size, weight and power usage of the optical portion of the pADC. The eADC is generally a high resolution ADC which adds to the size, weight and power usage of the electronic portion of the pADC.

Accordingly, there is a need for an analog-to-digital conversion system and method configured for reduced size, cost, weight and/or power consumption. Further still, there is a need for an improved photonic analog-to-digital converter (pADC) system and method which provides multi-amplitude demodulation. Yet further, there is a need for an ADC with a simplified optical processor that provides amplitude demodulation. Still further there is a need for a pADC that does not require a high resolution eADC. There is further a need for a pADC system and method that provides multi-amplitude signal processing. There is also a need for a flash photonic multi-amplitude converter. There is further a need for a high speed flash photonic processor for amplitude modulated signals with reduced circuit size.

It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.

An exemplary embodiment relates to a photonic processor. The photonic processor includes a first input configured to receive an amplitude-modulated optical pulse signal. The photonic processor also includes a second input configured to receive an optical reference signal. The photonic processor also includes stages. Each stage is configured to receive a version of the amplitude-modulated optical pulse signal and an amplitude attenuated version of the optical reference signal. Each stage is comprised of a reference path, a signal path, a first coupler, a second coupler and a balanced photo detector. The first coupler receives the version of the amplitude-modulated optical pulse signal and provides a stage amplitude-modulated optical pulse signal to the signal path. The signal path is coupled to a first input of the balanced photo detector. The second coupler receives the amplitude attenuated version of the optical reference signal and provides a stage optical reference signal to the reference path. The referenced path is coupled to a second input of the balanced photo detector. The stage optical reference signal is amplitude attenuated in accordance with a position of the stage. The balanced photo detector provides an electronic output signal corresponding to an amplitude relationship of the stage optical reference signal and the stage amplitude-modulated optical signal.

Another embodiment relates to a photonic processor including a first input configured to receive a phase-modulated optical pulse signal, a second input configured to receive an optical reference signal, and a number of stages. Each stage is configured to receive an amplitude-modulated optical pulse signal and an attenuated version of the optical reference signal. The attenuated version of the optical reference signal is attenuated in accordance with a position of the stage. Each stage is comprised of a reference path, a signal path, and a directional coupler coupled to the reference path and the signal path and used to compare the amplitude-modulated optical pulse signal to the amplitude reference along the optical reference stage. The stage also is comprised of a balanced photo detector that provides an electronic output signal corresponding to an amplitude relationship of the attenuated version of the optical reference signal and the amplitude-modulated optical pulse signal for the stage.

Another exemplary embodiment relates to a method of multi-amplitude processing. The method includes receiving an amplitude-modulated optical pulse signal at a plurality of stages, and receiving a respective amplitude attenuated version of an optical reference signal at the plurality of stages. The respective amplitude attenuated version of the optical reference signal is amplitude attenuated in accordance with a position of the stage. The method also includes comparing the respective amplitude attenuated version of the optical reference signal with the amplitude-modulated optical pulse signal at each stage, and providing an electronic output signal corresponding to an amplitude relationship of the amplitude attenuated optical reference signal and the amplitude-modulated optical pulse signal at each stage.

Yet another exemplary embodiment relates to a multi-amplitude photonic processor including an amplitude modulator configured to receive an electronic radio frequency signal and an optical clock signal and to provide an amplitude-modulated optical signal on a first signal path. The optical clock is provided on a second signal path. The processor also includes a first signal optical coupler for providing a first level of power of the amplitude-modulated optical signal to a first input of a first detector, and a first reference optical coupler for providing a first level of power of the optical clock signal to a second input of the first photo detector. The first photo detector is a balanced photo detector. The processor also includes a second signal optical coupler for providing a second level of power of the amplitude-modulated optical signal to a third input of a second photo detector. The second level of power of the amplitude-modulated optical signal is approximately equal to the first level of power of the amplitude-modulated optical signal. The processor also includes a second reference optical coupler for providing a second level of power of the optical clock signal to a fourth input of the second photo detector. The second level of power of the optical clock signal is less than the first level of power of the optical clock signal. The second photo detector is a balanced photo detector. The processor includes a first output associated with the first photo detector, and a second output associated with the second photo detector. The first output and the second output provide a thermometer electronic output signal corresponding to an amplitude relationship of the optical clock signal and the amplitude-modulated optical pulse signal.

Embodiments of the invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals denote the same or similar elements and in which:

FIG. 1 is a general schematic block diagram of a photonic analog-to-digital converter (pADC) including a photonic processor according to an exemplary embodiment;

FIG. 2A is a more detailed schematic block diagram of the photonic processor for the photonic analog-to-digital converter (pADC) illustrated in FIG. 1 , according to another exemplary embodiment;

FIG. 2B is a more detailed schematic block diagram of the photonic processor for the photonic analog-to-digital converter (pADC) illustrated in FIG. 1 , according to another exemplary embodiment;

FIG. 3A is a more detailed schematic block diagram of the photonic processor for the photonic analog-to-digital converter (pADC) illustrated in FIG. 1 , according to another exemplary embodiment;

FIG. 3B is a more detailed schematic block diagram of the photonic processor for the photonic analog-to-digital converter (pADC) illustrated in FIG. 1 , according to another exemplary embodiment; and

FIG. 4 is a side view schematic representation of a package for the photonic analog-to-digital converter illustrated in FIG. 1 , according to yet another exemplary embodiment.

Before describing in detail the particular improved system and method, it should be observed that the invention includes, but is not limited to, a novel structural combination of components and not in the particular detailed configurations thereof. Accordingly, the structure, software, methods, functions, control and arrangement of components have been illustrated in the drawings by readily understandable block representations and schematic drawings, in order not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art, having the benefit of the description herein. Further, the invention is not limited to the particular embodiments depicted in the exemplary diagrams, but should be construed in accordance with the language in the claims.

At least one embodiment relates to a photonic processor for use in a photonic analog-to-digital converter (pADC), for example, a pADC provided in a wide band or narrowband RF receiver. However, the pADC systems and methods of the specification can be utilized in any type of conversion applications. The pADC can be used for different types of receiving applications, including but not limited to applications associated with cellular phones, wireless devices, radios (e.g., software defined radio, radio receivers capable of wide band or narrowband receiver operations, etc.), radars (e.g., a digital radar synthetic aperture radar (SAR)), sensors, etc. Advantageously, certain embodiments of systems and methods described herein reduce the need for expensive eADCs and provide multi-amplitude demodulation.

Performing the sampling process using amplitude modulated optical pulses, as contrasted to simply relaying the RF signal to an electronic ADC for sampling, provides advantages in certain embodiments. Optical sampling allows the sampling to occur using an ultra-low jitter optical pulse source in one embodiment. Various embodiments can achieve one or more of the benefits described above.

FIG. 1 shows a photonic analog-to-digital converter (pADC) 100 according to one embodiment. In one embodiment, pADC 100 is embodied as an M-bit pADC based upon N outputs 220 A-E from a photonic processor 130 . N is an integer from 2 to a number where circuit design efficiency is practicable. In one embodiment, N can be 2, 3, 4, 5, 6, 7, 8, 9, . . . 16, 17, 18, 19, . . . 32, . . . 64. pADC 100 utilizes multi-amplitude demodulation and provides a thermometer type signal at outputs 220 A-E (e.g., S 01 -S 0N ) which can be converted to a data signal (e.g., B 1 -B M ) by a digital logic circuit 160 at outputs 161 A-D in one embodiment.

Although specific techniques, parameters, bit values and coordinates for the various conversion systems and methods described herein, they are not disclosed in a limiting fashion. Various adjustments to the types of signals, frequency of operation, types of modulation, etc. can be made without departing from the scope of the invention.

In one embodiment, pADC 100 includes an optical clock 145 for supplying a high repetition rate stream of optical pulses with very low clock jitter (e.g., an optical clock signal or optical reference signal). Examples of this type of device include a mode locked laser (MLL) and a coupled opto-electronic oscillator (COEO). Any type of suitable optical clock can be utilized.

In one embodiment, pADC 100 receives an RF voltage or signal V(t) at an RF input 110 . The RF signal can be a modulated signal representing data. RF input 110 can be any terminal or circuit for receiving an input signal. In one embodiment, RF input 110 is coupled to a wide band antenna (not shown in FIG. 1 ). The input signal at RF input 110 is provided to an amplitude modulator 120 , which performs amplitude modulation in response to the optical pulse signal (e.g., optical clock signal) from splitters or couplers 111 and 112 and in response to the input signal. Amplitude modulator 120 outputs an amplitude modulated optical pulse stream or signal at an output 115 . Amplitude modulator 120 can be a linear amplitude modulator, such as an electro-absorption modulator.

In one embodiment, pADC 100 or processor 130 includes optical splitters or couplers 111 and 112 coupled to optical clock 145 . An optical or photonic processor 130 is connected to receive signals (e.g., the amplitude modulated signal) on a signal path or output 115 from amplitude modulator 120 . Photonic processor 130 is also coupled to a signal path or an output 140 associated with the optical clock signal (e.g., a reference signal) from clock 145 via couplers 111 and 112 . The reference signal is coherent with the amplitude modulated signal in one embodiment.

Processor 130 provides electronic signals associated with the data associated with the RF signal V(t) in electronic format to an electronic logic circuit 160 . Processor 130 can be configured as an optical amplitude demodulator in one embodiment. In one embodiment, processor 130 provides signals S 01 , S 02 , S 03 , S 04 , . . . , S 0N . The electronic signals are provided to digital logic circuit 160 .

In one embodiment, logic circuit 160 provides a data signal. Processor 130 and logic circuit 160 can operate as low resolution flash eADC. Using photonic processor 130 requires less power (e.g., micro watts of peak optical power) than a conventional eADC.

Digital logic circuit 160 can be embodied as an encoder, one or more logical gates, a processor, ASIC, etc. Digital logic circuit 160 provides a data signal including at least one bit. In one embodiment, circuit 160 provides bits B 1 , B 2 , B 3 , B M as a data signal. The bit signals are provided as an electronic data signal. Digital logic circuit 160 is a clocked digital encoder in one embodiment and can receive an electronic clock signal converted from the optical pulse signal at output 140 in one embodiment. A photo detector 270 can be configured to provide an electronic clock output 274 for processor 130 and logic circuit 160 .

According to various embodiments, pADC 100 can be utilized in a variety of applications including high capacity optical communications receivers, telecommunication receivers, Department of Defense (DOD) wide receiver technology, digital radar, sensors, etc. In one embodiment, pADC 100 can be implemented using optical and electric circuit components as described herein including processors, FPGAs, ASICS, opto-electronic circuits, etc.

Clock 145 can be an optical clock as described in U.S. patent application Ser. No. 13/240,226. As shown in FIG. 1 , the signal path at output 115 and the reference path at output 140 are separate paths that do not directly intersect or directly connect with each other, but whereby both paths provide input signals to processor 130 according to one embodiment.

Processor 130 according to an embodiment as shown in FIG. 1 can operate at 8 GHz and have a bandwidth in the C-band of 1530-1550 nanometers. In one embodiment, processor 130 can be comprised of separate stages; each stage can provide one of signals S O1 -S ON at outputs 220 A-E corresponding to an amplitude associated with the amplitude modulated signal. Reference amplitudes for the stages can be hard coded into processor 130 . The optical pulse width can be 2 picoseconds.

Each stage in processor 130 can include a balanced photo detector for providing the signal at outputs 220 A-E. The stages can be arranged in a ladder structure for flash ADC operation in one embodiment. In one embodiment, flash ADC structures can be building blocks for more sophisticated ADC technology such as successive approximation register (SAR) ADCs. The current output responsivity at each of outputs 220 A-E (S 01 -S 0N ) without waveguide losses is approximately 0.8 A/W in one embodiment.

With reference to FIG. 2A , an embodiment of photonic processor 130 includes a number of stages 202 A-E. As discussed above, the number of stages 202 A-E can be any number of stages. In one embodiment, stage 220 A corresponds to stage 1 , stage 220 B corresponds to stage 2 , stage 220 C corresponds to stage n, stage 220 D corresponds to stage N−1 and stage 220 E corresponds to stage N. In one embodiment, n is equal to 3 and N is equal to 5, although N can be any integer 3 or more in certain embodiments. Each of stages 220 A-E includes a respective output 220 A-E which provides an electronic signal for reception by digital logic circuit 160 ( FIG. 1 ) in one embodiment.

Stage 202 A includes a coupler 206 A and a coupler 216 A. Stage 202 A also includes a photo detector 204 A. Couplers 206 A and 216 A can each have a 1550 nm optical input in one embodiment. Photo detector 204 A is comprised of a photo diode 205 A and a photo diode 207 A. Photo detector 204 A is coupled to output 220 A. Each of stages 202 B, C, D and E include similar elements denoted with a respective reference numeral suffix. Stages 202 A-E can have an optical ladder structure using large scale nanophotonic phased array technology in one embodiment. Diodes 205 A-E and 207 A-E can be germanium or InGaAs photo diodes.

Coupler 206 A is coupled to an optical signal path 240 coupled to output 115 which provides the amplitude modulated signal from amplitude modulator 120 . Coupler 206 A provides an amplitude or power associated with the amplitude modulated signal on path 240 to a signal path coupled to a first input of optical detector 204 A. Coupler 216 A provides an amplitude or power level of the reference signal at a reference path 280 A to a reference path coupled to a second input of detector 204 A. The first input of detector 204 A can be associated with photo diode 205 A, and the second input of photo detector 204 A can be associated with photo diode 207 A. Photo diodes 207 A and 205 A are balanced and form a balanced photo detector 204 A in one embodiment.

Couplers 206 A-D are configured to provide an equal power level of P n of the amplitude modulated signal provided to the first input of respective detectors 204 A-E in one embodiment. In one embodiment, P n =P m /N for all from 1 to N, where P m is the power of the amplitude-modulated signal from modulator 120 . In one embodiment, n is the stage number, and N is the total number of stages.

Couplers 216 A-D are configured to provide a respective power level P Rn for the reference signal to the second input of respective detectors 204 A-E. In one embodiment, P Rn equals P R *n/(N+1) where n is the stage number, N is the total number of stages 202 A-E and P R is a reference power level. Photo detector 204 A and coupler 203 A compare the power level P n of the amplitude modulated signal to a threshold power level P Rn using the first input of detector 204 A and the second input of detector 202 A to provide an electronic signal representing the amplitude level associated with the amplitude modulated optical signal at output 220 A. Stages 202 B-E provides similar electric signals representing the amplitude level associated with the amplitude modulated optical signal using photo detectors 202 B-E.

The power level provided by each of couplers 206 A-D for respective stages 202 A-E is approximately equal in one embodiment. The power level on each of paths 240 , and 241 A-D and efficiency of each coupler 206 A-D is chosen to provide the appropriate power level P n . In one embodiment, the power level on each of paths 280 A-E for the reference signal and the efficiency of each coupler 216 A-D is chosen to provide the appropriate power level P Rn to respective couplers 203 A-E and detectors 202 A-E.

The output provided by processor 130 can be a flash/thermometer output, where the amplitude is along a length of a reference line that can be compared to a single threshold. Each of stages 202 A-E provides an indication of the amplitude compared to an appropriate reference level. If the amplitude is larger than the respective reference signal, each of detectors 204 A-E provides a signal at respective outputs 220 A-E which is different than when the amplitude is less than the reference power level in one embodiment. Accordingly, processor 130 operates according to an amplitude ladder in one embodiment. Processor 130 can be provided as a planer lightweight circuit (PLC) have multiple stages 202 A-E. Waveguides for the PLC can be mode-matched to a panda fiber.

With reference to FIG. 2B , an embodiment of photonic processor 130 is similar to the embodiment of photonic processor 130 discussed with reference to FIG. 2A . However, each stage 202 A-E includes a receiver 211 A-E for receiving for cathode and anode inputs from respective diodes 205 A-E and 207 A-E. Receiver 211 A-E provides output signals at outputs 220 A-E. In one embodiment, receivers 211 A-E can be combined into a single circuit receiving inputs from each of detectors 204 A-E and providing an output signal for each of outputs 220 A-E.

With reference to FIG. 3A , an embodiment of photonic processor 130 includes a number of stages 202 A-C. The number of stages 202 A-C can be any number of stages. In one embodiment, stage 220 A corresponds to stage 1 , stage 220 B corresponds to stage 2 , and stage 220 C corresponds to stage N. In one embodiment, N is equal to 3, although N can be any integer 3 or more in certain embodiments. Each of stages 220 A-C includes a respective output 220 A-C which provides an electronic signal for reception by digital logic circuit 160 ( FIG. 1 ) in one embodiment. Processor 130 in FIG. 3A includes similar elements to processor 130 in FIG. 2A .

Coupler 206 A is coupled to an optical signal path 240 coupled to output 115 which provides the amplitude modulated signal from amplitude modulator 120 . Coupler 206 A provides an amplitude or power associated with the amplitude modulated signal on path 240 to a signal path coupled to a first input of optical detector 204 A via coupler 203 A. Coupler 216 A provides an amplitude or power level of the reference signal at a reference path 280 A to a second input of detector 204 A via coupler 203 A. The first input of detector 204 A can be associated with photo diode 205 A, and the second input of photo detector 204 A can be associated with photo diode 207 A. Photo diodes 207 A and 205 A are balanced and form a balanced photo detector 204 A in one embodiment.

Couplers 206 A-B are configured to provide an equal power level of P n of the amplitude modulated signal to couplers 203 A-C coupled to the first input of respective detectors 204 A-C in one embodiment. Couplers 216 A-B and optical attenuating elements 208 A and 208 B are configured to provide a respective power level P Rn for the reference signal couplers 203 A-C to the second input of respective detectors 202 A-C. In one embodiment, P Rn equals P n *n/(N+1), where n is the stage number and N is the total number of stages 202 A-C. In one embodiment, elements 208 A and 208 B are fixed attenuators providing attenuation of −1.25 dB and −1.30 dB, respectively. However, any attenuator levels are possible. Attenuator levels are related to the number of stages in one embodiment. An attenuating element can be provided between element detector 202 C and 208 B in one alternative embodiment. Photo detector 204 A and coupler 203 A compare the power level P n of the amplitude modulated signal to a threshold power level P Rn from coupler 216 A to provide an electronic signal representing the amplitude level associated with the amplitude modulated optical signal at output 220 A. Stages 202 B-C provides similar electric signals representing the amplitude level associated with the amplitude modulated optical signal using photo detectors 202 B-C.

The power level provided by each of couplers 206 A-B for respective stages 202 A-C is approximately equal in one embodiment. The power level on each of paths 240 , and 241 A-B and efficiency of each coupler 206 A-B is chosen to provide the appropriate power level P n . In one embodiment, attenuation elements 208 A-B and the efficiency of each coupler 216 A-B is chosen to provide the power level P Rn to respective couplers 203 A-C and detectors 202 A-C on paths 280 B-D.

Optical attenuating elements element 208 A can be part of stage 202 A, part of a reference path 280 A, part of a reference path 280 B, or part of coupler 216 A. Optical attenuating element 280 B can be part of stages 202 B-C and paths 280 B-C. Optical attenuating elements 208 A-B can include a respective trim or adjust inputs.

With reference to FIG. 3B , an embodiment of photonic processor 130 is similar to the embodiment of photonic processor 130 discussed with reference to FIG. 3A . Each stage 202 A-C includes a respective receiver 211 A-C for receiving inputs from the cathodes and anodes of respective diodes 205 A-C and 207 A-C. Receivers 211 A-C provide output signals at outputs 220 A-C.

With reference to FIG. 4 , a package 302 for pADC 100 can include an external mechanical package 305 which surrounds multi-amplitude photonic processor 130 , an electronics chip 161 for electronics portions of processor 130 and circuit 160 , and a thermo electric cooler 303 . Electronics chip 161 is preferably next to the thermal electric cooler 303 . In one embodiment, heterogeneously integrated silicon platform for package 300 and pADC 100 can lower power dissipation, lower parasitics, lower manufacturing costs, and allow phase shifts, attenuation elements, path lengths, etc. to be hard coded in lithography.

It is understood that while the detailed drawings, specific examples, material types, thicknesses, dimensions, and particular values given provide a preferred exemplary embodiment of the present invention, the preferred exemplary embodiment is for the purpose of illustration only. The method and apparatus of the invention is not limited to the precise details and conditions disclosed. For example, although specific types of optical component, dimensions and angles are mentioned, other components, dimensions and angles can be utilized. Also, while an optical conversion with balanced detectors system and method have been described above with respect to inclusion in a wide band or a narrowband receiver, it can be implemented in other types of high-frequency band receivers, such as receivers operating up to hundreds of GHz. Various changes may be made to the details disclosed without departing from the spirit of the invention which is defined by the following claims