Photonic parallel analog-to-digital converter

• US 6,525,682 B2
• Filed: 05/03/2001
• Issued: 02/25/2003
• Est. Priority Date: 05/03/2001
• Status: Expired due to Fees

• Current Assignee: HRL Laboratories LLC (The Boeing Co.)
• Original Assignee: HRL Laboratories LLC (The Boeing Co.)
• Inventors: Walden, Robert H., Yap, Daniel
• Primary Examiner(s): YOUNG, BRIAN K
• Application Number:
  US09/848,499
• Publication Number:
  US 20020163454A1
• Time in Patent Office: 663 Days
• Field of Search: 341/137, 341/120, 341/155, 341/159, 250/225
• US Class Current: 341/137
• CPC Class Codes: G02F 7/00 Optical analogue/digital converters
• CPC Class Codes: H03M 3/468 Interleaved, i.e. using multiple converters or converter parts for one channel, e.g. using Hadamard codes, pi-delta-sigma converters

Abstract

A photonically sampled analog-to-digital converter using parallel channels of sampling and quantizing. The parallel combination achieves cancellation of the spurs that result from the nonlinear transfer function of the samplers. The samplers feed a dual-detector optoelectronic receiver that has differential inputs for suppression of laser intensity noise. The outputs of the multiple photonic samplers are averaged to reduce the effects of shot or thermal noise from the optoelectronic receiver of a sampler. The errors produced by the quantization process can be reduced by using a delta-sigma modulator-based analog-to-digital convertor as the quantizer which provides noise-spectrum shaping and filtering.

Claims

1. An analog to digital converter having an analog input and a digital output, said analog to digital converter comprising:

an optical pulse source;

a plurality of channels, each channel comprising;

an optical encoding sampler circuit, coupled to said optical pulse source and said analog input, sampling said analog input to produce a sampled optical signal and encoding said sampled optical signal with a encoding code sequence to produce an encoded optical signal;

an optical to electric converter, coupled to said optical encoding sampler, converting said encoded optical signal to an electronic signal;

a quantizer, coupled to said optical to electric converter, producing a digital signal from said electronic signal; and

a digital decoder, coupled to said quantizer, decoding said digital signal with a decoding code sequence to produce a decoded digital signal; and

a summer, coupled to said digital decoder in each channel of said plurality of channels, digitally summing each decoded digital signal from each channel to produce said digital output.

2. An analog to digital converter according to claim 1, further comprising a Nyquist filter in each channel of said plurality of channels, said Nyquist Filter positioned between said optical to electric converter and said quantizer, and having an input coupled to said optical to electric converter and an output coupled to said quantizer.

3. An analog to digital converter according to claim 1, further comprising a gain element in each channel of said plurality of channels, said gain element located between said quantizer and said digital decoder, said gain element digitally modifying an amplitude of a digital signal output by said quantizer.

4. An analog to digital converter according to claim 1, further comprising a digital noise filter in each channel of said plurality of channels, said digital noise filter located between said quantizer and said digital decoder, said digital noise filter digitally filtering a digital signal output by said quantizer.

5. An analog to digital converter according to claim 3, wherein said gain element digitally modifies the amplitude of the digital signal to improve linearity and to decrease noise spurs in the digital output.

6. An analog to digital converter according to claim 3, wherein said channels in said plurality of channels are grouped as channel pairs and said analog input is divided between the optical encoding sampler circuits in each channel pair according to a first prescribed ratio and the two optical encoding sampler circuits are controlled to produce sampled optical signals with a phase shift of 180°

between the sampled optical signals of the two optical encoding sampler circuits, and the gain elements in each channel pair digitally modify the amplitude of the digital signal output in each channel in the pair according to a second prescribed ratio.

7. An analog to digital converter according to claim 6, wherein said gain elements compensate for the division of the analog input between the optical encoding sampler circuits so as to improve linearity and decrease noise spurs in the digital output.

8. An analog to digital converter according to claim 3, wherein said channels in said plurality of channels are grouped as one or more channel groups, each channel group comprising two or more channels, and said analog input is divided between the channels in each channel group according to a prescribed ratio, and the gain elements in each channel adjust a channel signal output to maintain the prescribed ratio.

9. An analog to digital converter according to claim 1, wherein said optical encoding sampler circuit comprises:

a single input, dual-output Mach-Zehnder interferometer, the single input of the Mach-Zehnder interferometer coupled to the optical pulse source; and

a directional coupler switch coupled to the outputs of the Mach-Zehnder interferometer.

10. An analog to digital converter according to claim 1, wherein said optical encoding sampler circuit comprises:

a directional coupler switch having two inputs and two outputs, one of the inputs being coupled to the optical pulse source; and

a dual input, dual-output Mach-Zehnder interferometer, the dual inputs of the Mach-Zehnder interferometer coupled to the outputs of the directional coupler switch.

11. An analog to digital converter according to claim 1, wherein said optical encoding sampler circuit comprises an integrated optical encoding sampler.

12. An analog to digital converter according to claim 1, wherein said optical to electrical converter is a photodetector.

13. An analog to digital converter according to claim 1, wherein said encoding code sequence is a Hadamard sequence and said decoding code sequence is a Hadamard sequence.

14. An analog to digital converter according to claim 1, wherein digital decoder comprises a digital multiplier which multiplies said digital signal by said decoding code sequence.

15. An analog to digital converter according to claim 1, wherein the quantizer comprises a delta-sigma modulator-based analog-to-digital conversion circuit.

16. An analog to digital converter having an analog input and a digital output, the analog to digital converter comprising:

means for producing optical pulses;

a plurality of channels, each channel comprising;

means for sampling and encoding the analog input with the optical pulses, thereby producing an encoded optical signal;

means for converting the encoded optical signal to an electric signal;

means for quantizing the electric signal, thereby producing a digital signal; and

means for decoding the digital signal with a decoding code sequence, thereby producing a decoded digital signal; and

means for summing together each decoded digital signal from each channel in the plurality of channels, thereby producing the digital output.

17. An analog to digital converter according to claim 16, wherein the means for sampling and encoding the analog input comprises:

a dual output Mach-Zehnder interferometer coupled to the optical pulses and controlled by the analog input; and

a directional coupler switch coupled to the outputs of the Mach-Zehnder interferometer and controlled by an encoding signal.

18. An analog to digital converter according to claim 16, wherein the means for sampling and encoding the analog input comprises:

a directional coupler switch coupled to the optical pulses and controlled by an encoding signal; and

a dual input, dual output Mach-Zehnder interferometer coupled to the outputs of the directional coupler switch and controlled by the analog input.

19. An analog to digital converter according to claim 16, wherein the means for sampling and encoding the analog input comprises an integrated optical encoding sampler.

20. An analog to digital converter according to claim 16, further comprising means for filtering the electric signal.

21. An analog to digital converter according to claim 16, further comprising means for filtering the digital signal.

22. An analog to digital converter according to claim 16, wherein said analog signal is encoded with a Hadamard sequence.

23. An analog to digital converter according to claim 16, wherein the means for quantizing the electric signal comprises a delta-sigma modulator.

24. An analog to digital converter according to claim 16, wherein the means for decoding the digital signal comprises a digital multiplier, the digital multiplier multiplying the digital signal by a decoding code sequence.

25. An analog to digital converter according to claim 16, further comprising means for digitally adjusting the digital signal.

26. An analog to digital converter according to claim 25, wherein said channels in said plurality of channels are grouped as channel pairs and said analog input is divided between the means for sampling and encoding the analog input in each channel pair according to a first prescribed ratio and the means for sampling and encoding the analog input are controlled to produce sampled optical signals with a phase shift of 180°

between the sampled optical signals of the means for sampling and encoding the analog input, and the means for digitally adjusting the digital signal in each channel pair digitally modify the amplitude of the digital signal in each channel in the pair according to a second prescribed ratio.

27. An analog to digital converter according to claim 26, wherein said means for digitally adjusting the digital signal compensate for the division of the analog input between the optical encoding sampler circuits so as to improve linearity and decrease noise spurs in the digital output.

28. An analog to digital converter according to claim 26, wherein said channels in said plurality of channels are grouped as one or more channel groups, each channel group comprising two or more channels, and said analog input is divided between the channels in each channel group according to a prescribed ratio, and said means for digitally adjusting the digital signal in each channel adjust a channel signal output to maintain the prescribed ratio.

29. A method for converting an analog input signal to a digital output signal comprising the steps of:

providing optical pulses;

coupling the optical pulses to a plurality of converter channels;

converting the optical pulses coupled to each channel converter channel to encoded optical samples of the analog input signal

converting the encoded optical samples to an electric signal in each converter channel;

quantizing the electric signal to produce a quantized digital signal in each converter channel;

decoding the quantized digital signal with a decoding code sequence to produce a decoded digital signal in each converter channel; and

summing together all the decoded digital signals from each converter channel to produce the digital output signal.

30. The method according to claim 29, wherein the step of converting the optical pulses further comprises the steps of:

sampling the analog input signal with the optical pulses to produce a sampled optical signal in each converter channel; and

encoding the sampled optical signal with an encoding code sequence to produce encoded optical samples in each converter channel.

31. The method according to claim 29, wherein the step of converting the optical pulses further comprises the steps of:

encoding the optical pulses with an encoding code sequence to produce encoded optical pulses in each of the converter channels; and

sampling the analog input signal with the encoded optical pulses to produce encoded optical samples in each of the converter channels.

32. The method according to claim 29, further comprising the step of filtering the electric signal in each channel.

33. The method according to claim 29, further comprising the step of digitally filtering the quantized digital signal in each converter channel.

34. The method according to claim 29, wherein said quantized digital signal has a value and said method further comprises the step of digitally increasing or decreasing the value of said quantized digital signal in each converter channel.

35. The method according to claim 34, wherein said converter channels in said plurality of converter channels are grouped as channel pairs and said method further comprises the steps of:

dividing said analog input into a first analog input and a second analog input, said first analog input and said second analog input having magnitudes differing according to a first prescribed ratio;

applying said first analog input to one converter channel of each channel pair;

applying said second analog input to the other converter channel of each channel pair with a 180°

phase shift in relation to the first analog input; and

digitally increasing or decreasing the value of the quantized digital signal in one converter channel of the channel pair in relation to the quantized digital signal in the other converter channel of the channel pair according to a second prescribed ratio.

36. The method according to claim 35, further comprising the step of digitally increasing or decreasing the value of the quantized digital signal of at least one of the converter channels of each channel pair to compensate for errors in dividing said analog input into a first analog input and a second analog input.

37. The method according to claim 34, wherein said converter channels in said plurality of converter channels are grouped as one or more channel groups, each channel group comprising two or more converter channels, and said method further comprises the steps of:

dividing said analog input into two or more divided analog inputs, each of said divided analog inputs having magnitudes differing according to a prescribed ratio;

applying each one of said divided analog inputs to a corresponding one converter channel in each channel group; and

digitally increasing or decreasing the value of the quantized digital signal in each converter channel within each channel group to maintain the prescribed ratio.