Electro-optic system

• US 5,199,086 A
• Filed: 01/17/1991
• Issued: 03/30/1993
• Est. Priority Date: 01/17/1991
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Johnson, Leonard M., Hutchinson, Warren K., Sonnenschein, Alexander
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Wise, Robert E.
• Application Number:
  US07/642,516
• Time in Patent Office: 803 Days
• Field of Search: 350/96.13, 350/96.14
• US Class Current: 385/2
• CPC Class Codes: G02F 1/225 in an optical waveguide structure
• CPC Class Codes: G02F 2/002 using optical mixing
• CPC Class Codes: G02F 2203/19 linearised modulation; reduction of harmonic distortions
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5051 using a series, i.e. cascade, combination of modulators

Abstract

An electro-optic system for mixing and/or transmitting electrical signals using an optical carrier is described in which the electrical signals are applied to an electro-optic intensity modulator with a nonlinear transfer function, preferably of the Mach-Zehnder interferometric type biased at the transmission null point. The modulator output optical signal is transmitted to a receiver where it is detected and voltage products of the applied electrical signals are recovered.

Claims

1. An electro-optic mixer for mixing at least two electrical signals to obtain voltage product terms of said electrical signals comprising:

a) an electro-optic intensity modulator in which the optical output is light transmission which varies non-linearly in accordance with the voltages of the electrical signals applied to it;

b) a source of light coupled to said modulator;

c) electrode means for applying electrical signals to said modulator;

d) bias means for setting the operating point of said modulator where the light transmission versus electrical signal voltage transfer function is comprised of even-order non-linearities; and

e) an optical detector for receiving the optical output of the modulator to provide an electrical output signal containing the desired product terms.

2. The mixer of claim 1 in which the light source is operated at constant power.

3. The mixer of claim 1 including transmission means for coupling the optical output to the detector.

4. The mixer of claim 1 including an optical amplifier between the modulator and detector for amplifying the optical signal prior to detection.

5. The mixer of claim 1 including an integrator responsive to said electrical output signal to form a correlator.

6. The mixer of claim 1 wherein the operating point is at a transmission minimum point.

7. The mixer of claim 1 wherein a separate electrode means is provided for each applied electrical signal.

8. The mixer of claim 1 wherein the electrical signals are combined before being applied to the modulator.

9. The mixer of claim 1 wherein the modulator is an interferometric type having three separate electrodes for applying electrical signals to the modulator, one such signal being a bias signal for setting the operating point.

10. The mixer of claim 9 wherein the operating point is at a transmission null.

11. The mixer of claim 9 wherein the operating point is at a transmission maximum.

12. The mixer of claim 9 including a detector for detecting the output of the modulator and generating an electrical signal proportionate thereto and an integrator for integrating the electrical signal over time.

13. The mixer of claim 1 wherein one of the electrical signals is a time varying electrical signal and one of the electrical signals is a constant amplitude sinusoidal electrical signal and one of the electrical signals is a bias signal for setting the operating point.

14. The mixer of claim 13 including an optical detector for detecting the mixed signal and an electrical mixer for mixing the detected signal with a constant amplitude sinusoidal electrical signal to recover the time varying signal.

15. The mixer of claim 1 wherein the modulator is an interferometric type.

16. An electro-optic mixer for mixing n electrical signals together to obtain voltage product terms of said signals comprising:

a) n electro-optic intensity modulators connected in series, including a first modulator and a last modulator in said series, with the light output of a first modulator coupled to the input of the next modulator until the last modulator is reached, each such modulator biased at an operating point which produces a linear signal response;

electrode means for applying the signals to be mixed separately to a respective one of the n modulators.

17. The mixer of claim 16 wherein "n" is 2 or more.

18. The mixer of claim 16 including a detector for detecting the light output of the last modulator and generating an electrical signal proportional thereto.

19. The mixer of claim 18 including an optical amplifier located before the detector for amplifying the output of the last modulator.

20. The mixer of claim 16 wherein the modulators are of the Mach-Zender type and are biased at the half-power point.

21. An electro-optic mixer for mixing at least two applied electrical signals comprising:

a) an electro-optic light intensity modulator having an input and an output port, a non-linear varying optical transmission versus applied voltage transfer characteristic curve; and

(i) electrode means for applying one or more such electrical signals to said modulator, and

(ii) bias means for biasing said modulator to operate about a point on said curve having even-order nonlinearities and a corresponding transmission minimum; and

b) a source of constant optical power coupled to said input port.

22. The mixer of claim 21 wherein the modulator is taken from the group comprising interferometric modulators, Fabry-Perot interferometric modulators and optical directional couplers.

23. The mixer of claim 21 wherein the electrode means includes a set of at least three separate electrodes, one for each of the two electrical signals and one for biasing.

24. The mixer of claim 21 in which the optical signal is amplified prior to detection.

25. The mixer of claim 21, including an optical detector coupled to said output port for providing an electrical output signal comprising the voltage product of the applied electrical signals.

26. An electro-optic mixer in which electrical signals to be mixed are applied to an electro-optic intensity modulator, with said modulator biased to operate about a symmetric transmission null point such that the optical transmission of the modulator varies as a function of the applied voltages and detector means for detecting the optical transmission and generating an electrical output signal corresponding to the product of the mixed electrical signals.

27. The mixer of claim 26 wherein the modulator is an interferometric modulator.

28. The mixer of claim 26 wherein the modulator has three separate electrodes for applying signals to the modulator, one for each of the mixed signals and one for a bias signal.

29. The mixer of claim 26 wherein the output of the modulator is an optical signal which is detected and converted to a time varying electrical output signal.

30. The mixer of claim 29 wherein the detected output signal is integrated to form a correlator.

31. The mixer of claim 29 wherein the optical signal is amplified prior to detection.

32. The mixer of claim 26 wherein the optical signal is coupled to an optical transmission means which couples the signal to be detected.

33. A method of mixing at least two electrical signals together to obtain voltage product terms of said electrical signals comprising:

a) coupling the signals to an electro-optic modulator having an input port and an output port and a non-linearly varying optical power transmission characteristic which varies in accordance with the voltages of the electrical signals applied to it and;

b) biasing said modulator to operate about a transmission minimum point; and

c) coupling optical power from an optical source to the input port; and

d) wherein the output of the modulator is an optical signal which is detected by an optical detector and converted to an electrical output signal containing the voltage product terms.

34. The method of claim 33 wherein the optical signal is coupled to an optical transmission line which couples the output signal to the detector.

35. The method of claim 33 wherein the optical signal is amplified prior to detection.

36. An electro-optic mixer for mixing at least two electrical signals together to obtain voltage product terms of said signals comprising:

a) an electro-optic interferometric modulator having an input port and an output port and three electrodes electrically isolated from each other, said modulator exhibiting a non-linearly varying optical power transmission versus applied electrode voltages transfer characteristic curve;

(i) one such electrode for applying a first such signal to said modulator,

(ii) another such electrode for applying a second of such signals to said modulator, and

(iii) the third such electrode for applying a biasing signal to said modulator so as to bias said modulator to operate about an operating point of said curve wherein the light transmission versus electrical signal voltage transfer function is comprised of even-order non-linearities; and

b) a source of optical power for applying constant optical power to said input port.

37. The mixer of claim 36 wherein the modulator is a Mach Zehnder type interferometric modulator.

38. The mixer of claim 36 wherein the output of the modulator is a time varying optical signal which is detected by an optical detector and converted to an electrical output signal.

39. The mixer of claim 38 wherein the electrical output signal is coupled to an integrator and integrated over time.

40. The mixer of claim 38 wherein the optical signal is amplified before being detected.

41. The mixer of claim 38 including transmission means for coupling the optical signal to the detector.