Opto-electronic oscillator having a positive feedback with an open loop gain greater than one

• US 5,723,856 A
• Filed: 08/01/1995
• Issued: 03/03/1998
• Est. Priority Date: 08/01/1995
• Status: Expired due to Term

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Maleki, Lutfollah, Yao, Xiaotian Steve
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): Lee, John R.
• Application Number:
  US08/510,064
• Time in Patent Office: 945 Days
• Field of Search: 250/205, 250/214.1, 250/214 R, 250/227.11, 250/227.12, 250/227.17, 250/227.18, 250/227.21, 359/315, 359/316, 359/317, 359/318, 359/319, 359/320, 359/181, 359/187, 359/184, 372/12, 372/18
• US Class Current: 250/227.11
• CPC Class Codes: G02F 1/0123 Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops
• CPC Class Codes: G02F 2203/56 Frequency comb synthesizer

Abstract

An electro-optical oscillator includes an electro-optical modulator having an electrical input port that accepts an electrical control signal and an optical output port. The electro-optical modulator is operable to generate at the optical output port an optical signal that oscillates at a frequency related to the electrical control signal. The oscillator also includes a photodetector that converts a portion of the optical signal from the optical output port of the electro-optical modulator to an electrical signal and provides the electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal. An open loop gain of a feedback loop including the optical output port, the photodetector and the electrical input port is greater than one.

Claims

1. An opto-electronic oscillator comprising:

an electric-optical modulator having an electrical input port that accepts an electrical control signal and an optical output port, wherein the electric-optical modulator is operable to generate at the optical output port an optical signal being modulated at an oscillation frequency related to the electrical control signal,

a photodetector operable to convert a portion of the optical signal from the optical output port of the electro-optical modulator to an electrical signal and to provide the electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal, the electrical signal from the photodetector oscillating at the same oscillation frequency at which the optical signal at the optical output port is modulated, and

an active feedback loop connecting the optical output port, the photodetector and the electrical input port, said feedback loop having a positive feedback with an open loop gain which is greater than one.

2. The electro-optical oscillator of claim 1, wherein the opto-electronic modulator further comprises an electrical bias port and the frequency at which the optical output signal oscillates is affected by a voltage applied to the bias port.

3. The electro-optical oscillator of claim 1, further comprising an optical fiber having a length longer than a predetermined length to effect a delay time in the active feedback loop, thereby causing the optical signal at the optical output port to have a spectral linewidth at the oscillation frequency below a desired linewidth having a first relation with the delay time and a phase noise below a desired noise level having a second relation with the delay time, the opto-electronic oscillator operable to receive the portion of the optical output signal and to transport said portion to the photodetector.

4. The electro-optical oscillator of claim 3, further comprising a fiber stretcher coupled to the optical fiber, wherein the fiber stretcher includes an electrical input port and is operable to modify a length of the optical fiber in response to a signal applied to the electrical input port.

5. The electro-optical oscillator of claim 4, wherein the opto-electronic oscillator is configured so that the output signal oscillates at a higher frequency and a lower frequency, the oscillator further comprising a second feedback loop operable to combine a low frequency portion of the output signal with a low frequency reference signal to produce a combined electrical signal, and the combined electrical signal being applied to the electrical input port of the fiber stretcher to affect the length of the optical fiber.

6. The electro-optical oscillator of claim 5, wherein the oscillator produces an electrical-output signal at an electrical output that oscillates at the same frequency as the optical output signal, and wherein the feedback loop includes:

a low pass filter connected to the electrical output and operable to produce a filtered electrical signal from which a high frequency component of the electrical output signal has been removed,

an amplifier connected to an output of the low pass filter and operable to produce an amplified filtered electrical signal,

a combiner having a first input port connected to an output of the amplifier, a second input port for receiving the low frequency reference signal, and an output port for producing the combined electrical signal.

7. The electro-optical oscillator of claim 1, further comprising an RF coupler operable to receive the electrical signal produced by the photodetector and to produce an electrical output signal from the received electrical signal.

8. The electro-optical oscillator of claim 7, further comprising an optical coupler operable to combine an external optical control signal with the portion of the optical output signal to produce a combined optical signal, and wherein the photodetector is operable to convert the combined optical signal to an electrical signal and to supply the electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal.

9. The electro-optical oscillator of claim 8, wherein the RF coupler is further operable to combine an external electrical control signal with the electrical signal produced by the photodetector to produce a combined electrical signal, to supply the combined electrical signal as the electrical output signal, and to supply the combined electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal.

10. The electro-optical oscillator of claim 9, wherein the electro-optical modulator further comprises an electrical bias port and the frequency at which the optical output signal oscillates is affected by a voltage applied to the bias port.

11. The electro-optical oscillator of claim 10, further comprising an optical fiber operable to receive the portion of the optical output signal and to transport it to the photodetector.

12. The electro-optical oscillator of claim 11, further comprising a fiber stretcher coupled to the optical fiber, wherein the fiber stretcher includes an electrical input port and is operable to modify a length of the optical fiber in response to a signal applied to the electrical input port.

13. The electro-optical oscillator of claim 12, further comprising an RF amplifier operable to amplify the electrical signal produced by the photodetector to produce an amplified electrical signal, and to supply the amplified electrical signal to the RF coupler.

14. The electro-optical oscillator of claim 12, further comprising a band pass filter disposed in the active feedback loop between the photodetector and the electrical input port of the electrical-optical modulator, operable to effect a filtered electrical signal from the combined electrical signal produced by the RF coupler and to supply the filtered electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal, the filtered electrical signal having a characteristic in frequency domain that is related to the band pass filter.

15. The electro-optical oscillator of claim 8, further comprising:

a remote source of an electrical reference signal,

a laser diode operable to convert the electrical reference signal to an optical reference signal, and

an optical fiber operable to supply the optical reference signal to the optical coupler as the external optical control signal.

16. The electro-optical oscillator of claim 15, wherein:

the remote source is operable to produce the electrical reference signal at a first frequency and at a power sufficient to cause the laser diode to produce harmonics of the first frequency in the optical reference signal, and

the electro-optical oscillator is configured to oscillate in a frequency range that includes a second frequency that is an integer multiple of the first frequency so that the optical reference signal causes the output signal to oscillate at the second frequency.

17. The electro-optical oscillator of claim 8, wherein:

the external optical control signal comprises a stream of digital data at a clock rate having a first frequency, and

the electro-optical oscillator is configured to oscillate in a frequency range that includes the first frequency so that the external optical control signal causes the output signal to oscillate at the first frequency and thereby causes the electro-optical oscillator to extract a clock signal from the stream of data.

18. A data recovery system including the electro-optical oscillator of claim 17, and further comprising:

an optical delay line operable to delay the stream of digital data for a time sufficient to permit the electro-optical oscillator to extract the clock signal from the stream of data,

an optical-to-electrical converter operable to convert the delayed stream of digital data to an electrical data stream, and

a data recovery circuit operable to receive the electrical output signal of the electro-optical oscillator and the electrical data stream and to produce digital data therefrom.

19. The electro-optical oscillator of claim 8, wherein:

the external optical control signal comprises a carrier signal having a first frequency and noise, and

the electro-optical oscillator is configured to oscillate in a frequency range that includes the first frequency so that the external optical control signal causes the output signal to oscillate at the first frequency and thereby causes the electro-optical oscillator to extract the clock signal from the noise.

20. The electro-optical oscillator of claim 7, further comprising a combining circuit operable to combine the electrical output with an electrical reference signal to produce a combined electrical control signal and to supply the combined electrical control signal to a control port to control the oscillation frequency of the output signal to implement a phase locked loop.

21. The electro-optical oscillator of claim 20, wherein the electro-optical modulator further comprises an electrical bias port and the frequency at which the optical output signal oscillates is affected by a voltage applied to the bias port, and wherein the electrical bias port is the control port to which the combined electrical control signal is supplied.

22. The electro-optical oscillator of claim 20, further comprising:

an optical fiber operable to receive the portion of the optical output signal and to transport it to the photodetector, and

a fiber stretcher coupled to the optical fiber, wherein the fiber stretcher includes an electrical input port and is operable to modify a length of the optical fiber in response to a signal applied to the electrical input port, and wherein the electrical input port is the control port to which the combined electrical control signal is supplied.

23. The electro-optical oscillator of claim 20, further comprising an electrical reference source operable to produce the electrical reference signal.

24. The electro-optical oscillator of claim 20, further comprising:

a fiber delay line for producing a delayed version of the optical output signal of the electro-optical modulator, and

a second photodetector operable to convert the delayed version of the optical output signal to an electrical signal and to supply the electrical signal to the combining circuit as the electrical reference signal.

25. A data downlink system including the electro-optical oscillator of claim 20, and further comprising:

a second electro-optical modulator operable to receive the optical output signal of the electro-optical oscillator and to modulate the optical output signal using an electrical control signal from a downlink receiver to produce a modulated optical signal, and

a low speed photodetector operable to extract the electrical control signal from the modulated optical signal.

26. The electro-optical oscillator of claim 1, further comprising an RF coupler operable to receive the electrical signal produced by the photodetector, to combine an external electrical control signal with the electrical signal produced by the photodetector to produce a combined electrical signal, and to supply the combined electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal.

27. The electro-optical oscillator of claim 26, further comprising:

a fiber delay line for producing a delayed version of the optical output signal of the electro-optical modulator, and

a second photodetector operable to convert the delayed version of the optical output signal to a delayed electrical signal and to supply the delayed electrical signal to the RF coupler as the external electrical control signal.

28. The electro-optical oscillator of claim 27, wherein a length of the fiber delay line is greater than one kilometer.

29. The electro-optical oscillator of claim 1, further comprising an optical coupler operable to combine an external optical control signal with the portion of the optical output signal to produce a combined optical signal, and wherein the photodetector is operable to convert the combined optical signal to an electrical signal and to supply the electrical signal to the electrical input port of the electro-optical modulator as the electrical control signal.

30. The electro-optical oscillator of claim 1, further comprising a laser source for supplying a laser signal to the electro-optical modulator, wherein the laser source, the electro-optical modulator and the photodetector are all implemented on a single integrated circuit substrate.

31. The opto-electronic oscillator of claim 1, wherein the active feedback loop comprises an optical delay line.

32. The opto-electronic oscillator of claim 1, wherein the active feedback loop comprises an RF delay line.

38. An opto-electronic oscillator as in claim 14, wherein the RF coupler includes an electrical output port for exporting an electrical signal, and an electrical input port for injecting an external electrical signal into the active feedback loop and enabling the RF coupler to combine the electrical signal from the photodetector and the external electrical signal, the external electrical signal oscillating at an injection frequency which is a subharmonic of the oscillating frequency of the active feedback loop, thereby producing a signal gain and frequency multiplication in an electrical output signal at the electrical output port with respect to the injected external electrical signal.

39. An opto-electronic oscillator as in claim 12, wherein the RF coupler includes an electrical output port for exporting an electrical signal, and an electrical input port for injecting an external electrical signal into the active feedback loop and enabling the RF coupler to combine the electrical signal from the photodetector and the external electrical signal, the opto-electronic oscillator further comprising:

an electrical signal generator connected to the electrical input port in the RF coupler, operating to produce a periodic signal having a signal period in frequency domain that is substantially equal to the mode spacing of the oscillation in the active feedback loop or a multiplication thereof, the periodic signal being injected at the input port as the external electrical signal; and

mode locking means for forcing different modes oscillate with a certain phase relation with respect to one another in a way that is determined by the injected periodic signal, producing a periodic pulsed signal in at least one of optical form and electrical form.

33. A method of generating an oscillatory optical signal comprising the steps of:

modulating an optical signal with an electrical control signal in an electro-optical modulator to produce a modulated optical output signal having a frequency related to the electrical control signal,

forming an active feedback loop having a positive feedback with an open loop gain greater than one to effect the oscillation,

converting a portion of the modulated optical output signal to an electrical signal that propagates in the feedback loop, and

supplying the electrical signal produced in the converting step to the electro-optical modulator through the feedback loop as the electrical control signal.

40. A method as in claim 33, further comprising:

filtering the electrical signal in the active feedback loop at a center frequency with a predetermined bandwidth with a band pass filter; and

effecting a single-mode oscillation by the filtering.

41. A method as in claim 33, further comprising:

producing a phase delay in the electrical control signal with the active feedback loop; and

increasing the phase delay larger than a predetermined delay value to effect a delay time in the active feedback loop to cause the optical signal to have a spectral linewidth at the oscillation frequency below a desired linewidth having a first relation with the delay time and a phase noise below a desired noise level having a second relation with the delay time.

34. A photonic device, comprising:

an electro-optical modulator having an input port for signals of a predetermined form, and having an output port for signals of a second predetermined form, one of said first and second predetermined forms being an electrical oscillating signal at an oscillating frequency and the other of said first and second predetermined forms being an optical oscillating signal that is modulated at said oscillating frequency; and

a converter operable to convert said first predetermined form signal to said second predetermined form signal; and

an element operable to conduct said first predetermined form signal to said converter and to conduct said second predetermined form signal to said input port, to form a feedback loop which uses both said electrical and optical signal and synchronizes to both, said feedback loop having a positive feedback with an open loop gain greater than one.

35. A system as in claim 34, further comprising optical and electrical injection ports, respectively allowing injection of optical and electrical injection signals, and to which said output signal is injection locked.

36. A system as in claim 34 further comprising an element operable to delay an output signal of said modulator by a time which is effective to lock said output to a past state.

37. A system as in claim 36 wherein said first predetermined output is optical, and wherein said element is a fiber delay line of at least 1 kilometer in length.