Method and system for first-order RF amplitude and bias control of a modulator

• US 6,687,451 B1
• Filed: 09/27/2000
• Issued: 02/03/2004
• Est. Priority Date: 09/27/2000
• Status: Expired due to Fees

• Current Assignee: Alcatel SA (Nokia Corporation)
• Original Assignee: Alcatel SA (Nokia Corporation)
• Inventors: Sikora, John K.
• Primary Examiner(s): NGO, HUNG NHAT
• Application Number:
  US09/670,769
• Time in Patent Office: 1,224 Days
• Field of Search: 359/237, 359/245, 398/23, 398/115, 398/195, 398/198, 398/182, 398/183, 398/187
• US Class Current: 398/187
• 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 2203/19 linearised modulation; reduction of harmonic distortions
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/50572 to control the modulating signal amplitude including amplitude distortion
• CPC Class Codes: H04B 10/50575 to control the modulator DC bias
• CPC Class Codes: H04B 10/58 Compensation for non-linear transmitter output

Abstract

A method and system are disclosed for robustly (using first order effects) controlling the bias point and radio frequency (RF) amplitude level of a modulator for an optical transmitter. The method comprises the steps of extracting an output dither signal component of a digital optical output signal from the optical transmitter to drive a feedback loop; measuring the output dither signal component in the feedback loop for comparison to an input dither signal to the modulator; comparing the output dither signal to the input dither signal to determine their difference; and, based on the difference between them, maintaining the bias point and the RF amplitude level at an optimum value by varying an input voltage to the modulator via the feedback loop. One embodiment of the system of this invention comprises a laser for providing an input light, a modulator to modulate the input light and generate a digital optical output signal, a radio frequency (RF) feedback loop to control an RF input voltage to the modulator, a bias feedback loop to control a bias input voltage to the modulator, an RF amplitude dither circuit to provide an RF input dither signal to the RF voltage input, and a bias dither circuit to provide a bias input dither signal to the bias voltage input. The modulator can be a Mach-Zehnder modulator.

Claims

1. A method for robustly controlling the bias point and radio frequency (RF) amplitude level of a modulator for an optical transmitter, comprising:

extracting an output dither signal component of a digital optical output signal from said optical transmitter to drive a feedback loop;

measuring said output dither signal component in said feedback loop for comparison to an input dither signal to said modulator;

comparing said output dither signal to said input dither signal to determine their difference; and

based on said difference, maintaining said bias point and said RF amplitude level at an optimum value by varying an input voltage to said modulator via the feedback loop.

2. The method of claim 1, wherein said optical transmitter comprises:

a laser for providing an input light;

said modulator to modulate said input light and generate said digital optical output signal;

a radio frequency (RF) feedback loop to control an RF input voltage to said modulator;

a bias feedback loop to control a bias input voltage to said modulator;

an RF amplitude dither circuit to provide an RF input dither signal to said RF voltage input; and

a bias dither circuit to provide a bias input dither signal to said bias voltage input.

3. The method of claim 2, wherein said bias voltage input can be controlled by said RF dither and wherein said RF voltage input can be controlled by said bias dither.

4. The method of claim 2, wherein said bias feedback loop and said RF feedback loop automatically compensate for changes over time in said optical transmitter caused by temperature or vibration.

5. The method of claim 2, wherein said bias dither and said RF dither are in a relationship where one is half the frequency of the other.

6. The method of claim 2, wherein said bias dither is at 250 hertz and said RF dither is at 500 hertz.

7. The method of claim 2, wherein there are no frequency components of said RF dither and said bias dither that overlap.

8. The method of claim 2, further comprising inputting a pedestal voltage into said RF feedback loop to further adjust said RF voltage input.

9. The method of claim 2, wherein said laser is operated at a constant input current.

10. The method of claim 1, wherein said feedback loop is an RF amplitude feedback loop, said output dither signal is an RF output dither signal, said input dither signal is an RF input dither signal, and said input voltage is an RF amplitude input voltage.

11. The method of claim 10, wherein said RF amplitude input voltage can be controlled by a bias dither.

12. The method of claim 1, wherein said feedback loop is a bias feedback loop, said output dither signal is a bias output dither signal, said input dither signal is a bias input dither signal, and said input voltage is a bias input voltage.

13. The method of claim 12, wherein said bias input voltage can be controlled by an RF input dither signal.

14. The method of claim 1, wherein said modulator is a Mach-Zehnder modulator.

15. The method of claim 1, wherein said optical transmitter is used to transmit optical data along a digital lightwave communications system.

16. The method of claim 1, wherein said feedback loop automatically compensates for changes over time in said RF amplitude or bias point caused by temperature or vibration.

17. The method of claim 1, further comprising inputting a pedestal voltage into said feedback loop to further adjust said input voltage.

18. The method of claim 1, wherein said measuring step further comprises measuring first-order linear effects of said output dither component to vary said input voltage.

19. The method of claim 1, wherein said feedback loop is a RF amplitude feedback loop.

20. The method of claim 1, wherein said feedback loop is a bias feedback loop.

21. The method of claim 1, wherein said optical transmitter is a SONET format optical transmitter within a SONET optical transmission system.

22. The method of claim 21, wherein said SONET optical transmission system is an OC48 or OC192 SONET system.

23. The method of claim 1, wherein said optical transmitter is a pseudo-random bit sequence (“

PRBS”

) formats optical transmitter.

24. The method of claim 1, wherein said optical transmitter is an optical transmitter employing a forward error correction scheme.

25. The method of claim 1, wherein said preferred value for said bias point is at quadrature, and wherein said preferred value for said RF amplitude level is V_π

.

26. A system for robustly controlling the bias point and RF amplitude level of a modulator for an optical transmitter, comprising:

a laser for providing an input light;

a modulator to modulate said input light and generate said digital optical output signal;

a splitter to split said digital optical output signal into a transmitter output signal and an output dither signal component;

a photodiode to provide a feedback signal;

a radio frequency (RF) feedback loop to control an RF amplitude input voltage to said modulator;

a bias feedback loop to control a bias input voltage to said modulator;

an RF amplitude dither circuit to provide an RF input dither signal to said RF amplitude input voltage;

a bias dither circuit to provide a bias input dither signal to said bias input voltage; and

an RF input amplifier to receive and amplify an RF data input signal.

27. The system of claim 26, wherein said modulator is a Mach-Zehnder modulator.

28. The system of claim 26, wherein said optical transmitter is used to transmit optical data along a digital lightwave communications system.

29. The system of claim 26, wherein said bias input voltage can be controlled by said RF input dither signal, and wherein said RF amplitude input voltage can be controlled by said bias input dither signal.

30. The system of claim 26, wherein said bias feedback loop and said RF feedback loop automatically compensate for changes over time in said RF amplitude input voltage and in said bias input voltage caused by temperature or vibration.

31. The system of claim 26, wherein said bias input dither signal and said RF input dither signal are in a relationship where one is half the frequency of the other.

32. The system of claim 26, wherein said bias input dither signal is at 250 hertz and wherein said RF input dither circuit is at 500 hertz.

33. The system of claim 26, wherein there are no frequency components of said RF input dither signal and said bias input dither signal that overlap.

34. The system of claim 26, wherein said RF feedback loop further comprises:

a first RF amplifier to amplify said photodiode feedback signal;

a filter to filter out residual digital output signal data and provide an RF output dither signal;

a second RF amplifier to amplify said RF output dither signal;

a synchronous detector to rectify said RF output dither signal into a DC output signal; and

an RF error amplifier to amplify and filter said synchronous detector DC output signal to provide a gain control signal to said RF input amplifier.

35. The system of claim 34, wherein said RF feedback loop further comprises a pedestal voltage input into said RF error amplifier to further adjust said RF amplitude input voltage.

36. The system of claim 34, wherein said synchronous detector further comprises an operational amplifier and an analog switch.

37. The system of claim 36, wherein said operational amplifier is either an inverting or a non-inverting amplifier in synchronization with said bias input dither signal.

38. The system of claim 26, wherein said bias feedback loop further comprises:

a first bias amplifier to amplify said photodiode feedback signal;

a filter to filter out residual digital output signal data and provide a bias output dither signal;

a second bias amplifier to amplify said bias output dither signal;

a synchronous detector to rectify said bias output dither signal into a DC output signal; and

a bias error amplifier to amplify and filter said synchronous detector DC output signal to provide a bias input voltage to said modulator.

39. The system of claim 38, wherein said synchronous detector further comprises an operational amplifier and an analog switch.

40. The system of claim 39, wherein said operational amplifier is either an inverting or a non-inverting amplifier in synchronization with said RF input dither signal.

41. The system of claim 26, wherein said laser is operated at a constant input current.

42. The system of claim 26, wherein said optical transmitter is a SONET format optical transmitter within a SONET optical transmission system.

43. The system of claim 42, wherein said SONET optical transmission system is an OC48 or OC192 SONET system.

44. The method of claim 26, wherein said optical transmitter is a pseudo-random bit sequence (“

PRBS”

) formats optical transmitter.

45. The method of claim 26, wherein said optical transmitter is an optical transmitter employing a forward error correction scheme.

46. A system for robustly controlling the bias point and RF amplitude level of a modulator for an optical transmitter, comprising

a laser for providing an input light;

a modulator to modulate said input light and generate said digital optical output signal;

a splitter to split said digital optical output signal into a transmitter output signal and an output dither signal component;

a photodiode to provide a feedback signal;

a radio frequency (RF) feedback loop to control an RF amplitude input voltage to said modulator;

a bias feedback loop to control a bias input voltage to said modulator;

a dither circuit to provide an input dither signal to either said RF amplitude input voltage or said bias input voltage;

synchronized selectors to select either said RF amplitude input voltage or said bias input voltage to receive said input dither signal; and

an RF input amplifier to receive and amplify an RF amplitude input signal.

47. The system of claim 46, wherein said modulator is a Mach-Zehnder modulator.

48. The system of claim 46, wherein said RF feedback loop further comprises:

a first RF amplifier to amplify said photodiode feedback signal;

a filter to filter out residual digital output signal data and provide an RF output dither signal;

a second RF amplifier to amplify said RF output dither signal;

a synchronous detector to rectify said RF output dither signal into a DC output signal; and

an RF error amplifier to amplify and filter said synchronous detector DC output signal to provide a gain control signal to said RF input amplifier.

49. The system of claim 48, wherein said RF feedback loop further comprises a pedestal voltage input into said RF error amplifier to further adjust said RF input voltage.

50. The system of claim 48, wherein said synchronous detector further comprises an operational amplifier and an analog switch.

51. The system of claim 50, wherein said operational amplifier is either an inverting or a non-inverting amplifier in synchronization with said input dither signal.

52. The system of claim 46, wherein said bias feedback loop further comprises:

a first bias amplifier to amplify said photodiode feedback signal;

a filter to filter out residual digital output signal data and provide a bias output dither signal;

a second bias amplifier to amplify said bias output dither signal;

a synchronous detector to rectify said bias output dither signal into a DC output signal; and

a bias error amplifier to amplify and filter said synchronous detector DC output signal to provide a bias signal to said modulator.

53. The system of claim 52, wherein said synchronous detector further comprises an operational amplifier and an analog switch.

54. The system of claim 53, wherein said operational amplifier is either an inverting or a non-inverting amplifier in synchronization with said input dither signal.

55. The system of claim 46, wherein said laser is operated at a constant input current.

56. The system of claim 46, wherein said optical transmitter is a SONET format optical transmitter within a SONET optical transmission system.

57. The system of claim 56, wherein said SONET optical transmission system is an OC48 or OC192 SONET system.

58. The method of claim 46, wherein said optical transmitter is a pseudo-random bit sequence (“

PRBS”

) formats optical transmitter.

59. The method of claim 46, wherein said optical transmitter is an optical transmitter employing a forward error correction scheme.