Optical transmitter

Abstract

An optical, transmitter with a stabilized operating point for an optical transmitter, especially for that including a Mach-Zehnder optical modulator, is disclosed. An optical modulator is provided which modulates a light beam from a light source according to a driving voltage from a drive circuit. An operating point stabilizing circuit is provided which detects a drift in the characteristic curve from the optical modulator and controls the optical modulator to bring the operating point to a specified position. Deterioration in the waveform of the output optical signal due to the operating point drift is thereby prevented without depending on the input signal. Further, by performing an operating point shift under specific conditions, deterioration in the waveform due to wavelength dispersion can be effectively prevented, whether the sign of the wavelength dispersion is positive or negative.

What is claimed is: 
     
       1. An optical transmitter comprising: a laser diode;   a drive circuit for generating a drive voltage in accordance with an input signal;   a Mach-Zehnder optical modulator responsive to the driving voltage for modulating a light beam from said laser diode, thereby converting the input signal into an optical signal;   a low-frequency oscillator for outputting a predetermined low-frequency signal;   a low-frequency superposing circuit, operatively connected to said low-frequency oscillator, for amplitude-modulating the input signal by superposing the low-frequency signal on a "0" logic level and a "1" logic level of the input signal such that a phase of the low-frequency signal at the "0" logic level and a phase of the low-frequency signal at the "1" logic level are opposite to each other and outputting the modulated signal to said drive circuit;   an optical branching circuit for branching the optical signal output from said Mach-Zehnder optical modulator;   a photoelectric converting circuit for converting one of the optical signals branched by said optical branching circuit to an electrical signal;   a phase detector circuit, operatively connected to said low-frequency oscillator, for detecting a frequency component of the low-frequency signal included in the electrical signal and comparing a phase of the frequency component with a phase of the low-frequency signal, thereby outputting a D.C. signal having a polarity corresponding to the phase difference and at a level corresponding to a drift in an operational characteristic curve of said Mach-Zehnder optical modulator; and   a bias control circuit, operatively connected to said drive circuit and said phase detector circuit, for feedback-controlling a bias for said optical modulator so that the D.C. signal becomes zero.   
     
     
       2. An optical transmitter according to claim 1, wherein an operating point of said Mach-Zehnder optical modulator is controlled by changing a phase of lightwave in optical waveguides of said Mach-Zehnder optical modulator. 
     
     
       3. An optical transmitter according to claim 2, wherein said low-frequency superposing circuit includes: an amplitude modulator for modulating an amplitude of the input signal with the low-frequency signal; and   a filter for removing a low-frequency component from the amplitude modulated signal.   
     
     
       4. An optical transmitter comprising: a laser diode;   a drive circuit for generating drive voltage in accordance with an input signal;   a Mach-Zehnder optical modulator responsive to the driving voltage for modulating a light beam from said laser diode, thereby converting the input signal into an optical signal;   a low-frequency oscillator for outputting a predetermined low-frequency signal;   a low-frequency superposing circuit, operatively connected to said low-frequency oscillator, for amplitude-modulating the input signal with the low-frequency signal and outputting the modulated signal to said drive circuit;   an optical branching circuit for branching the optical signal output from said Mach-Zehnder optical modulator;   a photoelectric converting circuit for converting one of the optical signals branched by said optical branching circuit to an electrical signal;   a phase detector circuit, operatively connected to said low-frequency oscillator, for detecting a frequency component of the low-frequency signal included in the electrical signal and comparing a phase of the frequency component with a phase of the low-frequency signal, thereby outputting a D.C. signal having a polarity corresponding to the phase difference and at a level corresponding to a drift in an operational characteristic curve of said Mach-Zehnder optical modulator;   a bias control circuit, operatively connected to said drive circuit and said phase detector circuit, for feedback-controlling a bias for said optical modulator so that the D.C. signal becomes zero; and   an operating point shifting circuit responsive to an operating point switching signal for shifting an operating point of said Mach-Zehnder optical modulator by half a cycle on the operational characteristic curve.   
     
     
       5. An optical transmitter according to claim 4, wherein said Mach-Zehnder optical modulator has optical waveguides divided into two paths which have different phase-modulating efficiencies. 
     
     
       6. An optical transmitter according to claim 5, wherein said optical transmitter includes a first polarity inverting circuit for inverting a polarity of the input signal according to the shift of the operating point provided by said operating point shifting circuit. 
     
     
       7. An optical transmitter according to claim 6, wherein said operating point shifting circuit is a second polarity inverting circuit responsive to the operating point switching signal for inverting a polarity of the low-frequency signal input to said phase detector circuit. 
     
     
       8. An optical transmitter according to claim 6, wherein said operating point shifting circuit is a third polarity inverting circuit responsive to the operating point switching signal for inverting a polarity of the low-frequency signal input to said low-frequency superposing circuit. 
     
     
       9. An optical transmitter according to claim 6, wherein said operating point shifting circuit is a fourth polarity inverting circuit responsive to the operating point switching signal for inverting a polarity of a feedback loop in said photoelectric converting circuit, phase detector circuit, or bias control circuit. 
     
     
       10. An optical transmitter according to claim 4, wherein the operating point is controlled by changing a phase of lightwave in optical waveguides of said Mach-Zehnder optical modulator. 
     
     
       11. An optical transmitter according to claim 4, wherein said low-frequency superposing circuit includes: an amplitude modulator for modulating an amplitude of the input signal with the low-frequency signal; and   a filter for removing a low-frequency component from the amplitude modulated signal. .Iadd.   
     
     
       12.  An optical transmitter operatively connected to receive an input light beam, comprising: an optical modulator responsive to a modulated input signal for modulating the input light beam to produce an optical signal;   a signal generator for outputting a low-frequency signal;   a low-frequency superposing circuit, operatively connected to said signal generator, for producing the modulated input signal by superposing the low-frequency signal on different levels of an input signal so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal;   a detector circuit, operatively connected to said optical modulator, for outputting a control signal indicating detection of a frequency component of the low-frequency signal included in the optical signal; and   a bias control circuit, operatively connected to said detector circuit, for controlling a bias of said optical modulator based on the control signal..Iaddend..Iadd.13. An optical transmitter as recited in claim 12, wherein the different levels of the input signal comprise first and second different levels..Iaddend..Iadd.14. An optical transmitter as recited in claim 13, wherein the different first and second levels comprise the minimum and maximum amplitude levels, respectively, of the input signal..Iaddend..Iadd.15. An optical transmitter as recited in claim 13, wherein the different first and second levels comprise "1" and "0" logic levels, respectively..Iaddend..Iadd.16. A circuit coupled to an optical modulator for modulating an input light beam based on a modulated input signal, to produce an optical signal, comprising:   a low-frequency superposing circuit for producing the modulated input signal by superposing a low-frequency signal on different levels of an input signal so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal;   a detector circuit, operatively connected to said optical modulator, for outputting a control signal indicating detection of a frequency component of the low-frequency signal included in the optical signal; and   a bias control circuit, operatively connected to the optical modulator and said detector circuit, for controlling a bias for the optical modulator   
     
     
        based on the control signal..Iaddend..Iadd.17.  A circuit as recited in claim 16, wherein the different levels of the input signal comprise first and second different levels..Iaddend..Iadd.18. A circuit as recited in claim 17, wherein the different first and second levels comprise the minimum and maximum amplitude levels, respectively, of the input signal..Iaddend..Iadd.19. A circuit as recited in claim 17, wherein the different first and second levels comprise "1" and "0" logic levels, respectively..Iaddend..Iadd.20. A method for modulating an input light beam, comprising the steps of: modulating an input light beam based on a modulated input signal to produce and output an optical signal;   superposing a low-frequency signal on different levels of an input signal to produce the modulated input signal, so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal;   detecting a frequency component of the low-frequency signal included in the optical signal output by the optical modulator;   outputting a control signal indicating detection of the frequency component; and   controlling a bias for the optical modulator based on the control   
     
     
        signal..Iaddend..Iadd.21.  A method as recited in claim 20, wherein the different levels of the input signal comprise first and second different levels..Iaddend..Iadd.22. A method as recited in claim 21, wherein the different first and second levels comprise the minimum and maximum amplitude levels, respectively, of the input signal..Iaddend..Iadd.23. A method as recited in claim 21, wherein the different first and second levels comprise "1" and "0" logic levels, respectively..Iaddend..Iadd.24. An optical transmitter comprising: a light source;   an optical modulator, responsive to a modulated input signal for modulating a light beam from said light source to produce an optical signal;   a low-frequency oscillator for outputting a predetermined low-frequency signal;   a low-frequency superposing circuit, operatively connected to said low-frequency oscillator, for producing the modulated input signal by superposing the low-frequency signal on a "0" logic level and a "1" logic level of an input signal such that a phase of the superposed low-frequency signal at the "0" logic level and a phase of the superposed low-frequency signal at the "1" logic level are opposite to each other and outputting the modulated input signal to said optical modulator;   an optical branching circuit for branching the optical signal output from said optical modulator;   a photoelectric converting circuit for converting one of the optical signals branched by said optical branching circuit to an electrical signal;   a detector circuit, operatively connected to said photoelectric converting circuit, for outputting a control signal indicating detection of a frequency component of the low-frequency signal included in the electrical signal; and   a bias control circuit, operatively connected to said detector circuit, for feedback-controlling a bias of said optical modulator based on the control   
     
     
        signal..Iaddend..Iadd.25.  An optical transmitter comprising: a light source;   an optical modulator responsive to a modulated input signal for modulating a light beam from said light source to produce an optical signal; and   an operating point stabilizing circuit which detects a drift in an operational characteristic curve of said optical modulator and controls said optical modulator so that the operating point thereof is brought to a position having a desired relationship with the operational characteristic curve, said operating point stabilizing circuit comprising: a low-frequency oscillator for outputting a desired low-frequency signal,   a low-frequency superposing circuit for amplitude-modulating different levels of an input signal with the low-frequency signal and for outputting the modulated input signal to said optical modulator,   a low-frequency signal detector circuit for detecting a frequency component of the low-frequency signal included in the optical signal output from said optical modulator and comparing the phase of the detected frequency component with the phase of the desired low-frequency signal, thereby detecting a direction of the operating point drift, and   a control circuit for controlling the operating point of said optical modulator so that it moves in the same direction as the drift direction..Iaddend..Iadd.26. An optical transmitter as recited in claim 25, wherein the different levels of the input signal comprise first and second different levels..Iaddend..Iadd.27. An optical transmitter as recited in claim 26, wherein the different first and second levels comprise the minimum and maximum amplitude levels, respectively, of the input signal..Iaddend..Iadd.28. An optical transmitter as recited in claim 26, wherein the different first and second levels comprise "1" and "0" logic levels, respectively..Iaddend..Iadd.29. An optical transmitter according to claim 25, wherein said light source is a laser diode..Iaddend..Iadd.30. An optical transmitter according to claim 29, wherein said optical modulator is a Mach-Zehnder optical modulator..Iaddend..Iadd.31. An optical transmitter having an optical modulator of which bias is controlled and which modulates an input light beam based on a modulated input signal to produce an optical signal, comprising:     a first circuit superposing a low-frequency signal on different levels of an input signal to produce a modulated input signal, so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal; and   a second circuit operatively connected to the optical modulator and detecting a frequency component of the low-frequency signal included in the optical signal to control the bias of the optical modulator..Iaddend..Iadd.32. A circuit, operatively coupled to an optical modulator of which bias is controlled and which modulates an input light beam based on a modulated input signal to produce an optical signal, the circuit comprising:   a first circuit superposing a low-frequency signal on different levels of an input signal to produce a modulated input signal, so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal; and   a second circuit operatively connected to the optical modulator and detecting a frequency component of the low-frequency signal included in the optical signal to control the bias of the optical modulator..Iaddend..Iadd.33. A method for controlling a bias of an optical modulator which modulates an input light beam, based on a modulated input signal, to produce an optical signal, comprising:   superposing a low-frequency signal on different levels of an input signal to produce a modulated input signal, so that the superposed low-frequency signal has different phases, respectively, at the different levels of the modulated input signal; and   detecting a frequency component of the low-frequency signal included in the optical signal to control the bias of the optical modulator..Iaddend.