Nova Patents
USRE36088E

Optical transmitter

Claim Score by NHIP

Read claim 4, the broadest

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.

Term

Term ended

Expired 1 August 2017, 9.1 years ago.

  1. Priority
  2. Filed
  3. Granted
  4. Expired
  5. Today

12 claims: 2 independent, 10 dependent

  1. 1.
    1. An optical transmitter comprising:PA1 a laser diode;PA1 a drive circuit for generating a drive voltage in accordance with an input signal;PA1 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;PA1 a low-frequency oscillator for outputting a predetermined low-frequency signal;PA1 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;PA1 an optical branching circuit for branching the optical signal output from said Mach-Zehnder optical modulator;PA1 a photoelectric converting circuit for converting one of the optical signals branched by said optical branching circuit to an electrical signal;PA1 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 PA1 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. 4.
    4. An optical transmitter comprising:PA1 a laser diode;PA1 a drive circuit for generating drive voltage in accordance with an input signal;PA1 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;PA1 a low-frequency oscillator for outputting a predetermined low-frequency signal;PA1 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;PA1 an optical branching circuit for branching the optical signal output from said Mach-Zehnder optical modulator;PA1 a photoelectric converting circuit for converting one of the optical signals branched by said optical branching circuit to an electrical signal;PA1 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;PA1 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 PA1 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.
Independent claims2