EP0444688A2
Expired / Lapsed
EP
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Optical transmitter
Assignee: Fujitsu Ltd
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Filing: 1991-02-28
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Granted: —
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Expires: 2011-02-28
Abstract
Abstract An art for stabilizing the operating point suitable for an optical transmitter, especially for that including a Mach-Zehnder optical modulator (6), is disclosed. According to this art, deterioration in the waveform of the output optical signal due to the operating point drift can be 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. <IMAGE>
Inventors
- 👤 Naoki Kuwata
- 👤 Hiroshi Nishimoto
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-09 07:22
CPC Classifications
G; G02; G02F; G02F1/00; G02F1/01; G02F1/0121; G02F1/0123; H; H04; H04B; H04B10/00; H04B10/50; H04B10/501; H04B10/503; H04B10/505; H04B10/5057; H04B10/50575; H04B10/564; H04B10/572; H04B10/58; H04B10/588; G02F1/21; G02F1/212; G02F1/225; G02F2203/00; G02F2203/20; G02F2203/21
Claims Analysis
Total Claims: 10
Claim 1
Independent Claim
243 words
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 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 delivering 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 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 of 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 for feedback-controlling a bias for said optical modulator so that the D.C. signal becomes zero
Claim 2
Dependent Claim
32 words
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.
Claim 3
Dependent Claim
229 words
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 for amplitude-modulating the input signal with the low-frequency signal and delivering 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 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 of 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 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.
Claim 4
Dependent Claim
24 words
An optical transmitter according to claim 3, wherein said Mach-Zehnder optical modulator has optical waveguides divided into two paths which have different phase-modulating efficiencies.
Claim 5
Dependent Claim
40 words
An optical transmitter according to claim 4, 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.
Claim 6
Dependent Claim
40 words
An optical transmitter according to claim 5, 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.
Claim 7
Dependent Claim
40 words
An optical transmitter according to claim 5, 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.
Claim 8
Dependent Claim
46 words
An optical transmitter according to claim 5, 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.
Claim 9
Dependent Claim
27 words
An optical transmitter according to claim 3, wherein the operating point is controlled by changing a phase of lightwave in optical waveguides of said Mach-Zehnder optical modulator.
Claim 10
Dependent Claim
43 words
An optical transmitter according to claim 2 or 3, 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.
References Cited (Backward Citations)
No citation records indexed.
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