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US 5,317,443 AGranted
Optical transmitter
AssigneeFujitsu Limited
InventorHiroshi Nishimoto
Life of this patent
Estimating the term…
Abstract
An optical transmitter operates on the DPSH-IM (direct phase-shift and self-homodyne intensity modulation) principle. The optical transmitter slightly amplitude-modulates the modulation current of a laser diode using a low frequency signal and detects the modulated component from the optical output. These features stabilize the operating point and improve the transmission waveform.
Claims21
An optical transmitter comprising:
- a laser diode;
- a bias circuit for supplying a bias current to said laser diode;
- a driving circuit for supplying said laser diode with a modulation current in accordance with an input signal, said modulation current being supplied in such a manner that the integral value of the oscillation frequency of said laser diode, varied in one time slot by said modulation current, reaches a phase amount of one of k.sub..pi. and -k.sub..pi., k being at least 1;
- a Mach-Zehnder optical interferometer for converting angle-modulated light coming from said laser diode into intensity-modulated light, the interferometer having a delay time difference corresponding to 1/k of one time slot;
- an optical branching circuit for branching the signal light from said Mach-Zehnder interferometer;
- a first oscillator for slightly amplitude-modulating said modulation current using a first low frequency signal;
- a first amplitude detector for detecting the amplitude and polarity of the frequency component of said first low frequency signal contained in the signal light branched by said optical branching circuit;
- and
- an operating point control circuit for increasing or decreasing, depending on the polarity detected by said first amplitude detector, the phase difference given upon interference by multiplying said oscillation frequency by said delay time difference, whereby control of said bias current is effected so that the amplitude detected by said first amplitude detector reaches zero.
6 more elements
An optical transmitter comprising:
- a laser diode;
- a bias circuit for supplying a bias current to said laser diode;
- a driving circuit for supplying said laser diode with a modulation current in accordance with an input signal, said modulation current being supplied in such a manner that said laser diode is frequency-modulated or phase-modulated;
- an optical branching circuit for branching the signal light from said laser diode;
- a Mach-Zehnder optical interferometer for converting the signal light branched by said optical branching circuit into intensity-modulated light;
- a first oscillator for slightly amplitude-modulating said modulation current using a first low frequency signal;
- a first amplitude detector for detecting the amplitude and polarity of the frequency component of said first low frequency signal contained in the intensity-modulated light from said Mach-Zehnder optical interferometer;
- an operating point control circuit for increasing or decreasing said bias current depending on the polarity detected by said first amplitude detector, whereby control is effected of said bias current so that the amplitude detected by said first amplitude detector reaches zero;
- a second oscillator for slightly modulating said bias current using a second low frequency signal having a frequency different from that of said first low frequency signal;
- a second amplitude detector for detecting the amplitude and polarity of the frequency component of said second low frequency signal contained in the intensity-modulated light from said Mach-Zehnder optical interferometer;
- and
- a driving amplitude control circuit for increasing or decreasing the amplitude of said modulation current depending on the polarity detected by said second amplitude detector, whereby control is effected so that the amplitude detected by said second amplitude detector reaches zero.
9 more elements
An optical transmitter comprising:
- a laser diode;
- a bias circuit for supplying a bias current to said laser diode;
- a Mach-Zehnder optical modulator for intensity-modulating the light from said laser diode;
- a driving circuit for supplying said Mach-Zehnder optical modulator with a modulation signal in accordance with an input signal;
- an optical branching circuit for branching the signal light from said Mach-Zehnder optical modulator;
- a first oscillator for slightly amplitude-modulating said modulation signal for said Mach-Zehnder optical modulator using a first low frequency signal;
- a first amplitude detector for detecting the amplitude and polarity of the frequency component of said first low frequency signal contained in the signal light branched by said optical branching circuit;
- an operating point control circuit for increasing or decreasing the bias voltage of said Mach-Zehnder optical modulator depending on the polarity detected by said first amplitude detector, whereby control is effected so that the amplitude detected by said first amplitude detector reaches zero;
- a second oscillator for slightly modulating the bias voltage of said Mach-Zehnder optical modulator using a second low frequency signal having a frequency different from that of said first low frequency signal;
- a second amplitude detector for detecting the amplitude and polarity of the frequency component of said second low frequency signal contained in the signal light branched by said optical branching circuit;
- and
- a driving amplitude control circuit for increasing or decreasing the amplitude of said modulation signal fed to said Mach-Zehnder optical modulator depending on the polarity detected by said second amplitude detector, whereby control is effected so that the amplitude detected by said second amplitude detector reaches zero.
9 more elements
An optical transmitter comprising:
- a laser diode;
- a bias circuit for supplying a bias current to said laser diode;
- a driving circuit for supplying said laser diode with a modulation current in accordance with an input signal, said modulation current being supplied in such a manner that the integral value of the oscillation frequency of said laser diode, varied in one time slot by said modulation current, reaches a phase amount of one of k.sub..pi. and -k.sub..pi., k being at least 1;
- a Mach-Zehnder optical interferometer for converting angle-modulated light coming from said laser diode into intensity-modulated light, the interferometer having a delay time difference corresponding to 1/k of one time slot;
- an optical branching circuit for branching the signal light from said Mach-Zehnder interferometer;
- a first oscillator for slightly amplitude-modulating said modulation current using a first low frequency signal;
- a first amplitude detector for detecting the amplitude and polarity of the frequency component of said first low frequency signal contained in the signal light branched by said optical branching circuit;
- and
- an operating point control circuit for increasing or decreasing, depending on the polarity detected by said first amplitude detector, the phase difference given upon interference by multiplying said oscillator frequency by said delay time difference, whereby control is effected of said time delay difference of said Mach-Zehnder optical interferometer so that the amplitude detected by said first amplitude detector reaches zero.
6 more elements
Dependencies are inferred from the claim text.