초록 ▼

An optical transmitter has a Mach-Zehnder modulator, and a feedback loop that controls the bias voltage supplied to the modulator according to the insertion loss of the modulator, to compensate for DC drift in the attenuation characteristic of the modulator. The feedback loop can be switched to prov

An optical transmitter has a Mach-Zehnder modulator, and a feedback loop that controls the bias voltage supplied to the modulator according to the insertion loss of the modulator, to compensate for DC drift in the attenuation characteristic of the modulator. The feedback loop can be switched to provide negative feedback on either the positive or negative slope of the attenuation characteristic. Switching the slope switches the polarity of the chirp generated by the modulator, enabling positive or negative chirp to be selected, whichever produces the least dispersion penalty. Since switching the slope also switches the polarity of the modulated signal, an optical transmission system using this optical transmitter has an additional circuit that selectively inverts the transmitted or received signal.

대표청구항 ▼

[ What is claimed is:] [1.] An optical transmitter for generating a modulated optical signal from an electrical input signal, the optical transmitter comprising:a light source generating an unmodulated optical signal;a Mach-Zehnder modulator, coupled to said light source, modulating said unmodulated

[ What is claimed is:] [1.] An optical transmitter for generating a modulated optical signal from an electrical input signal, the optical transmitter comprising:a light source generating an unmodulated optical signal;a Mach-Zehnder modulator, coupled to said light source, modulating said unmodulated optical signal according to a driving signal and a bias voltage, thereby generating a modulated optical signal with chirp, said Mach-Zehnder modulator having an attenuation characteristic with DC drift, said chirp having a polarity that depends on said bias voltage;a driver circuit, coupled to said Mach-Zehnder modulator, generating said driving signal from said electrical input signal;a first photodetector, coupled to said Mach-Zehnder modulator, monitoring average power of said modulated optical signal;a second photodetector, coupled to said light source, monitoring power of said unmodulated optical signal;a feedback circuit, coupled to said Mach-Zehnder modulator, generating said bias voltage, compensating for said DC drift by adjusting said bias voltage, operating in a first mode and a second mode, in the first mode increasing said bias voltage when the average power of said modulated signal increases in relation to the power of said unmodulated signal, in the second mode decreasing the bias voltage when the average power of said modulated signal increases in relation to the power of said unmodulated signal; anda switching circuit, coupled to said feedback circuit, switching the feedback circuit between the first mode and the second mode, thereby switching the polarity of said chirp.