Patent ID: 7215894

Claim:
An optical transmitter device comprising: a light source (LD); a driver circuit for generating a driving voltage corresponding to an input signal; a Mach-Zehnder (MZ) optical modulator for modulating output light from said light source in accordance with the driving voltage and outputting the modulated light as an optical signal; an operating point stabilizing circuit for detecting drift of an operation characteristic curve of said MZ optical modulator and controlling a bias voltage applied to said MZ optical modulator such that an operating point of said MZ optical modulator is situated at a fixed position with respect to the operation characteristic curve; a bottom voltage calculation unit for calculating periodically a bottom voltage of the operation characteristic curve from a half-wave voltage of the operation characteristic curve and the bias voltage; and wherein the bottom voltage calculation unit generates and sends a shutdown instruction for said LD and shuts down the output light of said LD in response to a shutdown instruction signal from an external source; a switch for forcing said MZ optical modulator to be driven at the bottom voltage calculated by said bottom voltage calculator and shutting down the output of the optical signal in response to the shutdown instruction signal from an external source.