Abstract: A variable optical wavelength filter for making it possible to control the movement of an optical wavelength filter during a momentary loss of the optical input signal in the same way as when the optical input signal is present. Even when the input of the second direct current signal to an integrator is interrupted because of the momentary loss of the optical input signal, the integrator is able to place a dielectric multilayer filter at the position directly before the momentary loss of the optical input signal because of sustaining the voltage from directly before the momentary loss of the optical input signal.

Abstract: A variable optical wavelength filter for making it possible to control the movement of an optical wavelength filter during a momentary loss of the optical input signal in the same way as when the optical input signal is present. Even when the input of the second direct current signal to an integrator is interrupted because of the momentary loss of the optical input signal, the integrator is able to place a dielectric multilayer filter at the position directly before the momentary loss of the optical input signal because of sustaining the voltage from directly before the momentary loss of the optical input signal.

Abstract: An apparatus for controlling a laser diode (135) includes a photodiode (136) which monitors an output light (P.sub.pump) of the laser diode (135) and a control signal generator (132) which generates a control signal in response to an output of the photodiode (136). The apparatus also includes a driver circuit (134) which drives the laser diode (135) in response to the control signal so that the laser diode (135) generates a constant power of light.