A variable attenuator is inserted into an optical input part and a feedback control is performed so that an optical input to an amplifying optical fiber becomes constant. Further, a control for changing a total optical output and an optical input to the amplifying optical fiber is performed on the basis of channel number information obtained from a supervisory signal. When light to/from an intermediate optical component is detected and the absence of the component is detected, pumping is suppressed, thereby avoiding occurrence of an optical surge at the time of connection of the optical component and a signal indicative of detachment of the optical component is generated.

The present invention relates to a method for transmitting a supervisory optical signal. A first optical amplifier for a first band (e.g., a conventional band: C band) determining a noise characteristic and a gain efficiency is provided. A second optical amplifier for a second band (e.g., a long wavelength band: L band) determining a noise characteristic and a gain efficiency is provided. The supervisory optical signal is processed in relation to one of the first and second optical amplifiers superior in at least one of the noise characteristic and the gain efficiency to the other. According to this method, undue degradation in the noise characteristic and the gain efficiency can be prevented to maintain a good transmission quality of a main signal.

A method of controlling an optical wavelength division multiplexing transmission apparatus achieves stable wavelength division multiplexing optical transmission by switching a control mode of an optical amplification section in accordance with an input state of optical signals of various wavelengths. Accordingly, the method involves, upon startup of the optical wavelength division multiplexing transmission apparatus, initial setting of information such as the wavelengths being used and the number of wavelengths being used, setting the amount of optical attenuation for each wavelength to a maximum value, and setting an optical amplification unit to ALC. Then, upon input of an optical signal corresponding to the wavelengths being used, the amount of optical attenuation corresponding to the input optical wavelength is controlled so that the levels of the input optical signals analyzed by the spectral analysis unit are approximately constant; and moreover, so that the input optical level per single wavelength input into the optical amplification unit is of a level which corresponds with the number of wavelengths being used. Then the apparatus shifts to normal operating conditions. Next, when the number of wavelengths being input varies, the optical amplifier is switched to AGC, and after the power level of the optical signal of each wavelength is adjusted, the optical amplifier is switched to ALC again.

A variable dispersion optical unit is used to vary the relative dispersion between an input signal and an output signal of the optical unit under control of a control circuit. The control unit uses an optical homodyne detector to beat the input optical signal with the output optical signal to generate a control signal to adjust the relative dispersion a variable dispersive element. The variable dispersive optical element may be selected from a group of dispersive elements including at least a Bragg grating, a Fabry-Perot filter, a Mach-Zehnder filter, and a waveguide routing element. The variable dispersive optical unit can be used as part of a variable wavelength selective optical circuit (e.g., a tunable wavelength filter) where the varying dispersion characteristics is used to control the selection of the wavelength of the tunable wavelength filter, or is used to select the drop/add wavelength(s) of an Add/Drop circuit.

A multiplex optical communication system transmitting a multiplex optical signal between terminal equipments each including converters producing optical signals combined to the multiplex optical signal, through repeater equipment including an optical amplifier, under controlling the amplifier so that when a converter is added or removed, a time constant of the amplifier is equal to a time constant of the added or removed converter, or the amplifier produces output under constant output level control before adding or removing the converter and after the amplifier produces final output and constant gain control during the added or removed converter increases or decreases output. The time constant control is performed to the amplifier by making the amplifier repeat start and stop of increasing or decreasing output step by step in accordance with prescribed objective values corresponding to half way output of the optical amplifier.