Abstract: An EDFA 1 is comprised of a pre-amp 2 and a post-amp 3, which are connected in series, a photocoupler 4, a long wave pass filter (LWPF) 5 and an LWPF controller 6. The photocoupler 4 is connected to an input-stage of the EDFA 1 and enters the light signal in the LWPF controller 6. The LWPF 5 is located between the pre-amp 2 and the post-amp 3, and its cutoff wavelength is adjustable. A short-wavelength-part of the light signal which is amplified by the pre-amp 2, is cut off by the LWPF 5. The LWPF controller 6 detects an input signal power of the input light signal and adjusts the cutoff wavelength of the LWPF 5 based on the detected input signal power. Gain of light signals which are output from the post-amp 3, are equalized ranging the extensive input signal power and covering wide signal-wavelength bands.

Abstract: Each optical repeater in an optical transmission system includes an optical amplifier, and a control circuit for controlling the optical transmission. Furthermore, each optical repeater transmits management information to the next optical repeater. The control circuit checks the status of the optical amplifier and obtains monitoring information. The control circuit generates management information in accordance with the monitoring information and the management information transmitted from the upstream side optical repeater. The management information is converted to an optical signal and transmitted to the next optical repeater. A receiving station receives the management information. The management information includes an ID code for an optical repeater which has failed and the status of the failure.

Abstract: In a variable high-voltage generating circuit for an avalanche photodiode, a pulse oscillating circuit generates pulses, a current driving circuit current-amplifies the pulses, a Cockcroft-Walton circuit boosts and rectifies the pulses from the current driving circuit, and a low-pass filter is connected at the output of the Cockcroft-Walton circuit. The avalanche photodiode is connected in series with a first resistor, and the series connection is connected to receive the output of the low-pass filter. A series connection of a transistor and a second resistor is connected in parallel with the series connection of the avalanche diode and the first resistor. A control circuit applies a variable signal to the base of the transistor, thereby to vary the voltage applied across the APD.