Abstract: A signal transmission system includes an interface unit that monitors the operating condition of each device. The signal transmission system also includes a simulator that simulates the transmission quality of the system in response to the operating condition of each device, and controls each device so as to optimize the transmission quality.

Abstract: In an optical filter module and an optical amplification apparatus for a wavelength division multiplex (WDM light), the respective wavelength intervals of the plural passing center wavelengths and the filter characteristics in the vicinities of the respective center wavelengths can be set independently, whereby the BPF1 is provided with different passing center wavelengths .lambda.1, .lambda.2 . . . .lambda.n and different filter characteristics. And, when the WDM signal light including wavelengths .lambda.1, .lambda.2 . . . .lambda.n is inputted into the port 2-1a, the .lambda.k (k=1,2 . . . n) component is transmitted by the corresponding BPF1-k, and the transmitted light is multiplexed sequentially and outputted from the port 2-nb and the other wavelength components (noise light components) are reflected by all of the BPFs and outputted from the port 2-nc. When the WDM signal light including wavelengths except for .lambda.1, .lambda.2 . . . .lambda.

Abstract: In an optical heterodyne receiver having a local oscillating light source controlled by a driver circuit and which uses a polarity diversity technique thereby producing first and second intermediate frequency signals, an automatic frequency control circuit (AFC) controls the driver circuit by generating an error signal. The AFC includes two power regulators for generating respective fixed power signals based on the two intermediate frequency signals.