Abstract: The present invention provides an input interruption detecting circuit of an optical receiver that realizes a circuit structure that avoids erroneous synchronization due to an influence of a synchronous noise, and enables an accurate detection of an input interruption, with respect to input interruption detection based on the synchronous or asynchronous state of a PLL circuit. To this end, the input interruption detecting circuit of an optical receiver according to the invention is one which is provided with a noise superimposition section which superimposes an asynchronous noise having frequency components that make the PLL circuit asynchronous, onto a signal input to a phase comparator of the PLL circuit, in a circuit structure for performing input interruption detection based on the synchronous state of a PLL circuit.

Abstract: The invention relates to a modulation controlling circuit for supplying a modulating signal to an external modulator that outputs a modulated optical signal, while setting a proper operating point in the external modulator. The external modulator that operates in cooperation with the modulation controlling circuit of the invention enables high-quality modulation at a wide bit rate range. Therefore, in an optical transmission system or a measuring instrument to which the invention is applied, the characteristics and the performance are improved and the operation is made stable without loss of advantages of the external modulator.