FREQUENCY CHARACTERISTIC MEASURING INSTRUMENT FOR PHOTODETECTING CIRCUIT

摘要

PURPOSE:To speedily measure the frequency characteristics of the photodetecting circuit by accurately control and sweep the frequency of a beat signal obtained by making reference light and laser light to be controlled incident on the photodetecting element and synchronizing the sweeping for the frequency response of the photodetecting circuit by a spectrum analyzer at the same time. CONSTITUTION:The beat signal corresponding to the frequency difference between the reference light 108 and the output light of a laser diode 101 to be controlled is obtained from circuit 81 to be measured. The beat signal and the output of a 1st local oscillator 111 are mixed by a mixer 110 to generate an intermediate frequency signal. The frequency of the laser diode 101 is controlled by using a PLL to hold the frequency of the intermediate frequency signal constant. In this state, the oscillation frequency of the local oscillator 111 is swept to sweep the oscillation frequency of the diode 101. Consequently, the frequency of the beat signal is tracked by a heterodyne type PLL at all times, so the beat frequency can accurately be swept in a short time and the level of the detected beat signal is recorded. The sweep needs to be performed only once when the spectrum analyzer and the sweeping speed of the beat frequency are made coincident, and started and measured at the same time.