Frequency changer and photodetector - has modulated photodiode bias to reduce optical modulation frequency and match impedance

摘要

The photodetector consists of an avalanche photodiode (1) and a series current amplifier (2). The negative bias applied to the photodiode is modulated by a signal which differs in frequency by less than the optical modulation frequency and the difference frequency to which the optical signal is transposed is then amplified. The peak to peak level of the voltage modulation is maintained within the operating rate of the diode. By reducing the output signal frequency the impedance matching between the diode and amplifier is improved and so is the signal to noise ratio. The modulator comprises a voltage amplifier (31) having an npn transistor (32) mounted in common base configuration and operating in class A. mode.