Examination to eliminate undesirable phase delay of an avalanche photodiode (APD) for intensity‐modulated light

Undesirable phase delay in avalanche photodiode (APD) between input light and output current is noticeable when modulation frequency of the incident light is over several hundreds of MHz. The degree of the delay increases with a decrease of the incident intensity. To clarify this phenomenon, relationships among the phase delay, output current, and the multiplication factor (current gain) of APD have been investigated. From this investigation, we found that the multiplication factor of APD increased with a decrease of the incident intensity, resulting in a decrease of the cutoff frequency of the APD. Supposing that the cutoff frequency defines the signal delay, the amount of the phase delay has been quantified with respect to the multiplication factor. Such a phase delay can be eliminated by superimposing a bias light on the modulation light so that the total intensity remains constant. To confirm this, a servo control system using an intensity modulated laser diode and unmodulated laser diodes (LDs) was c...