Photodetectors with TIAs, however, offer a trade-off between sensitivity and response. Transimpedance amplifiers (TIAs) have been widely used in readout circuits of photodetectors owing to their low cost and easy operation. The bandwidth of the input-referred noise current was 7 kHz, which is consistent with rise times below 10 pA. The 50% rise time for a light pulse input increased from 3.1 μs at a photocurrent of 10 nA to 15 μs at photocurrents below 10 pA. The time response of the system was found to deteriorate with decreasing photocurrent. The measured noise equivalent power of the system was 1.7 fW/Hz 1/2 at 10 Hz and 1.3 μm, which is consistent with the derived expression. An optical-feedback photodetection system with an InGaAs pin photodiode was fabricated.
The expression for the input-referred noise current of the optical-feedback photodetection system was derived, and the trade-off between sensitivity and response, which is a characteristic of TIA, was found to considerably improve. The output light was detected by a photodiode with a TIA having a small feedback resistance. The current gain has no theoretical limit. The current gain corresponds to the ratio of the photocurrent created by the output light to that created by the feedback light because the feedback current value is identical to the input photocurrent value generated by an input light to be measured. Current amplification was implemented by extracting an output light from the same light source that emitted the feedback light. A photodetection system with an optical-feedback circuit accompanied by current amplification was fabricated to minimize the drawbacks associated with a transimpedance amplifier (TIA) with a very high resistance feedback resistor.
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