Precision distance measurement using a two-photon absorption process in a silicon avalanche photodiode with saw-tooth...

We present a novel configuration of a precision laser distance measurement based on the two-photon absorption (TPA) photocurrent from a silicon avalanche photodiode (Si-APD). The proposed system uses saw-tooth phase modulation, known as serrodyne modulation, in order to shift the frequency of the reference light from that of the probe light. It suppresses the coherent interference noise between the probe and the reference. The serrodyne modulation also enables lock-in detection of the TPA photocurrent. Furthermore, it contributes to the reduction of the system components. The precision measurement is experimentally demonstrated by measuring a fiber length difference of 2.6 m with a standard deviation of 27 μm under constant temperature. The high-precision displacement measurement is also demonstrated by measuring the temperature-induced change in the optical path length difference of a fiber interferometer.