Quadratic detection of an enhanced self-mixing laser Doppler signal with two-photon absorption

Abstract

A new detection scheme is proposed for an enhanced self-mixing laser Doppler velocimeter (SLDV) using two-photon absorption (TPA), which is based on the TPA-induced quadratic photoconductivity dependence of a photodiode on the incident radiance at 1550 nm with self-mixing interference (SMI). The harmonics of the SMI signal in the frequency spectrum are enhanced simultaneously, as for a fiber-Bragg-grating-based enhanced SLDV. The quadratic response induced by TPA can suppress the undesirable harmonics, making their amplitude into the same level as the noise. From the experimental results presented in this paper, the TPA effect in a commercial Si photodiode implements a sort of ultra-wide bandwidth filter without passively adjusting the cutoff frequencies for an unknown Doppler frequency, which simplifies Doppler signal processing. The signal-to-noise ratio (SNR) is also improved to a considerable level of 15 dB for a Doppler frequency of 4 kHz due to the amplification of an erbium-doped fiber amplifier. This paper provides guidance for the design of the enhanced SLDV with optical filtration.