An all-fiber tunable ultralow frequency shifter based on cascaded acousto-optic tunable filters

Abstract

Optical heterodyne detection has advantages of high sensitivity and accuracy, and it has been widely used in optical communication and sensing. The kernel part of a heterodyne detection system is a phase or frequency shifter, which is usually based on the reflection of a mirror mounted on a piezoelectric transducer (PZT) or the diffraction of an acousto-optic modulator (AOM). The ultralow-frequency heterodyne interferometry and holography usually need a precise low frequency shift, which requires two AOMs with a high-quality, high-frequency and high-power signal generator. In this work, an all-fiber tunable ultralow frequency shifter was demonstrated based on cascaded acousto-optic tunable filters (AOTF). One of the AOTFs would down-shift the frequency of the light ∼ 1MHz the same as its driven radio frequency (RF) by coupling LP01 mode of the fiber to LP 11 mode, while the other AOTF shared the same etched fiber would up-shift the frequency when the LP 11 mode is coupled back. The optical frequency shift is determined by the difference in the two driven RF of the two AOTFS. With the driving frequency and power of the two AOTFs being around 0.918MHz and 15 dBm, the frequency shift could be tuned from 1 Hz to 100 Hz by tuning the frequency of the RF signal applied on the acousto-optic tunable filters.