Frequency-shifted laser feedback interferometry in non-planar ring oscillators

Abstract

Laser feedback interferometry (LFI) has a wide range of applications such as displacement, distance and velocity measurements. LFI has been realized in many types of lasers but has never been reported in non-planar ring oscillators (NPROs) to the best of our knowledge. Here, we present a new type of LFI based on an NPRO laser. The intrinsic resistance to optical feedback in NPROs is broken under weak-magnetic-intensity condition, where stable bidirectional lasing is initiated in the ring cavity. The interference signal, i.e., the beat of the bidirectional lasing is with frequency in the range of a few hundred kilohertz, which is mainly determined by the applied magnetic intensity in NPRO. Frequency-shifted LFI is thus constructed in NPRO without using acousto-optic modulators as mostly used in conventional LFI. A theoretical model based on dual-frequency rate equations and Lang–Kobayashi equation is presented to describe the mechanism of LFI in NPRO. In the end, micro-vibrational measurements are demonstrated to prove the potential application, where vibration-detection amplitude limit is sub-picometer, and the detection frequency range from kilohertz to a few hundred kilohertz is achieved. Benefiting from the characteristics of tiny footprint, ruggedized structure, long lifetime and ultralow-noise of NPRO lasers, NPRO-based LFI may find important applications in industry, scientific research, military and aerospace.