Optical Heterodyne Vibrometer Utilizing the Inter-Mode Beat of a Two-Mode Stabilized He-Ne Laser

Laser Doppler vibrometer was built by using a two-mode stabilized He-Ne laser. The laser supports two orthogonally-polarized axial-modes which generate an inter-mode beat frequency of 724 MHz. These two modes serve as sensing and reference light. To detect instantaneous Doppler frequency shift, a novel demodulation scheme was proposed. The frequency shift is detected by a delayed self-homodyne technique. The delay is provided via an optical fiber. The performance of the demodulator was investigated theoretically and experimentally.