Improvement of Noise Performance in the Phase Locking Laser Doppler Vibrometer with a Low Carrier Frequency via Harmonic Locking

A heterodyne laser Doppler vibrometer (LDV) with a Bragg cell has a stationary signal carrier at a frequency of at least 35 MHz. The expensive Bragg cell with the restricted shift frequency is not an optimal solution to meet the requirements for many measurement scenarios. For vibrations with low frequencies and small amplitudes, a tens-of-megahertz carrier frequency wastes not only bandwidth at the photodetector but also requires a fast and expensive analog-to-digital converter (ADC). Compared to the Bragg-cell-based LDVs, LDVs with an optical phase-locked loop (OPLL) enable a selectable carrier and, thus, can provide a low carrier frequency. However, obviously problems arise when the carrier frequency is smaller than the laser linewidth and OPLL bandwidth. We accidentally were able to offset-lock the OPLL on the harmonic component of the heterodyne oscillator signal and explored this phenomenon, which enables carrier frequencies much smaller than the laser linewidth of the locked lasers at a smaller noise level. In this paper, we demonstrate a carrier frequency down to 3 MHz with a 3.3 dB better signal-to-noise-ratio (SNR) compared to the traditional locking technology. Our findings may enable very cost-efficient LDVs.