Absolute distance measurement using optical feedback with heterodyne detection has been demonstrated by sweeping the optical frequency of a single longitudinal mode (SLM) Yb:Er glass laser. This technique allows enhancing the sensitivity of the laser response to self-mixing thanks to resonant excitation close to the relaxation oscillation frequency peak. The experimental results on a non-cooperative target are in good agreement with the theory. The shape of the resulting signal is analysed both in the temporal and frequency domains considering the specific dynamic of a class B solid-state laser.
The optical feedback into a class B laser can be used as optical velocimeter. This self-mixing technique is simple, self-aligned and very sensitive on low cooperative target. However, the resulting frequency beating only allows deducing the longitudinal speed component along the laser beam. In some cases, such geometry becomes unpractical compared to classical laser Doppler velocimetry (LDV). By using two beams geometry, we have demonstrated the possibility to simultaneously measure both transverse and longitudinal components of the speed vector. This new self-mixing scheme is demonstrated using a single-frequency diode pumped Yb3+:Er3+ phosphate glass laser selected for its inherent very high sensitivity to optical feedback. The principle is validated on a rotating disc with diffusing surface and the tangential linear speed of the disc is precisely measured from 1m/s up to 10m/s without knowing the exact orientation of the disc. Moreover, the technique is spatially selective thanks to the peculiar dynamical response of the laser showing three characteristic beating frequencies in the power spectra when the target is precisely located at the focus point. The dynamic and resolution of the optical sensor are discussed depending on the characteristics of the laser and the geometry of the optical design.
Accurate and highly sensitive speed measurements have been successfully demonstrated by a self-aligned optical feedback velocimetry technique using the self-mixing modulation effect in a double-clad Er-Yb-doped fiber laser.