Cross-Polarized Dual-Frequency VECSEL at 1.5 μm for Fiber-Based Sensing Applications

We have realized a dual-frequency vertical external cavity surface emitting laser (VECSEL) at 1.5 μm. Laser emission of two orthogonally polarized cavity modes is obtained by inserting a birefringent crystal into the VECSEL cavity. We have examined the influence of the different intracavity elements on the laser emission. It is shown that optimizing the free spectral range and the bandwidth of the intracavity Fabry-Perot etalon is of practical importance to achieve a stable single longitudinal laser emission for each of the two orthogonal polarizations. The optimization of the output power has also been investigated, and it is concluded that up to 50 mW output power can be expected in dual-frequency operation by adjusting the reflectivity of the output coupling mirror of the VECSEL cavity. The influence of different parameters on the stability of the dual-frequency emission has been studied. It is concluded that mechanical vibrations are the main cause of radio-frequency (RF) signal instability in our free-running VECSEL cavity. The design of a compact or mono-block cavity may allow meeting the stability requirements for optical fiber sensors based on Brillouin scattering.