Dual-polarization modes and self-heterodyne noise in a single-frequency 21-μm microchip Ho,Tm:YAG laser

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Abstract
We have observed, for the first time to our knowledge, residual stress-induced birefringent dual-polarization modes in a diode-pumped, single-frequency, microchip cw 2.0914-μm Ho,Tm:YAG laser. The two orthogonally polarized modes were separated in frequency by approximately 12 MHz and, when photodetected, produced a 12-MHz self-heterodyne beat signal. The self-heterodyne signal could be eliminated by an external λ/4 plate and a polarizer to filter out one of the modes, but this required regulation of the pump laser power to better than 1%. These results have important implications for use of the microchip Ho,Tm:YAG laser in a heterodyne detection coherent lidar.