Ultrafast Laser Inscription and ∼2 μm Laser Operation of Y-Branch Splitters in Monoclinic Crystals

We report on the first active surface Y-branch waveguide in the ~2 μm spectral range. Depressed-cladding rectangular-cross-section surface waveguides with a splitting ratio of 1 × 2 are fabricated by femtosecond direct laser writing in a thulium (Tm
³⁺
) doped monoclinic double tungstate crystal. Confocal laser microscopy and μ-Raman spectroscopy reveal well preserved crystallinity of the waveguide core. Under high-brightness laser pumping at 0.8 μm, a simultaneous continuous-wave laser operation in both arms is achieved resulting in a total output power of 0.46 W at ~1.84 μm with a slope efficiency of 40.6% and a laser threshold of 0.28 W. The laser output is linearly polarized and spatially multimode (TE12/TE22) with a power splitting ratio between arms of 52.1/47.9%). The waveguide propagation losses at 1.84 μm are ~1.6 dB/cm and the loss from the Y-junction is 0.1 dB. The fabricated waveguides represent a route towards advanced photonic micro-structures such as a Mach-Zehnder interferometer for bio-sensing at ~2 μm.