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.This work was supported in part by the Spanish Government (Projects No. MAT2016-75716-C2-1-R (AEI/FEDER,UE), TEC 2014- 55948-R, FIS2017-87970-R), in part by Junta de Castilla y León (Project No. SA287P18), in part by Generalitat de Catalunya (Project No. 2017SGR755), and in part by the Ministry of Science and Higher Education of the Russian Federation (Goszadanie no. 2019-1080). The work of E. Kifle was supported by the Generalitat de Catalunya under Grant 2016FI_B00844, Grant 2017FI_B100158 and Grant 2018FI_B200123