Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-7-12-4258
Timestamp: 2019-04-22 12:01:50+00:00

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We report on the fabrication, µ-Raman characterization, and continuous-wave laser operation of a channel waveguide with a hexagonal optical-lattice-like cladding fabricated in monoclinic Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. µ-Raman spectroscopy indicates preservation of the crystalline quality in the core region and an anisotropic residual stress field. When pumped by a Ti:Sapphire laser at 802 nm, the Tm:KLu(WO4)2 buried channel waveguide laser generated 136 mW at 1843.7 nm with a slope efficiency of 34.2% and a threshold as low as 21 mW, which are the record characteristics for femtosecond-laser-written Tm crystalline waveguide lasers. The variation of the output coupling resulted in discrete wavelength tuning of the laser emission from 1785 to 1862 nm. The propagation losses in the waveguide are ~1.2 ± 0.3 dB/cm.
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Vazquez de Aldana, J. R.
Fig. 1 (a,b) Bright-field microscope images of the facet cross-section of the fs-DLW hexagonal cladding Tm:KLuW waveguide.
Fig. 2 (a) 3F4 → 3H6 luminescence of Tm3+ ions from the fs-DLW Tm:KLuW waveguide for light polarizations E || Nm and Np, λexc = 802 nm; the noise at 1.8-1.9 µm is due to the water absorption in air, and (b) polarized Raman spectra of the waveguide core region measured in backscattering geometry, λexc = 514 nm.
Fig. 3 Micro-Raman mapping of the facet cross-section of the fs-DLW hexagonal cladding Tm:KLuW waveguide at 907.6 cm−1: (a) peak intensity, (b) full width at half maximum (FWHM) and (c) phonon energy shift. Similar maps are given for the Raman band at 686.5 cm−1 in (d) peak intensity, (e) FWHM and (f) phonon energy shift.
Fig. 4 Variation of the Raman intensity (a) and Raman peak shift (b) along the cross-section of the fs-DLW hexagonal cladding Tm:KLuW waveguide, as shown in the inset of (a), for the 907.6 cm−1 and 686.5 cm−1 Raman bands. The Raman peak shift in (b) is calculated with respect to the bulk (non-damaged) material.
Fig. 5 (a) Scheme of the fs-DLW Tm:KLuW channel waveguide laser: PM – pump mirror, OC – output coupler, ND – neutral density filter, F – long-pass filter, CL – collimating lens; (b) top view of the pumped Tm:KLuW waveguide showing blue upconversion luminescence.
Fig. 6 Input-output dependences of the fs-DLW hexagonal cladding Tm:KLuW channel waveguide laser: (a) TOC = 1.5%...30% at 1.8–2.1 μm, (b) TOC = 80% (“bandpass” OC) or 89% (without an OC), η – slope efficiency.
Fig. 7 Typical emission spectra for the fs-DLW hexagonal cladding Tm:KLuW channel waveguide laser (measured at the maximum Pabs).
Fig. 8 fs-DLW hexagonal cladding Tm:KLuW channel waveguide laser: (a) modified Caird analysis for TOC = 1.5%...30% (circles – experimental data, line – their linear fit) (b) absorption saturation in the waveguide under non-lasing conditions: red circles – experimental data from the pump-transmission measurement, blue circle – small-signal absorption from the spectroscopic data, solid curve – rate-equation modelling.
Fig. 9 Spatial profile of the output laser mode from the fs-DLW hexagonal cladding Tm:KLuW channel waveguide laser (TOC = 30%, Pabs = 188 mW): (a) 2D profile measured in the far-field, E is the laser polarization; (b) spatial overlap of the waveguide cross-section and the reconstructed beam profile at the output facet of the waveguide; (c) the corresponding 1D profiles of the laser mode in the horizontal (|| Nm) and vertical (|| Np) directions.

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