Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-8745
Timestamp: 2019-04-23 12:47:00+00:00

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We report the generation of mid-infrared (~2 µm) high repetition rate (MHz) sub-100 ns pulses in buried thulium-doped monoclinic double tungstate crystalline waveguide lasers using two-dimensional saturable absorber materials, graphene and MoS2. The waveguide (propagation losses of ~1 dB/cm) was micro-fabricated by means of ultrafast femtosecond laser writing. In the continuous-wave regime, the waveguide laser generated 247 mW at 1849.6 nm with a slope efficiency of 48.7%. The laser operated at the fundamental transverse mode with a linearly polarized output. With graphene as a saturable absorber, the pulse characteristics were 88 ns / 18 nJ (duration / energy) at a repetition rate of 1.39 MHz. Even shorter pulses of 66 ns were achieved with MoS2. Graphene and MoS2 are therefore promising for high repetition rate nanosecond Q-switched infrared waveguide lasers.
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Fig. 1 Confocal laser microscope images of the laser-grade-polished end-face of the Tm:KLuW WG: (a) transmission mode, polarized light (P || Np), λ = 405 nm; (b) back-propagating luminescence, unpolarized light, natural color, λexc = 405 nm; (c) transmission mode, crossed polarizers (P || Np, A || Nm), λ = 488 nm.
Fig. 2 Confocal laser microscope images (top view) of the central part of the circular cladding WG in Tm:KLuW: (a,b) transmission mode, polarized light (P || Ng), λ = 405 nm; (c) transmission mode, crossed polarizers (P || Ng, A || Nm), λ = 488 nm.
Fig. 3 Laser set-up: ND – gradient neutral-density filter, PM – pump mirror, SA – saturable absorber, OC – output coupler, F – cut-off filter.
Fig. 4 Graphene- and MoS2-SAs: (a) small-signal transmission spectra, (b) Raman spectra, λexc = 514 nm. The arrow in (a) corresponds to the laser wavelength.
Fig. 5 CW Tm:KLuW channel WG laser: (a) input-output dependences, η – slope efficiency; (b) typical laser emission spectra (measured at Pabs = 0.52 W).
Fig. 6 Graphene and MoS2 PQS Tm:KLuW channel WG lasers: (a) input-output dependences, η – slope efficiency, (b) typical laser emission spectra measured at maximum Pabs, (c) calibrated 1D intensity profiles of the laser mode at the output face of the WG along the Nm and Np axes: symbols – experimental data, curves – Gaussian fits, inset – 2D mode profile, the white circle indicates the cladding (graphene SA, TOC = 30%, Pabs = 0.35 W). The laser polarization is E || Nm.
Fig. 7 Graphene and MoS2 PQS Tm:KLuW channel WG laser: (a) pulse duration (FWHM), (b) pulse repetition frequency (PRF) and (c) pulse energy. Symbols: experimental data; curves – numerical calculation using the model from  for graphene-SA.
Fig. 8 Oscilloscope traces of (a) the typical pulse trains and (b) the shortest Q-switched pulses from the graphene and MoS2 PQS Tm:KLuW channel WG lasers.
Table 3 Output Characteristics of the PQS Tm:KLuW Channel WG Lasers.
(1) α'(I)=α ' NS + α ' S 1+(I/ I sat ) , α ' SA =1− T SA =α ' NS +α ' S .
Output Characteristics of the PQS Tm:KLuW Channel WG Lasers.

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