Fs-laser-written thulium waveguide lasers Q-switched by graphene and MoS₂

Abstract

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 MoS 2 . 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 MoS 2 . Graphene and MoS 2 are therefore promising for high repetition rate nanosecond Q-switched infrared waveguide lasers. IntroductionWaveguide (WG) lasers emitting in the spectral range of ~2 μm are of interest for spectroscopy, telecom and environmental sensing applications [1]. This is because their emission is eye-safe and matches the absorption lines of various atmospheric and biomolecules. The laser operation at ~2 μm is achieved using such trivalent rare-earth ions as Thulium (Tm 3+ ) or Holmium (Ho 3+ ). In the former case, the transition of interest is 3 F 4 → 3 H 6 . Due to the large Stark splitting of the Tm 3+ ground-state, its emission is wavelength-tunable. The Tm 3+ ions feature strong absorption at ~0.8 μm ( 3 H 6 → 3 H 4 transition) allowing for their efficient pumping by AlGaAs laser diodes and Ti:Sapphire lasers. Moreover, the crossrelaxation for adjacent Tm 3+ ions, 3 H 4 + 3 H 6 → 3 F 4 + 3 F 4 , can potentially raise the pump quantum efficiency up to 2.Efficient continuous-wave (CW) thulium WG lasers are known [2,3]. Several methods have been used to fabricate WGs based on Tm 3+ -doped materials, e.g., liquid phase epitaxy (LPE) in combination with ion beam etching [2], optical bonding [4], ion diffusion [5], reactive co-sputtering [6] and femtosecond direct laser writing (fs-DLW) also referred as ultrafast laser inscription (ULI) [7]. In the latter approach, the output of an ultrafast (fs) An overview of PQS thulium WG lasers reported to date is presented in Table 1. Regarding planar WG lasers produced by LPE, a "slow" SA (Cr 2+ :ZnS) was employed in a Tm:KY(WO 4 ) 2 laser yielding 1.2 μs / 0.12 μJ (duration / energy) pulses at a low repetition rate of 10 kHz [24]. The use of a "fast" SWCNT-based SA in a similar laser lead to much shorter pulses of 83 ns / 33 nJ at 1.39 MHz [25]. However, both these lasers generated spatially multimode output.

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

“…In 2013, the researchers fabricated a graphene SAM and applied it to a single‐chip waveguide laser to obtain the mode‐locked pulses with a pulse width of 1.06 ps and a repetition rate of 1.5 GHz, as shown in Figure . Since then, waveguide lasers using layered materials such as graphene, Bi 2 Se 3 , TMDCs, and BP have been further developed, as shown in Table . It can be seen that, for Q‐switching operation, the minimum pulse width and maximum pulse energy are 25.2 ns and 310 nJ, respectively.…”

Watt-level ultrafast laser inscribed thulium waveguide lasers

“…An estimation of the refractive index change of À0.6 Â 10 À3 and the propagation losses of 1.2 AE 0.3 dB/cm at the wavelength of~1.84 μm were provided. Furthermore, the power scaling of Tm WG laser with a circular cladding was also demonstrated [112], yielding 247 mW at 1849.6 nm with a slope efficiency of 48.7%. The Tm WG lasers operated at the fundamental transverse mode.…”

“…Micro-Raman and micro-luminescence mapping of the WG end-facet were also reported. In both Tm and Ho fs-DLW WG lasers, the output was linearly polarized (E || N m ) [112,113]. For Ho WG, it was spatially multimode [113].…”

Low-dimensional materials as saturable absorbers for pulsed waveguide lasers

“…With graphene SA, Q-switched waveguide laser has been achieved at wavelength of 1831.8 nm and repetition frequency of 1.13 MHz [55]. Based on MoS 2 thin film and circular cladding Tm:KLuW waveguides, 1.58 MHz Q-switched laser have been demonstrated at 1845.0 nm with pulse duration down to 66 ns [57]. Sb 2 Te 3 thin film has also been employed as a SA for the passive Q-switched laser at ∼2 µm based on evanescent field interaction.…”