A pulsed high energy and narrow bandwidth tunable Tm:YLF laser at the milli-Joule level is demonstrated. The spectral bandwidth was narrowed down to 0.15 nm FWHM, while 33 nm of tunability range between 1873 nm and 1906 nm was achieved using a pair of YAG Etalons. The laser was actively Q-switched using an acousto-optic modulator and mJ level pulse energy was measured along the whole tuning range at a repetition rate of 1 kHz. Up to 1.97 mJ of energy per pulse was achieved at a pulse duration of 37 ns at a wavelength of 1879 nm, corresponding to a peak-power of 53.2 kW and at a slope efficiency of 36 %. The combination of both high energy pulsed lasing and spectral tunability, while maintaining narrow bandwidth across the whole tunability range, enhances the laser abilities, which could enable new applications in the sensing, medical and material processing fields.

This paper presents a KGW Raman laser with an external-cavity configuration at the 2 µm region. The Raman laser is pumped by an actively Q-switched Tm:YLF laser, especially designed for this purpose emitting at 1880 nm. Due to the KGW bi-axial properties, the Raman laser is able to lase separately at two different output lines, 2197 nm and 2263 nm. The output energies and pulse durations that were achieved for these two lines are 0.15 mJ/pulse at 21 ns and 0.4 mJ/pulse at 5.4 ns, respectively. To the best of our knowledge, this is the first time that the KGW crystal, which is well known for its wide use in shorter wavelengths, is demonstrated in a Raman laser in the 2 µm region. According to the achieved results and due to the KGW properties, it appears to be a suitable crystal for energy scaling and efficient Raman conversion in this spectral range. An estimation of the Raman gain coefficient for this wavelength is provided as well. IntroductionIn the past few years, there has been a growing demand for high-power lasers emitting at wavelength beyond 2 μm. Lasers at these "retina-safe" wavelengths have several potential applications, including LIDAR, biomedicine [1], polymer material processing [2], defense applications, and gas sensing [3]. Therefore, 2 µm (SWIR) coherent sources have drawn much attention, as is evident from the recent efforts to develop high power lasers that cover this spectral range [3][4][5].Within this context, solid-state Raman lasers lend themselves to an interesting approach. They are efficient and useful high-brightness sources that extend the spectral span of existing lasers and fill the spectral gaps between them [6-9]. Compared to OPO, Raman lasers have advantages such as narrow linewidth, avoid of phase matching constraints, pulse length shortening and beam quality improvement through Raman beam cleanup [10]. However, solid-state Raman lasers are mostly implemented in the visible and ~1 µm regions and only rarely in the SWIR region. The reasons for this are two fold, first because the stimulated-Raman-scattering (SRS) gain coefficient drops theoretically approximately according to inverse wavelength, resulting in lower efficiency compared to NIR Raman lasers [6]. The second reason is the lack of suitable high power pump lasers that have sufficient gain over the Raman stimulated threshold. Recent developments in high-power pulsed Tm-doped and Hodoped solid-state lasers in the 2 µm region [5,[11][12][13][14] expands the availability of pump sources that can be used for efficient Raman laser conversion in this spectral range. Those lasers can be designed to have high peak power, linear polarization and narrow bandwidth, characteristics which are often required to match the Raman gain properties [6,8,15,16].The first demonstrations of SRS conversion in 2 µm using Tm:KY(WO 4 ) 2 and BaWO 4 crystals were reported more than a decade ago [17,18]. However, these reports lack

We present an all-passive efficient KGW Raman laser with an external-cavity configuration in the 2 µm spectral regime. The Raman laser was pumped by a passively Q -switched Tm:YAP laser emitting at 1935 nm. Due to the bi-axial properties of the KGW crystal, the laser exhibits stimulated Raman emission at two separate spectral lines: 2272 nm and 2343 nm. The output energies achieved at these two lines are 340 µJ/pulse and 450 µJ/pulse, accordingly. The seed to Raman laser conversion efficiencies achieved of 19.2% and 23.5%, respectively, are comparable to actively Q -switched laser arrangements. To the best of our knowledge, this is the first time an efficient Raman laser in the 2 µm regime is demonstrated in a completely passive configuration.

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