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We report on a highly efficient continuous-wave (CW) and Q-switched Tm:CaGdAlO4 (Tm:CGA) laser at ~1.96 µm in-band pumped by a high power Raman fiber laser at ~1.7 µm. Over 7.2 W of CW output power with a polarization extinction ratio (PER) of 22 dB has been achieved, corresponding to a slope efficiency of up to 60%. For Q-switching, the shortest pulse duration of 75 ns was obtained at 10 kHz pulse repetition frequency (PRF) and over 5.2 W of average output power has been generated at 50 kHz PRF. The prospects for further improvement in continuous-wave and Q-switched performance are discussed.
© 2017 Optical Society of America
1. Introduction
Solid-state lasers emitting at ~2 µm wavelength region are now the research highlights owing to their widespread applications in lidar, atmospheric remote-sensing, optical communications and medicine, etc. Pulsed laser sources with high average output power and in the nanosecond timescale are deemed to be potentially useful for organic materials processing and nonlinear frequency conversion [1–5]. Among which, linearly polarized pulsed lasers with high polarization extinction ratio (PER) are especially desired pump sources for generating mid-infrared (3-5 µm) lasers via optical parametric oscillation (OPO), on account of their advantages of high absorption efficiency and simplified cavity structures [6, 7].
Up to now, various crystals doped with Tm3+-ions (Tm:YAG, Tm:YLF, Tm:CYA, etc.) have been investigated as the gain media for 2 µm laser generation due to their unique thermal, mechanical and optical properties. Benefitting from the anisotropic structures, Tm:YLF and Tm:CYA crystals have been demonstrated to be outstanding bulk media for high PER linearly polarized laser emission [8–10]. Particularly, the Tm:CYA crystal, characterized by the disordered structure, performs strong energy storage capability in Q-switched mode of operation. The relatively low thermal conductivity (~3.6 W/m·K in a-cut orientation), however, limits its possibility for further power scaling [11]. By substitution of Y3+ ions with Gd3+ ions, a novel Tm3+-doped CaGdAlO4 (CGA) crystal can be grown by the standard Czochralski method [12]. The Tm:CGA crystal not only has excellent thermo-mechanical properties (thermal conductive is ~6.9 W/m·K in a-cut orientation), but also covers the spectroscopic properties of the Tm:CYA crystal. Associated with the relatively broad and flat gain spectrum that caused by the inhomogeneous spectrum broadening resulted from randomly occupying the neighboring sites by Ca2+ and Gd3+ (Tm3+) ions between Al3+ layers, the Tm:CGA crystal is believed to be a promising candidate for both Q-switched and mode-locked laser generation at ~2 μm, just as in the case of the Nd3+, Yb3+, or Er3+-doped CGA crystals for ~1 μm and ~1.5 μm lasers [13–15]. Very recently, mode-locked Tm:CGA lasers in the picosecond and femtosecond regimes have been experimentally realized with average output powers of hundreds miliwatts by employing semiconductor saturable absorber mirrors as the mode lockers [16, 17]. Lasing characteristics of Tm:CGA crystal in nanosecond timescale commonly produced by Q-switched technique, however, are still unknown. Moreover, as far as we have known, the so far reported Tm:CGA lasers were all pumped by typical laser diode at ~800 nm, which inevitably induced a relatively large quantum defect heating and leading to a relatively low laser conversion efficiency.
In this letter, we report on high power and efficient operation of a Tm:CGA laser at ~1.96 μm in both CW and Q-switched regimes. The crystal is uncoated and in-band pumped with a home-constructed high-power Raman fiber laser at ~1.7 µm. In the CW mode of operation, over 7.2 W of output power with a PER of 22 dB has been generated, corresponding to a slope efficiency of ~60%. Preliminary experiments on Q-switched mode of operation were carried out using a z-shaped resonator with an acoustic-optics Q-switch, and the lasing characteristics at different PRFs have been investigated. Stable pulse trains of 10-50 kHz PRFs have been produced with over 5.2 W of average output power at 50 kHz PRF. At 10 kHz of PRF, stable pulses of 75 ns duration have been produced with a peak power of 2.9 kW.
2. Continuous-wave laser operation
Absorption spectrum of the Tm:CGA crystal is shown in Fig. 1(a). It can be seen that the absorption bandwidth is quite broad and covers a wavelength range of 1550-1900 nm, corresponding to the transition of Tm3+ ions from the ground state 3H6 to the excited state 3F4. The absorption peak is at ~1735 nm and the absorption bandwidth (FWHM) is ~170 nm. Pump light for this transition, however, is not readily available since it is beyond the efficient gain bandwidth of common rare earth ions. Taking advantages of stimulated Raman scattering known as the third order nonlinear frequency conversion, high power output at this wavelength region can be realized via high power Er doped solid-state/fiber laser pump sources. Owing to the wide Raman gain spectra, the stimulated Raman scattering lasers can, in principle, span the whole absorption bandwidth of Tm:CGA by choosing a suitable pump wavelength. In our experiment, the Raman fiber laser was pumped by a high power tunable Er, Yb fiber laser at 1564 nm. The fiber used for Raman laser generation was a 3 km nonpolarization-maintained graded index multimode fiber with a core diameter of 50 µm and a numerical aperture of 0.21. The laser generated a maximum output power of ~20 W at 1693 nm, which matches well with the absorption of the Tm:CGA crystal, as shown in Fig. 1(a).
Fig. 1 (a): Absorption spectrum of Tm:CGA and emission wavelength of the pump fiber laser. (b): Emission spectra of Tm:CGA and optical absorption spectra of water molecules.
With the high power Raman source pumping, we measured the emission spectra of the Tm:CGA crystal using a long wavelength optical spectrum analyzer (AQ6375, Yokogawa Electric Corp., wavelength range: 1200-2400 nm), as shown in Fig. 1(b). The emission spectra span from 1580 nm to 2080 nm with a bandwidth (FWHM) of ~150 nm. This is due to the inhomogeneous spectrum broadening and splitting resulting from the disordered structure in the crystal. It is worth noting that the curve is obviously not smooth in the wavelength range of 1850-1950 nm, this should be attributed to the water molecules absorption in the air.
Figure 2 exhibits the in-band pumped Tm:CGA laser configuration in our experiment. The pump light was collimated and then focused onto the Tm:CGA crystal by two plano-convex lenses with focal length of 26 mm and 100 mm. The laser resonator consisted of two plane dichroic mirrors labelled as M1 and M2. The input mirror (M1) is high transmission (>98%) at 1570-1715 nm and high reflectivity (>99%) at 1800-2100 nm. The output mirror (M2) is high reflectivity (>98%) at 1570-1715 nm and a transmission of 20% at 1850-2150 nm. The a-cut Tm:CGA crystal sample is 8 mm in length and 4 mm × 4 mm in cross section, with both surfaces polished but uncoated. The pump light in the Tm:CGA crystal was estimated to have e−2 diameter of ~200 μm. The optical axis of whole laser resonator wasn’t completely parallel but at a 5-degree angle to the pump light to avoid that the reflected light from the uncoated crystal surface influenced the pump Raman fiber laser.
Under nonlasing condition and without the existence of the output coupler, single-pass small signal absorption was estimated to be ~70% and the absorption decreased to ~60% at maximum incident pump power due to grand-state bleaching. Pump absorption under lasing condition was estimated according to the small-signal absorption, 70%, at all pump levels. Lasing characteristics of the Tm:CGA crystal in CW mode of operation was evaluated first. Figure 3(a) illustrates output power as a function of the absorbed pump power. It can be seen that the output power increased linearly with the absorbed pump power and a maximum output power of 7.2 W has been generated, corresponding to a slope efficiency of 59.8%. Further power scaling should be possible with an anti-reflection coated Tm:CGA and higher pump source at ~1.7 µm. Due to the disorder structure, the spectrum of CW Tm:CGA laser exhibits a randomly multi-peaks structure without specific center wavelength and range, and even at a fixed pump power the spectrum were different. A typical spectrum of CW Tm:CGA laser is shown in the inset of Fig. 3(a). The center wavelength is at ~1959 nm and the spectrum spans a wavelength range of ~9 nm from 1954 nm to 1963 nm. The laser beam quality factors M2 of the laser are about 1.5 and 1.4 in x and y directions, respectively.
Fig. 3 (a): Output power of CW Tm:CGA laser versus the absorbed pump power. Inset: Spectrum of CW Tm:CGA laser. (b): Polarization characteristic of CW Tm:CGA laser output.
Output of the a-cut Tm:CGA laser exhibited a linearly polarized characteristic due to the birefringence nature of the crystal. In our experiment, a half-wave plate and a Glan prism was employed to investigate the polarization state. The output light of Tm:CGA laser was collimated and launched on the half-wave plate and Glan prism. We rotated the half-wave plate carefully and measured the power values after Glan prism. Figure 3(b) shows the measured power versus the rotated angle of the half-wave plate under an absorbed pump power of 5 W (we defined the direction passing through the maximum power as 0°). The output polarization state was found to be quite stable and almost linearly polarized with a PER of as high as 22 dB. At the maximum output power of 7.2 W, the PER of the laser output decreased to ~20 dB owing to the thermal induced birefringence effect, but still kept at a rather high level.
3. Q-switched operation
To evaluate lasing characteristics of the Tm:CGA crystal in Q-switched pulsed regime, we constructed a z-shaped folded resonator formed by four mirrors as shown in Fig. 4. M1, M4 are the same mirrors as the M1 employed in CW experiment, and M2 here is identical to the output coupler in Fig. 2. M3 was a concave mirror (R = 100 mm) with high reflectivity (reflectivity>99%) at 1850-2150 nm and high transmission (transmission >99%) at 1500-1700 nm. The whole resonator length was ~250 mm and a 2 µm acoustic-optics Q-switch (Gooch & Housego, QS027-2D-B5) was inserted in between mirror M2 and M4. Using the ABCD matrix propagation theory, the mode beam e−2 diameter at the center position of the Tm:CGA crystal was around 200 µm, which matched well with the pump light.
Laser characteristics of Tm:CGA for Q-switched mode of operation were evaluated at PRFs of 10 kHz, 20 kHz and 50 kHz and stable pulses trains at these repetition rates have been generated with pulse durations varying from 75 to 480 ns. Figure 5 shows the dependence of pulse width and peak power on absorbed pump power at 10 kHz of PRF. The pulse width varies from 195 to 75 ns and peak power increases from 0.33 to 2.93 kW when the absorbed pump power increases to 5.8 W.
Pulse energy versus absorbed pump power at different PRFs is plotted in Fig. 6(a). At 10 kHz of PRF, the output pulse energy is ~0.22 mJ for an absorbed pump power of 5.8 W. At 50 kHz of PRF, a maximum average output power of 5.2 W has been generated, with a slope efficiency of 41.7% (as illustrated in the inset of Fig. 6(a)). The PER in pulse mode of operation was ~21 dB. Compared to CW operation, the spectrum of Q-switched Tm:CGA laser is also random, but more longitudinal modes appear with a broader wavelength spanning from 1945 nm to 1970 nm just as shown in Fig. 6(b). The beam quality factors M2 of Q-switch Tm:CGA laser are about 1.2 and 1.1 in x and y directions and much better than CW laser (illustrated in the inset of Fig. 6(b)). Figure 7 shows a typical pulse train of 10 kHz PRF and single-pulse envelope of 75 ns of pulse duration and the pulse-to-pulse amplitude fluctuation in the experiment was measured to be less than 5%.
Fig. 6 (a): Single pulse energy verse absorbed pump power at different PRFs. Inset: Output power versus absorbed pump power at 50 kHz of PRF. (b): Typical spectrum (Inset: Beam quality) of Q-switched Tm:CGA laser.
4. Conclusion
In summary, we have demonstrated high power and efficient operation of a Tm:CGA laser at ~1.96 µm in both CW and Q-switched regimes. In the CW mode of operation, over 7.2 W of output power with a PER of 22 dB has been generated, corresponding to a slope efficiency of ~60%. For Q-switched mode of operation, stable pulse trains of 10-50 kHz PRFs have been produced with 5.2 W of average output power at 50 kHz PRF. At 10 kHz of PRE, stable pulses of 75 ns duration have been produced with a peak power of 2.9 kW. Further improvements in laser performance in both CW and Q-switched mode of operation should be possible with anti-reflection coated laser crystal and higher pump power at ~1.7 μm.
Funding
This work is supported by the National Natural Science Foundation of China (NSFC) (61505072, U1430111), and a project funded by the Priority Academic Development of Jiangsu Higher Education Institutions (PAPD).
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