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A compact continuous wave (CW) and actively Q-switched (AQS) Ho:LuVO4 laser pumped by a 1.94 μm Tm: YAP laser is demonstrated. The performance of the laser was investigated by changing the output coupler. The maximum output power of 4.1 W at 2058.43 nm in CW regime is obtained at the maximum absorbed pump power of 12.3 W. The minimum pulse width of 29.3 ns was obtained at Pulse Repetition Frequency (PRF) of 20 kHz with the same output coupler corresponding to a peak power of 6.9 kW. The maximal output power is 4.1 W with center wavelength of 2058.43 nm at PRF of 40 kHz, corresponding to slope efficiency of 43.0% with respect to absorbed pump power. The M2 factors measured by the traveling knife-edge method are 1.04 in parallel a-axis and 1.08 in parallel c-axis with diffraction limited beam quality.
© 2015 Optical Society of America
1. Introduction
Lasers emitting in the 2 μm region are very attractive due to a various potential applications, such as medical surgery, detection of pollutant and coherent Doppler wind lidars [1–3]. In recent years, Tm3+/ Ho3+ lasers based on the different hosts has been widely investigated. The main doped hosts includes an oxide (YAG, YAP, etc), a fluoride (YLF, LLF, etc.), and a vanadate (YVO4, GdVO4, etc.), etc. Due to the excellent crystal quality on the oxide and fluoride hosts, the Tm3+/Ho3+ doped oxide and fluoride lasers have got a series of remarkable achievements [4–6]. Comparatively, vanadate hosts i.e. YVO4, GdVO4, and LuVO4 have lower crystal symmetry and large phonon coupling energy. Therefore, vanadate hosts doped with rare earth ions has a wider pump absorption band, a greater absorption cross section and laser emission cross section [7,8]. Besides, the vanadate crystal host family are isostructural to ZrSiO4 with tetragonal group I41/amd [9] with high density (for example, LuVO4: 6.33 g/cm3, YVO4: 4.22 g/cm3, and GdVO4: 5.47 g/cm3) [10]. The study of 2 μm vanadate laser mostly focused on Tm3+ doped and Tm3+, Ho3+ co-doped laser [11,12]. However, only a few Ho doped lasers are proposed. At 2011, Newburgh et al. reported Ho: YVO4 laser operation for the first time under liquid nitrogen refrigeration (77 k) [13]. In the same year, Li et. al. reported Ho: YVO4 laser operation at room temperature [14]. Recently our group has conducted extensive laser researches characteristics research on Ho:YVO4 and Ho:GdVO4. In 2014, we reported Ho:GdVO4 laser working in Q switched mode. The minimum pulse width of 4.7 ns and the maximum pulse energy of 0.9 mJ were obtained [15]. In 2015, using Tm doped fiber laser as the pump source, we reported Ho: YVO4 Q-switched laser operation and obtained the maximum average output power of 11.4 W. The corresponding minimum pulse width (FWHM) was measured to be 19.3 ns [16].
Recently, a new member of the doped vanadate crystals, Ho:LuVO4, was discovered as a suitable laser crystal [17]. The Ho:LuVO4 crystal has a large absorption cross section in the 1.94 μm band of π-polarization (0.72 × 10−20 cm2) and σ-polarization (1.06 × 10−20 cm2), which indicates the feasibility to get lasing by using 1.94 μm pump sources. For π-polarization, the Ho:LuVO4 crystal has a large emission cross section (1.84 × 10 −20 cm2 for 2041nm and 1.7 × 10- 20 cm2 for 2054nm). For σ-polarization, the Ho:LuVO4 crystal has a large emission cross section at the wavelength of 2008nm: 1.03 × 10−20 cm2. Up to now, the CW laser performance of Ho:LuVO4 crystal has been investigated, however, Q-switched Ho:LuVO4 laser has not been reported.
In this paper, we investigated the room temperature CW and actively Q-switched (AQS) performances of the Ho:LuVO4 laser end-pumped by a high-power Tm:YAP laser. In CW operation, the maximum output power of 4.1 W at 2058.43 nm with slope efficiency of 43.0% for output coupler T = 20% is obtained under the absorbed pump power of 12.3 W. In AQS operation, with the same pump power, a maximum peak power of 6.9 kW and minimum pulse width of 29.3 ns was obtained at the PRF of 20 kHz for output coupler T = 20%. Furthermore, a comparative analysis of laser performance with different output couplers is first carried out.
2. Experimental setup
The experimental setup of the Q-switched Ho:LuVO4 laser is shown in Fig. 1. To evaluate the lasing performance of Ho:LuVO4 crystal in room temperature, a diode-pumped Tm:YAP laser with emission wavelength of 1.94 µm was utilized for pumping which emits 25 W of output power with beam quality M2 of∼3.4. The Ho:LuVO4 crystal was a-cut, and the c-axis was paralleled with the polarization direction of the pump laser (Tm:YAP laser). The Ho:LuVO4 crystal was orientated with c-axis perpendicular to the image plane of Fig. 1. The Ho:LuVO4 crystal with 0.5 at. % Ho3+ concentration was grown by a standard Czochralski technique. The Ho:LuVO4 crystal for the experiment was 20 mm in length and 3 × 3 mm2 in cross-section. The lasing facets of the crystal have antireflection coatings, which centered at 1.94 μm (R < 0.5%) and 2 μm (R < 0.3%) respectively. The crystal was warped with silver foil and held in the copper heat sink for good thermal conduction. The temperature was controlled at 17 °C by a thermoelectric cooler. The single-pass absorption of pump radiation was measured to be 55%. A simple L-shaped resonator was used for the Ho:LuVO4 laser with cavity length of about 110 mm. As shown in Fig. 1, the flat mirror M1 had high reflectivity (R>99.5%) at 2 µm and high transmission (T>96.2%) at 1.94 µm. The flat 45°dichroic mirror (M2) had high reflectivity for both p-polarization and s-polarization (R>97.7%) at 2 µm and high transmission (T>97%) at 1.94 µm. The output coupler M3 was plane-concave with radius curvature of 120 mm. The calculated diameter of the TEM00 mode was about 380 µm based on cold resonator. The pump spot diameter was focused to a 320 µm in the center of the Ho:LuVO4 crystal with a 75-mm-focal length lens. A 35-mm-long acousto-optic (AO) Q-switch with an active aperture of 1 mm was used inside the cavity when working in Q-switched mode.
3. Experimental results and discussion
The output power of Ho:LuVO4 laser as a function of the absorbed pump power is shown in Fig. 2 with the output coupler transmittance of 10%, 20% and 30%. The output power of Ho:LuVO4 laser in the experiment was measured using Coherent PM30 power meter. In CW mode, at the absorbed pump power of 12.3W, the maximum output powers of 3.2 W, 4.1 W and 4.1 W were obtained for T = 10%, T = 20% and T = 30% respectively, which indicated corresponding optical-optical conversion efficiencies to be 26%, 33.3% and 33.3%. The slope efficiencies were 34.3%, 43.0% and 43.5%, respectively [see Fig. 2(a)]. Moreover, the output laser was detected by a Glan prism, to be found that the polarization state of the light was π polarized. In Q-switched regime, the output performance of Q-switched Ho:LuVO4 laser was investigated at PRF of 40 kHz. For T = 10%, the laser achieved 3.1 W output power under the absorbed pump power of 12.3 W, corresponding to optical-optical conversion efficiency of 25.2% and a slope efficiency of 33.0%. For T = 20% and T = 30%, the laser achieved 4.1W and 3.9 W output power under the absorbed pump power of 12.3 W, which indicated optical-optical conversion efficiencies of 33.3% and 31.7%, corresponding to slope efficiency of 43.0% and 42.8% [see Fig. 2(b)]. These results indicate that the difference of slope efficiencies with the T = 20% and T = 30% was small.
Fig. 2 Output power of Ho:LuVO4 laser versus absorbed pump power, (a) CW and (b) Q-switched at PRF of 40 kHz.
The CW and Q-switched laser spectrum recorded with Spectrum Analyzer (BRISTOL INSTRUMENTS 721) at the absorbed pump power of 12.3 W is shown in Fig. 3. The CW lasers with three different output coupler T = 10%, T = 20% and T = 30% have central wavelengths of 2058.59 nm, 2058.43 nm and 2058.13 nm, respectively. With the increase of the output coupler transmittance, the CW laser brings the shorter wavelength. The Q-switched laser spectrum obtained at 40 kHz is showed in Fig. 3(b). Compared with CW operation, the central wavelengths of Q-switched lasers keep the same with central wavelengths of 2058.59 nm, 2058.43 nm and 2058.13 nm for the different output coupler T = 10%, T = 20% and T = 30% were obtained. Also it was found that the output wavelength of the laser crystal was not sensitive to temperature changes.
The pulse temporal trace was recorded by a Lecroy digital oscilloscope (600 MHz bandwidth) with a fast PIN photodiode. At the fixed PRF of 40 kHz, the dependence of laser pulse width on absorbed pump power was measured and shown in Fig. 4. As shown in Fig. 4, the pulse width shortens sharply when the absorbed pump power increases, and the difference of pulse widths in two output coupler T = 20% and T = 30% is slight. The maximum energies per pulse of 0.08 mJ, 0.1 mJ and 0.1 mJ, corresponding to the peak powers of approximately 1.5 kW, 2.3 kW and 2.2 kW were obtained for output coupler T = 10%, T = 20% and T = 30%, respectively.
Furthermore, at transmittance of T = 20% we measured the dependence of laser pulse widths on different repetition rates when absorbed pump power was 12.3 W, as shown in Fig. 5. The pulse width increased from 29.6 ns to 59 ns as the repetition rate was tuned from 20 kHz to 60 kHz. Correspondingly, the peak power of the laser output decreased from 6.9 kW to 1.2 kW. Figures 6 show the typical oscilloscope trace of expanded shape of a single pulse. The minimum pulse width was 45.3 ns and 29.3 ns at PRF of 40 kHz and 20 kHz when the absorbed pump power was 12.3 W, respectively.
Fig. 5 The dependence of pulse width on the different repetition rate at absorbed pump power of 12.3W.
Figures 7 show the measured beam radius under maximum output power for T = 20% at various distances from the lens with f = 100 mm located about 115 mm after the output mirror (M1). Using the 90/10 knife edge method for Ho:LuVO4 laser beam quality measurement, we achieved the beam quality factors of M2a = 1.04 (parallel a-axis) and M2c = 1.08 (parallel c-axis), which clearly indicated nearly diffraction-limited beam propagation (TEM00). The inset of Fig. 7 is the measured beam intensity distribution by a pyroelectric camera.
4. Conclusions
In conclusion, to our knowledge, we first demonstrated an acousto-optic Q-switched Ho:LuVO4 laser with a diode-pumped Tm:YAP emitting at 1.94 µm. The influence of the different output couplers on laser performances is comparatively analyzed. A maximal CW output power of 4.1 W is obtained for the output coupling transmittance of T = 20% and T = 30%. A maximal CW centric wavelength of 2058.59 nm is obtained for the output coupling transmittance of T = 10%. For Q-switched operation, the minimum pulse width was 29.3 ns at PRF of 20 kHz when the absorbed pump power of 12.3 W for output coupler T = 20%, corresponding to a peak power of 6.9 kW. The maximum output power was 4.1 W with center wavelength of 2056.43 nm at PRF of 40 kHz, corresponding to slope efficiency of 43.0% relative to absorbed pump power. The centric wavelength shifts to the shorter value with the increase of output coupling transmittance. Furthermore, the M2a factor of 1.04 in parallel a-axis and M2c factor of 1.08 in parallel c-axis with diffraction limited beam quality. Our experimental results indicated that Q-switched Ho:LuVO4 laser, which have high peak power and short pulse width [18], will provide an excellent pump source for mid-IR optical parametric oscillators.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 61308009, No.61405047), China Postdoctoral Science Foundation funded project (No. 2013M540288), Fundamental Research funds for the Central Universities (Grant No. HIT. NSRIF. 2014044, Grant No. HIT. NSRIF. 2015042) and Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310)
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