We demonstrated continuous-wave (CW) and Q-switched operation of a room-temperature Ho:YAlO3 laser that is resonantly end-pumped by a diode-pumped Tm:YLF laser at 1.91 µm. The CW Ho:YAlO3 laser generated 5.5 W of linearly polarized (E‖c) output at 2118 nm with beam quality factor of M 2≈1.1 for an incident pump power of 13.8 W, corresponding to optical-to-optical conversion efficiency of 40%. Up to 1-mJ energy per pulse at pulse repetition frequency (PRF) of 5 kHz, and the maximum average power of 5.3-W with FWHM pulse duration of 30.5 ns at 20 kHz were achieved in Q-switched mode.
©2008 Optical Society of America
2 µm Holmium lasers resonantly pumped by 1.9 µm thulium emission are of great interest for applications including remote sensing and nonlinear frequency conversion to 3~12 µm spectral region [1,2]. Direct resonant pumping Ho 5I7 manifold offers the advantages of high slope efficiency (82% in Ho:LuAg) , minimal heating due to low quantum defect of less than 10% between pump and laser. Resonantly pumped Ho lasers based on YAG and YLF host materials have been extensively investigated for generation of high power and high pulse energy 2-µm laser by Budni et al. , Lippert et al. , and Dergachev et al. .
Yttrium aluminium oxide (YAlO3) is an attractive laser host for holmium due to its natural birefringence combined with good thermal (thermal conductivity of 11 W/m.K) and mechanical properties similar to those of YAG . Because of the birefringence of character of the YAlO3, thermally induced birefringence does not degrade the laser performance at high power level, and linearly polarized output is favorable for acousto-optic Q-switched operation with high PRF.
In this paper, we report the spectral characteristics and room-temperature laser actions of 1.9-µm-pumped Ho:YAlO3 in both CW and Q-switched modes.
2.1 Energy levels
Figure 1 shows the Stark-split ground and first excited states in Ho3+:YAlO3 at 2 K, chosen primarily because of availability from Ref. . The 2.12-µm laser transition in our experiment, which originates from the two lowest, near degenerate levels of 5I7 (5186 and 5187 cm-1), terminates at 474 cm-1 level of the ground-state 5I8 manifold.
Table 1 shows a summary of the population fraction of 5I7 and 5I8 ions in the pump and laser levels for Ho:YAlO3, Ho:YAG and Ho:YLF at 300K , where f l and f u are the population fraction of 5I7 and 5I8 manifolds in the lower and upper laser Stark levels, f a and f b are the population fraction of the lower and upper Stark levels of the pump transition, f T=f l/(f l+f u) is the fraction of ions needed to be excited to the upper laser level to obtain optical transparency on the laser transition , and f x=(1+f b/f a) accounts for both ground-state depletion (the “1”) and population buildup in the upper manifold (f b/f a) . As seen from Table 1, the terminal level of 5I8 manifold is only ~1.19%-populated at 300 K for Ho:YAlO3 (1.72% for Ho:YAG and 2.4% for Ho:YLF), which means that the laser is a nearly-four-level system. We estimate that only ~10.7% of the Ho3+ ions need to be excited for achievement of transparency at the emission wavelength in YAlO3, significantly less than that required in YAG (14.7%) or YLF (21.5%).
2.2 Absorption and emission
YAlO3 crystallizes in the orthorhombic space group D16 2h-Pbnm, and this low symmetry (as compared to cubic YAG) has an important consequence that the absorption and fluorescence properties of rare-earth ions doped YAlO3 are polarization dependent .
The laser crystals used here were grown by the Czochralski technique. The polarized absorption spectrum of Ho 5I8→5I7 band in YAlO3 host was recorded on a sample with 1 at. % holmium (2.0×1020 ions/cm3) using a Shimadzu UV-3100PC spectrophotometer with a resolution of 1-nm, and is shown in Fig. 2. The absorption peaks near 1.9 µm are located at 1883, 1916, 1928 and 1947 nm, and their corresponding absorption cross sections are 0.39×10-20 0.73×10-20, 0.66×10-20 and 0.59×10-20 cm2, respectively. The absorption peaks around 2.0 µm lie at 1976, 1998 and 2038 nm with corresponding absorption cross sections of 0.85×10-20, 0.48×10-20 and 0.28×10-20 cm2, and FWHM linewidth of 24, 15 and 17 nm, respectively. The broad absorption band of Ho:YAlO3 with multiple intense peaks from 1850 to 2050 nm allows great flexibility in the selection of Tm-doped ‘bulk’ or fiber lasers for in-band pumping.
Figure 3 shows the polarized fluorescence spectrum from Ho3+ 5I7→5I8 transitions in Ho:YAlO3, which was excited by a 1.91 µm Tm:YLF laser. A 300 mm WDM1-3 monochromator with a 600 lines/mm grating blazed for 2.0 µm was used to scan across the spectrum (0.8 nm resolution). The fluorescence was monitored by an InGaAs detector with a SRS830 lock-in amplifier for signal extraction. As shown in Fig. 3, the locations and relative intensity of fluorescence emission peaks are orientation dependent, and the 2.12 µm emission peak is in polarization E‖ c with spectral linewidth of 17 cm-1 (8nm). According to Ref. , the effective emission cross section is 0.82×10-20 cm-2 at 2.12 µm (E‖ c) in Ho:YAlO3, compared with that of 1.13×10-20 cm2 at 2.09 µm in Ho:YAG  and 1.55×10-20 cm2 (π polarization) at 2.05 µm in Ho:YLF . The lifetime of Ho 5I7 manifold for YAlO3 was 8.1 ms, measured by using a 2.05-µm Tm,Ho:GdVO4 laser (10 Hz PRF, 20-ns pulse duration) as an exciting source, comparing with 8 ms for Ho:YAG and 12 ms for Ho:YLF.
3. Experimental arrangement
The pump laser schematic diagram is shown in Fig. 4. To evaluate the lasing performance of Ho:YAlO3 crystal at room temperature, a diode-pumped Tm:YLF laser with emission wavelength of 1.91 µm was utilized as a pump source, since other high power lasers coinciding with the absorption peaks of Ho:YAlO3 are not available to us. By use of dual-end-pumped configuration, and a Tm (4 at. %):YLF crystal with size of 3×3×12 mm3, more than 15 W of output power with beam quality M2 of ~1.1 was obtained under the incident LD power of 43 W.
The Ho:YAlO3 crystal is 20 mm in length and 4×4 mm in cross section, doped with 1 at. % Ho and antireflection coated at the pump and laser wavelengths. The absorption coefficient of Ho:YAlO3 at 1.91 µm is about 0.5 cm-1, implying nearly 63% single-pass and 86% double-pass pump absorption by the crystal. The gain medium is wrapped in indium foil and clamped in a copper heatsink held at a temperature of 15°C with a thermoelectric cooler.
The L-shaped Ho:YAlO3 laser cavity, also shown in Fig. 4, consists of a plane mirror with R>99.5% at 2.1 µm, a 45° dichroic mirror with R>99.5% at 2.1 µm and T>98% at 1.91 µm, and an output coupler with a curvature radius of 100 mm. The physical cavity length is about 67 mm, resulting in an estimated resonant mode diameter of ~360 µm in the crystal with an analysis of cold resonator with no lasing. By use of a 200 mm focal length mode-matching lens, pump spot size of ~360 µm in diameter is formed in the Ho crystal.
A 20-mm long acousto-optic Q-switch with an acoustic aperture of 0.8 mm is the central component for repetitively Q-switched operation, and both of surfaces are AR-coated and flat. The material is crystal quartz with 99.6% transmission for 2.1 µm operation. It is rated for 15-W radio frequency input power at frequency of 40.7 MHz. The modulation loss (with vertical polarization) is greater than 55%, which is adequate to hold off the intracavity laser actions.
4. Experimental results and discussion
To calculate intracavity loss L and quantum efficiency η q, the output of Ho:YAlO3 laser dependence on absorbed pump power in a single-pass pumping configuration is shown in Fig. 5 for output transmissions of T=14, 20, 29, and 51%. L and η q values can be derived from the measured slope efficiency η s and the formula η s=η q ν L/ν P·T/(L+T), where ν L and ν P are laser and pump frequency . A least-squares fit to this formula yields η q=73±4% and L=5.9±1.5%. Ho:YAlO3 laser produced linearly polarized output along crystallographic c-axis, which was measured to have a contrast ratio of >20 dB.
To obtain higher output power, the Ho:YAlO3 laser in a double-pass pumping configuration was investigated by using an output coupler with T=29%. The CW laser data obtained are shown in Fig. 6. The highest output power was 5.5 W for 13.8 W of incident pump power, corresponding to an optical-to-optical conversion efficiency of 40%. A linear regression fit to the data yields a slope efficiency of 47% and threshold pump power of 1.8 W. The total diode-to-holmium conversion efficiency achieved is 14%, which is greater than 10% observed in the Tm, Ho-codoped YAlO3 laser operating at room temperature . Codoped YAlO3 suffers from the effects of low Tm-Ho transfer efficiency and reversible transfer at room temperature. In addition, the strong cooperative upconversion losses lead to a significant reduction in the effective upper level lifetime (4 ms for Tm,Ho:YAlO3 compared with 8 ms for Ho:YAlO3). Resonant laser pumping of the Ho using the Tm emission around 1.9 µm has the advantage of creating very low quantum defect heating so that very high lasing efficiencies are attainable.
At a Q-switch frequency of 10 kHz, we achieved an approximate 27.5-ns FWHM pulse. The energy per pulse was 0.52 mJ, corresponding to a peak power of 18.9 kW. As shown in Table 2, the pulse duration increase from 24.5 ns at 5 kHz to 30.5 ns at 20 kHz, and average output power increase from 5.0 W to 5.3 W. Repetitively Q-switched operation at 5 kHz yielded the maximum output energy per pulse of 1.0 mJ with a peak power of 40.8 kW. With reduced upconversion losses, resonantly pumping Ho:YAlO3 laser allows efficient energy extraction in short-pulse regime owing to the lower lifetime reduction under intense pumping than Tm-sensitized system. However, below the 1-kHz region, the modulation loss provided by the AO Q-switch is insufficient to hold off the intracavity lasing. Acousto-optic modulator with the length of greater than 40-mm and an RF drive power of 50 W will be required for obtaining higher energy pulses.
The propagation characteristics of the laser beam at the output power level of 5.5 W were evaluated utilizing the standard M2 measurement technique . The pyroelectric camera to observe the 2-µm laser beam profile (inset in Fig. 7) has low spatial resolution (124×124 number of elements, 85×85 µm2 pixel size) and low signal-to-noise ratio, so the second moment beam diameter could not be accurately measured as Ref. . Using the scanning knife-edge technique, we measured the 1/e 2 beam diameter at several positions through an auxiliary waist formed by an f/25.4, 200 mm focal length lens, positioned 270 mm from the output coupler. The data were fitted by least-squares analysis to standard Gaussian beam propagation equations to determine the M 2 parameter (Fig. 7). The fit yields M 2=1.09±0.02, clearly indicating nearly diffraction-limited beam propagation.
The spectral output of the Ho:YAlO3 laser was recorded with a Burleigh WA-650 spectrum analyzer combined with a WA-1500 wavemeter (0.7 pm resolution), and is shown in Fig. 8. The emission line is centered on 2118 nm with a FWHM linewidth of less than 0.3 nm, and no other emission oscillates with it simultaneously. The Ho:YAlO3 cavity emission wavelength does not depend on the reflectivity of the output coupler and only 2118 nm emission line was observed for output transmission from 14% to 51%.
The broad absorption band and large absorption cross sections in Ho:YAlO3, corresponding to 5I8-5I7 transition, will contribute to a variety of pumping scheme using Tm-doped ‘bulk’ or fiber lasers, and thus moderate the requirements for the in-band pumping wavelengths. We demonstrated efficient operation of room-temperature Ho:YAlO3 laser resonantly pumped by a 1.91-µm Tm:YLF laser. In CW mode, the maximum output powers of 5.5 W and a slope efficiency of 47% with respect to the incident pump power were achieved with a near diffraction-limited beam quality of M 2~1.1. In Q-switched mode with high PRF, we achieved the maximum 1 mJ energy per pulse at 5 kHz, and 5.3-W average output power with pulse width of 30.5 ns at 20 kHz. The Ho:YAlO3 laser with high peak power can be used as a pump source of ZnGeP2 optical parametric oscillator for mid-infrared radiation generation.
The work is supported by the program of excellent team in Harbin Institute of Technology, China.
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