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We report on a diode-pumped, cryogenic and room temperature operation of a Tm,Ho:YAlO3 (c-cut) laser. In a temperature of 77 K, an optical-optical conversion efficiency of 27% and a slope efficiency of 29% were achieved with the maximum continuous-wave (CW) output power of 5.0 W at 2.13 µm. Acousto-optic switched operation was performed at pulse repetition frequency (PRF) from 1 kHz to 10 kHz, the highest pulse energy of 3.3 mJ in a pulse duration of 40 ns was obtained. In room temperature (RT), the maximum CW power of Tm,Ho:YAlO3 laser was 160 mW with a slope efficiency of 11% corresponding to the absorbed pump power.
©2008 Optical Society of America
1. Introduction
Solid state lasers emitting in the 2 µm region are of great interest for eye-safe applications like Doppler radar wind sensing [1], range finding or water vapor profiling [2]. Also, high-power quasi-continuous wave (QCW) 2-µm lasers with high peak power are effective pump sources of optical parametric oscillators (OPOs) for frequency conversion in the 3–12 µm range [3, 4]. Short pulse (<30 ns), high peak power is required to drive nonlinear frequency shifters for the generation of tunable mid-infrared output, while Doppler based coherent systems needed longer pulsewidth for measurement accuracy [5]. So, many host crystals for 2 µm radiation must be carefully chosen to satisfy the requirement of diverse laser systems [6, 7]. It shall be convenient to achieve different lasing performance from the same host with anisotropic properties.
Yttrium aluminium oxide (YAlO3) crystallizes in the orthorhombic space group Pbnm (D 16 2h) and this low symmetry (as compared to cubic YAG) has two important consequences: the luminescence is anisotropic and the laser emission is linearly polarized [8]. The linear polarization output leads up to efficient modulation loss being attainable with a compact acousto-optical Q-switch. YAlO3 is an attractive laser host for thulium and holmium doping due to its natural birefringence combined with good thermal and mechanical properties similar to those of YAG [9]. And then, thermally induced birefringence does not degrade the laser performance due to its birefringence of character.
The Tm-Ho codoped laser crystals for generation of 2 µm radiation exhibits certain significant properties: two-for-one pumping of the Ho ions via cross relaxation, 6~15ms fluorescence lifetime, 0.5~1.8×10-20cm2 emission cross section, and energy storage capability [10]. J. Yu, et al., reported 600 mJ Q-switched pulse energy in Tm,Ho:YLF [11] and greater than 1 J/pulse energy output in Tm,Ho:LuLF [7]. An output power in excess of 250 mW was achieved in Tm,Ho:KYF4 by G. Galzerano, et al., [12], and 130 mW of output power was attained in Tm,Ho:LuAG by K. Scholle, et al. [13]. Up to present, however, there are few papers reporting on 2 µm laser actions based on Ho-doped YAlO3 host. CW output power of 270 mW and optical conversion efficiency of 14% have been demonstrated by I. F. Elder, et al., using a diode-pumped b-axis Tm(4.2%),Ho(0.28%):YAlO3 with emission wavelength of 2.12 µm operating at room temperature [14]. Up to 136 mJ pulse energy has been obtained from a lamp pumped Er,Tm,Ho:YAlO3 laser operating at 77K by M. J. Weber, et al [15].
In this paper, we demonstrate CW and acousto-optic Q-switched operation of diode-pumped Tm,Ho co-doped YAlO3 (c-cut) laser.
2. Spectroscopy
The laser crystals used here were grown at Laser & Optoelectronic Functional Material R&D Center, Shanghai Institute of Optics and Fine Mechanics, China by the Czochralski technique, the growth direction being along the crystalline c-axis for YAlO3 (Pbnm notation). The polarized absorption spectrum of Tm 3 H 6→3 H 4 band was recorded on a c-cut sample with 5 at. % thulium (1.0×1021 ions/cm3), 0.3 at. % holmium (0.6×1020 ions/cm3) using a Shimadzu UV-3100PC spectrophotometer with a resolution of 0.1 nm, and is shown in Fig. 1. The absorption line of 795 nm become stronger and sharper at 77 K compared to that at room temperature, the absorption coefficient at 77 K is a factor of 1.6 greater than that at 300 K, and absorption spectral width is only 1 nm compared to 3 nm at room temperature. The pump absorption efficiency is sensitive to diode emission wavelength that must be held at constant value by controlling its operating temperature.
Figure 2 shows the polarized fluorescence spectrum from 1.85 to 2.15 µm of the Tm 3F4→3H6 and Ho 5I7-5I8 transitions in Tm, Ho:YAlO3 (c-cut) recorded at room temperature. 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. The spectrum has not been corrected for grating and detector response. As shown in Fig. 2, the location and relative intensity of fluorescence emission peaks is orientation dependent, and the 2.13 µm emission peak is in polarization E‖ a with spectral linewidth of 17 cm-1 (8nm). According to Ref. [16], the effective stimulated emission cross section of 0.82×10-20 cm-2 occurs at 2118 nm in polarization on E‖ c-axis. The emission cross section for E‖ a polarization near 2.12 µm is only 0.36 as large as that for E‖ c, ~0.31×10-20 cm2 [17].
To reduce the radiation trapping, a Tm,Ho:YAlO3 sample with 0.8 mm thick and anti-reflection coating at both end surfaces in the wavelength range from 1900 nm to 2150 nm was used to measure the Ho 5I7 manifold fluorescence lifetime. The lifetimes were acquired by exciting the crystal at 800 nm with a pulsed diode laser (10 Hz pulse repetition frequency, 0.1 ms duration). The transient luminescence signal at 2.1 µm was registered by an InGaAs photodiode at the exit slit of a 0.3-m monochromator, and then accumulated with a digital oscilloscope. The measured fluorescence decay lifetime of the coupled first excited states of Tm,Ho:YAlO3 were 4.0±0.2 ms at room temperature and 5.9±0.2 ms at 77 K.
3. Experimental arrangement
The schematic diagram of cryogenic Tm,Ho:YAlO3 laser setup is shown in Fig. 3. The laser crystal, c-cut along the growth direction, doped with 5 at. % Tm3+ and 0.3 at. % Ho3+, had a dimension of 4mm×4mm×8mm(in length), whose two end surfaces were AR-coated at both 795 nm (R<0.5%) and 2130 nm (R<0.3%). This Tm,Ho:YAlO3 crystal was wrapped in indium foil and held in a copper heat-sink connected with a small dewar filled with 300 ml liquid-N2. The windows of dewar are water-free fused silica with 3 mm thick and 25 mm in diameter, and AR-coated with transmissivity of about 99.4% at 2.13 µm and greater than 99% near 795 nm. The pump source was a fiber-coupled laser diode arrays which deliver greater than 20 W content within fiber core of 0.4 mm and numerical number of 0.22. The emission wavelength of the diode is 798 nm with full-width half-maximum (FWHM) spectral width of 2 nm at 25-°C and temperature tuned to around 795 nm for maximal absorption by Tm3+ in YAlO3 host. The collimated output of the fiber was split in half for dual end-pumping, and each half was relay-imaged into the gain medium with a magnification of 1.57. With the choice of focused pump beam diameter of 0.63 mm, the Rayleith length (2πnw 2 p/M 2 λp, where n (1.9) was the refractive of YAlO3, wp was the pump beam radius, M 2 is the pump beam quality factor being equal to 175, λp was the pump wavelength) inside the Tm,Ho:YAlO3 crystal was calculated to be ~8.5 mm.
The compact plano-concave L-shaped resonator of the cryogenic Tm,Ho:YALO3 laser consists of mirrors M1, M2 and M3. M1 is a 400 mm radius of curvature concave input-coupling mirror with 99.5%R at the laser wavelength and 95%T at the diode wavelength. M3 is a flat output coupler with a reflectivity of 70% at 2.1µm. M2 is a 45°dichroic mirror with reflectivity of 99.5% at 2.1 µm and a transmissivity of 95% at 795 nm. The physical length of the resonator was kept at 55 mm when operating in CW mode, and 125 mm in Q-switched mode. For Q-switched operation, infrared Fused Silica (water free) acousto-optic Q-switch was inserted between the output coupler and M2, and oriented such that the optical polarization and acoustic wave vector were mutually orthogonal for optimum scattering. The physical length of the Q-switch was 44 mm, and the acoustic aperture was 1 mm. Operating the switch at 27.16 MHz and 50-W radio-frequency power, modulation loss of greater than 80% was achieved. Pulsewidth measurements were performed with the use of a room temperature mercury cadmium telluride photovoltaic detector with a rise time of 0.2 ns.
The laser in Fig. 3 could not work in room temperature due to serious Ho 5 I 8→5 I 7 reabsorption loss for the 8-mm-long crystal. For operation in room temperature, a 2-mm-thick Tm,Ho:YAlO3 sample with the same dopants was utilized in the experiment. It was cooled at 274 K to reduce pump threshold intensity. The resonator was a linear one with a physical length of 17-mm, which consisted of a plano-plane input mirror (HR at 2100 nm & HT at 795 nm) and a 50-mm curvature radius output mirror with a transmissivity of 2% at 2.1 µm. The fiber-coupled diode light was transformed to 0.2-mm beam diameter inside the medium with a reimaging optics. No AO Q-switch was inserted in the laser cavity.
4. Experimental results and discussion
Power performance of diode pumped cryogenic Tm, Ho:YAlO3 laser is shown in Fig. 4. The maximum continuous wave output power obtained was 5 W with incident pump power of 18.8 W, corresponding to the optical efficiency of 27%. A slope efficiency of 29% and a threshold pump power of 1.8 W were yielded by linear fit of the experimental data with respect to the incident diode power. Laser emission was linearly polarized along crystallographic a-axis. The power fluctuation of Tm, Ho:YAlO3 laser increased from ±1% at lower power to ±4% at the maximum output, and would not increase with greater than 20 W of pumping power into the active medium. According to our measured results, the thermal conductivity of Tm-doped YAP along c-axis decrease with the increase of Tm concentration, from 9.96 W/m.K at 1% Tm doping level to 5.79 W/m.K at 5% Tm:YAlO3, and those value along a-axis and b-axis are 5.26 and 5.06 W/m.K, respectively. The lower thermal conductivity and higher thermal optical coefficient of 10.1×10-6/K (a-axis) compared with 7.3×10-6/K of YAG lead to serious thermal lensing effect in Tm,Ho:YAlO3 [18], and the laser performance with more output power was deteriorated.
The performance of room temperature operation of CW Tm,Ho:YAlO3 laser is shown in Fig. 5. The maximum output power obtained was 160 mW at a crystal temperature of 1 °C with absorbed pump power of 2.3 W, corresponding to the optical efficiency of 7%. The slope efficiency of 11% was achieved with respect to absorbed pump power. The threshold intensity for the Tm,Ho:YAlO3 laser at room temperature was near 3.8 kW/cm2 (1.2 W incident LD power), compared to 0.15 kW/cm2 at 77 K.
The green upconversion fluorescence of 545 nm attributed to Ho3+5S2 to 5I8 transition was evident under intense pumping at both 77 K and room temperature. Upcoversion effects deplete the Ho laser upper level of 5I7 multifold population density, and increase the intracavity loss leading to lower conversion efficiency.
Figure 6 shows the output laser spectrum of cryogenic Tm,Ho:YAlO3 laser. The emission wavelength of Ho:YAlO3 laser is centered at 2.13 µm with spectral linewidth of 1.2 nm. In room temperature operation, the radiation wavelength with polarization direction along a-axis was shifted to 2.10 µm, at which the emission cross section was ~0.36×10-20 cm2, being higher than that at 2.13 µm. According to the energy levels of Ho:YAlO3 [14], the output laser wavelength of 2.1 µm is assigned to the transition from the two lowest, near degenerate levels of 5I7 (5186 and 5187 cm-1) to the highest level of 5I8 at 499 cm-1.
At a Q-switch frequency of 1 kHz, we achieved an approximate 40-ns FWHM pulse. The energy per pulse was 3.3 mJ, corresponding to a peak power of 82.5 kW. As shown in Table 1, the pulse duration increase from 40 ns at 1 kHz to 184 ns at 10 kHz, and average output power increase from 3.3 to 4.0 W with respect to pump power of 16.8 W. The Q-switch insertion loss of 2% and strong thermal lensing (the measured thermal focusing length was 450 mm at pump power of 17 W) account for the higher pump threshold power of 3 W and lower conversion efficiency of 20% at pulse repetition frequency of 1 kHz. Although the low gain of E‖a in Tm,Ho:YAlO3 is preferable for high energy storage, the 4~6 ms upper laser level lifetime and optically induced coating or material damage would limit the pulse energy scaling under continuous wave pumping.
Table 1. Performance of Q-switched cryogenic Tm, Ho:YAlO3 laser
5. Conclusion
In conclusion, spectroscopic properties and lasing performance of a c-cut Tm,Ho:YAP were investigated. At 77 K, a CW output power of 5 W was achieved with slope efficiency of 29% at 2.13 µm, and a Q-switched energy of 3.3 mJ in the pulse duration of 40 ns was obtained at pulse repetition rate of 1 kHz. Room temperature operation of Tm,Ho:YAlO3 laser was also demonstrated, and the maximum CW power of 160 mW with a slope efficiency of 11% was attained at the wavelength of 2.10 µm.
Acknowledgments
The work is supported by the program of excellent team in Harbin Institute of Technology, China.
References and Links
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