1. Introduction

The interest in all-solid-state laser operating in the eye-safe spectral region near 2µ m is well acknowledged for medical and remote-sensing applications [1–2]. Since the first demonstration of rare-earth laser operation in YAG obtained at cryogenic temperature, considerable progress has been recorded. At present, room-temperature, diode-pumped 2µ m laser(continuous and Q-switched) [3–4] are integrated in systems for ground-based or airborne lidar measurements, additional applications for which short duration optical pulses at 2µ m are required include altimetry, topographical, and nonlinear optical studies. Tm, Ho codoped lasers are conducive to operation in Q-switched mode due to their long fluorescence lifetime of 8~16 ms compared to 230µ s in neodymium doped YAG. However, in many hosts studied to date upconversion is a deleterious influence, manifest as an effective lifetime reduction, with concomitant reduction of the energy storage capacity and loss of conversion efficiency [5]. High excited state densities lead up to unconversion processes, so we have elected to operate at 77 K where the excited state densities for fixed gain are minimized. The absorption cross section of thulium in GdVO₄ is considerably stronger than in YAG and YLF [6], the absorption spectrum, shown in Fig. 1, is broader (770–820 nm) and shifted closer to the emission wavelength of commercially available AlGaAs laser diodes. In this experiment, we used a 794 nm laser diode as pumping source. Our previous experimental results with YLF crystals have shown fracture when subjected to pump densities greater than 5kW/cm². GdVO₄ host has good thermal-mechanical property and can experience >10 kW/cm² pump density, confirmed by our experiment, allowing to scale up holmium laser average power by endpumping fabrication without the requirement of diffusion bonded composite rod. The Boltzmann coupling lifetime of thulium ³ F ⁴ and holmium ⁵ I ⁷ in GdVO₄ host is 3 ms that is shorter than 9 ms for Tm,Ho:YAG and 15 ms for Tm,Ho:YLF. The shorter lifetime and moderate emission cross section are very favorable for the Q-switched short pulse generation operated at repetition rate up to 10 kHz.

 

Fig. 1. Room temperature transmission spectrum of an a-cut 2mm-long, AR-coated around 794 nm 5% Tm, 0.5% Ho:GdVO₄ sample, measured with Shimadzu UV-3101PC scanning photometer. The calculated absorption coefficients are 21cm^-1 at 799 nm and 13 cm^-1 at 797.5 nm for π and σ polarizations. Thulium doped in GdVO₄ host also exhibits >4 cm^-1 absorption coefficient in the range from 792 to 794 nm.

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Work by Morris demonstrated the generation of room temperature continuous wave (cw) 4.6 mW output at 2.048µm with 135mW absorbed pump power in Tm:Ho:GdVO₄ [7]. At 77K, cw output power (4.2 W) was obtained by Yao from diode-pumped Tm, Ho:GdVO₄ laser [8]. With a 3% Tm, 0.3% Ho:GdVO₄ crystal an output energy of 31.2 mJ and a slope efficiency of 14.5 % were attained by Atsushi Sato at room temperature [9]. The slope efficiency of 10% in Tm:GdVO₄ laser pumped by a laser diode was achieved by Mikhailov et al. [10]. Wyss also demonstrated 1.4 W radiation at 1.95µm with 32W laser diode pumping [11]. In this paper, we describe the performance of a acouto-optically Q-switched Tm, Ho-codoped:GdVO₄ laser with high PRF in the range from 3 kHz to 50 kHz, which has generated up to 3.9 W of QCW average output power at 2.048 µm.

2. Experimental setup

 

Fig. 2. Experimental setup of AO Q-switched Tm,Ho:GdVO₄ laser

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The diode-pumped laser geometry is illustrated in Fig. 2. The Tm, Ho:GdVO₄ crystal is end-pumped by a fiber-coupled laser diode arrays which deliver maximum 13.6 W content within fiber core of 0.4 mm and numerical number of 0.3. The emission wavelength of the diode is 793 nm with full-width half-maximum(FWHM) spectral width of 1.65 nm at 25 °C and temperature tuned to 794 nm for optimal absorption and uniform thermal distribution along the length of Tm, Ho:GdVO₄ crystal. The pumping light is refocused into crystal with approximately beam diameter of 0.6 mm for optimal overlap between the pump and the laser beam, by use of an imaging telescope that is composed of a double AR-coated plano-convex lenses with focal length of 35 mm and 50 mm. The vanadate crystal is a-cut with dimension of 4×4 mm² in cross section and 7-mm in length, and has dopant concentrations of 5 % atomic thulium and 0.5 % atomic holmium. The faces are polished plane and parallel, and antireflection coated at both 794 nm and 2048 nm with reflectivity <0.5 %. The input mirror is a plano-concave one with 400 mm radius of curvature. It is antireflection coated at 794 nm on the entrance face (R<0.2 %), and on the second surface coated with reflectivity of greater than 99.5% at 2048 nm and transmissivity of >96% at 794-nm. The output coupler is a plano-plane water-free fused silica mirror with 40% transmissivity at 2050 nm. The total cavity length is about 110 mm. This Tm,Ho:GdVO₄ crystal is wrapped in indium foil and held in a copper heat-sink connected with a small dewar filled with 350 ml liquid-N₂. The windows of dewar are CaF₂ with 3 mm thick and 25 mm in diameter, and AR-coated with transmissivity of about 99.4% at 2050 nm and greater than 99% near 794 nm.

Infrared Fused Silica (water free) acousto-optic Q-switch is located between the crystal and the output coupler and oriented such that the optical polarization and acoustic wave vector are mutually orthogonal for optimum scattering. A 44 mm long acousto-optic Q-switch is the central component for high peak power laser generation at tens of kilohertz pulse repetition frequency (PRF), and both of surfaces are AR-coated and flat. The material is SiO2 (infrasil) with 99.6% transmission for 2.05 µm operation. The acceptance and separation Angle is 6.7mrad and 9.3mrad respectively. It is rated for 50 W maximum radio frequency (RF) input power at frequency of 27.12 MHz. The loss Modulation (with vertical polarization) is greater than 80% at 50W RF power, which is adequate to prevent lasing action. At 10 W RF power transferred to Q-switch transducer, 50% loss modulation is sufficient to generate stable pulses at 10 kHz repetition rates, which demonstrated by our experiments.

3. Experimental results and discussions

The CW and QCW laser data obtained using above configuration are shown in Fig. 3. The measured pumping threshold power was 1.8 W. In Q-switched operation at 10 kHz repetition frequency the highest QCW output power was 3.9 W for 13.6 W of pump, corresponding to an optical-optical conversion efficiency of 29%. Under the same pump power, the maximum output power was 4.2 W in CW operation mode. A linear regression fit to the data yields a slope of 35%. The diode power at 794 nm was absorbed about 82% by the 7mm-long Tm,Ho:GdVO₄ crystal with no lasing. Corrections for absorbed diode power yielded a slope efficiency of 37%, with corresponding optical-to-optical efficiency of 45%. From Fig. 3, it is not seen to “roll-over” at this pump level as the pumped region center temperature becomes too high. The lasing wavelength of Tm,Ho:GdVO₄ laser is centered at 2048.6 nm within FWHM linewidth of 1.4 nm, measured with a 30 cm monochromator (WDG30, 300 lines/mm grating blazed at 2-µm).

 

Fig. 3. Output power of Tm, Ho:GdVO₄ laser versus pumping power

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At 77 K, the Ho laser transition is close to four-level, and the slope efficiency can be written as:

Where, η _a is the pump light absorption efficiency, η _q is the pump quantum efficiency, T is the output coupler transmissivity, L is the intracavity round loss including reabsorption, ν_l is the laser frequency, and ν _p is the pump frequency. The measured L value of 11 % is adopted here. According to Eq. (1) and the measured 37% slope efficiency, the calculated ηq is up to 1.5 due to the efficient cross relaxation between Thulium ions in GdVO4 host, compared to 1.48 in YLF and 1.78 in YAG [12].

 

Fig. 4. Tm,Ho:GdVO₄ output power and unpolarized absorption coefficient as a function of pump wavelengths

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As shown in Fig. 4, the average output power from Tm,Ho:GdVO₄ laser increase with the pump wavelengths from 792 nm to 794 nm, corresponding to the unpolarized absorption coefficient increasing from 4 cm^-1 to 5.5 cm^-1. With the focused pump beam size of 0.6 mm the confocal distance(2${\pi \mathit{\text{nw}}}_p^2$ /M ² λ_p , where n(2.1) is the refractive of Tm,Ho:GdVO₄, w_p is the pump beam radius, M ² is the pump beam quality factor being equal to 237, λ_p is the pump wavelength) is calculated to be 5.7 mm inside the gain medium. The calculated effective pump absorption length is 3.7 mm for the 792 pump wavelength nm and 2.7 mm for the 794 nm. The shorter effective absorption length is significantly favorable for high intensity pumping to generate efficient output, especially with respect to the pumping source with bad beam quality.

The diode emission wavelength of 794 nm is far from the peak absorption peak of thulium ions in GdVO₄ host, which is located around 798–799 nm. If diodes with longer wavelengths around 800 nm for higher absorption are utilized, the higher conversion efficiency should be attained. On the other hand, more absorbed pumping power over shorter medium length could lead to serious thermal loading which will reduce output power, so optimizations of Tm, Ho doped concentrations and optically pumping schemes are also needed.

The effective upper laser level lifetime of 3 ms in Tm, Ho-coupled system allows efficient energy storage and short width pulse generation compared to other host such as YLF and YAG, but it leads to longer pulse width at higher modulation frequency. At a Q-switch frequency of 10 kHz we achieved an approximate 40-ns FWHM pulse. The energy per pulse was 0.39 mJ, corresponding to a peak power of 10 kW. As shown in Fig. 5, the pulse duration increase from 23 ns at 3 kHz to 100 ns at 50 kHz, and average output power increase from 3.2W to 3.9 W, but change little with more than 10 kHz PRF. Experiments at 3 kHz yielded 1.1 mJ energy per pulse with a peak power of 46 kW.

 

Fig. 5. Pulse duration and energy per pulse of Tm,Ho:GdVO₄ laser dependence on PRF.

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Pulsewidth measurements were performed with the use of a room temperature mercury cadmium telluride photovoltaic detector with a rise time of 0.2 ns and a TDS-3012B 300-MHz digital oscilloscope. As shown in Fig. 6(a), the pulse temporal profile is symmetric, and no evident inflection in the trailing edge is observed as Ref. [5]. No multiple pulses were generated within Q-switch RF off-time of 2 µs, as shown in Fig. 6(c), even within longer time of 10 µs, which indicated that no higher order transverse modes were present in the spatial beam, verified by a infrared imaging plate. The stable pulse train, shortest pulsewidth and only one pulse with no tail in longer duration of high-Q state provide the evidences that the laser was operating in TEM₀₀ mode and good resonator alignment [13].

Reducing RF off-duration to about 200 ns, the amplitude of laser pulses were abruptly boosted, and at the same time pulsewidth became shorter with duration of less than 30 ns, shown in Fig. 6(d). The pulse series were still stable with measured repetition rates of 5 kHz. The resonator high-Q duration was not long enough to allow every laser pulse being built up at a driving cycle of Q-switch, only after the inversion population was accumulated to be larger enough, higher gain could allow laser pulse evolving during limited time. From which, it is estimated that the build-up time of the giant pulse is about 200 ns at 10 kHz. As shown in Fig. 6(b), no stable pulse series can be obtained under condition where RF off-time is less than 120 ns.

 

Fig. 6. (a) Pulse temporal profile of Q-switched Tm,Ho:GdVO₄ laser with 10 kHz PRF; (b) Unstable pulses series overlapped with Q-switch RF off-time <120 ns; (c) Single laser pulse with 40 ns pulsewidth at 10 kHz repletion rates and RF voltage envelope with 2 µs off-time; (d) Pulse temporal profile with higher pulse amplitude and shorter pulsewidth compared to (c) when the RF off-time is tuned to about 200 ns.

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4. Conclusion

In summary, the diode pumped cryogenic Tm, Ho:GdVO₄ laser was demonstrated in Q-switched operation mode. The highest energy per pulse of 1.1 mJ in 23 ns was attained with a peak power of 46 kW at 3-kHz. Average power of 3.9 W, optical-to-optical efficiency of 29 %, and slope efficiency of 35 % have been achieved at PRF 10 kHz. To reduce the pulsewidth at tens of kilohertz PRF, optimizations of the diode-pumped Tm,Ho:GdVO₄ laser should be performed on improving beam quality of fiber-coupled laser diode with wavelengths closer to absorption peak around 798~799 nm, and adopting dual-end pumping configuration for less reabsorption loss and higher gain in the 7mm-long crystal.