A CW Side-Pumped Tm:YLF Laser
Alex Dergachev, Kevin Wall, and Peter F. Moulton
Q-Peak, Inc., 135 South Road, Bedford, MA 01730
dergachev@qpeak.com
Abstract: We report on a broadly tunable (1905 - 2067 nm) diode-pumped Tm:YLF laser producing cw output powers of >18 W (37% slope efficiency) in multimode operation and 12 W in the TEM00 mode.
OCIS codes: (140.3480) Lasers, diode-pumped; (140.3600) Lasers, tunable; (140.3070) Infrared and far-infrared lasers
1. Introduction
As has been demonstrated previously [1, 2], Tm-doped YLF and YAG crystals are efficient 2-mm laser materials suitable for diode-laser pumping. A CW output power of 115 W was reported by Honea et al. from a 2% Tm:YAG laser end-pumped with up to 360 W from a diode-array stack [1]. For Tm:YLF, a CW output power of 36 W was reported by Budni et al. [2] where two 3% Tm:YLF rods in a folded resonator were end-pumped by four fiber-coupled diode laser bars with a combined power of 120 W.
The principal objective of this work was to investigate the feasibility of a side-pumped geometry for room-temperature, 2-mm Tm:YLF lasers. In previous work, an efficient, multi-pass, side-pumped geometry has been successfully demonstrated for Nd:YLF [3], Nd:YVO4 [4], and Er:YLF [5] lasers. The design we used for the Tm:YLF laser is similar. In contrast to prior systems, an issue of importance in the development of a side-pumped Tm-laser in 1900 - 2000 nm range is the quasi-three level nature of the laser transition. The laser transition, 3F4 to 3H6, terminates on the ground manifold. With a side-pumped design and room-temperature operation, there will be an increase in threshold caused by absorption from the laser mode passing through un-pumped regions in the crystal. In addition, maintaining a low crystal temperature is important in minimizing re-absorption losses at wavelengths longer the zero-phonon line. As we discuss below, our particular design minimized un-pumped regions as well as the temperature rise in the pumped region. Also different from the other materials, the multiplicity of levels in the Tm lower laser manifold as well as the strong phonon broadening of the levels leads to a very broad gain bandwidth. In this work we report what we believe is the largest continuous tuning range yet demonstrated for a Tm:YLF laser.
2. Side-pumped Tm:YLF laser development
In the present work we have developed a CW Tm:YLF laser based on a 3.5% Tm:YLF slab side-pumped by two diode laser bars. We used aluminum-free, 40-W, 1-cm-active-length, diode pump lasers at a wavelength of 792 nm, which overlaps a strong absorption of Tm:YLF. Schematic sketches of the laser set-ups are shown in Fig. 1. The length of the crystal was set as short as possible to minimize the un-pumped volume of the crystal. Two resonators were designed to either single-pass or 5-pass the laser mode through the Tm:YLF slab. The pump laser's polarization was such that we pumped the s‑absorption in Tm:YLF.
The c-axis of the Tm:YLF slab was parallel to the gain sheet and the crystal dimensions were 2.5 x 6 x 22 mm, where the 2.5-mm dimension was perpendicular to the gain sheet and the 22-mm dimension was parallel to the propagation of the laser mode. We extracted the heat from the two faces parallel to the gain sheet (the largest faces) by bonding both surfaces of the crystal to a water-cooled heatsink. This minimizes the distance from the heat generation to the heat sink and maximizes the area of heat transfer, resulting in a lower overall crystal temperature. The fact that the heat deposition is spread over a larger volume than in an end-pumping geometry also results in lower crystal temperatures in the pumped regions.
Fig. 1. Schematic layout of the side-pumped Tm:YLF laser with single pass (a) and 5-pass (b) geometry (DL – diode Laser, OC – output coupler, HR – high reflector, BRF – birefringent filter, r – radius).
The diode-laser bars were coupled to the Tm:YLF through a single cylindrical lens attached directly to each bar package, to minimize the divergence of the pump light in the plane perpendicular to the linear emitter. The bars were offset on opposite sides of the Tm:YLF crystal to create a sheet of gain in the crystal. The pump faces of the crystal had segmented dielectric coatings (AR/HR) to allow double-pass pump absorption. The relatively uniform pump power density in the crystal minimized excess heating and loss due to up-conversion.
For the single-pass experiments (Fig. 1a), we used a near-confocal resonator based on two concave mirrors with 10-cm radii of curvature. All of the laser experiments were performed at ambient atmosphere conditions without dry gas purging to eliminate absorption due to water vapor or CO2. A typical output power vs. input pump power curve for a 98.5% reflective output coupler is shown in Fig. 2. An output power of ~18 W was achieved with a pump diode laser power of ~80 W. The threshold was 27 W and the slope efficiency was 37%. The laser simultaneously operated on 4 to 5 lines near 1960 nm. The output beam profile was diffraction limited in the vertical plane (normal to the plane of Fig. 1) and highly multimode (>10 times diffraction limited) in the horizontal plane. The laser emission was s-polarized.
In order to improve the beam quality, we redesigned the resonator to provide five passes of the laser beam through the Tm:YLF slab, to better match the fundamental mode size in the horizontal dimension to the gain region. A schematic sketch of the 5-pass laser resonator is shown in Fig. 1b. Output couplers with transmission ranging from 3% to 13% at 1900 – 2100 nm were tried. As an example, Fig. 2 shows the output power vs. input pump power using an 87% reflective output coupler. The laser operated in TEMoo mode (M2 < 1.3) producing more than 12 W of output power with a threshold of 37 W and a slope efficiency of 28%.
3. Tunable operation of Tm:YLF laser
Wavelength tuning of a Tm:YLF laser (operated at liquid nitrogen temperature) from 1850 to 1920 nm was reported by Ketteridge et al. [6]. In present work, we investigated tunable operation of the Tm:YLF laser at room temperature. An intracavity 2-plate birefringent filter (see Fig.1b) was used as the tuning element. Two flat output couplers with transmission of 13 - 15% and 3-5% in 1900 - 2100 nm were used. The laser was operated with TEMoo spatial mode. The measured tuning is shown in Fig. 3. We were able to continuously tune the Tm:YLF laser from 1905 to 2067 nm.
Fig. 2. Output power for a side-pumped CW Tm:YLF in multimode and TEMoo operation.
Fig. 3. Wavelength tuning of Tm:YLF laser with a 2-plate birefringent filter (TEMoo operation) for two values of output coupler transmission.
4. Conclusion and future work
In conclusion, we have demonstrated efficient operation of a side-pumped CW Tm:YLF laser, and demonstrated tunable operation from 1905-2067 nm. Efforts are underway to increase the TEMoo output power by improved matching of the laser mode to the pumped volume and optimization of output coupling. In the future, we would expect higher performance through the use of un-doped end-caps bonded to the laser crystal, and perhaps through optimization of the Tm doping level. The predicted heating in Tm:YLF is lower than that of Nd:YLF, and since we have been able to pump a similar size Nd:YLF crystal with 80 W of cw power [3], we would expect that the Tm:YLF laser could operate at an even higher pump powers.
The demonstrated tuning range of the room-temperature Tm:YLF laser makes it suitable as a pump for Ho-doped crystals and as a possible source for laser surgery, overlapping a strong liquid-water-absorption region in tissue. In addition, the laser wavelength tunes through absorption lines of water vapor and CO2, suggesting uses in atmospheric and process-control measurements.
5. Acknowledgments
This work was supported by the US Air Force under Contract F08630-01-C-0037.
6. References
1. E. C. Honea, R. J. Beach, S. B. Sutton, J. A. Speth, S. C. Mitchell, J. A. Skidmore, M. A. Emanuel, S. A. Payne, “115-W Tm:YAG diode-pumped solid-state laser”, IEEE J. Quantum Electron. 33, 1592-1600 (1997).
2. P. A. Budni, M. L. Lemons, J. R. Mosto, and E. P. Chicklis, “High-power/high-brightness diode-pumped 1.9-µm Thulium and resonantly pumped 2.1-µm Holmium lasers,” IEEE J. on Selected Topics in Quantum Electron., 6, 629-635 (2000).
3. K. J. Snell, D. Lee, K. F. Wall, and P. F. Moulton, “Diode-pumped, high-power CW and modelocked Nd:YLF lasers”, OSA Trends in Optics and Photonics Vol. 34, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds., (Optical Society of America, Washington, D.C., 2000), pp. 55-59.
4. K. J. Snell and D. Lee, “High average power, high repetition rate side-pumped Nd:YVO4 slab laser”, OSA Trends in Optics and Photonics Vol. 26, Advanced Solid State Lasers, Martin M. Fejer, Hagop Injeyan and Ursula Keller, eds., (Optical Society of America, Washington, D.C., 1999), pp. 295-297.
5. A. Yu. Dergachev, J. H. Flint, and P. F. Moulton, “1.8-W CW Er:YLF diode-pumped laser,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington, D.C., 2000), pp. 564-565.
6. P. A. Ketteridge, P. A. Budni, M. G. Knights and E. P. Chicklis, “All solid-state 7 watt CW, tunable Tm:YLF laser” OSA Trends in Optics and Photonics Vol. 10, Advanced Solid State Lasers, Clifford R. Pollock and Walter R. Bosenberg, eds., (Optical Society of America, Washington, D.C., 1997), pp. 197-198.
