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Abstract
We report on a high-average power, pulsed source at 497.5 nm based on the frequency doubling of an acousto-optically Q-switched, cryogenically-cooled Yb:YLiF4 (Yb:YLF) laser. The Yb:YLF was resonantly pumped by a laser diode module into the 960 nm absorption band and generated a diffraction limited output beam at 995 nm. At this wavelength, the laser delivered up to 50 W of average Q-switched output power at a pulse repetition frequency (PRF) of 10 kHz with a pulse duration of 60 ns and a slope efficiency of 69%. At a PRF of 500 Hz, this laser yielded 14 ns long, 31 mJ pulses with the slope efficiency of ~40%. Frequency doubling was achieved using an LBO crystal with a conversion efficiency approaching 50% at both 500 and 1000 Hz PRF. The maximum achieved pulse energy at 497.5 nm was 14.3 mJ at 500 Hz PRF (average power over 7 W). To the best of our knowledge, this is the highest reported pulse energy and average power for an all-solid-state laser operating under 500 nm.
© 2017 Optical Society of America
1. Introduction
High average power, all-solid-state, Q-switched lasers, operating in the blue wavelength range of 450 – 500 nm, are important for a variety of applications in laser projection and imaging, spectroscopy, underwater communications and LIDAR detection systems in oceanography because of the high water transparency (~10−4 cm−1) in this band [1].
Lasing in this wavelength range can be achieved either directly or indirectly. The first method utilizes laser transitions of rare-earth ions such as Pr3+ (480 nm, 3P0 → 3H4 transition in Pr:YLiF4 (YLF) and Pr:LiLuF4 crystals [2]) or Tm3+ (488 nm, 1G4 → 3H6 transition in Tm:YAG [3]). But these solid state lasers are inefficient in Q-switching due to the short upper laser level lifetime, especially of Pr-doped media. In addition, they require either pump sources emitting in the blue range or rely on two- or three stage up-conversion or two-step pumping [4]. Recently developed GaN laser diodes [5] can be used for pumping when they achieve sufficient power and brightness.
Indirect methods of generating Q-switched laser outputs in the blue range are generally associated with some sort of nonlinear frequency conversion [6–8]. An excellent summary of their implementation in all-solid-state lasers is presented in [9]. Among frequency doubling (and tripling), sum-frequency generation, and optical parametric oscillation, the second harmonic conversion of the sub-1-µm emission from Nd- and Yb-doped crystalline lasers is the most attractive and reliable option. Indeed, the most studied laser material, Nd:YAG, can lase at 946 nm [10], Nd:GSAG - at 944 nm [11], and Nd:GdVO4 at 912 nm [12]. In addition, some Ytterbium-doped materials can lase under 1 μm as well - Yb:YLF at 995 nm [13] and Yb:CaF2 - at 992 nm [14]. A straightforward second harmonic generation can easily be achieved with many nonlinear crystals [15] with conversion efficiencies as high as 60% [16].
The advantage of the Nd-doped lasers is that they operate at room temperature, while both Ytterbium-doped crystals require deep cooling to de-populate lower laser level 4F7/2 to achieve efficient lasing at sub-1-µm wavelengths. On the other hand, lasing in the vicinity of 940 nm in Nd-doped media has a few shortcomings. First of all, it is hindered by the higher lasing threshold due to (i) the three-level nature of the 94x nm lasers utilizing 4F3/2 – 4I9/2 transition and (ii) its relatively low cross-section, compared to the high cross-section of the major 4F3/2 – 4I11/2 laser transition at 1.06 μm. Competition from the stronger transition significantly affects laser performance in the Q-switched mode through the increased influence of the amplified spontaneous emission (ASE). Other problems, which the most of high average power Nd-based lasers face, are strong thermal lensing and thermally induced birefringence. All these issues can be avoided by using cryogenically cooled Yb:YLF, that was originally developed for ultra-short pulse amplification at 1020 nm [17] due to its wide bandwidth luminescence. It was recently shown that the cryo-cooled Yb:YLF is nearly an ideal laser material for high-average-power, Q-switched operation at 995 nm with high beam quality [13,18,19].
While the majority of laser transitions in Nd-doped media experience very strong competition from the 1.06 μm one, this is not the case with cryogenically-cooled Yb:YLF where its 995 nm emission completely dominates the 1.02 μm band [18]. Yb:YLF can be resonantly diode-pumped into a strong and rather wide absorption line of Yb3+ centered at 960 nm, ensuring a very low quantum defect (~3.5%). Natural birefringence of a uniaxial YLF crystal provides linearly polarized laser output and eliminates stress-induced depolarization loss which degrades the performance of high power Nd:YAG lasers. A considerable (4-5 times [20,21]) improvement in thermo-optic properties of YLF at cryogenic temperatures and its weak and negative thermal lensing helps to achieve high beam quality even in high-average power operation.
Spectroscopic features of cryogenically cooled Yb:YLF are all very favorable for powerful, sub-1-µm Q-switched operation: a large emission cross-section (~4.6 · 10−20 cm2) of the 995 nm laser transition, a long upper laser level 4F5/2 lifetime (~2 ms) and strong absorption at 960 nm. The latter two ensure sufficient energy storage. Cryogenically-cooled, high power Yb:YLF lasers emitting at 995 nm have been successfully demonstrated in both the CW [18] and Q-switched [19] modes of operation.
In this paper, we report on a frequency doubled, cryogenically cooled Q-switched Yb:YLF laser. It delivered ~50 W of average power at PRFs of 500 Hz – 10 kHz with diffraction limited beam quality at 995 nm. Frequency doubling was achieved with ~50% efficiency for PRFs of 500 Hz – 1 kHz, 40% for PRFs of 2 kHz and 22% for 5 kHz. Maximum achieved pulse energy at 497.5 nm was 14.3 mJ at 500 Hz PRF (average power over 7 W). To the best of our knowledge, this pulse energy and average output power are the highest reported for an all-solid-state laser in the blue wavelength band.
2. Spectroscopy
The emission and absorption spectra of Yb:YLF at room and cryogenic temperatures have been studied by several research groups [13,22,23]. An energy-level diagram of 2F7/2 and 2F5/2 manifolds of Yb3+ in LiYF4 was carefully determined in [22], and presented in Fig. 1(a). Due to some discrepancies that we found in the emission data, we had to re-visit the low temperature fluorescence of Yb(1%):YLF for both π- and σ-polarizations with an Optical Spectrum Analyzer (Yokogawa 6370C, resolution - 50 pm) along with the 4F5/2 lifetime. Our emission cross-section data are presented in Fig. 1(b, c).
Fig. 1 (a) An energy level diagram of Yb3+:LiYF4 single crystal (source [22]:);(b) Yb(1%):YLF emission cross-sections at 77K; (c) Yb:YLF absorption spectrum at 77K. Absorption was measured in a 1.1 mm thick Yb(1%):YLF using Nicolet 8700 FT-IR spectrometer with 0.125 cm−1 resolution. An inset in the upper, left corner depicts exponential decay of the 2F5/2 manifold at 77 and 300K temperatures.
A Yb3+:LiYF4 single crystal was grown by the Czochralski technique with a nominal doping concentration of 1% at. (NYb = 1.44·1020 cm−3). The measured 2F5/2 manifold lifetime at 77° K was almost 2 msec, which is in agreement with [22,23]. The peak emission cross-section of the 995 nm, π-polarized transition was found to be ~5.7·10−20 cm2 - a somewhat stronger value than that reported in [18], presumably due to the higher spectral resolution of our setup.
3. Laser experiments
3.1. Experimental set-up
A cryogenically cooled 15 mm long, Yb(1%):YLF slab with transverse dimensions of 2.5 × 8 mm (height × width) was used as an active element in our laser experiments. Both faces of the crystal were anti-reflection (AR) coated for 960 - 995 nm. A slab was wrapped in indium foil and clamped to a cold plate of a boil-off liquid nitrogen cryostat. The c-axis of the crystal (which defines the π-polarization of the laser output) was normal to the laser axis and the cold plate. A simplified experimental setup is shown in Fig. 2(a).
Fig. 2 (a) Optical schematic of a cryogenically cooled Q-switched Yb:YLF laser at 995 nm with second harmonic generation at 497.5 nm. (b) Spectral overlay of the diode pump spectrum and Yb:YLF σ-polarized absorption spectrum at 77K.
The Yb:YLF crystal was longitudinally pumped into the 960 nm absorption band (an estimated peak of the σ-polarized absorption cross section is σabs ~2.7 × 10−20 cm2) by a CW laser diode module. Its astigmatic, elliptically-shaped, linearly polarized output beam had the divergences of θx ~16 and θy ~5 mrad. The full spectral width of the pump was ~4 nm. Figure 2(b) shows an overlay of the pump source spectrum with the 960 nm absorption band of the Yb:YLF at 77K. Even with significant narrowing of the band, the 15 mm long crystal absorbed approximately 82% of the incident pump power.
The pump was free-space coupled into the crystal by an optical system consisting of two orthogonally placed cylindrical lenses with focal distances of fY = 400 mm and fX = 200 mm. The resulting pump beam had an almost constant diameter of ~1.6 mm along the entire crystal length.
The linear polarization of the pump made it possible to separate the pump and laser beams with an air-spaced, polarization beam splitting cube (PBS). It transmitted the σ-polarized pump and reflected the π-polarized laser emission at 995 nm, see Fig. 2(a). The folded laser cavity was formed by two mirrors: a highly reflective (HR) rear cavity mirror with the radius of curvature of 300 cm and a flat output coupler (OC) with the reflectivity of 35%. The cavity length varied from 33 cm to 98 cm. An acousto-optic Q-switch was placed near the HR mirror.
The second harmonic conversion of a linearly polarized 995 nm beam into the 497.5 nm was achieved with the LBO crystal (type I, dimensions of 5 × 5 × 15 mm). It was placed outside the laser cavity right after the output coupler. Due to the close proximity of the LBO crystal to the OC and the small divergence of the laser output, its beam diameter inside the LBO crystal was approximately equal to the exit beam size at the OC ~1.6 mm.
The temperature of the LBO crystal was kept at 25° C. The second harmonic was separated from the residual fundamental wavelength by a dichroic mirror (HT at 995 nm at 45° incidence, HR at 497.5 nm).
Pulse temporal shape and its duration were recorded with a fast, Si-based detector (PDA-10A) and a fast oscilloscope with a 1 GHz bandwidth (Tektronix TDS5104B).
3.2 Experimental results
We studied the performance of a cryogenically-cooled, 995 nm Yb:YLF laser with two different cavity lengths - 33 cm and 98 cm. In both cases we achieved CW operation first and then moved on to Q-switching. The PRF varied from 500 Hz to 10 kHz. In Fig. 3, the average laser output in both modes of operation is plotted against the absorbed pump power.
Fig. 3 CW and Q-switched output power of a cryogenically-cooled Yb:YLF laser in a range of PRFs (shown in the legend) for two different cavity lengths: (a) LCAV = 33 cm, (b) LCAV = 98 cm.
With the cavity length of LCAV = 33 cm, see Fig. 3(a), the laser yielded up to 70 W of CW power with an ~82% slope efficiency relative to the absorbed pump power. To the best of our knowledge, it is the highest slope efficiency reported for an Yb:YLF laser operated under 1 µm. The laser spectrum was centered at 995.1 nm with the linewidth of ~0.25 nm at the maximum CW power of 70 W.
In a Q-switched mode with the PRF of 500 Hz, the pulse duration became very short (less than 8 ns), and in order to avoid damaging optical coatings and the crystals we had to limit the average output power to 6.5 W (slope efficiency ~42%). Experimental data for the 995 nm output are summarized in the Table. 1:
Table 1. Experimental data reflecting the performance of the cryogenic Yb:YLF laser at 995 nm - average power, pulse-width, slope efficiency obtained for two cavity lengths: 33 cm and 98 cm (in parentheses).
When we increased the length of the laser cavity to 98 cm, the pulse duration became longer allowing harder pumping, and at the same 500 Hz PRF we achieved 15.2 W of average output power with a 14 nsec pulse width. The maximum intra-cavity fluence in this case did not exceed 3.5 – 4 J/cm2. A gradual reduction of the laser efficiency from 69% to about 40% at a lower PRF can be explained by the depletion of the number of excited ions in the 2F5/2 state between pulses caused by an amplified spontaneous emission (ASE). Notice that the efficiencies of the CW and 10 kHz PRF operations were very close to each other (~74% and ~69% respectively).
Figure 4 shows how the pulse width of a Q-switched laser at 995 nm with the 98 cm cavity length depends on the output pulse energy. For 500 Hz, 1 and 2 kHz PRFs, the pulse widths reached 20 nsec. For faster rates of 5 and 10 kHz they were 35 and 60 ns at maximum output powers of 48 and 50 W, respectively.
Fig. 4 (a) Pulse widths of the Q-switched Yb:YLF laser at 995 nm vs. the output pulse energy for different PRFs. Pulse widths were measured at the Full Width Half Maximum (FWHM). (b) - a close-up of the area framed by a red rectangle in Fig. 4 (a) for pulse energies exceeding 10 mJ. The inset depicts an oscilloscope trace of the 22 mJ pulse at 1 kHz PRF.
The laser beam had a nearly perfect Gaussian distribution up to ~35 W of average power (see Fig. 5). Measured angular divergence was ~1.4 mrad, which corresponded to TEM00 mode of the laser cavity. Above 35 W, the laser beam quality started deteriorating and its divergence increased to ~3 mrad at the average output of 50 W.
Fig. 5 Far-field laser beam intensity distribution for 15.5W (at 500 Hz), 30W (2 kHz) and 50W (10 kHz) laser outputs at 995 nm.
The results of the efficient frequency doubling of our Q-switched, Yb:YLF laser are presented in Fig. 6. It depicts the measured second harmonic pulse energy at 497.5 nm as a function of the incident pulse energy at 995 nm for different PRFs.
Fig. 6 Second harmonic pulse energy at 497.5 nm as a function of the incident pulse energy at 995 nm for different PRFs as indicated in the legend.
As we mentioned above, for frequency doubling we used a 15 mm long LBO crystal (type I) at room temperature with no focusing optics due to the high intensity and nearly perfect quality of the fundamental 995 nm beam. Such a simple optical arrangement provided maximum conversion efficiency, when this laser operated at a PRF of 500 Hz and 1 kHz - 47% and 49%, respectively. This conversion efficiency is nearly the maximum achievable for the 15 mm long LBO crystal and the Gaussian beam with the intensity of 30 - 50 MW/cm2 (~50%) [15]. The maximum output was 14.3 mJ/pulse and 13 mJ/pulse at 500 and 1000 Hz PRF, correspondingly. The frequency conversion data is summarized in Table 2.
Table 2. Second harmonic generation in LBO crystal for different PRFs
The data presented in the Table 2 has been taken with the same optical setup - no focusing optics before the LBO frequency doubler for all PRFs. This arrangement allowed us to maximize the blue output at PRFs of 500-1000 Hz, which, in fact, were our target PRFs in this study. We believe that for PRFs of 5 kHz and higher, better conversion efficiencies can be achieved by focusing the fundamental laser beam into the LBO crystal.
4. Conclusions
In conclusion, we report on a high-average power, pulsed source based on a frequency doubling of an acousto-optically Q-switched, cryogenically-cooled Yb:YLF laser. It was resonantly pumped by a laser diode module into the 960 nm absorption band and yielded a diffraction limited output beam at 995 nm. The laser delivered up to 50 W of the average Q-switched output power at 10 kHz with a pulse duration of 60 ns and slope efficiency of 69%. At the PRF of 500 Hz, this laser yielded 14 ns long, 31 mJ pulses with the slope efficiency of ~40%. Frequency doubling to a 497.5 nm was achieved using an LBO crystal with conversion efficiency approaching 50% at the 500 and 1000 Hz PRF. The maximum achieved pulse energy at 497.5 nm was 14.3 mJ at 500 Hz PRF (average power over 7 W). To the best of our knowledge, this is the highest reported pulse energy and average power for an all-solid-state laser operating under 500 nm.
References
1. W. S. Pegau, D. Gray, and J. R. Zaneveld, “Absorption and attenuation of visible and near-infrared light in water: dependence on temperature and salinity,” Appl. Opt. 36(24), 6035–6046 (1997). [CrossRef] [PubMed]
2. C. Kränkel, D.-T. Marzahl, F. Moglia, G. Huber, and P. W. Metz, “Out of the blue: semiconductor laser pumped visible rare-earth doped lasers,” Laser Photonics Rev. 10(4), 548–568 (2016). [CrossRef]
3. B. P. Scott, F. Zhao, R. S. F. Chang, and N. Djeu, “Upconversion-pumped blue laser in Tm:YAG,” Opt. Lett. 18(2), 113–115 (1993). [CrossRef] [PubMed]
4. D. N. Patel, B. R. Reddy, and S. K. Nash-Stevenson, “Photon-avalanche upconversion in thulium-doped lutetium aluminum garnet,” Appl. Opt. 38(15), 3271–3274 (1999). [CrossRef] [PubMed]
5. K. Kyosuke, O. Akihito, Y. Tomoo, O. Hiroaki, and K. Kazushige, “High efficiency and high-power GaN-based laser diodes,” Review of Laser Engineering 35(2), 69–72 (2007). [CrossRef]
6. W. D. Kulatilaka, T. N. Anderson, and R. P. Lucht, “Development and application of an injected-seeded, nanosecond, pulsed OPO for high resolution spectroscopy,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science and Photonic Applications Systems Technologies, Technical Digest (CD) (Optical Society of America, 2005), paper CThY6.
7. S. Johansson, S. Bjurshagen, C. Canalias, V. Pasiskevicius, F. Laurell, and R. Koch, “An all solid-state UV source based on a frequency quadrupled, passively Q-switched 946 nm laser,” Opt. Express 15(2), 449–458 (2007). [CrossRef] [PubMed]
8. L. Bao, C. Zhao, and W. Wang, “494.5 nm generation by sum-frequency mixing of diode pumped neodymium lasers,” Optik (Stuttg.) 125(20), 5909–5911 (2014). [CrossRef]
9. F. Chen, D. J. Li, J. Guo, and X. Yu, “Research on all-solid-state blue lasers,” Optik (Stuttg.) 126(19), 1778–1781 (2015). [CrossRef]
10. T. J. Axenson, N. P. Barnes, D. J. Reichle Jr, and E. E. Koehler, “High-energy Q-switched 946 nm solid-state diode pumped laser,” J. Opt. Soc. Am. B 19(7), 1535 (2002). [CrossRef]
11. F. S. Ermeneux, R.W. Equall, R. L. Hutcheson, R. L. Cone, R. Moncorge, N. P. Barnes, H. G. Gallagher, and T. P. Han, “Nd-doped scandium garnets for compositional tuning to 944.1 nm and improved laser performance around 945 nm,” OSA TOPS26, Advanced Solid State Lasers, 242 (1999).
12. J. Gao, X. Yu, F. Chen, X. D. Li, R. P. Yan, Z. Zhang, J. H. Yu, and Y. Z. Wang, “Pulsed 456 nm deep-blue light generation by acousto-optical Q-switching and intracavity frequency doubling of Nd:GdVO4,” Laser Phys. Lett. 5(8), 577–581 (2008). [CrossRef]
13. J. Kawanaka, K. Yamakawa, H. Nishioka, and K. Ueda, “Improved high-field laser characteristics of a diode-pumped Yb:LiYF4 crystal at low temperature,” Opt. Express 10(10), 455–460 (2002). [CrossRef] [PubMed]
14. S. Ricaud, D. N. Papadopoulos, A. Pellegrina, F. Balembois, P. Georges, A. Courjaud, P. Camy, J. L. Doualan, R. Moncorgé, and F. Druon, “High-power diode-pumped cryogenically cooled Yb:CaF2 laser with extremely low quantum defect,” Opt. Lett. 36(9), 1602–1604 (2011). [CrossRef] [PubMed]
15. V. G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan, “Handbook of nonlinear optical crystals,” 2nd edition Springer-Verlag, 414 (1997).
16. L. Deyra, I. Martial, J. Didierjean, F. Balembois, and P. Georges, “3 W, 300 μJ, 25 ns pulsed 473 nm blue laser based on actively Q-switched Nd:YAG single-crystal fiber oscillator at 946 nm,” Opt. Lett. 38(16), 3013–3016 (2013). [CrossRef] [PubMed]
17. J. Kawanaka, H. Nishioka, N. Inoue, and K. Ueda, “Tunable continuous wave Yb:YLF laser operation with diode-pumped chirped-pulse amplification system,” Appl. Opt. 40(21), 3542–3546 (2001). [CrossRef] [PubMed]
18. L. E. Zapata, D. J. Ripin, and T. Y. Fan, “Power scaling of cryogenic Yb:LiYF4 lasers,” Opt. Lett. 35(11), 1854–1856 (2010). [CrossRef] [PubMed]
19. D. E. Miller, J. R. Ochoa, and T. Y. Fan, “Cryogenically cooled, 149 W, Q-switched, Yb:LiYF4 laser,” Opt. Lett. 38(20), 4260–4261 (2013). [CrossRef] [PubMed]
20. R. L. Aggarwal, D. J. Ripin, J. R. Ochoa, and T. Y. Fan, “Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80-300 K temperature range,” J. Appl. Phys. 98(10), 103514 (2005). [CrossRef]
21. D. Rand, D. Miller, D. J. Ripin, and T. Y. Fan, “Cryogenic Yb3+-doped materials for pulsed solid-state laser applications,” Opt. Mater. Express 1(3), 434–450 (2011). [CrossRef]
22. A. Bensalah, Y. Guyot, M. Ito, A. Brenier, H. Sato, T. Fukuda, and G. Boulton, “Growth of Yb3+-doped YLiF4 laser crystal by the Czochralski method. Attempt of Yb3+ energy level assignment and estimation of the laser potentiality,” Opt. Mater. 26(4), 375–383 (2004). [CrossRef]
23. T. Y. Fan, D. J. Ripin, R. L. Aggarwal, J. R. Ochoa, B. Chann, M. Tilleman, and J. Spitzberg, “Cryogenic Yb3+ -doped solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 448–459 (2007). [CrossRef]
