December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS
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A 41-W ZnGeP2 optical parametric oscillator pumped by a Q-switched Ho:YAG laser Bao-Quan Yao, Ying-Jie Shen, Xiao-Ming Duan,* Tong-Yu Dai, You-Lun Ju, and Yue-Zhu Wang National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 15001, China *Corresponding author: [email protected] Received September 8, 2014; revised October 20, 2014; accepted October 20, 2014; posted October 20, 2014 (Doc. ID 222569); published November 18, 2014 We reported a high-power ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumped by a Q-switched Ho:YAG laser. The maximum output power of the ZGP OPO was 41.2 W at 107.0 W incident Ho pump power, corresponding to a slope efficiency of 44.6%. The ZGP OPO produced 16-ns mid-IR pulse laser in the 3.74–3.98 μm and 4.38–4.76 μm spectral regions simultaneously. The beam quality was measured to be M2 < 4.37. The continuous wave maximum average output power of the Ho:YAG laser was 128 W, corresponding to a slope efficiency of 65.8%. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3540) Lasers, Q-switched; (190.4970) Parametric oscillators and amplifiers. http://dx.doi.org/10.1364/OL.39.006589
High power mid-infrared (3–5 μm) laser sources are of interest for a variety of applications such as remote sensing [1–3], defense and civilian applications, and they are key components in directive systems for countermeasures against infrared sensors. For broadband applications, such as some defense applications, it may even be possible to have both the signal and idler wavelengths in the useful spectral range. The most attractive 3–5-μm sources are ZnGeP2 (ZGP)-based optical parametric oscillators (OPOs) pumped at 2 μm owing to the ZGP crystal having high optical nonlinearity, high damage threshold, good thermal conductivity, and an optical transparency range of 2–12 μm. In addition, ZGP is also commercially available in high quality. Various work on the ZGP OPO was reported in recent years. Output powers over 14 W from a single OPO based on ZGP was reported in 1999 [4]. Lippert et al. reported a ZnGeP2 -based OPO with 22 W of output power in the 3–5-μm range using a novel V-shaped 3-mirror ring resonator [5]. More than 30 W output power from a ZGP OPO was reported [6] in 2007, but without giving any details. Up to 27 W has been demonstrated from a double-pass standing wave OPO, which was limited by the optical isolator in the system [7]. Hemming et al. reported a ZGP OPO laser in a walk-off geometry with output power of 27 W and a modulated peak output power of 99 W at a reduced duty cycle of 25% [8], and recently demonstrated a 30.2 W of mid-IR output from a ring resonator with two ZGP crystals [9]. In the present work, we obtain more than 41 W of output power from a ZGP OPO with a rectangle ring resonator consisting of four flat mirrors, to our knowledge, which is the highest report for a ZGP OPO laser pumped by a Ho:YAG oscillator. To achieve high power, ZGP must be pumped at wavelength above 2 μm due to high absorption at shorter wavelengths. A major challenge of a high-power midIR system can be the development of an efficient pulsed high-power 2-μm pump laser. The bulk Ho laser allows Q-switched operation with high pulse energies that can be suitable for pumping a ZGP OPO. We have reported a high-power and high-beam-quality Q-switched Ho:YAG laser with over 100 W output power [10]. To make a 0146-9592/14/236589-04$15.00/0
high-power mid-infrared OPO source, a second challenge is to avoid reflection from the OPO into the pump laser. Some kind of isolation should be required to avoid the feedback from an OPO with a standing wave cavity. Unfortunately, Faraday rotators available at 2 μm have lower damage threshold and power handling capacity. The promising solution is to use a ring resonator. We use a four-mirror ring resonator with two ZGP crystals, which can allow two Q-switched Ho:YAG beams to be pumped into two ZGP crystals, respectively. In this report, we report a high-power ZGP OPO laser pumped by a Q-switched Ho:YAG laser that has been double-end-pumped by four diode-pumped Tm:YLF lasers at room temperature. With the pulse repetition frequency of 20 kHz, the average output power of ZGP OPO laser of 41.2 W was achieved, corresponding to a slope efficiency of 44.6% and optical-to-optical conversion efficiency of 38.5%. The beam quality was measured to be M2 < 4.37. The schematic diagram of experimental setup is shown in Fig. 1. As shown in Fig. 1, two Ho:YAG crystals were used in the resonator, each of which was dual-endpumped by two orthogonally polarized Tm:YLF lasers. Each of the Tm:YLF lasers involved one slab crystal that had a cross-section of 1.5 mm × 6 mm and a length of 25 mm. The maximum output power of each of the four diode-end-pumped Tm:YLF lasers was about 60 W, and the beam quality factor of M2 was less than three at
Fig. 1. Schematic diagram of experimental setup. © 2014 Optical Society of America
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the highest output power. The Ho laser resonator consists of a plane mirror M1 (with T > 99.8% at 1.9 μm and R > 99.5% at 2.1 μm), two 45° dichroic mirrors M2 (with T > 97.7% at 1.9 μm and R > 99.8% at 2.1 μm), and an output coupler (M3) with 300 mm curvature radius and transmission of 68.4% at 2.1 μm. Four thin-film polarizers (TFPs) were employed in the experiment to avoid the Tm:YLF lasers from being influenced by each other. A 30-mm-long Brewster-cut acousto-optic Q-switch (AOM) with an acoustic aperture of 1.8 mm was employed in the cavity. The radio-frequency (RF) input power at frequency of 40.7 MHz is rated for 25 W. A 0.1-mm-thickness YAG uncoated etalon was employed in the resonator to force the Ho laser to emit at a single wavelength. A Ho3 -doping concentration of 0.8 at. % was chosen in our experiment, which was employed to diminish the upconversion losses [11]. The dimension of the Ho:YAG crystals is 5 mm in diameter and 45 mm in length. Both end faces of the crystals were antireflection-coated at the pump and laser wavelengths. The crystals were wrapped in indium foil and mounted into a copper block, which was held at 18°C by the water-cooled heat sink. The physical length of resonator was 240 mm resulting in an estimated TEM00 beam radius of about 310 μm in the resonator. For simplicity, the simple telescopic lens system (TLS) consisting of collimating and focusing lenses were not shown in the picture. After passing the TLS, the Tm:YLF pump spot size of about 900 μm in diameter was focused into the Ho crystal at the Tm pump power of 60 W. The ZGP OPO with a rectangle configuration is also shown in Fig. 1, which consists of four 45° flat mirrors. The output coupler (M7) has a transmission of 50% at 3–5 μm for the s-polarized component and highly transparent (T > 95.0%) at 2.1 μm for the p-polarized component. The other three plane mirrors (M6) were highly transparent (T > 95.0%) at 2.1 μm for the p-polarized component and highly reflective (R > 99.8%) at 3–5 μm for the s-polarized component. The OPO is then doubly resonant and single-pass pumped. After passing the TLS (not shown in Fig. 1), the Ho:YAG pump spot size of approximately 1 mm in diameter was focused into the ZGP crystal at the incident Ho pump power of 107.0 W. The physical length of the ring cavity was about 108 mm, which was length matched to the Ho laser that had an optical round-trip length of about 584 mm [12]. The Ho pump beam was changed from s-polarized component to p-polarized component by a 2.1 μm λ1∕2 plate. Due to the limited damage threshold of the ZGP crystal coating, the Ho pump power was divided into two parts by a 45° flat beam splitter (M5) with half transparent at 2.1 μm for the p-polarized component. Two 6 mm × 6 mm × 23.0 mm ZGP crystals cut for Type I phase-matching (θ 55° and φ 0° relative to the optical axis) were employed in the experiment. The absorption coefficients of the two ZGP crystals (School of Chemical Engineering & Technology, HIT) at 2.1 μm was measured to be α 0.05, 0.03 cm−1 . The ZGP crystals were wrapped in indium foil and mounted into a copper block with water cooling, and both end faces of them were antireflection-coated at the pump and laser wavelengths. A 5° dichroic mirror (M8) with highly reflective
Fig. 2. Output power of the CW Ho:YAG laser.
(R > 99.9%) for pump wavelength and highly transparent (T > 99.9%) for the mid-IR laser was employed in the experiment. The Ho:YAG laser operation was achieved at both CW and Q-switched modes. Under CW mode, the slope efficiency as a function of the Tm pump power is shown in Fig. 2. The maximum output power of 128.0 W was achieved in relation to the Tm pump power of 210.8 W, corresponding to a slope efficiency of 65.8% and an optical-to-optical efficiency of 60.7%. While in Q-switched operation, the maximum output power of 118.0 W at 20 kHz was achieved at 209.5 W Tm:YLF pump powers, which corresponded to a slope efficiency of 61.7% and an optical conversion of 56.3%. The result is shown in Fig. 3. We achieved maximum output energy per pulse of 5.9 mJ and a peak power of approximately of 245.8 kW. The temporal trace of the laser pulse (inset of Fig. 3) shows a pulse width of 23 ns (FWHM). The pulse duration shows a steady decrease as the Tm pump powers up to levels of 103.5 W. The Ho:YAG laser beam quality was measured to be M2 ∼ 2 at the highest output power. Without the etalon in the resonator, the laser operated simultaneously at 2090 and 2097 nm, as shown in Fig. 4, which is not desired for pumping an OPO. For the Ho laser operating at one single wavelength, a 0.1-mmthickness YAG uncoated etalon was inserted in the Ho resonator. The output wavelength was tuned to 2090 nm by adjusting the angle of the etalon, which showed higher performance than 2097 nm.
Fig. 3. Output performance of the Q-switched Ho:YAG laser with 20 kHz. The inset is the oscilloscope trace for the pulse energy of 5.9 mJ.
December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS Without etalon with 0.1 mm YAG etalon
1.2
Intensity (arb.units)
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1.0 0.8 0.6 0.4 0.2 0.0 2080
2085
2090 2095 2100 Wavelength (nm)
2105
2110
Fig. 4. Output spectrum of the Q-switched Ho:YAG laser with and without etalon.
Fig. 6.
Output spectrum of the ZGP OPO.
Beam radius (mm)
2.0 Experimental data 2 M [email protected] µm 2 M [email protected] µm
1.6 1.2 0.8 0.4 0.0 20
40
60
80
100
120
Distance from lens (mm) Fig. 7. Fig. 5. Output performance of the ZGP OPO with a four-mirror ring resonator. The inset is the oscilloscope trace at the highest output power.
As shown in Fig. 5, the average 3–5-μm output power as well as the pulse width were measured as a function of the incident Ho:YAG pump power. The output power of the ZGP OPO is the combined sum of the signal (3.94 μm) and the idler (4.50 μm), both in the mid-IR region. The threshold pump power of the ring OPO was about 14.5 W. The maximum average output power of the ZGP OPO at the pulse repetition frequency of 20 kHz was 41.2 W, corresponding to the slope efficiency of 44.6%. To our knowledge, this is the highest average output power of a ZGP OPO pumped by a Q-switched Ho:YAG oscillator. The shortest pulse width was about 16 ns at the Ho incident pump power of 107.0 W, the typical oscilloscope trace of which is shown in the inset in Fig. 5. The output spectrum of the ZGP OPO was measured by a 300-mm WDG15-Z monochromator and a HgCdTe detector, which is shown in Fig. 6. The whole ZGP OPO spectrum measurement process takes about 10 min. The spectrum was measured several times, and all of them were observed as the same profile, in other words, the ZGP OPO spectrum was stable over the measured time. The output spectrum consists of several clusters with central wavelength of 3.94 μm for the signal and 4.50 μm for the idler, which can be tuned by rotation of the ZGP crystals. Such a cluster structure usually occurs for doubly resonant OPOs pumped by a single longitudinal mode laser. We observed a broad output spectrum envelop with a FWHM of approximately 240 nm for the signal and 380 nm for the idler. Figure 7 shows propagation characteristics of the ZGP OPO beam
Beam quality of the ZGP OPO.
at the highest output power of 41.2 W. By using 90/10 knife-edge technique, we measured the 1∕e2 beam radius with different positions after passing a CaF2 lens with a focal length of 100 mm. By fitting Gaussian beam standard expression to these data, the fit yields M2 ∼ 4.37 for the output wavelength of 3.94 μm and M2 ∼ 3.83 for 4.50 μm. In conclusion, an efficient ZnGeP2 OPO pumped by a Ho:YAG laser was demonstrated. The continuous wave maximum output power of Ho:YAG laser was about 128.0 W, which corresponded to a slope efficiency of 65.8%. The maximum output power of the ring OPO laser was 41.2 W, corresponding to a slope efficiency of about 44.6% with respect to the incident pump power. The pulse width of the OPO at the highest output power was as short as 16 ns. The mid-IR laser with beam quality factor of M2 < 4.37 was achieved. This work was supported by National Natural Science Foundation of China (Nos. 61308009, 61405047), Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310) and Fundamental Research funds for the Central Universities (Grant Nos. HIT.NSRIF. 2014044, 2015042). References 1. W. Yu, K. Numata, H. Riris, J. B. Abshire, G. Allan, X. Sun, and M. A. Krainak, in Laser Applications to Chemical, Security and Environmental Analysis (Optical Society of America, 2008), paper LMC5. 2. Y. He, P. Wang, R. T. White, and B. J. Orr, Opt. Photon. News 13(5), 56 (2002). 3. A. K. Y. Ngai, S. T. Persijn, and F. J. M. Harren, in Conference on Lasers and Electro-Optics (Optical Society of America, 2007), paper CWE2.
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4. E. C. Cheung, S. Palese, H. Injeyan, C. Hoefer, J. Ho, R. Hilyard, H. Komine, J. Berg, and W. Bosenberg, in Advanced Solid-State Lasers (Optical Society of America, 1999), pp. 514–517. 5. E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, Opt. Express 18, 26475 (2010). 6. P. G. Schunemann, in Conference on Lasers and Electro-Optics (Optical Society of America, 2007), paper CThL3. 7. A. Hemming, J. Richards, A. Davidson, N. Carmody, S. Bennetts, N. Simakov, P. Davies, and J. Haub, in Conference on Lasers and Electro-Optics/Pacific Rim (Optical Society of America, 2011), pp. 624–626.
8. A. Hemming, J. Richards, A. Davidson, N. Carmody, N. Simakov, S. Bennetts, N. Simakov, and J. Haub, Opt. Express 21, 10062 (2013). 9. A. Hemming, J. Richards, A. Davidson, N. Carmody, S. Bennetts, M. Hughes, N. Simakov, P. Davies, and J. Haub, in Conference on Lasers and Electro-Optics (Optical Society of America, 2013), paper CW1B. 10. Y. J. Shen, B. Q. Yao, X. M. Duan, G. L. Zhu, W. Wang, Y. L. Ju, and Y. Z. Wang, Opt. Lett. 37, 3558 (2012). 11. E. Lippert, S. Nicolas, G. Arisholm, K. Stenersen, and G. Rustad, Appl. Opt. 45, 3839 (2006). 12. G. Arisholm, E. Lippert, G. Rustad, and K. Stenersen, Opt. Lett. 25, 1654 (2000).
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A 41-W ZnGeP2 optical parametric oscillator pumped by a Q-switched Ho:YAG laser Bao-Quan Yao, Ying-Jie Shen, Xiao-Ming Duan,* Tong-Yu Dai, You-Lun Ju, and Yue-Zhu Wang National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 15001, China *Corresponding author: [email protected] Received September 8, 2014; revised October 20, 2014; accepted October 20, 2014; posted October 20, 2014 (Doc. ID 222569); published November 18, 2014 We reported a high-power ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumped by a Q-switched Ho:YAG laser. The maximum output power of the ZGP OPO was 41.2 W at 107.0 W incident Ho pump power, corresponding to a slope efficiency of 44.6%. The ZGP OPO produced 16-ns mid-IR pulse laser in the 3.74–3.98 μm and 4.38–4.76 μm spectral regions simultaneously. The beam quality was measured to be M2 < 4.37. The continuous wave maximum average output power of the Ho:YAG laser was 128 W, corresponding to a slope efficiency of 65.8%. © 2014 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3540) Lasers, Q-switched; (190.4970) Parametric oscillators and amplifiers. http://dx.doi.org/10.1364/OL.39.006589
High power mid-infrared (3–5 μm) laser sources are of interest for a variety of applications such as remote sensing [1–3], defense and civilian applications, and they are key components in directive systems for countermeasures against infrared sensors. For broadband applications, such as some defense applications, it may even be possible to have both the signal and idler wavelengths in the useful spectral range. The most attractive 3–5-μm sources are ZnGeP2 (ZGP)-based optical parametric oscillators (OPOs) pumped at 2 μm owing to the ZGP crystal having high optical nonlinearity, high damage threshold, good thermal conductivity, and an optical transparency range of 2–12 μm. In addition, ZGP is also commercially available in high quality. Various work on the ZGP OPO was reported in recent years. Output powers over 14 W from a single OPO based on ZGP was reported in 1999 [4]. Lippert et al. reported a ZnGeP2 -based OPO with 22 W of output power in the 3–5-μm range using a novel V-shaped 3-mirror ring resonator [5]. More than 30 W output power from a ZGP OPO was reported [6] in 2007, but without giving any details. Up to 27 W has been demonstrated from a double-pass standing wave OPO, which was limited by the optical isolator in the system [7]. Hemming et al. reported a ZGP OPO laser in a walk-off geometry with output power of 27 W and a modulated peak output power of 99 W at a reduced duty cycle of 25% [8], and recently demonstrated a 30.2 W of mid-IR output from a ring resonator with two ZGP crystals [9]. In the present work, we obtain more than 41 W of output power from a ZGP OPO with a rectangle ring resonator consisting of four flat mirrors, to our knowledge, which is the highest report for a ZGP OPO laser pumped by a Ho:YAG oscillator. To achieve high power, ZGP must be pumped at wavelength above 2 μm due to high absorption at shorter wavelengths. A major challenge of a high-power midIR system can be the development of an efficient pulsed high-power 2-μm pump laser. The bulk Ho laser allows Q-switched operation with high pulse energies that can be suitable for pumping a ZGP OPO. We have reported a high-power and high-beam-quality Q-switched Ho:YAG laser with over 100 W output power [10]. To make a 0146-9592/14/236589-04$15.00/0
high-power mid-infrared OPO source, a second challenge is to avoid reflection from the OPO into the pump laser. Some kind of isolation should be required to avoid the feedback from an OPO with a standing wave cavity. Unfortunately, Faraday rotators available at 2 μm have lower damage threshold and power handling capacity. The promising solution is to use a ring resonator. We use a four-mirror ring resonator with two ZGP crystals, which can allow two Q-switched Ho:YAG beams to be pumped into two ZGP crystals, respectively. In this report, we report a high-power ZGP OPO laser pumped by a Q-switched Ho:YAG laser that has been double-end-pumped by four diode-pumped Tm:YLF lasers at room temperature. With the pulse repetition frequency of 20 kHz, the average output power of ZGP OPO laser of 41.2 W was achieved, corresponding to a slope efficiency of 44.6% and optical-to-optical conversion efficiency of 38.5%. The beam quality was measured to be M2 < 4.37. The schematic diagram of experimental setup is shown in Fig. 1. As shown in Fig. 1, two Ho:YAG crystals were used in the resonator, each of which was dual-endpumped by two orthogonally polarized Tm:YLF lasers. Each of the Tm:YLF lasers involved one slab crystal that had a cross-section of 1.5 mm × 6 mm and a length of 25 mm. The maximum output power of each of the four diode-end-pumped Tm:YLF lasers was about 60 W, and the beam quality factor of M2 was less than three at
Fig. 1. Schematic diagram of experimental setup. © 2014 Optical Society of America
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the highest output power. The Ho laser resonator consists of a plane mirror M1 (with T > 99.8% at 1.9 μm and R > 99.5% at 2.1 μm), two 45° dichroic mirrors M2 (with T > 97.7% at 1.9 μm and R > 99.8% at 2.1 μm), and an output coupler (M3) with 300 mm curvature radius and transmission of 68.4% at 2.1 μm. Four thin-film polarizers (TFPs) were employed in the experiment to avoid the Tm:YLF lasers from being influenced by each other. A 30-mm-long Brewster-cut acousto-optic Q-switch (AOM) with an acoustic aperture of 1.8 mm was employed in the cavity. The radio-frequency (RF) input power at frequency of 40.7 MHz is rated for 25 W. A 0.1-mm-thickness YAG uncoated etalon was employed in the resonator to force the Ho laser to emit at a single wavelength. A Ho3 -doping concentration of 0.8 at. % was chosen in our experiment, which was employed to diminish the upconversion losses [11]. The dimension of the Ho:YAG crystals is 5 mm in diameter and 45 mm in length. Both end faces of the crystals were antireflection-coated at the pump and laser wavelengths. The crystals were wrapped in indium foil and mounted into a copper block, which was held at 18°C by the water-cooled heat sink. The physical length of resonator was 240 mm resulting in an estimated TEM00 beam radius of about 310 μm in the resonator. For simplicity, the simple telescopic lens system (TLS) consisting of collimating and focusing lenses were not shown in the picture. After passing the TLS, the Tm:YLF pump spot size of about 900 μm in diameter was focused into the Ho crystal at the Tm pump power of 60 W. The ZGP OPO with a rectangle configuration is also shown in Fig. 1, which consists of four 45° flat mirrors. The output coupler (M7) has a transmission of 50% at 3–5 μm for the s-polarized component and highly transparent (T > 95.0%) at 2.1 μm for the p-polarized component. The other three plane mirrors (M6) were highly transparent (T > 95.0%) at 2.1 μm for the p-polarized component and highly reflective (R > 99.8%) at 3–5 μm for the s-polarized component. The OPO is then doubly resonant and single-pass pumped. After passing the TLS (not shown in Fig. 1), the Ho:YAG pump spot size of approximately 1 mm in diameter was focused into the ZGP crystal at the incident Ho pump power of 107.0 W. The physical length of the ring cavity was about 108 mm, which was length matched to the Ho laser that had an optical round-trip length of about 584 mm [12]. The Ho pump beam was changed from s-polarized component to p-polarized component by a 2.1 μm λ1∕2 plate. Due to the limited damage threshold of the ZGP crystal coating, the Ho pump power was divided into two parts by a 45° flat beam splitter (M5) with half transparent at 2.1 μm for the p-polarized component. Two 6 mm × 6 mm × 23.0 mm ZGP crystals cut for Type I phase-matching (θ 55° and φ 0° relative to the optical axis) were employed in the experiment. The absorption coefficients of the two ZGP crystals (School of Chemical Engineering & Technology, HIT) at 2.1 μm was measured to be α 0.05, 0.03 cm−1 . The ZGP crystals were wrapped in indium foil and mounted into a copper block with water cooling, and both end faces of them were antireflection-coated at the pump and laser wavelengths. A 5° dichroic mirror (M8) with highly reflective
Fig. 2. Output power of the CW Ho:YAG laser.
(R > 99.9%) for pump wavelength and highly transparent (T > 99.9%) for the mid-IR laser was employed in the experiment. The Ho:YAG laser operation was achieved at both CW and Q-switched modes. Under CW mode, the slope efficiency as a function of the Tm pump power is shown in Fig. 2. The maximum output power of 128.0 W was achieved in relation to the Tm pump power of 210.8 W, corresponding to a slope efficiency of 65.8% and an optical-to-optical efficiency of 60.7%. While in Q-switched operation, the maximum output power of 118.0 W at 20 kHz was achieved at 209.5 W Tm:YLF pump powers, which corresponded to a slope efficiency of 61.7% and an optical conversion of 56.3%. The result is shown in Fig. 3. We achieved maximum output energy per pulse of 5.9 mJ and a peak power of approximately of 245.8 kW. The temporal trace of the laser pulse (inset of Fig. 3) shows a pulse width of 23 ns (FWHM). The pulse duration shows a steady decrease as the Tm pump powers up to levels of 103.5 W. The Ho:YAG laser beam quality was measured to be M2 ∼ 2 at the highest output power. Without the etalon in the resonator, the laser operated simultaneously at 2090 and 2097 nm, as shown in Fig. 4, which is not desired for pumping an OPO. For the Ho laser operating at one single wavelength, a 0.1-mmthickness YAG uncoated etalon was inserted in the Ho resonator. The output wavelength was tuned to 2090 nm by adjusting the angle of the etalon, which showed higher performance than 2097 nm.
Fig. 3. Output performance of the Q-switched Ho:YAG laser with 20 kHz. The inset is the oscilloscope trace for the pulse energy of 5.9 mJ.
December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS Without etalon with 0.1 mm YAG etalon
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Intensity (arb.units)
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1.0 0.8 0.6 0.4 0.2 0.0 2080
2085
2090 2095 2100 Wavelength (nm)
2105
2110
Fig. 4. Output spectrum of the Q-switched Ho:YAG laser with and without etalon.
Fig. 6.
Output spectrum of the ZGP OPO.
Beam radius (mm)
2.0 Experimental data 2 M [email protected] µm 2 M [email protected] µm
1.6 1.2 0.8 0.4 0.0 20
40
60
80
100
120
Distance from lens (mm) Fig. 7. Fig. 5. Output performance of the ZGP OPO with a four-mirror ring resonator. The inset is the oscilloscope trace at the highest output power.
As shown in Fig. 5, the average 3–5-μm output power as well as the pulse width were measured as a function of the incident Ho:YAG pump power. The output power of the ZGP OPO is the combined sum of the signal (3.94 μm) and the idler (4.50 μm), both in the mid-IR region. The threshold pump power of the ring OPO was about 14.5 W. The maximum average output power of the ZGP OPO at the pulse repetition frequency of 20 kHz was 41.2 W, corresponding to the slope efficiency of 44.6%. To our knowledge, this is the highest average output power of a ZGP OPO pumped by a Q-switched Ho:YAG oscillator. The shortest pulse width was about 16 ns at the Ho incident pump power of 107.0 W, the typical oscilloscope trace of which is shown in the inset in Fig. 5. The output spectrum of the ZGP OPO was measured by a 300-mm WDG15-Z monochromator and a HgCdTe detector, which is shown in Fig. 6. The whole ZGP OPO spectrum measurement process takes about 10 min. The spectrum was measured several times, and all of them were observed as the same profile, in other words, the ZGP OPO spectrum was stable over the measured time. The output spectrum consists of several clusters with central wavelength of 3.94 μm for the signal and 4.50 μm for the idler, which can be tuned by rotation of the ZGP crystals. Such a cluster structure usually occurs for doubly resonant OPOs pumped by a single longitudinal mode laser. We observed a broad output spectrum envelop with a FWHM of approximately 240 nm for the signal and 380 nm for the idler. Figure 7 shows propagation characteristics of the ZGP OPO beam
Beam quality of the ZGP OPO.
at the highest output power of 41.2 W. By using 90/10 knife-edge technique, we measured the 1∕e2 beam radius with different positions after passing a CaF2 lens with a focal length of 100 mm. By fitting Gaussian beam standard expression to these data, the fit yields M2 ∼ 4.37 for the output wavelength of 3.94 μm and M2 ∼ 3.83 for 4.50 μm. In conclusion, an efficient ZnGeP2 OPO pumped by a Ho:YAG laser was demonstrated. The continuous wave maximum output power of Ho:YAG laser was about 128.0 W, which corresponded to a slope efficiency of 65.8%. The maximum output power of the ring OPO laser was 41.2 W, corresponding to a slope efficiency of about 44.6% with respect to the incident pump power. The pulse width of the OPO at the highest output power was as short as 16 ns. The mid-IR laser with beam quality factor of M2 < 4.37 was achieved. This work was supported by National Natural Science Foundation of China (Nos. 61308009, 61405047), Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310) and Fundamental Research funds for the Central Universities (Grant Nos. HIT.NSRIF. 2014044, 2015042). References 1. W. Yu, K. Numata, H. Riris, J. B. Abshire, G. Allan, X. Sun, and M. A. Krainak, in Laser Applications to Chemical, Security and Environmental Analysis (Optical Society of America, 2008), paper LMC5. 2. Y. He, P. Wang, R. T. White, and B. J. Orr, Opt. Photon. News 13(5), 56 (2002). 3. A. K. Y. Ngai, S. T. Persijn, and F. J. M. Harren, in Conference on Lasers and Electro-Optics (Optical Society of America, 2007), paper CWE2.
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4. E. C. Cheung, S. Palese, H. Injeyan, C. Hoefer, J. Ho, R. Hilyard, H. Komine, J. Berg, and W. Bosenberg, in Advanced Solid-State Lasers (Optical Society of America, 1999), pp. 514–517. 5. E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, Opt. Express 18, 26475 (2010). 6. P. G. Schunemann, in Conference on Lasers and Electro-Optics (Optical Society of America, 2007), paper CThL3. 7. A. Hemming, J. Richards, A. Davidson, N. Carmody, S. Bennetts, N. Simakov, P. Davies, and J. Haub, in Conference on Lasers and Electro-Optics/Pacific Rim (Optical Society of America, 2011), pp. 624–626.
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