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High-repetition-rate passively Q-switched Nd:GdTaO4 1066 nm laser under 879 nm pumping
Infrared Physics and Technology 102 (2019) 103025
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
High-repetition-rate passively Q-switched Nd:GdTaO4 1066 nm laser under 879 nm pumping
T
⁎
Yang Liua, Renpeng Yana, , Wentao Wua, Xudong Lia, Zhiwei Donga, Zhixiang Liub, Xiaolin Wenb, ⁎ Wenming Yaoc, Fang Pengd, Qingli Zhangd, Renqin Doud, Jing Gaoc,e, a
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China Shenzhen Aerospace Industry Technology Research Institute, Shenzhen 518000, China c Jiangsu Key Laboratory of Medical Optics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou, Jiangsu 215163, China d The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China e Suzhou Guoke Medical Science & Technology Development Co. Ltd, Suzhou 215163, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: LD pumped Passively Q-switched Nd:GdTaO4
We report an 879 nm laser diode directly pumped passively Q-switched Nd:GdTaO4 laser at 1066 nm. The continuous-wave output power at 1066 nm reaches 5.6 W under an incident pump power of 12.8 W, corresponding to an optical-to-optical efficiency of 43.8% and a slope efficiency of 48.0%. By using Cr4+:YAG as the saturable absorber, passively Q-switched laser operation is conducted, yielding a highest pulse repetition frequency of 185 kHz and a maximum average power of 3.6 W. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors at the maximum average output power is measured to be Mx2 = 2.2, My2 = 1.5 by using knife-edge method. The thermal focal length and internal loss for Nd:GdTaO4 crystal is also investigated.
1. Introduction
maximum output power of 2.5 W and an optical-to-optical efficiency of 34.6% [5]. In 2015, Zhang et al. showed an 808 nm LD pumped passively Q-switched Nd:GdTaO4 laser using graphene oxide as saturable absorber (SA), and obtained a pulse laser output with a maximum average output power of 0.382 W and a pulse width of 194 ns at 362 kHz [6]. Wang et al. presented a passively Q-switched mode locking Nd:GdTaO4 1066 nm laser with a maximum average output power of 0.156 W [7]. Compared with indirect 808 nm pumping, directly pumping has higher theoretical quantum efficiency and reduce thermal effect [8,9]. Cr4+:YAG has advantages of high damage threshold, good chemical stability, and excellent mechanical and optical properties and is widely used as a saturable absorber to shorten pulse width and boost peak power [10–12]. In this paper, we present an 879 nm LD pumped Nd:GdTaO4 laser in CW and passively Q-switched operation. The absorption spectrum of Nd:GdTaO4 near 879 nm is investigated and the highest absorption coefficient is about 4.2 cm−1 with a FWHM of 1.3 nm. When the incident pump power is 12.8 W, the maximum CW output power of 5.6 W at 1066 nm is achieved with an optical conversion efficiency of 48.0%. By using Cr4+:YAG as saturable absorber, The maximum average power
Diode-pumped Nd3+-doped lasers with advantages of compactness, high efficiency, and low cost are widely used in scientific research, industry, communication and so on. In recent years, Nd3+-doped crystals with different host materials have aroused great research interest for high efficiency and high power laser output [1–3]. Nd:GdTaO4 is a new laser medium with high optical quality and low symmetry. The low symmetry is beneficial to luminescence efficiency and realize polarized output. Nd:GdTaO4 has a relatively broad absorption band and large stimulated emission cross-section, which is suitable for laser diode (LD) pumped solid-state lasers. The full width at half maximum (FWHM) of the absorption band around 808 nm is 6 nm for Nd:GdTaO4. The broad absorption band can reduce the requirement of pump light in terms of bandwidth. The stimulated emission crosssection at 1066 nm is 3.9 × 10−19 cm2 and the fluorescence lifetime of 4 F3/2 level is 178.4 μs. Nd:GdTaO4 crystal was firstly grown by Peng et al. through Czochralski (Cz) method in 2014 [4]. In 2015, Peng et al. demonstrated the spectroscopic properties of Nd:GdTaO4 and a continuous-wave (CW) 808 nm LD pumped Nd:GdTaO4 laser with a
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Corresponding authors at: National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China (R. Yan). Jiangsu Key Laboratory of Medical Optics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou, Jiangsu 215163, China (J. Gao). E-mail addresses: [email protected] (R. Yan), [email protected] (J. Gao). https://doi.org/10.1016/j.infrared.2019.103025 Received 16 July 2019; Received in revised form 29 August 2019; Accepted 29 August 2019 Available online 30 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103025
Y. Liu, et al.
Fig. 1. Experimental setup of LD end-pumped passively Q-switched Nd:GdTaO4 1066 nm laser.
of 3.6 W is obtained with an optical-to-optical efficiency of 20% and a slope efficiency of 24.6%. The highest repetition frequency of pulse is 185 kHz and the corresponding pulse width is 27.5 ns. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors at the average output power of 3.6 W are measured to be Mx2 = 2.2, My2 = 1.5 by using traveling knife-edge method. The thermal effect of the Nd:GdTaO4 crystal is investigated while its internal losses is calculated to be δ = 0.078 cm−1. 2. Experimental setup Fig. 1 shows the experimental setup of LD end-pumped passively Qswitched Nd:GdTaO4/Cr4+:YAG 1066 nm laser. A wavelength-stabilized fiber-coupled LD (P4-100-0878.6, nLIGHT Inc.) at 879 nm with beam quality factor of 81.9 is employed as pumping source. The fiber has a diameter of 400 μm and a numerical aperture of 0.22. Pumping light is coupled to laser medium through a pair of lenses L1 and L2 with a focal length of 26.7 mm. Nd:GdTaO4 crystal with a Nd3+ doping concentration of 2 at.% and a dimension of 2 × 2 × 5 mm3 serves as the gain medium. The Rayleigh length of pump beam is calculated to be 11 mm, longer than the crystal length. The end surfaces of Nd:GdTaO4 are coated with high transmission (HT) at 879 nm and antireflection (AR) at 1066 nm. The laser crystal is wrapped with indium foil and placed in the water-cooled copper heat sink with a small amount of thermal grease in the contact surfaces to improve the capability of thermal dissipation. The temperature of water cooling is maintained at 18 °C. The laser cavity composes of two planes mirrors M1 and M2 with a geometric length of 60 mm. The mirror M1 is HT coated at 879 nm and highly reflective coating at 1066 nm. The mirror M2 is an output coupler mirror with partial transmissions of 10%, 20% and 30% at 1066 nm. Cr4+:YAG crystal is placed near the M2 mirror as a saturable absorber. Both surfaces of Cr4+:YAG is AR coated at 1066 nm. A pinhole is inserted in the cavity to reduce the influence of pump light on Cr4+:YAG. A fast photodiode (DET025AL, Thorlabs, Inc.) and a digital oscilloscope (DSO-X3034A, Agilent, Inc.) are used to recorder the Qswitched pulse.
Fig. 2. Absorption spectrum of Nd:GdTaO4 in the range of 860–900 nm.
total absorption efficiency of the Nd:GdTaO4 under 879 nm LD pumping is measured to be ~90%. We investigate the relationship of CW output power versus incident pump power with different output mirrors when the cavity length is 30 mm. Fig. 3 presents the dependence of output powers on pump power. The threshold pump power of 1066 nm laser with an output mirror of T = 10% is 0.83 W and it increases to 1.44 W with an output mirror of T = 30%. The maximum output power of 5.6 W is achieved at Pin = 12.8 W with the output coupler mirror of T = 20%, corresponding to an optical-to-optical efficiency of 43.8% and a slope efficiency of 48.0%. The efficiency of 879 nm LD pumped Nd:GdTaO4 laser is much higher than that under 808 nm pumping due to the lower quantum defect. Passive Q-switching operation is conducted when Cr4+:YAG crystal is inserted into the cavity with a cavity length of 60 mm. Fig. 4 shows
3. Results and discussions The absorption spectrum of Nd:GdTaO4 near 880 nm is measured by a fiber-coupled optical spectrometer analyzer (Ocean Optics HR4000, 790–910 nm) with a resolution of 0.1 nm. The spontaneous emission of LD working below threshold current has an emission bandwidth of 30 nm around 880 nm. The absorption spectrum is derived from the measured spontaneous emission spectrum with and without crystal in optical path. Fig. 2 shows the measured absorption spectrum of Nd:GdTaO4 crystal around 880 nm at room temperature. Four peaks in the absorption spectra of Nd:GdTaO4 are located at 872.7 nm, 879.2 nm, 887.8 nm, and 892.6 nm. The highest absorption peak of 879.2 nm is produced via 4I9/2 → 4F3/2 transition, corresponding to Strak levels value of 238 cm−1 and 11615 cm−1 [13]. The highest absorption coefficient is about 4.2 cm−1 with a FWHM of ~1.3 nm. The output spectrum width of wavelength-stabilized 879 nm LD is ~ 0.5 nm, which matches the absorption spectrum of Nd:GdTaO4 crystal well. The
Fig. 3. 1066 nm laser output power versus incident pump power with different output couplers. 2
Infrared Physics and Technology 102 (2019) 103025
Y. Liu, et al.
Fig. 6. The dependence of pulse width on incident pump power.
Fig. 4. The relationship between average output power and incident pump power in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser.
used, the output laser possesses higher average output power and repetition frequency, but the pulse width is longer compared with T0 = 80% Cr4+:YAG. The shortest pulse width of 15.2 ns is obtained at Pin = 7.0 W with T0 = 80% Cr4+:YAG, corresponding to a peak power of 2.34 kW at 33.7 kHz. Fig. 7 shows the temporal pulse train and pulse profile at 58.6 kHz by using a Cr4+:YAG crystal with T0 = 85%. The pulse energy stability at 58.6 kHz is calculated to be 94.6% while it becomes to be 80.7% at 185 kHz. The low stability is caused by the higher-order transverse mode and the serious thermal effect at high pump power [16]. It can be seen that the temporal single pulse profiles have good symmetry in growth and decay time. The beam quality factor in passively Q-switched Nd:GdTaO4/ Cr4+:YAG laser is also researched by using the traveling 90/10 knifeedge method [17]. Fig. 8 gives the beam radius variations versus location at the average output powers of Pav = 1.0 W and Pav = 3.6 W. The insertions of Fig. 8 are the beam spatial distribution of the Nd:GdTaO4/Cr4+:YAG 1066 nm laser measured by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.). By fitting the data to the standard Gaussian propagation expression, the beam quality factor at Pav = 1.0 W is calculated to be Mx2 = 1.2 and My2 = 1.1 with a good symmetry in both directions similar as the beam distribution. The beam quality factors degrade to Mx2 = 2.2 and My2 = 1.5 with asymmetry at Pav = 3.6 W, which is ascribed to the anisotropic thermal property of Nd:GdTaO4 crystal. The thermal conductivity of GdTaO4 single crystal along a, b, and c-axis is 7.3, 6.2, and 8.2 W/m K, and the thermal expansion coefficient along a, b, and c-axis is 6.17 × 10−6, 12.12 × 10−6, and 13.4 × 10−6 K−1 [5,18]. The output power saturation and beam quality degradation is related to thermal effects in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser [19,20]. The thermal focal length of the Nd:GdTaO4 crystal under 879 nm pumping is measured by steady cavity method [21]. The dependence of thermal focal length on incident pump power is illustrated in Fig. 9. As the pump power increases, the thermal focal length is shortened and the thermal effect becomes more serious. The thermal focal length of the Nd:GdTaO4 crystal is 130 mm at the incident pump power of 11.5 W, much shorter than that of Nd:YVO4 crystal [22,23]. In addition, the internal loss in Nd:GdTaO4 laser is calculated according to the theory of Findlay and Clay as following [24]:
the relationship between average output power and incident pump power in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser. The threshold pump power is 4.6 W using Cr4+:YAG with a lower initial transmission of T0 = 80% while it is 3.4 W with T0 = 85% SA. The average output power increases linearly when the incident pump power is lower than 14 W. While the average output power is hampered when the incident pump is higher than 14 W, which is maybe because of the thermal effect of Nd:GdTaO4. The maximum average output power of 3.6 W by employing the T0 = 85% SA is obtained at an incident pump power of 18 W, higher than 3.1 W with T0 = 80% SA. The stability of pulsed 1066 nm laser is measured and the power fluctuation is lower than 0.03 W. The maximum optical-to-optical efficiency is 20% and the slope efficiency is 24.6%. The variation of repetition frequency with incident pump power is illustrated in Fig. 5. The pulse repetition frequency of Nd:GdTaO4 laser increases with the incident pump power, which agrees with the theory of passive Q-switched lasers [14]. The highest repetition frequencies are 185 kHz and 132 kHz with the Cr4+:YAG of T0 = 85% and T0 = 80%. The maximum single pulse energy of 39.7 μJ is achieved with T0 = 80% Cr4+:YAG while it is 30.8 μJ with T0 = 85%. The pulse repetition rate of 185 kHz is close to the upper limit achievable for Cr4+:YAG saturable absorber, which has an upper state lifetime of ~ 4 µs. The variation of pulse width versus incident pump power is given in Fig. 6. The pulse width varies slightly with incident pump power, which is attributed to the unchanged modulation depth throughout the whole pump power range [15]. When Cr4+:YAG crystal with T0 = 85% is
Pt = Pt0 ⎡1 + ⎣
ln(1/ R) ⎤ 2δL ⎦
(1)
where Pt is the threshold pump energy, Pt0 is the threshold energy for zero output coupling, R is the reflectivity of output mirror, L is the length of crystal, δ is the total internal losses coefficient. We measured the threshold pump power of a CW Nd:GdTaO4 laser
Fig. 5. The dependence of pulse repetition rate on incident pump power. 3
Infrared Physics and Technology 102 (2019) 103025
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Fig. 7. The pulse train and the temporal pulse profile at 58.6 kHz.
for output mirror with different R, as shown in Fig. 10. By fitting these data with Eq. (1) we can get the internal loss δ = 0.078 cm−1. We notice that the total internal loss for Nd:GdTaO4 is higher compared with the most-used Nd:YAG with an internal loss of 0.01 cm−1 [19]. According to the passively Q-switched laser theory, high internal losses may increase the pulse repetition rate in some extent [25]. However, the high internal loss also reduces the laser output power and efficiency. If the internal loss of Nd:GdTaO4 can reduced to a normal level, the laser performance of passively Q-switched Nd:GdTaO4 laser would be improved significantly. 4. Conclusion In conclusion, we have investigated a high-repetition-rate passively Q-switched Nd:GdTaO4/Cr4+:YAG laser under 879 nm pumping. The absorption spectrum of Nd:GdTaO4 is measured and the highest absorption coefficient is ~4.2 cm−1 near 879 nm. The maximum output power of 5.6 W is obtained in CW 1066 nm laser with an optical-tooptical efficiency of 43.8% and a slope efficiency of 48.0%. Using the Cr4+:YAG with T0 = 85%, the average power of Q-switched 1066 nm laser reaches 3.6 W at 185 kHz with a pulse width of 27.5 ns, corresponding to an optical-to-optical efficiency of 20% and a slope efficiency of 24.6%. The pulse repetition rate of 185 kHz is close to the maximum obtainable repetition rate for Cr4+:YAG Q-switched lasers, which depends on the recovery time of SA. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors are measured to be Mx2 = 2.2, My2 = 1.5 at Pav = 3.6 W by using travelling knife-edge method. The thermal focal length is measured to be 130 mm at Pin = 11.5 W by the method of steady resonant cavity. The internal loss of Nd:GdTaO4 is calculated to be δ = 0.078 cm−1, which is much higher than Nd:YAG. If the internal
Fig. 9. Thermal focal length versus incident pump power.
loss of Nd:GdTaO4 crystal can be ameliorated, the laser performance of passively 1066 nm laser would be improved significantly. Declaration of Competing Interest The authors declare that they have no competing financial interests for this paper. Acknowledgements This work was supported by the National Natural Science
Fig. 8. Beam radius variation and laser intensity distributions of passively Q-switched NdGdTaO4 laser at the average output power of 1.0 W (a) and 3.6 W (b). 4
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Fig. 10. The relationship between threshold pump power and ln(1/R).
Foundation of China (NSFC) (61605032 and 61505042), Shenzhen Science and Technology Program (JSGG20170414141239041), Natural Science Foundation of Jiangsu Province (BK20180220), Key Project of Jiangsu Province (grant numbers BE2016090 and BE2016005-2), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (grant number IOSKL2016KF12), General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2015 M80263). References [1] S. Damodaraiah, V.R. Prasad, Y.C. Ratnakaram, Investigations on spectroscopic properties of Nd3+ doped alkali bismuth phosphate glasses for 1.053 um laser applications, Opt. Laser Technol. 113 (2019) 322–329. [2] H. Yu, J. Liu, H. Zhang, A.A. Kaminskii, Z. Wang, J. Wang, Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer, Laser Photon. Rev. 8 (6) (2014) 847–864. [3] G. Neelima, K. Venkata Krishnaiah, N. Ravi, K. Suresh, K. Tyagarajan, T. Jayachandra Prasad, Investigation of optical and spectroscopic properties of neodymium doped oxyfluoro-titania-phosphate glasses for laser applications, Scripta Mater. 162 (2019) 246–250. [4] F. Peng, W. Liu, Q. Zhang, H. Yang, C. Shi, R. Mao, D. Sun, J. Luo, G. Sun, Crystal growth, optical and scintillation properties of Nd3+ doped GdTaO4 single crystal, J. Cryst. Growth. 406 (2014) 31–35. [5] F. Peng, H. Yang, Q. Zhang, J. Luo, W. Liu, D. Sun, R. Dou, G. Sun, Spectroscopic properties and laser performance at 1,066 nm of a new laser crystal Nd:GdTaO4,
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Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
High-repetition-rate passively Q-switched Nd:GdTaO4 1066 nm laser under 879 nm pumping
T
⁎
Yang Liua, Renpeng Yana, , Wentao Wua, Xudong Lia, Zhiwei Donga, Zhixiang Liub, Xiaolin Wenb, ⁎ Wenming Yaoc, Fang Pengd, Qingli Zhangd, Renqin Doud, Jing Gaoc,e, a
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China Shenzhen Aerospace Industry Technology Research Institute, Shenzhen 518000, China c Jiangsu Key Laboratory of Medical Optics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou, Jiangsu 215163, China d The Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China e Suzhou Guoke Medical Science & Technology Development Co. Ltd, Suzhou 215163, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: LD pumped Passively Q-switched Nd:GdTaO4
We report an 879 nm laser diode directly pumped passively Q-switched Nd:GdTaO4 laser at 1066 nm. The continuous-wave output power at 1066 nm reaches 5.6 W under an incident pump power of 12.8 W, corresponding to an optical-to-optical efficiency of 43.8% and a slope efficiency of 48.0%. By using Cr4+:YAG as the saturable absorber, passively Q-switched laser operation is conducted, yielding a highest pulse repetition frequency of 185 kHz and a maximum average power of 3.6 W. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors at the maximum average output power is measured to be Mx2 = 2.2, My2 = 1.5 by using knife-edge method. The thermal focal length and internal loss for Nd:GdTaO4 crystal is also investigated.
1. Introduction
maximum output power of 2.5 W and an optical-to-optical efficiency of 34.6% [5]. In 2015, Zhang et al. showed an 808 nm LD pumped passively Q-switched Nd:GdTaO4 laser using graphene oxide as saturable absorber (SA), and obtained a pulse laser output with a maximum average output power of 0.382 W and a pulse width of 194 ns at 362 kHz [6]. Wang et al. presented a passively Q-switched mode locking Nd:GdTaO4 1066 nm laser with a maximum average output power of 0.156 W [7]. Compared with indirect 808 nm pumping, directly pumping has higher theoretical quantum efficiency and reduce thermal effect [8,9]. Cr4+:YAG has advantages of high damage threshold, good chemical stability, and excellent mechanical and optical properties and is widely used as a saturable absorber to shorten pulse width and boost peak power [10–12]. In this paper, we present an 879 nm LD pumped Nd:GdTaO4 laser in CW and passively Q-switched operation. The absorption spectrum of Nd:GdTaO4 near 879 nm is investigated and the highest absorption coefficient is about 4.2 cm−1 with a FWHM of 1.3 nm. When the incident pump power is 12.8 W, the maximum CW output power of 5.6 W at 1066 nm is achieved with an optical conversion efficiency of 48.0%. By using Cr4+:YAG as saturable absorber, The maximum average power
Diode-pumped Nd3+-doped lasers with advantages of compactness, high efficiency, and low cost are widely used in scientific research, industry, communication and so on. In recent years, Nd3+-doped crystals with different host materials have aroused great research interest for high efficiency and high power laser output [1–3]. Nd:GdTaO4 is a new laser medium with high optical quality and low symmetry. The low symmetry is beneficial to luminescence efficiency and realize polarized output. Nd:GdTaO4 has a relatively broad absorption band and large stimulated emission cross-section, which is suitable for laser diode (LD) pumped solid-state lasers. The full width at half maximum (FWHM) of the absorption band around 808 nm is 6 nm for Nd:GdTaO4. The broad absorption band can reduce the requirement of pump light in terms of bandwidth. The stimulated emission crosssection at 1066 nm is 3.9 × 10−19 cm2 and the fluorescence lifetime of 4 F3/2 level is 178.4 μs. Nd:GdTaO4 crystal was firstly grown by Peng et al. through Czochralski (Cz) method in 2014 [4]. In 2015, Peng et al. demonstrated the spectroscopic properties of Nd:GdTaO4 and a continuous-wave (CW) 808 nm LD pumped Nd:GdTaO4 laser with a
⁎
Corresponding authors at: National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China (R. Yan). Jiangsu Key Laboratory of Medical Optics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou, Jiangsu 215163, China (J. Gao). E-mail addresses: [email protected] (R. Yan), [email protected] (J. Gao). https://doi.org/10.1016/j.infrared.2019.103025 Received 16 July 2019; Received in revised form 29 August 2019; Accepted 29 August 2019 Available online 30 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103025
Y. Liu, et al.
Fig. 1. Experimental setup of LD end-pumped passively Q-switched Nd:GdTaO4 1066 nm laser.
of 3.6 W is obtained with an optical-to-optical efficiency of 20% and a slope efficiency of 24.6%. The highest repetition frequency of pulse is 185 kHz and the corresponding pulse width is 27.5 ns. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors at the average output power of 3.6 W are measured to be Mx2 = 2.2, My2 = 1.5 by using traveling knife-edge method. The thermal effect of the Nd:GdTaO4 crystal is investigated while its internal losses is calculated to be δ = 0.078 cm−1. 2. Experimental setup Fig. 1 shows the experimental setup of LD end-pumped passively Qswitched Nd:GdTaO4/Cr4+:YAG 1066 nm laser. A wavelength-stabilized fiber-coupled LD (P4-100-0878.6, nLIGHT Inc.) at 879 nm with beam quality factor of 81.9 is employed as pumping source. The fiber has a diameter of 400 μm and a numerical aperture of 0.22. Pumping light is coupled to laser medium through a pair of lenses L1 and L2 with a focal length of 26.7 mm. Nd:GdTaO4 crystal with a Nd3+ doping concentration of 2 at.% and a dimension of 2 × 2 × 5 mm3 serves as the gain medium. The Rayleigh length of pump beam is calculated to be 11 mm, longer than the crystal length. The end surfaces of Nd:GdTaO4 are coated with high transmission (HT) at 879 nm and antireflection (AR) at 1066 nm. The laser crystal is wrapped with indium foil and placed in the water-cooled copper heat sink with a small amount of thermal grease in the contact surfaces to improve the capability of thermal dissipation. The temperature of water cooling is maintained at 18 °C. The laser cavity composes of two planes mirrors M1 and M2 with a geometric length of 60 mm. The mirror M1 is HT coated at 879 nm and highly reflective coating at 1066 nm. The mirror M2 is an output coupler mirror with partial transmissions of 10%, 20% and 30% at 1066 nm. Cr4+:YAG crystal is placed near the M2 mirror as a saturable absorber. Both surfaces of Cr4+:YAG is AR coated at 1066 nm. A pinhole is inserted in the cavity to reduce the influence of pump light on Cr4+:YAG. A fast photodiode (DET025AL, Thorlabs, Inc.) and a digital oscilloscope (DSO-X3034A, Agilent, Inc.) are used to recorder the Qswitched pulse.
Fig. 2. Absorption spectrum of Nd:GdTaO4 in the range of 860–900 nm.
total absorption efficiency of the Nd:GdTaO4 under 879 nm LD pumping is measured to be ~90%. We investigate the relationship of CW output power versus incident pump power with different output mirrors when the cavity length is 30 mm. Fig. 3 presents the dependence of output powers on pump power. The threshold pump power of 1066 nm laser with an output mirror of T = 10% is 0.83 W and it increases to 1.44 W with an output mirror of T = 30%. The maximum output power of 5.6 W is achieved at Pin = 12.8 W with the output coupler mirror of T = 20%, corresponding to an optical-to-optical efficiency of 43.8% and a slope efficiency of 48.0%. The efficiency of 879 nm LD pumped Nd:GdTaO4 laser is much higher than that under 808 nm pumping due to the lower quantum defect. Passive Q-switching operation is conducted when Cr4+:YAG crystal is inserted into the cavity with a cavity length of 60 mm. Fig. 4 shows
3. Results and discussions The absorption spectrum of Nd:GdTaO4 near 880 nm is measured by a fiber-coupled optical spectrometer analyzer (Ocean Optics HR4000, 790–910 nm) with a resolution of 0.1 nm. The spontaneous emission of LD working below threshold current has an emission bandwidth of 30 nm around 880 nm. The absorption spectrum is derived from the measured spontaneous emission spectrum with and without crystal in optical path. Fig. 2 shows the measured absorption spectrum of Nd:GdTaO4 crystal around 880 nm at room temperature. Four peaks in the absorption spectra of Nd:GdTaO4 are located at 872.7 nm, 879.2 nm, 887.8 nm, and 892.6 nm. The highest absorption peak of 879.2 nm is produced via 4I9/2 → 4F3/2 transition, corresponding to Strak levels value of 238 cm−1 and 11615 cm−1 [13]. The highest absorption coefficient is about 4.2 cm−1 with a FWHM of ~1.3 nm. The output spectrum width of wavelength-stabilized 879 nm LD is ~ 0.5 nm, which matches the absorption spectrum of Nd:GdTaO4 crystal well. The
Fig. 3. 1066 nm laser output power versus incident pump power with different output couplers. 2
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Fig. 6. The dependence of pulse width on incident pump power.
Fig. 4. The relationship between average output power and incident pump power in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser.
used, the output laser possesses higher average output power and repetition frequency, but the pulse width is longer compared with T0 = 80% Cr4+:YAG. The shortest pulse width of 15.2 ns is obtained at Pin = 7.0 W with T0 = 80% Cr4+:YAG, corresponding to a peak power of 2.34 kW at 33.7 kHz. Fig. 7 shows the temporal pulse train and pulse profile at 58.6 kHz by using a Cr4+:YAG crystal with T0 = 85%. The pulse energy stability at 58.6 kHz is calculated to be 94.6% while it becomes to be 80.7% at 185 kHz. The low stability is caused by the higher-order transverse mode and the serious thermal effect at high pump power [16]. It can be seen that the temporal single pulse profiles have good symmetry in growth and decay time. The beam quality factor in passively Q-switched Nd:GdTaO4/ Cr4+:YAG laser is also researched by using the traveling 90/10 knifeedge method [17]. Fig. 8 gives the beam radius variations versus location at the average output powers of Pav = 1.0 W and Pav = 3.6 W. The insertions of Fig. 8 are the beam spatial distribution of the Nd:GdTaO4/Cr4+:YAG 1066 nm laser measured by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.). By fitting the data to the standard Gaussian propagation expression, the beam quality factor at Pav = 1.0 W is calculated to be Mx2 = 1.2 and My2 = 1.1 with a good symmetry in both directions similar as the beam distribution. The beam quality factors degrade to Mx2 = 2.2 and My2 = 1.5 with asymmetry at Pav = 3.6 W, which is ascribed to the anisotropic thermal property of Nd:GdTaO4 crystal. The thermal conductivity of GdTaO4 single crystal along a, b, and c-axis is 7.3, 6.2, and 8.2 W/m K, and the thermal expansion coefficient along a, b, and c-axis is 6.17 × 10−6, 12.12 × 10−6, and 13.4 × 10−6 K−1 [5,18]. The output power saturation and beam quality degradation is related to thermal effects in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser [19,20]. The thermal focal length of the Nd:GdTaO4 crystal under 879 nm pumping is measured by steady cavity method [21]. The dependence of thermal focal length on incident pump power is illustrated in Fig. 9. As the pump power increases, the thermal focal length is shortened and the thermal effect becomes more serious. The thermal focal length of the Nd:GdTaO4 crystal is 130 mm at the incident pump power of 11.5 W, much shorter than that of Nd:YVO4 crystal [22,23]. In addition, the internal loss in Nd:GdTaO4 laser is calculated according to the theory of Findlay and Clay as following [24]:
the relationship between average output power and incident pump power in passively Q-switched Nd:GdTaO4/Cr4+:YAG 1066 nm laser. The threshold pump power is 4.6 W using Cr4+:YAG with a lower initial transmission of T0 = 80% while it is 3.4 W with T0 = 85% SA. The average output power increases linearly when the incident pump power is lower than 14 W. While the average output power is hampered when the incident pump is higher than 14 W, which is maybe because of the thermal effect of Nd:GdTaO4. The maximum average output power of 3.6 W by employing the T0 = 85% SA is obtained at an incident pump power of 18 W, higher than 3.1 W with T0 = 80% SA. The stability of pulsed 1066 nm laser is measured and the power fluctuation is lower than 0.03 W. The maximum optical-to-optical efficiency is 20% and the slope efficiency is 24.6%. The variation of repetition frequency with incident pump power is illustrated in Fig. 5. The pulse repetition frequency of Nd:GdTaO4 laser increases with the incident pump power, which agrees with the theory of passive Q-switched lasers [14]. The highest repetition frequencies are 185 kHz and 132 kHz with the Cr4+:YAG of T0 = 85% and T0 = 80%. The maximum single pulse energy of 39.7 μJ is achieved with T0 = 80% Cr4+:YAG while it is 30.8 μJ with T0 = 85%. The pulse repetition rate of 185 kHz is close to the upper limit achievable for Cr4+:YAG saturable absorber, which has an upper state lifetime of ~ 4 µs. The variation of pulse width versus incident pump power is given in Fig. 6. The pulse width varies slightly with incident pump power, which is attributed to the unchanged modulation depth throughout the whole pump power range [15]. When Cr4+:YAG crystal with T0 = 85% is
Pt = Pt0 ⎡1 + ⎣
ln(1/ R) ⎤ 2δL ⎦
(1)
where Pt is the threshold pump energy, Pt0 is the threshold energy for zero output coupling, R is the reflectivity of output mirror, L is the length of crystal, δ is the total internal losses coefficient. We measured the threshold pump power of a CW Nd:GdTaO4 laser
Fig. 5. The dependence of pulse repetition rate on incident pump power. 3
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Fig. 7. The pulse train and the temporal pulse profile at 58.6 kHz.
for output mirror with different R, as shown in Fig. 10. By fitting these data with Eq. (1) we can get the internal loss δ = 0.078 cm−1. We notice that the total internal loss for Nd:GdTaO4 is higher compared with the most-used Nd:YAG with an internal loss of 0.01 cm−1 [19]. According to the passively Q-switched laser theory, high internal losses may increase the pulse repetition rate in some extent [25]. However, the high internal loss also reduces the laser output power and efficiency. If the internal loss of Nd:GdTaO4 can reduced to a normal level, the laser performance of passively Q-switched Nd:GdTaO4 laser would be improved significantly. 4. Conclusion In conclusion, we have investigated a high-repetition-rate passively Q-switched Nd:GdTaO4/Cr4+:YAG laser under 879 nm pumping. The absorption spectrum of Nd:GdTaO4 is measured and the highest absorption coefficient is ~4.2 cm−1 near 879 nm. The maximum output power of 5.6 W is obtained in CW 1066 nm laser with an optical-tooptical efficiency of 43.8% and a slope efficiency of 48.0%. Using the Cr4+:YAG with T0 = 85%, the average power of Q-switched 1066 nm laser reaches 3.6 W at 185 kHz with a pulse width of 27.5 ns, corresponding to an optical-to-optical efficiency of 20% and a slope efficiency of 24.6%. The pulse repetition rate of 185 kHz is close to the maximum obtainable repetition rate for Cr4+:YAG Q-switched lasers, which depends on the recovery time of SA. The shortest pulse width of 15.2 ns is achieved with a peak power of 2.34 kW at 33.7 kHz. The beam quality factors are measured to be Mx2 = 2.2, My2 = 1.5 at Pav = 3.6 W by using travelling knife-edge method. The thermal focal length is measured to be 130 mm at Pin = 11.5 W by the method of steady resonant cavity. The internal loss of Nd:GdTaO4 is calculated to be δ = 0.078 cm−1, which is much higher than Nd:YAG. If the internal
Fig. 9. Thermal focal length versus incident pump power.
loss of Nd:GdTaO4 crystal can be ameliorated, the laser performance of passively 1066 nm laser would be improved significantly. Declaration of Competing Interest The authors declare that they have no competing financial interests for this paper. Acknowledgements This work was supported by the National Natural Science
Fig. 8. Beam radius variation and laser intensity distributions of passively Q-switched NdGdTaO4 laser at the average output power of 1.0 W (a) and 3.6 W (b). 4
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Fig. 10. The relationship between threshold pump power and ln(1/R).
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