Experimental and theoretical study of a passively Q ... - OSA Publishing

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Vol. 32, No. 5 / May 2015 / Journal of the Optical Society of America B

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Experimental and theoretical study of a passively Q-switched Nd:LuAG laser at 1.3 μm with a V3+:YAG saturable absorber CHENG LIU,1 SHENGZHI ZHAO,1 GUIQIU LI,1 KEJIAN YANG,1,2,* DECHUN LI,1 TAO LI,1 WENCHAO QIAO,1 TIANLI FENG,1 XINTIAN CHEN,1 XIAODONG XU,3 LIHE ZHENG,3 AND JUN XU3 1

School of Information Science and Engineering, Shandong University, 27 Shanda South Road, Jinan 250100, China State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, China 3 Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Shanghai 201800, China *Corresponding author: [email protected] 2

Received 10 December 2014; revised 20 March 2015; accepted 22 March 2015; posted 3 April 2015 (Doc. ID 229202); published 1 May 2015

We report the characteristics of a diode-pumped continuous-wave (CW) and passively Q-switched Nd:LuAG laser emitting 1.3 μm wavelength. A maximum average output power of 954 mW was obtained under CW operation, giving a slope efficiency of 14.7%. With an V3 :YAG crystal wafer employed as saturable absorber, the passively Q-switched Nd:LuAG laser produced a minimum pulse duration of 17 ns under a repetition rate of 8 kHz, and a maximum single pulse energy of 18.9 μJ. A rate equation model is introduced to theoretically analyze the results obtained in the experiment, in which the Gaussian spatial distribution of the intracavity photon density and the longitudinal distribution of the photon density along the cavity axis are taken into account. The results of numerical calculations of the rate equations are consistent with the experimental results. The results indicated the Nd:LuAG crystal as a promising gain medium for achieving short pulses with high energy at 1.3 μm. © 2015 Optical Society of America OCIS codes: (140.3460) Lasers; (140.3540) Lasers, Q-switched; (140.3480) Lasers, diode-pumped. http://dx.doi.org/10.1364/JOSAB.32.001001

1. INTRODUCTION Diode-pumped passively Q-switched solid-state lasers have been widely used in the fields of industrial material processing, scientific research, and military applications due to the advantages of simplicity, compactness, low cost, and high efficiency. The lasers operating at 1.3 μm, especially those with pulsed sources, have wide applications in many fields, such as medicine, fiber optics, and spectroscopy. Lutetium aluminum garnet (Lu3 Al5 O12 , LuAG) is an isostructure of YAG. Like YAG it can be grown as high-quality single crystals, and has high thermal conductivity, and excellent physical and chemical properties [1–3]. It also has a wider bandgap and smaller lattice compared with YAG. Due to the above advantages, LuAG crystal has become an ideal laser host for rare earth (RE) ion doping, such as Nd:LuAG [4], Tm:LuAG [5], Yb:LuAG [6,7], Ho:LuAG [8], Tm:Ho:LuAG [9], and Er:LuAG [10]. For Nd:LuAG, Xu et al. have reported a CW output power of 3.8 W at 1064 nm [11], and a pulse energy of 50.6 μJ was obtained by Di et al. with a Cr4 :YAG [12]. In the spectral region of 1.3 μm, the emission cross section of about 5 × 10−20 cm2 in Nd:LuAG [13] was found to be lower than 7 × 10−20 cm2 of Nd:YAG, 0740-3224/15/051001-06$15/0$15.00 © 2015 Optical Society of America

2.8 × 10−19 cm2 of Nd:YVO4 , and 1.8 × 10−19 cm2 of Nd:GdVO4 [14], which determines a larger oscillation threshold for Nd:LuAG crystal working at 1.3 μm. However, the long upper-level lifetime of 277 μs in Nd:LuAG [11], which is larger than those of other Nd-doped crystals, such as 230 μs in Nd: YAG [15], 98 μs in Nd:YVO4 [16], 100 μs in Nd:GdVO4 [14], and 250 μs in Nd:LGGG [17], would help to generate high-energy and short Q-switched pulses. Unfortunately, to date, an Nd:LuAG laser around 1.3 μm has not been reported, to our best knowledge. For passively Q-switched lasers at 1.3 μm, a variety of saturable absorbers have been investigated, such as semiconductor saturable absorber mirrors (SESAMs) [18], V 3 :YAG [19], Co2 -doped crystals [20], and graphene [21]. However, graphene and Co2 -doped crystals usually generate wider pulses than the others, while SESAMs have lower damage thresholds and need complicated and expensive fabrication, which limit its application. V 3 :YAG was the most commonly used saturable absorbers at 1.3 μm, due to the virtues of a broad absorption band ranging from 0.75 to 1.44 μm [22], high damage threshold, fast relaxation process, and low saturable energy and

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residual absorption [23]. In this paper, for the first time as far as we know, we demonstrate the characteristics of diode-pumped CW and passively Q-switched Nd:LuAG lasers at 1338 nm. When the Nd:LuAG laser ran in the CW regime, a maximum average output power of up to 954 mW was obtained under incident pump power of 7.69 W, corresponding to a slope efficiency of 14.7%. By using a V 3 :YAG crystal wafer with small signal transition of 96% as a saturable absorber, a passively Q-switched Nd:LuAG laser was realized with a maximum output power of 133 mW. The minimum pulse duration was 17 ns under a repetition rate of 8 kHz, corresponding to a single-pulse energy of ∼16.6 μJ. 2. EXPERIMENTAL DETAILS The experimental setup of the diode-end-pumped Nd:LuAG laser in a 5 cm long cavity is schematically shown in Fig. 1. The 3 mm × 3 mm × 10 mm Nd:LuAG laser crystal was grown by the Czochralski technique at Shanghai Institute of Ceramics in China, doped with 1.0 at. % Nd3 ions. The laser crystal was wrapped in indium foil and mounted in a copper block cooled by water to 18°C. The pump source was a fibercoupled laser diode (Coherent, FAP system) with a maximum output power of 30 W at 808 nm. The pump light was focused into the laser crystal through a 1∶1 imaging module with a pump spot diameter of 400 μm. M1 was a concave mirror with a curvature of 200 mm, which was antireflection (AR) coated at 808 nm, and high-reflection (HR) coated at 1.3 μm. Output couplers (OCs) with different transmissions of 5%, 10%, and 15% at 1.3 μm were employed in our experiment. A laser powermeter (MAX 500AD, Coherent) was used to measure the average output power. The output spectra were recorded by a laser spectrometer that had a resolution bandwidth of 0.4 nm (APE WaveScan, APE Inc.). 3. EXPERIMENTAL RESULTS First, CW running performance of the diode-pumped Nd: LuAG laser was investigated. Figure 2 shows the average output powers versus the incident pump powers for different OCs. The threshold incident pump powers were 0.9 W, 1.9 W, and 2.8 W for OCs of T 5%, 10%, and 15%, respectively. By using the OC of T 5%, a maximum CW output power of 954 mW was obtained, giving a slope efficiency of 14.7%, and no power saturation was observed within the experimental pump power range. With OCs of T 10% and 15%, maximum CW output powers of 540 and 662 mW were achieved, respectively, corresponding to slope efficiencies of 10.9% and 8.25%. Meanwhile, the intrinsic loss and quantum efficiency of the Nd:LuAG crystal could be calculated according to the formula for a four-level laser system [15]:

Fig. 1. Schematic setup of the diode-end-pumped Nd:LuAG laser.

Fig. 2. Average output powers under different OCs from CW Nd:LuAG laser.

P in;th

δ − I nR AhνL ; ηt ηa ηq ηs ηb στ 2

where P in;th is the threshold incident pump power for CW operation, δ is the intrinsic loss, R is the reflectivity of the output coupler, hνL is the emitted photon energy, A is the laser mode area, ηt is pump light transfer efficiency, which is more than 90% in our system, ηa 1 − exp−α l is absorption efficiency, α is the absorption coefficient, l is the gain medium length, ηq is the quantum efficiency, ηs λp∕λs is quantum defect efficiency, λp is the pump wavelength, λs is the lasing wavelength, ηb is beam overlap efficiency, which reaches about 1 due to the nearly equal radii of pump mode and lasing mode in our experiment, σ is the stimulated-emission cross section, and τ is the upper-level lifetime. With the measured threshold incident pump powers under different output coupling rates, the calculated intrinsic loss and quantum efficiency were about 0.01 and 0.7, respectively. The intrinsic loss was found to be smaller than 0.03 for Nd:YAG crystal [24], 0.021 for Nd:LGGG [17], 0.024 for Nd:GAGG [25], and 0.012 for Nd:GdLuYVO4 [26] when working at 1.3 μm. For clarity, the intrinsic loss, emission cross section, lifetime, and quantum yield of the Nd:LuAG crystal in this work and those of the other Nd-doped crystals mentioned above at 1.3 μm are summarized in Table 1. The gain in the cavity was low due to the lower emission cross section at 1.3 μm compared with Nd:LuAG crystal emitting at 1.06 μm; the loss in the cavity would be lower with the smaller transmission of OCs, so that the output power with the OC of T 5% was higher than the OC of T 15%. In a free-running regime, the emitted output wavelength was found to locate at 1336.6 nm; however, dual-wavelength operation simultaneously at 1334.7 and 1338.7 nm could also be achieved by aligning the OC, as shown in Fig. 2. According to the results, a wavelength-switchable Nd:LuAG laser could be easily obtained with a suitable wavelength selector. In order to get good laser power performance, the OC of T 5% was chosen for the passively Q-switched Nd:LuAG laser. After inserting the V 3 :YAG wafer (AR coated at ∼1.3 μm with a small signal transmission of T 0 96%) into


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Table 1. Intrinsic Loss, Emission Cross Section, Lifetime, and Quantum Yield of Nd:LuAG and the Other Nd-Doped Crystals at 1.3 μm Crystals Nd:LuAG Nd:YAG Nd:GdLuYVO4 Nd:LGGG Nd:GAGG a

Intrinsic Loss

Emission Cross Section (×10−19 cm2 )

Upper-Level Lifetime (μs)

Quantum Yield

Ref.

0.01 0.03 0.012 0.021 0.024

0.5 0.7 0.47 0.37 0.274

277 230 96 250 228

0.7 – 1a 1a 1a

This work [24] [26] [17] [25]

These values were assumed by the authors to calculate the intrinsic loss in the corresponding references.

the laser cavity (seen in Fig. 1), stable passive I-switching operation was achieved when the incident pump power reached 3.4 W by aligning the cavity carefully. The pump power threshold of 3.4 W was much higher than that of CW operation, mainly induced by the additional insertion loss of V 3 :YAG. The average output power characteristics are summarized in Fig. 3, from which we can see that a maximum average output power of 133 mW was obtained at laser diode temperature of 22°C, giving a slope efficiency of 3.0%. At the maximum output power of 133 mW, the output power was recorded for 1 h, and a power instability of 0.416% was demonstrated, as shown in the upper inset of Fig. 3. Additionally, the dependence of 1.3 μm laser output power stability on the temperature and emission wavelength of the laser diode was measured. As shown in the lower inset of Fig. 3, the laser output power reached maximum when the laser diode temperature was set as 22°C, where the emission wavelength of the laser diode was about 808 nm, locating in the peak absorption band of the Nd:LuAG crystal centered at 809 nm with a FWHM of 5 nm [11]. When the cooling temperature for the laser diode was decreased below 22°C, the emission wavelength of the laser diode showed an obvious blueshift, and moved out of the absorption band of Nd:LuAG, which led to a serious degradation of laser output power. The temporal pulse characteristics of repetition rates and pulse durations (FWHM) were recorded by a fast PIN photodiode with a rise time of 400 ps and a digital oscilloscope

Fig. 3. Average output power of passively Q-switched Nd:LuAG laser. Upper inset: the output power versus time. Lower inset: the output power and laser diode wavelength versus laser diode temperature.

Fig. 4. (a) Pulse duration, (b) repetition rate, and (c) single pulse energy of passively Q-switched Nd:LuAG laser versus incident pump power.

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(1 GHz bandwidth, Tektronix DPO 7102), as scattered dots shown in Figs. 4(a) and 4(b). The repetition rates of the passively Q-switched Nd:LuAG laser increased almost linearly in a range from about 3 to 8 kHz with the increase of the incident pump power, and a maximum repetition rate of 8 kHz was obtained under incident pump power of 7.69 W. The pulse widths decreased versus the incident pump power, but tended to be invariable when the incident pump power was high, because the initial inversion ratio would be extremely high when the incident pump power was high, leading to a pulse width close to the limit equal to the cavity lifetime t c , based on the following equation: n t c nnti 1 − nfi t p ni ni ; nt − 1 − I n nt where ni is the initial population inversion density, nt is the threshold population inversion density, nf is the final populan tion inversion density, nnti is the initial inversion ratio, and 1 − nfi is the energy extraction efficiency depending on the initial inversion ratio [27]. On the other hand, according to the initial inversion density ni InR1 I nT12 L∕2σl , where T 0 0

is the small signal transmission, σ is laser stimulated emission cross section, l is the length of gain, and Ref. [28], the narrower pulse width is expected with a lower small signal transmission of V 3 :YAG. The single-pulse energy was calculated and is shown as scattered dots in Fig. 4(c). The fast increase of pulse

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Fig. 6. Simulated pulse profile (dotted line) and the oscilloscope pulse (solid line) at pump power of 7.69 W.

repetition rates does not affect the augment of single-pulse energy, because the repetition rates were determined only by the time required to bleach the V 3 :YAG saturable absorber, i.e., the time for energy accumulation. A maximum single pule energy of 18.9 μJ was obtained under incident pump power of 6.5 W. However, when the incident pump power was increased beyond 6.5 W, the single-pulse energy showed a saturation effect, which was induced mainly by the thermal effects due to the large quantum defects in Nd-doped crystals around 1.3 μm. At incident pump power of 7.69 W, minimum pulse duration of 17 ns was achieved under a repetition rate of 8 kHz, corresponding to a single-pulse energy 16.6 μJ. The achieved pulse width was shorter than the 20 ns of Nd:YAG [22], 21.7 ns of Nd:GdVO4 [29], and 54 ns of Nd:YVO4 [30] lasers working at 1.3 μm. The recorded pulse train and the single-pulse profile with duration of 17 ns under a repetition rate of 8 kHz are shown in Fig. 5(a) and by the solid line in Fig. 6, respectively. We also measured the pulse-to-pulse amplitude instability, and an amplitude instability of 4.25% was observed in 60 s; however, it increased to 8.87% in 1 h, as shown in the upper left and the upper right parts in Fig. 5(a), respectively. The thermal effects in the gain medium and the saturable absorber were considered as the source of the pulse-to-pulse amplitude instability. 4. THEORETICAL ANALYSIS To better understand the phenomenon, we used the coupled rate equations considering the Gaussian transversal and longitudinal distributions of the intracavity photon density to simulate the characteristics of the emitted pulses. The coupled rate equations modeling a passively Q-switched laser could be written as follows [31]: Z ∞ Z ∞ d ϕr; t 1 2σnr; tl ϕg r; t 2πrdr dt 0 0 tr −2σ g ns1 r; tl s ϕs r; t

Fig. 5. (a) Pulse train and pulse to pulse amplitude instability recorded by oscilloscope. Upper left: the pulse to pulse amplitude instability in 60 s. Upper right: in 1 h. (b) Simulated pulse train.

−2σ e ns0 − ns1 r; tl s ϕs r; t 1 ϕ r; t − Lϕr; t 2πrdr; −In R s

(1)


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d nr; t 2r 2 nr; t R in exp − 2 − σcnr; tϕg r; t − ; ωp dt τ (2) d ns1 r; t ns0 − ns1 r; t − σ g ns1 r; tcϕs r; t; dt τs

(3)

where nr; t is the average population inversion density, ns1 r; t and ns0 are the ground-state and total population densities of the V 3 :YAG saturable absorber, respectively, L is the intracavity roundtrip loss, l is the length of the Nd:LuAG gain medium, and σ g and σ e are the ground-state and excited-state absorption cross sections of the saturable absorber, respectively. l s is the length of the saturable absorber, t r 2n1 l 2n2 l s 2Lp − l − l s ∕c is the round-trip time of light in the resonator, and n1 and n2 are the refractive indices of Nd:LuAG gain medium and V 3 :YAG saturable absorber, respectively. Lp is the cavity length, c is the velocity of light in vacuum, τs is the excited-state lifetime of the saturable absorber, R in P in 1 − exp−αl ∕hvp πω2p l is the volume pumping rate, where P in is the incident pump power, α is the absorption coefficient of the Nd:LuAG crystal, hvp is the photon energy of the pump beam, and ωp is the average radius of the pump light in the gain medium. ϕr; t ϕ0; t exp−2r 2 ∕ω2l is the average intracavity photon density, where ϕ0; t is the photon density in the laser axis., ϕg r; t and ϕs r; t are the photon densities at the positions of the Nd:LuAG crystal and the V 3 :YAG saturable absorber, and ϕi r; t ω2l ∕ω2i ϕ 0; t exp−2r 2 ∕ω2i i g; s. ωg , ωs , and ωl are the radii of the TEM00 mode at the above positions and the average radius of the TEM00 mode in the cavity, respectively, which can be calculated by ABCD matrix theory, and the calculated values are shown in Table 2. According to the corresponding parameters shown in Table 2 by numerically solving Eqs. (1)–(3) on a computer, we can obtain the relationship between ϕ0; t and t, from which we can obtain the pulse width and the repetition rates of the generated pulses. The solid lines in Figs. 4(a) and 4(b) show the dependence of the pulse width and pulse repetition rate on the incident pump power, respectively. The single-pulse energy E can be given as [31] 1 2 1 ϕ ; (4) E πωl hvcIn 4 R int Table 2. Related Parameters for the Rate Equations [11,13] Parameter σ σg σe α τ n1 n2 τs ωp

Value

Parameter

Value

5 × 10−20 cm2 7.2 × 10−18 cm2 7.4 × 10−19 cm2 0.75 cm−1 277 μs 1.83 1.80 22 ns 200 μm

ns0 l ls Lp L ωs ωg ωl

2 × 1017 cm−3 1 cm 0.5 mm 5 cm 0.01 186 μm 207 μm 202 μm

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where ϕint is the integral over t from the beginning time t 1 to ending time t 2 of the single pulse. Using Eq. (4), we can obtain the theoretical pulse energy, as shown by the solid line in Fig. 4(c). We also show the simulated pulse train in Fig. 5(b) and the simulated pulse profile in Fig. 6 (dotted line). From Figs. 4 to 6, we can see that the experimental results are in agreement with the theoretical calculations. 5. CONCLUSIONS We have demonstrated the output characteristics of diodepumped CW and passively Q-switched Nd:LuAG lasers at 1.3 μm with V 3 :YAG crystal for the first time, to the best of our knowledge. The experimental results showed that maximum CW and Q-switched output powers of 954 and 133 mW were obtained, giving slope efficiencies of 14.7% and 3.0%, respectively. A minimum pulse duration of 17 ns under a repetition rate of 8 kHz was achieved, and a maximum pulse energy of 18.9 μJ was obtained at incident pump power of 6.5 W. To understand the results obtained in the experiment, we introduced a rate equation model in which the Gaussian spatial distribution of the intracavity photon density and the longitudinal distribution of the photon density along the cavity axis are taken into account. The numerical results of the rate equations are consistent with the experimental results. National Natural Science Foundation of China (NSFC) (61378022, 61205145, 61475088); Open Foundation of State Key Laboratory of Crystal Material of Shandong University (KF1403). REFERENCES 1. A. Brenier, Y. Guyot, H. Canibano, G. Boulon, A. Ródenas, D. Jaque, A. Eganyan, and A. G. Petrosyan, “Growth, spectroscopic, and laser properties of Yb3+-doped Lu3Al5O12 garnet crystal,” J. Opt. Soc. Am. B 23, 676–683 (2006). 2. H. Kalaycioglu, A. Sennaroglu, A. Kurt, and G. Ozen, “Spectroscopic analysis of Tm3+:LuAG,” J. Phys. 19, 036208 (2007). 3. Y. Kuwano, K. Suda, N. Ishizawa, and T. Yamada, “Crystal growth and properties of (Lu, Y)3Al5O12,” J. Cryst. Growth 260, 159–165 (2004). 4. X. D. Xu, J. Q. Di, W. D. Tan, J. Zhang, D. Y. Tang, D. Z. Li, D. H. Zhou, and J. Xu, “High efficient diode-pumped passively mode-locked Nd:LuAG laser,” Laser Phys. Lett. 9, 406–409 (2012). 5. N. P. Barnes, M. G. Jani, and R. L. Hutcheson, “Diode-pumped, roomtemperature Tm:LuAG laser,” Appl. Opt. 34, 4290–4294 (1995). 6. T. Kasamatsu, H. Sekita, and Y. Kuwano, “Temperature dependence and optimization of 970-nm diode-pumped Yb:YAG and Yb:LuAG lasers,” Appl. Opt. 38, 5149–5153 (1999). 7. J. Dong, K. Ueda, and A. A. Kaminskii, “Laser-diode pumped efficient Yb:LuAG microchip lasers oscillating at 1030 and 1047 nm,” Laser Phys. Lett. 7, 726–733 (2010). 8. A. Kaminskii, A. G. Petrosyan, and V. A. Fedorov, “Two-micron stimulated emission by crystals with Ho3+ ions based on the transition 5I7 → 5I8,” Sov. Phys. Dokl. 26, 846–848 (1981). 9. V. Kushawaha, Y. Chen, Y. Yan, and L. Major, “High-efficiency continuous-wave diode-pumped Tm:Ho:LuAG laser at 2.1 μm,” Appl. Phys. B 62, 109–111 (1996). 10. S. D. Setzler, K. J. Snell, T. M. Pollak, P. A. Bundi, Y. E. Young, and E. P. Chicklis, “5-W repetitively Q-switched Er:LuAG laser resonantly pumped by an erbium fiber laser,” Opt. Lett. 28, 1787–1789 (2003). 11. X. D. Xu, X. D. Wang, J. Q. Meng, Y. Cheng, D. Z. Li, S. S. Cheng, F. Wu, Z. W. Zhao, and J. Xu, “Crystal growth, spectral and laser

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