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High power diode-pumped passively Q-switched and mode-locking Nd:GdVO4 laser at 912 nm
Optics Communications 284 (2011) 635–639
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
High power diode-pumped passively Q-switched and mode-locking Nd:GdVO4 laser at 912 nm Fei Chen ⁎, Xin Yu, Xudong Li, Renpeng Yan, Cheng Wang, Deying Chen, Zhonghua Zhang, Junhua Yu National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 12 May 2010 Received in revised form 21 September 2010 Accepted 22 September 2010 Keywords: Diode-pumped lasers Q-switched and mode-locking Nd:GdVO4 912 nm
a b s t r a c t A high power diode-end-pumped passively Q-switched and mode-locking (QML) Nd:GdVO4 laser at 912 nm was demonstrated for the first time, to the best of our knowledge. A Z-type laser cavity with Cr4+:YAG crystals as the intracavity saturable absorber were employed in the experiments. Influence of the initial transmission (TU) of the saturable absorber on the QML laser performance was investigated. Using the TU = 95% Cr4+:YAG, as much as an average output power of 2.0 W pulsed 912 nm laser was produced at an absorbed pump power of 25.0 W, then the repetition rates of the Q-switched envelope and the mode-locking pulse were ~ 224 kHz and ~ 160 MHz, respectively. Whereas the maximum output power was reduced to 1.3 W using the TU = 90% Cr4+:YAG, we obtained a 100% modulation depth for the mode-locking pulses inside the Q-switched envelope. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diode-pumped passively Q-switched and mode-locking (QML) solid-state lasers by use of an intracavity saturable absorber is an efficient way to produce high repetition rate, high peak power laser pulses. Quasi-three-level Nd3+-doped lasers around 900 nm can be used as the pump sources for Ytterbium (Yb)-doped crystals and Yb-doped fibers, and it also can be used in differential absorption lidar (DIAL) for ozone measurements. Moreover, blue lasers can be generated availably by means of frequency-doubling technology [1–4]. With intracavity frequency-doubling to Nd3+-doped QML quasi-three-level lasers, high peak power and short pulsed blue lasers will be achieved, and the pulsed blue laser can be applied in the fields of biological and medical diagnostics, color displays, underwater imaging and communication. Cr4+:YAG crystal has a relative large absorption cross-section at 900–1200 nm with a long recovery time of about 4 μs. At a lower intracavity laser intensity, most of the Cr4+-ions are located in the ground-state, the excited-state absorption (ESA) is negligible, then the Cr4+:YAG functions as an effective intracavity saturable absorber only for Q-switching. While at sufficient higher intracavity laser intensity, all the Cr4+-ions are quickly excited to the first excited-state, from which further excitation by the strong ESA promotes an accumulation of the Cr4+-ions in the higher-lying level, leading to saturation of the ESA. Since the relaxation time of the ESA is relatively short (τes ~ 0.1 ns), it is possible to achieve a passively mode-locking operation with a Cr4+:YAG
⁎ Corresponding author. E-mail address: [email protected] (F. Chen). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.09.059
saturable absorber if the intracavity laser intensity is large enough to saturate the ESA. So Cr4+:YAG crystal can be used as an effective saturable absorber to generate nanosecond Q-switched and picosecond mode-locking laser pulses in the near infrared region. Nd:GdVO4 crystal has many excellent characteristics as a laser medium, such as a higher absorption coefficient at 808 nm diode-laser, a moderate emission cross-section at 1063 nm laser (σ 1063 / σ912 ≈ 11.5), good thermal property and polarized laser light emission [5,6]. These characteristics will be more favorable for the operation of 912 nm quasi-three-level laser and 456 nm deep-blue laser by frequency-doubling, and many results about continuous-wave 912 nm and 456 nm lasers have been reported [3,7]. Considering the gain competition with four-level lasers and severe thermal lensing effect during the pulsed operation of quasi-three-level lasers [8,9], Nd: GdVO4 crystal will be a highly performed laser medium for Q-switched 912 nm laser operation. In the past, much more attention had been paid on the Nd3+-doped QML lasers operating at 1064 nm and 1342 nm [10–16], and varieties of solid-state saturable absorbers had been investigated for generating QML lasers, such as Cr4+:YAG crystal, GaAs wafer [17,18] as well as semiconductor saturable absorption mirrors (SESAM) [19]. However, Nd3+-doped QML quasi-three-level laser were less reported. In 2006, P. Blandin reported a diode-pumped passively mode-locked 914 nm Nd: YVO4 laser by using a SESAM, with averaged output power of 87 mW at the repetition rate of 94 MHz [20]. In 2008, C. W. Xu demonstrated a stable mode-locked 912 nm Nd:GdVO4 laser by using the SESAM, with averaged output power of 128 mW at the repetition rate of 178 MHz [21]. In 2008, Q. N. Li presented a QML 946 nm Nd:YAG laser by using a Cr, Nd:YAG crystal as gain medium as well as saturable absorber, with the maximum averaged output power of 751 mW at the repetition rate
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F. Chen et al. / Optics Communications 284 (2011) 635–639
From analysis of the coupled rate equation, the criterion for a good passive Q-switch is given by [24]
Table 1 Main parameters and calculated results for selecting 912 nm passive Q-switch. TU
T
M
N
90%
6% 9% 12% 6% 9% 12%
56.1 51.0 46.7 41.1 35.7 31.5
2.99 2.99 2.99 2.99 2.99 2.99
95%
MN NN
ð2Þ
Basic parameters and calculated results are listed in Table 1 for selecting of the 912 nm passive Q-switch. We can see that, no matter which output coupler is used with the transmission ratio of T = 6%, T = 9% and T = 12% at 912 nm, for two kinds of Cr4+:YAG crystal with different initial transmission of TU = 95% and TU = 90%, values of M are always much larger than that of N.
L = 0.05, σgs = 3.9 × 10− 18 cm2, σ = 6.6 × 10− 20 cm2, A/As = 1.45, γ = 2, β = 0.36 [25]
3. Experimental setups of 135.13 MHz [22]. Based on the former research results, it's obvious that the output powers of Nd3+-doped mode-locking quasi-three-level lasers were rather low, this is owing to many essential difficulties in Nd3+-doped quasi-three-level laser operation, such as the small stimulated-emission cross-section and the significant ground-state reabsorption loss [23]. In this paper, a diode-end-pumped high power QML 912 nm Nd: GdVO4 laser was present for the first time. A thermal insensitive laser cavity was designed and the intracavity saturable absorber was optimized to scale the output power of 912 nm laser. Using the Cr4+: YAG with an initial transmission of TU = 95%, a maximum average output power of 2.0 W QML 912 nm laser was obtained at an absorbed pump power of 25.0 W, to the best of our knowledge, it's the highest output power of Nd3+-doped QML quasi-three-level lasers. When the TU = 90% Cr4+:YAG was used, the maximum output power was reduced, but 100% modulation depth for the mode-locking pulses inside the Q-switched envelope was obtained.
Fig. 1 shows the experimental setup for the QML 912 nm Nd:GdVO4 laser system. A fiber-coupled LD (HLU110F400, LIMO Inc.) was employed as the pump source, which delivered the maximum output power of 110 W at 808 nm from the end of a fiber with 400 μm core in diameter and a N.A. of 0.22. The pump beam was coupled into the gain medium by a series of coupling optics, and the beam spot radius generated in the crystal was ~100 μm. An a-cut conventional Nd:GdVO4 crystal with a Nd3+-doping concentration of 0.1 at.% and the dimensions of 3× 3× 6 mm3 had been chosen as the gain medium. The crystal was wrapped with 0.05 mm thick indium foil, mounted in a copper microchannel heat sink and maintained at 283±0.1 K by water cooling. After a good match between the pump wavelength and the absorption peak of the laser crystal was accomplished, ~60% pump power was absorbed by the laser medium. QML laser was obtained by placing the Cr4+:YAG crystals near the output coupler of the Z-type laser cavity. Diameters of Cr4+:YAG crystals with TU = 95% and TU = 90% are Ф10 mm, but thickness for them are 0.8 mm and 1.0 mm, respectively. To prevent the more efficient four-level transitions at 1064 nm and 1342 nm, all sides of these crystals was not only coated for high transmission (HT) at 912 nm (T N 99.8%), but also coated for anti-reflection (AR) at 1064 nm (Rb 1%) and 1342 nm (Rb 2%). The experiments was carried out with the Z-type cavity, which was built by a flat dichroic input mirror M1, two concave mirrors M2 and M3, and a flat output coupler M4. The radius of curvature for M2 and M3 were 300 mm and 100 mm, respectively. In experiments, L1 and L2 were kept at 165 mm and 50 mm, respectively, and total cavity length was 915 mm. The laser cavity was designed to allow mode matching between the laser beam and the pump beam in the Nd:GdVO4 crystal and to provide the proper spot size in the Cr4+:YAG crystal. The folded angle (α) was set to be ~5° to reduce the astigmatism. Considering the thermal focal lens in the gain medium, the laser beam radius in the laser crystal (ωL) and the saturable absorber (ωA) were calculated by the software of Lascad (LAS-CAD GmbH), which are shown in Fig. 2. We can see that the laser cavity is insensitive to the thermal lens and can stably operate at high pump level. The difference of laser mode radius between the tangential and sagittal plane is very small, so the astigmatism will be not serious. The beam radius in the Cr4+:YAG crystal was ~53 μm, and the large ratio of the mode sizes between the gain
2. Theoretical analysis For passively Q-switched laser operation, it is crucial to match the so-called “Q-switched criterion”. Defining the parameters of M and N, ! 1 TU2 σgs A ! M= σ As 1 1 +L ln 2 + ln 1−T TU ln
N=
γ 1−β
ð1Þ
Where TU is the initial transmission of the saturable absorber, T is the transmission ratio of the output coupler, L is the nonsaturable intracavity round-trip dissipative optical loss, σgs is the ground-state absorption cross-section of the saturable absorber, σ is the stimulatedemission cross-section of the gain medium, A/As is the ratio of the effective area in the gain medium and in the saturable absorber, γ is the inversion reduction factor (γ = 1 and γ = 2 correspond to four-level and three-level systems), and β is the ratio of the excited-state absorption to that of the ground-state absorption in the saturable absorber.
L1 M2
Photodiode
α Nd:GdVO4 LD
Coupling optics
CCD M4
M3
Digital oscilloscope
M1 Output laser
Beam analyzer
Cr:YAG L2
Filter
Beam splitter Power meter
Fig. 1. Experimental setup for the passively Q-switched and mode-locking 912 nm Nd:GdVO4 laser.
F. Chen et al. / Optics Communications 284 (2011) 635–639
100
2.0
Average output power(W)
90
Laser beam radius(μm)
637
80 70 60 50 40
ω LX sagittal
30
ω LY tangential
20
ω AX sagittal
10
ω AY tangential
0
100
200
300
400
1.5
1.0
0.5
0.0
0 500
T=12% T=9% T=6%
8
10
12
14
16
18
20
22
24
26
Absorbed pump power(W)
Thermal focal length(mm) Fig. 2. Laser beam radius in Nd:GdVO4 and Cr4+:YAG crystals versus the thermal focal length.
Fig. 4. Average output power for 912 nm laser versus the absorbed pump power with different output couplers.
medium and the absorber was sufficient to achieve high-quality Q-switching. However, it would be increased rapidly once the thermal focal length was smaller than 40 mm, and the output performance of QML laser was affected inevitably then, such as the pulse width and the repetition rate. Output laser spectra and the power were measured by a fiber spectrometer (HR4000, Ocean Optics Inc.) and a laser power meter PM30 (PM30, Coherent Inc.), respectively.
a passively Q-switched 1063 nm Nd:GdVO4 laser [11]. This instability was actually the intrinsic vulnerability of all passively Q-switched or passively QML lasers. From the upper inset of Fig. 3, it can be seen that the pulse-to-pulse amplitude fluctuation of the 912 nm Q-switched pulse train was less than ±20% at the repetition rate ~175 kHz. The lower inset of Fig. 3 shows that the mode-locking pulse train was enveloped within a ~200 ns Q-switched pulse. When the laser configuration, gain medium and saturation absorber were fixed, selection of an output coupler with proper transmissivity was important to achieve the high power passively Q-switched laser output. There was a trade-off between obtaining low lasing threshold, short pulse width and high average output power in this selection. Using a higher transmissivity of output coupler, shorter pulse width and lower repetition rate would be obtained, this was because the higher transmittance leads to a larger loss, the larger loss results in a narrower pulse width of the Q-switched envelope, and it required longer time to accumulate the energy to saturate the Cr:YAG, so the lower repetition rate of the Q-switched laser pulses would be induced. Contrarily, passively Q-switched laser with longer pulse width and higher repetition rate would be generated when an output coupler with lower transmittance was used. In experiments, to optimize the performance of QML 912 nm laser, output couplers with transmission ratio of T = 6%, T = 9%, and T = 12% and Cr4+:YAG
4. Results and discussion
1.0 0.8
2.0
0.6 0.4 0.2 0.0 -40
-30
-20
-10
0
10
20
30
40
Intensity(arb. unit)
Time (µs) 0.8 0.6 0.4
Average output power(W)
Intensity(arb. unit)
When the photon intensity in the laser cavity was lower, the Cr4+: YAG crystal would be an effective saturable absorber only for Qswitching, because the transition of the ESA to higher-lying levels was rather week. As the increase of the pump power, the intensity fluctuation in the resonator would be strong, a Q-switched and modelocking pulse could be generated. In our experiments, the Q-switched pulse train and mode-locked pulse train within the Q-switched pulse envelope were recorded by using a digital oscilloscope (DPO 7104, Tektronix Inc.) and a fast photodiode (DET 210, Thorlabs Inc.) with a rising time of ~1 ns. As displayed in Fig. 3, the phenomena of a pulse-topulse instability occurred in the Q-switched pulse train of the QML 912 nm laser at an absorbed pump power about 20 W, as it happened in
1.5
1.0
0.5
TU=90% TU=95%
0.2 0.0 10
0.0 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
12
14
16
18
20
22
24
26
Absorbed pump power(W)
Time (µs) Fig. 3. Temporal traces of the Q-switched and QML laser pulses of 912 nm Nd:GdVO4 lasers.
Fig. 5. Average output power for 912 nm laser versus the absorbed pump power with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively. The spatial beam profile for QML 912 nm laser at the maximum output power.
Repetition rate(kHz)
638
F. Chen et al. / Optics Communications 284 (2011) 635–639
240 200
TU=95%
160
TU=90%
120 80 40 0 10
12
14
16
18
20
22
24
26
Absorbed pump power(W) Pulse width(ns)
300 TU=95% TU=90%
250 200 150 100 50 10
12
14
16
18
20
22
24
26
Absorbed pump power(W) Fig. 6. Q-switched repetition rate and the pulse width versus the absorbed pump power with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively.
crystal with the initial transmission of TU = 95% were used. The dependence of average output power on the absorbed pump power is shown in Fig. 4. We can see that, using T = 9% output coupler, the lasing threshold was higher than those of T = 6%, but the output power was much better than that of T = 12% and T = 6%. At the absorbed pump power of 25.0 W, the maximum output power of 2.0 W was achieved, corresponding to an optical efficiency of 8% and a slope efficiency of 17.9%, to the best of our knowledge, it's the highest output power of Nd3+-doped QML quasi-three-level lasers. Fig. 5 gives a comparative study on the output power of QML 912 nm laser using saturable absorbers with different initial transmission. When the T = 9% output coupler was used, the dependence of the average output power on the absorbed pump power was presented in Fig. 5. Compared with TU = 90% Cr4+:YAG crystal, a lower pump threshold and a higher average output power were obtained using the TU = 95%. This is due to the lower intracavity loss of Cr4+:YAG with TU = 95%. At the absorbed pump power of 25.0 W, the maximum output power of QML 912 nm laser for TU = 95% and TU = 90% are 2.0 W and 1.34 W, respectively. The output powers are inclined to saturate in higher pump power field, this can be attributed to the influence of severe thermal lensing effect in Nd3+-doped quasithree-level laser operation. At the maximum average output power of
2.0 W, we also measured the typical spatial beam profile by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.), which shows that the laser intensity distribution was very symmetrical. By using the traveling 90/10 knife-edge method, beam quality factor of maximum output power was estimated to be M2 = 1.43. For the QML 912 nm Nd:GdVO4 laser with different intracavity saturable absorber of the TU = 90% and TU = 95%, the dependence of Q-switched repetition rate and the pulse width on the absorbed pump power are shown in Fig. 6. It can be seen that with the increase of absorbed pump power, the repetition rate of the Q-switched envelope pulses was increased, while the pulse duration was decreased. Using the TU = 95% Cr4+:YAG, when the absorbed pump power was increased from 11.3 W to 25.0 W, the repetition rate was increased from ~22 to ~224 kHz and the pulse width was decreased from ~283 to ~170 ns. When the TU = 90% Cr4+:YAG was employed at the same conditions, the repetition rate was increased from ~ 9 to ~95 kHz and the pulse width was decreased from ~111 to ~ 72 ns. At the absorbed pump power of 25.0 W, we obtained stable Q-switched mode-locked pulses. The repetition rate was ~ 160 MHz, in agreement with the cavity length. If the absorbed pump power is beyond 25.0 W, a series of strongly fluctuating narrow pulses would be observed instead of the Q-switched and mode-locked pulses. This phenomenon was originated from the bleaching of the Cr4+:YAG crystal due to the high intracavity intensity, and it indicated that it was impossible to achieve pure mode-locking by increasing the intensity on Cr4+:YAG crystal. The influence on QML 912 nm laser pulse width and modulation depth induced by the different saturable absorbers was also investigated. The temporal traces at the power level of 1.3 W were displayed Fig. 7. With the initial transmission TU = 95%, as shown in Fig. 7(a), the mode-locking modulation depth was ~ 70%, and the width of the Q-switched pulse envelope was ~ 230 ns. While using the TU = 90% Cr4+:YAG, as shown in Fig. 7(b) for comparison, the Q-switched laser pulse exhibited a modulation depth and pulse width of ~100% and ~105 ns, respectively. Results show that shorter pulse duration and higher modulation depth would be achieved by using the saturable absorber with lower initial transmission. This can be understood as follows. With a lower initial transmission, the saturable absorber had a larger density of Cr4+-ions involved in the saturable absorption, and the intracavity loss was larger. Accordingly, shorter Q-switched laser pulse duration was achieved. Moreover, Cr4+:YAG crystal with a lower initial transmission required a higher energy to saturate ESA, leading to the generation of a mode-locked pulse train with a higher modulation depth. 5. Conclusion
(a)
100 ns/div
TU=95% Modulation depth~70%
(b)
50 ns/div
TU=90%
In conclusion, we demonstrated a diode-pumped QML quasithree-level 912 nm Nd:GdVO4 laser by using Cr4+:YAG as the intracavity saturable absorber. When the Cr4+:YAG with initial transmission of TU = 95% was used, a maximum average output laser power of 2.0 W was obtained at an absorbed pump power of 25.0 W. Synchronously, a ~160 MHz mode-locked pulse train was enveloped in a ~283 ns Q-switched laser pulse. Contrastive results show that almost 100% modulation depth would be obtained for the mode-locking pulses inside the Q-switched envelope using the TU = 90% Cr4+:YAG. It was found that the saturable absorber with lower initial transmission would not only shorten the width of the Q-switched envelope, but also increase the modulation depth of the mode-locked pulse in the Q-switched envelope. Acknowledgements
Modulation depth~100%
Fig. 7. Temporal traces of QML laser pulses obtained with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively.
This work was partially supported by the National Scientific Foundation of China (60978016) and Technological Project of Heilongjiang Province No. GC06A116.
F. Chen et al. / Optics Communications 284 (2011) 635–639
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P. Zeller, P. Peuser, Opt. Lett. 25 (2000) 34. C. Czeranowsky, E. Heumann, G. Huber, Opt. Lett. 28 (2003) 432. Q. Zheng, Y. Yao, B. Li, D.P. Qu, L. Zhao, J. Opt. Soc. Am. B 26 (2009) 1238. Q. Zheng, Y. Yao, B. Li, K. Zhou, Y. Liu, L. Zhao, Appl. Opt. 48 (2009) 2979. H.J. Zhang, J.H. Liu, J.Y. Wang, C.Q. Wang, L. Zhu, Z.S. Shao, X.L. Meng, X.B. Hu, M.H. Jiang, J. Opt. Soc. Am. B 19 (2002) 18. J. Didierjean, E. Herault, F. Balembois, P. Georges, Opt. Express 16 (2008) 8995. J. Gao, X. Yu, F. Chen, X.D. Li, R.P. Yan, K. Zhang, J.H. Yu, Y.Z. Wang, Opt. Express 17 (2009) 3574. D. Krennrich, R. Knappe, B. Henrich, R. Wallenstein, J.A. L'huillier, Appl. Phys. B 92 (2008) 165. S. Bjurshagen, R. Koch, Appl. Opt. 43 (2004) 4753. P.K. Mukhopadhyay, M.B. Alsous, K. Ranganathan, S.K. Sharma, P.K. Gupta, J. George, T.P.S. Nathan, Opt. Commun. 222 (2003) 399. S. Zhang, E. Wu, H. Pan, H. Zeng, Appl. Phys. B 78 (2004) 335. S.P. Ng, D.Y. Tang, J. Kong, L.J. Qin, X.L. Meng, Z.J. Xiong, Appl. Phys. B 81 (2005) 511.
639
[13] J. Liu, C.H. Wang, S.H. Liu, W.M. Tian, L. Li, S.S. Liu, M. Liu, J. Modern Opt. 55 (2008) 1971. [14] J.H. Lin, H.R. Chen, H.H. Hsu, M.D. Wei, K.H. Lin, W.F. Hsieh, Opt. Express 16 (2008) 16538. [15] Y.F. Chen, S.W. Tsai, S.C. Wang, Opt. Lett. 25 (2000) 1442. [16] B.T. Zhang, J.L. He, H.T. Huang, C.H. Zuo, K.J. Yang, X.L. Dong, J.L. Xu, S. Zhao, Laser Phys. Lett. 6 (2009) 22. [17] Y.F. Chen, K.F. Huang, S.W. Tsai, Y.P. Lan, S.C. Wang, J. Chen, Appl. Opt. 40 (2001) 6038. [18] J. Liu, Y.G. Wang, W.M. Tian, L.Y. Gao, J.L. He, X.Y. Ma, Opt. Mater. 28 (2006) 970. [19] S.J. Zhang, E. Wu, H.F. Pan, H.P. Zeng, IEEE J. Quantum Electron. 40 (2004) 505. [20] P. Blandin, F. Druon, F. Balembois, P. Georges, Opt. Lett. 31 (2006) 214. [21] C.W. Xu, Z.Y. Wei, K.N. He, D.H. Li, Y.D. Zhang, Z.G. Zhang, Opt. Commun. 281 (2008) 4398. [22] Q.N. Li, S.M. Wang, S.F. Du, Y.X. Shi, J. Xing, D.X. Zhang, B.H. Feng, Z.G. Zhang, S.W. Zhang, Opt. Commun. 281 (2008) 2184. [23] W.P. Risk, J. Opt. Soc. Am. B 5 (1988) 1412. [24] G.H. Xiao, M. Bass, IEEE J. Quantum Electron. 33 (1997) 41. [25] X.Y. Zhang, A. Brenier, J.Y. Wang, H.J. Zhang, Opt. Mater. 26 (2004) 293.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
High power diode-pumped passively Q-switched and mode-locking Nd:GdVO4 laser at 912 nm Fei Chen ⁎, Xin Yu, Xudong Li, Renpeng Yan, Cheng Wang, Deying Chen, Zhonghua Zhang, Junhua Yu National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 12 May 2010 Received in revised form 21 September 2010 Accepted 22 September 2010 Keywords: Diode-pumped lasers Q-switched and mode-locking Nd:GdVO4 912 nm
a b s t r a c t A high power diode-end-pumped passively Q-switched and mode-locking (QML) Nd:GdVO4 laser at 912 nm was demonstrated for the first time, to the best of our knowledge. A Z-type laser cavity with Cr4+:YAG crystals as the intracavity saturable absorber were employed in the experiments. Influence of the initial transmission (TU) of the saturable absorber on the QML laser performance was investigated. Using the TU = 95% Cr4+:YAG, as much as an average output power of 2.0 W pulsed 912 nm laser was produced at an absorbed pump power of 25.0 W, then the repetition rates of the Q-switched envelope and the mode-locking pulse were ~ 224 kHz and ~ 160 MHz, respectively. Whereas the maximum output power was reduced to 1.3 W using the TU = 90% Cr4+:YAG, we obtained a 100% modulation depth for the mode-locking pulses inside the Q-switched envelope. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diode-pumped passively Q-switched and mode-locking (QML) solid-state lasers by use of an intracavity saturable absorber is an efficient way to produce high repetition rate, high peak power laser pulses. Quasi-three-level Nd3+-doped lasers around 900 nm can be used as the pump sources for Ytterbium (Yb)-doped crystals and Yb-doped fibers, and it also can be used in differential absorption lidar (DIAL) for ozone measurements. Moreover, blue lasers can be generated availably by means of frequency-doubling technology [1–4]. With intracavity frequency-doubling to Nd3+-doped QML quasi-three-level lasers, high peak power and short pulsed blue lasers will be achieved, and the pulsed blue laser can be applied in the fields of biological and medical diagnostics, color displays, underwater imaging and communication. Cr4+:YAG crystal has a relative large absorption cross-section at 900–1200 nm with a long recovery time of about 4 μs. At a lower intracavity laser intensity, most of the Cr4+-ions are located in the ground-state, the excited-state absorption (ESA) is negligible, then the Cr4+:YAG functions as an effective intracavity saturable absorber only for Q-switching. While at sufficient higher intracavity laser intensity, all the Cr4+-ions are quickly excited to the first excited-state, from which further excitation by the strong ESA promotes an accumulation of the Cr4+-ions in the higher-lying level, leading to saturation of the ESA. Since the relaxation time of the ESA is relatively short (τes ~ 0.1 ns), it is possible to achieve a passively mode-locking operation with a Cr4+:YAG
⁎ Corresponding author. E-mail address: [email protected] (F. Chen). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.09.059
saturable absorber if the intracavity laser intensity is large enough to saturate the ESA. So Cr4+:YAG crystal can be used as an effective saturable absorber to generate nanosecond Q-switched and picosecond mode-locking laser pulses in the near infrared region. Nd:GdVO4 crystal has many excellent characteristics as a laser medium, such as a higher absorption coefficient at 808 nm diode-laser, a moderate emission cross-section at 1063 nm laser (σ 1063 / σ912 ≈ 11.5), good thermal property and polarized laser light emission [5,6]. These characteristics will be more favorable for the operation of 912 nm quasi-three-level laser and 456 nm deep-blue laser by frequency-doubling, and many results about continuous-wave 912 nm and 456 nm lasers have been reported [3,7]. Considering the gain competition with four-level lasers and severe thermal lensing effect during the pulsed operation of quasi-three-level lasers [8,9], Nd: GdVO4 crystal will be a highly performed laser medium for Q-switched 912 nm laser operation. In the past, much more attention had been paid on the Nd3+-doped QML lasers operating at 1064 nm and 1342 nm [10–16], and varieties of solid-state saturable absorbers had been investigated for generating QML lasers, such as Cr4+:YAG crystal, GaAs wafer [17,18] as well as semiconductor saturable absorption mirrors (SESAM) [19]. However, Nd3+-doped QML quasi-three-level laser were less reported. In 2006, P. Blandin reported a diode-pumped passively mode-locked 914 nm Nd: YVO4 laser by using a SESAM, with averaged output power of 87 mW at the repetition rate of 94 MHz [20]. In 2008, C. W. Xu demonstrated a stable mode-locked 912 nm Nd:GdVO4 laser by using the SESAM, with averaged output power of 128 mW at the repetition rate of 178 MHz [21]. In 2008, Q. N. Li presented a QML 946 nm Nd:YAG laser by using a Cr, Nd:YAG crystal as gain medium as well as saturable absorber, with the maximum averaged output power of 751 mW at the repetition rate
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From analysis of the coupled rate equation, the criterion for a good passive Q-switch is given by [24]
Table 1 Main parameters and calculated results for selecting 912 nm passive Q-switch. TU
T
M
N
90%
6% 9% 12% 6% 9% 12%
56.1 51.0 46.7 41.1 35.7 31.5
2.99 2.99 2.99 2.99 2.99 2.99
95%
MN NN
ð2Þ
Basic parameters and calculated results are listed in Table 1 for selecting of the 912 nm passive Q-switch. We can see that, no matter which output coupler is used with the transmission ratio of T = 6%, T = 9% and T = 12% at 912 nm, for two kinds of Cr4+:YAG crystal with different initial transmission of TU = 95% and TU = 90%, values of M are always much larger than that of N.
L = 0.05, σgs = 3.9 × 10− 18 cm2, σ = 6.6 × 10− 20 cm2, A/As = 1.45, γ = 2, β = 0.36 [25]
3. Experimental setups of 135.13 MHz [22]. Based on the former research results, it's obvious that the output powers of Nd3+-doped mode-locking quasi-three-level lasers were rather low, this is owing to many essential difficulties in Nd3+-doped quasi-three-level laser operation, such as the small stimulated-emission cross-section and the significant ground-state reabsorption loss [23]. In this paper, a diode-end-pumped high power QML 912 nm Nd: GdVO4 laser was present for the first time. A thermal insensitive laser cavity was designed and the intracavity saturable absorber was optimized to scale the output power of 912 nm laser. Using the Cr4+: YAG with an initial transmission of TU = 95%, a maximum average output power of 2.0 W QML 912 nm laser was obtained at an absorbed pump power of 25.0 W, to the best of our knowledge, it's the highest output power of Nd3+-doped QML quasi-three-level lasers. When the TU = 90% Cr4+:YAG was used, the maximum output power was reduced, but 100% modulation depth for the mode-locking pulses inside the Q-switched envelope was obtained.
Fig. 1 shows the experimental setup for the QML 912 nm Nd:GdVO4 laser system. A fiber-coupled LD (HLU110F400, LIMO Inc.) was employed as the pump source, which delivered the maximum output power of 110 W at 808 nm from the end of a fiber with 400 μm core in diameter and a N.A. of 0.22. The pump beam was coupled into the gain medium by a series of coupling optics, and the beam spot radius generated in the crystal was ~100 μm. An a-cut conventional Nd:GdVO4 crystal with a Nd3+-doping concentration of 0.1 at.% and the dimensions of 3× 3× 6 mm3 had been chosen as the gain medium. The crystal was wrapped with 0.05 mm thick indium foil, mounted in a copper microchannel heat sink and maintained at 283±0.1 K by water cooling. After a good match between the pump wavelength and the absorption peak of the laser crystal was accomplished, ~60% pump power was absorbed by the laser medium. QML laser was obtained by placing the Cr4+:YAG crystals near the output coupler of the Z-type laser cavity. Diameters of Cr4+:YAG crystals with TU = 95% and TU = 90% are Ф10 mm, but thickness for them are 0.8 mm and 1.0 mm, respectively. To prevent the more efficient four-level transitions at 1064 nm and 1342 nm, all sides of these crystals was not only coated for high transmission (HT) at 912 nm (T N 99.8%), but also coated for anti-reflection (AR) at 1064 nm (Rb 1%) and 1342 nm (Rb 2%). The experiments was carried out with the Z-type cavity, which was built by a flat dichroic input mirror M1, two concave mirrors M2 and M3, and a flat output coupler M4. The radius of curvature for M2 and M3 were 300 mm and 100 mm, respectively. In experiments, L1 and L2 were kept at 165 mm and 50 mm, respectively, and total cavity length was 915 mm. The laser cavity was designed to allow mode matching between the laser beam and the pump beam in the Nd:GdVO4 crystal and to provide the proper spot size in the Cr4+:YAG crystal. The folded angle (α) was set to be ~5° to reduce the astigmatism. Considering the thermal focal lens in the gain medium, the laser beam radius in the laser crystal (ωL) and the saturable absorber (ωA) were calculated by the software of Lascad (LAS-CAD GmbH), which are shown in Fig. 2. We can see that the laser cavity is insensitive to the thermal lens and can stably operate at high pump level. The difference of laser mode radius between the tangential and sagittal plane is very small, so the astigmatism will be not serious. The beam radius in the Cr4+:YAG crystal was ~53 μm, and the large ratio of the mode sizes between the gain
2. Theoretical analysis For passively Q-switched laser operation, it is crucial to match the so-called “Q-switched criterion”. Defining the parameters of M and N, ! 1 TU2 σgs A ! M= σ As 1 1 +L ln 2 + ln 1−T TU ln
N=
γ 1−β
ð1Þ
Where TU is the initial transmission of the saturable absorber, T is the transmission ratio of the output coupler, L is the nonsaturable intracavity round-trip dissipative optical loss, σgs is the ground-state absorption cross-section of the saturable absorber, σ is the stimulatedemission cross-section of the gain medium, A/As is the ratio of the effective area in the gain medium and in the saturable absorber, γ is the inversion reduction factor (γ = 1 and γ = 2 correspond to four-level and three-level systems), and β is the ratio of the excited-state absorption to that of the ground-state absorption in the saturable absorber.
L1 M2
Photodiode
α Nd:GdVO4 LD
Coupling optics
CCD M4
M3
Digital oscilloscope
M1 Output laser
Beam analyzer
Cr:YAG L2
Filter
Beam splitter Power meter
Fig. 1. Experimental setup for the passively Q-switched and mode-locking 912 nm Nd:GdVO4 laser.
F. Chen et al. / Optics Communications 284 (2011) 635–639
100
2.0
Average output power(W)
90
Laser beam radius(μm)
637
80 70 60 50 40
ω LX sagittal
30
ω LY tangential
20
ω AX sagittal
10
ω AY tangential
0
100
200
300
400
1.5
1.0
0.5
0.0
0 500
T=12% T=9% T=6%
8
10
12
14
16
18
20
22
24
26
Absorbed pump power(W)
Thermal focal length(mm) Fig. 2. Laser beam radius in Nd:GdVO4 and Cr4+:YAG crystals versus the thermal focal length.
Fig. 4. Average output power for 912 nm laser versus the absorbed pump power with different output couplers.
medium and the absorber was sufficient to achieve high-quality Q-switching. However, it would be increased rapidly once the thermal focal length was smaller than 40 mm, and the output performance of QML laser was affected inevitably then, such as the pulse width and the repetition rate. Output laser spectra and the power were measured by a fiber spectrometer (HR4000, Ocean Optics Inc.) and a laser power meter PM30 (PM30, Coherent Inc.), respectively.
a passively Q-switched 1063 nm Nd:GdVO4 laser [11]. This instability was actually the intrinsic vulnerability of all passively Q-switched or passively QML lasers. From the upper inset of Fig. 3, it can be seen that the pulse-to-pulse amplitude fluctuation of the 912 nm Q-switched pulse train was less than ±20% at the repetition rate ~175 kHz. The lower inset of Fig. 3 shows that the mode-locking pulse train was enveloped within a ~200 ns Q-switched pulse. When the laser configuration, gain medium and saturation absorber were fixed, selection of an output coupler with proper transmissivity was important to achieve the high power passively Q-switched laser output. There was a trade-off between obtaining low lasing threshold, short pulse width and high average output power in this selection. Using a higher transmissivity of output coupler, shorter pulse width and lower repetition rate would be obtained, this was because the higher transmittance leads to a larger loss, the larger loss results in a narrower pulse width of the Q-switched envelope, and it required longer time to accumulate the energy to saturate the Cr:YAG, so the lower repetition rate of the Q-switched laser pulses would be induced. Contrarily, passively Q-switched laser with longer pulse width and higher repetition rate would be generated when an output coupler with lower transmittance was used. In experiments, to optimize the performance of QML 912 nm laser, output couplers with transmission ratio of T = 6%, T = 9%, and T = 12% and Cr4+:YAG
4. Results and discussion
1.0 0.8
2.0
0.6 0.4 0.2 0.0 -40
-30
-20
-10
0
10
20
30
40
Intensity(arb. unit)
Time (µs) 0.8 0.6 0.4
Average output power(W)
Intensity(arb. unit)
When the photon intensity in the laser cavity was lower, the Cr4+: YAG crystal would be an effective saturable absorber only for Qswitching, because the transition of the ESA to higher-lying levels was rather week. As the increase of the pump power, the intensity fluctuation in the resonator would be strong, a Q-switched and modelocking pulse could be generated. In our experiments, the Q-switched pulse train and mode-locked pulse train within the Q-switched pulse envelope were recorded by using a digital oscilloscope (DPO 7104, Tektronix Inc.) and a fast photodiode (DET 210, Thorlabs Inc.) with a rising time of ~1 ns. As displayed in Fig. 3, the phenomena of a pulse-topulse instability occurred in the Q-switched pulse train of the QML 912 nm laser at an absorbed pump power about 20 W, as it happened in
1.5
1.0
0.5
TU=90% TU=95%
0.2 0.0 10
0.0 -2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
12
14
16
18
20
22
24
26
Absorbed pump power(W)
Time (µs) Fig. 3. Temporal traces of the Q-switched and QML laser pulses of 912 nm Nd:GdVO4 lasers.
Fig. 5. Average output power for 912 nm laser versus the absorbed pump power with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively. The spatial beam profile for QML 912 nm laser at the maximum output power.
Repetition rate(kHz)
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F. Chen et al. / Optics Communications 284 (2011) 635–639
240 200
TU=95%
160
TU=90%
120 80 40 0 10
12
14
16
18
20
22
24
26
Absorbed pump power(W) Pulse width(ns)
300 TU=95% TU=90%
250 200 150 100 50 10
12
14
16
18
20
22
24
26
Absorbed pump power(W) Fig. 6. Q-switched repetition rate and the pulse width versus the absorbed pump power with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively.
crystal with the initial transmission of TU = 95% were used. The dependence of average output power on the absorbed pump power is shown in Fig. 4. We can see that, using T = 9% output coupler, the lasing threshold was higher than those of T = 6%, but the output power was much better than that of T = 12% and T = 6%. At the absorbed pump power of 25.0 W, the maximum output power of 2.0 W was achieved, corresponding to an optical efficiency of 8% and a slope efficiency of 17.9%, to the best of our knowledge, it's the highest output power of Nd3+-doped QML quasi-three-level lasers. Fig. 5 gives a comparative study on the output power of QML 912 nm laser using saturable absorbers with different initial transmission. When the T = 9% output coupler was used, the dependence of the average output power on the absorbed pump power was presented in Fig. 5. Compared with TU = 90% Cr4+:YAG crystal, a lower pump threshold and a higher average output power were obtained using the TU = 95%. This is due to the lower intracavity loss of Cr4+:YAG with TU = 95%. At the absorbed pump power of 25.0 W, the maximum output power of QML 912 nm laser for TU = 95% and TU = 90% are 2.0 W and 1.34 W, respectively. The output powers are inclined to saturate in higher pump power field, this can be attributed to the influence of severe thermal lensing effect in Nd3+-doped quasithree-level laser operation. At the maximum average output power of
2.0 W, we also measured the typical spatial beam profile by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.), which shows that the laser intensity distribution was very symmetrical. By using the traveling 90/10 knife-edge method, beam quality factor of maximum output power was estimated to be M2 = 1.43. For the QML 912 nm Nd:GdVO4 laser with different intracavity saturable absorber of the TU = 90% and TU = 95%, the dependence of Q-switched repetition rate and the pulse width on the absorbed pump power are shown in Fig. 6. It can be seen that with the increase of absorbed pump power, the repetition rate of the Q-switched envelope pulses was increased, while the pulse duration was decreased. Using the TU = 95% Cr4+:YAG, when the absorbed pump power was increased from 11.3 W to 25.0 W, the repetition rate was increased from ~22 to ~224 kHz and the pulse width was decreased from ~283 to ~170 ns. When the TU = 90% Cr4+:YAG was employed at the same conditions, the repetition rate was increased from ~ 9 to ~95 kHz and the pulse width was decreased from ~111 to ~ 72 ns. At the absorbed pump power of 25.0 W, we obtained stable Q-switched mode-locked pulses. The repetition rate was ~ 160 MHz, in agreement with the cavity length. If the absorbed pump power is beyond 25.0 W, a series of strongly fluctuating narrow pulses would be observed instead of the Q-switched and mode-locked pulses. This phenomenon was originated from the bleaching of the Cr4+:YAG crystal due to the high intracavity intensity, and it indicated that it was impossible to achieve pure mode-locking by increasing the intensity on Cr4+:YAG crystal. The influence on QML 912 nm laser pulse width and modulation depth induced by the different saturable absorbers was also investigated. The temporal traces at the power level of 1.3 W were displayed Fig. 7. With the initial transmission TU = 95%, as shown in Fig. 7(a), the mode-locking modulation depth was ~ 70%, and the width of the Q-switched pulse envelope was ~ 230 ns. While using the TU = 90% Cr4+:YAG, as shown in Fig. 7(b) for comparison, the Q-switched laser pulse exhibited a modulation depth and pulse width of ~100% and ~105 ns, respectively. Results show that shorter pulse duration and higher modulation depth would be achieved by using the saturable absorber with lower initial transmission. This can be understood as follows. With a lower initial transmission, the saturable absorber had a larger density of Cr4+-ions involved in the saturable absorption, and the intracavity loss was larger. Accordingly, shorter Q-switched laser pulse duration was achieved. Moreover, Cr4+:YAG crystal with a lower initial transmission required a higher energy to saturate ESA, leading to the generation of a mode-locked pulse train with a higher modulation depth. 5. Conclusion
(a)
100 ns/div
TU=95% Modulation depth~70%
(b)
50 ns/div
TU=90%
In conclusion, we demonstrated a diode-pumped QML quasithree-level 912 nm Nd:GdVO4 laser by using Cr4+:YAG as the intracavity saturable absorber. When the Cr4+:YAG with initial transmission of TU = 95% was used, a maximum average output laser power of 2.0 W was obtained at an absorbed pump power of 25.0 W. Synchronously, a ~160 MHz mode-locked pulse train was enveloped in a ~283 ns Q-switched laser pulse. Contrastive results show that almost 100% modulation depth would be obtained for the mode-locking pulses inside the Q-switched envelope using the TU = 90% Cr4+:YAG. It was found that the saturable absorber with lower initial transmission would not only shorten the width of the Q-switched envelope, but also increase the modulation depth of the mode-locked pulse in the Q-switched envelope. Acknowledgements
Modulation depth~100%
Fig. 7. Temporal traces of QML laser pulses obtained with saturable absorber of the initial transmission TU = 90% and TU = 95%, respectively.
This work was partially supported by the National Scientific Foundation of China (60978016) and Technological Project of Heilongjiang Province No. GC06A116.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P. Zeller, P. Peuser, Opt. Lett. 25 (2000) 34. C. Czeranowsky, E. Heumann, G. Huber, Opt. Lett. 28 (2003) 432. Q. Zheng, Y. Yao, B. Li, D.P. Qu, L. Zhao, J. Opt. Soc. Am. B 26 (2009) 1238. Q. Zheng, Y. Yao, B. Li, K. Zhou, Y. Liu, L. Zhao, Appl. Opt. 48 (2009) 2979. H.J. Zhang, J.H. Liu, J.Y. Wang, C.Q. Wang, L. Zhu, Z.S. Shao, X.L. Meng, X.B. Hu, M.H. Jiang, J. Opt. Soc. Am. B 19 (2002) 18. J. Didierjean, E. Herault, F. Balembois, P. Georges, Opt. Express 16 (2008) 8995. J. Gao, X. Yu, F. Chen, X.D. Li, R.P. Yan, K. Zhang, J.H. Yu, Y.Z. Wang, Opt. Express 17 (2009) 3574. D. Krennrich, R. Knappe, B. Henrich, R. Wallenstein, J.A. L'huillier, Appl. Phys. B 92 (2008) 165. S. Bjurshagen, R. Koch, Appl. Opt. 43 (2004) 4753. P.K. Mukhopadhyay, M.B. Alsous, K. Ranganathan, S.K. Sharma, P.K. Gupta, J. George, T.P.S. Nathan, Opt. Commun. 222 (2003) 399. S. Zhang, E. Wu, H. Pan, H. Zeng, Appl. Phys. B 78 (2004) 335. S.P. Ng, D.Y. Tang, J. Kong, L.J. Qin, X.L. Meng, Z.J. Xiong, Appl. Phys. B 81 (2005) 511.
639
[13] J. Liu, C.H. Wang, S.H. Liu, W.M. Tian, L. Li, S.S. Liu, M. Liu, J. Modern Opt. 55 (2008) 1971. [14] J.H. Lin, H.R. Chen, H.H. Hsu, M.D. Wei, K.H. Lin, W.F. Hsieh, Opt. Express 16 (2008) 16538. [15] Y.F. Chen, S.W. Tsai, S.C. Wang, Opt. Lett. 25 (2000) 1442. [16] B.T. Zhang, J.L. He, H.T. Huang, C.H. Zuo, K.J. Yang, X.L. Dong, J.L. Xu, S. Zhao, Laser Phys. Lett. 6 (2009) 22. [17] Y.F. Chen, K.F. Huang, S.W. Tsai, Y.P. Lan, S.C. Wang, J. Chen, Appl. Opt. 40 (2001) 6038. [18] J. Liu, Y.G. Wang, W.M. Tian, L.Y. Gao, J.L. He, X.Y. Ma, Opt. Mater. 28 (2006) 970. [19] S.J. Zhang, E. Wu, H.F. Pan, H.P. Zeng, IEEE J. Quantum Electron. 40 (2004) 505. [20] P. Blandin, F. Druon, F. Balembois, P. Georges, Opt. Lett. 31 (2006) 214. [21] C.W. Xu, Z.Y. Wei, K.N. He, D.H. Li, Y.D. Zhang, Z.G. Zhang, Opt. Commun. 281 (2008) 4398. [22] Q.N. Li, S.M. Wang, S.F. Du, Y.X. Shi, J. Xing, D.X. Zhang, B.H. Feng, Z.G. Zhang, S.W. Zhang, Opt. Commun. 281 (2008) 2184. [23] W.P. Risk, J. Opt. Soc. Am. B 5 (1988) 1412. [24] G.H. Xiao, M. Bass, IEEE J. Quantum Electron. 33 (1997) 41. [25] X.Y. Zhang, A. Brenier, J.Y. Wang, H.J. Zhang, Opt. Mater. 26 (2004) 293.