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High repetition rate Q-switching performance in transversely diode-pumped Nd doped mixed gadolinium yttrium vanadate bounce laser Takashige Omatsu Department of Information and Image Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan [email protected]

Masahito Okida Institute of Applied Physics, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki, 305-8573, Japan

Ara Minassian and Michael J. Damzen Department of Physics, Imperial College London, Prince Consort Road, London SW72BW, UK

Abstract: Up to 650kHz high repetition active Q-switching was achieved with a pulse-width of 100 kHz Q-switched operation in transversely diode-pumped ceramic Nd3+:YAG laser in bounce geometry,” Opt. Commun. 249, 531-537 (2005). 10. T. Omatsu, Y. Ojima, A. Minassian and M. J. Damzen, “Power scaling of highly neodymium-doped YAG ceramic lasers with a bounce amplifier geometry,” Opt. Express 13 7011-7016 (2005) 11. J. H. Garcıa-Lopeza, V. Aboitesa, A. V. Kiryanova, M. J. Damzen, and A. Minassian, “High repetition rate Q-switching of high power Nd:YVO4 slab laser,” Opt. Commun. 218 155–160 (2003) 12. A. Minassian, B. A. Thompson, G. R. Smith, and M. J. Damzen, “104W diode-pumped TEM00 Nd:GdVO4 master-oscillator power-amplifier,” in Advanced Solid State Photonics Technical Digest (Optical Society of America) paper MF46,Vienna, Austria (2005) 13. A. Minassian, D. Sauder, G. R. Smith, B. A. Thompson, and M. J. Damzen, “Diode-pumped bounce geometry Nd:GdVO4 master oscillator power amplifier with high beam quality,” in Pacific Rim Conf. On

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Received 9 February 2006; revised 17 March 2006; accepted 18 March 2006

3 April 2006 / Vol. 14, No. 7 / OPTICS EXPRESS 2727

Lasers and Electro-Optics Technical Digest (Optical Society of America) paper, CTuI3-6, Tokyo, Japan (2005)

1. Introduction Neodymium-doped vanadate crystals including Nd:YVO4 and Nd:GdVO4 are very attractive for diode-pumping, because of their large absorption as well as large emission cross-sections. However, their very large emission cross-sections significantly limit their energy storage capacities, yielding a small pulse energy and low peak power in the Q-switching operation. Mixed vanadate crystals can allow adjustable laser parameters such as stimulated crosssection, fluorescence lifetime as well as thermal conductivity, and can produce the best crystal for Q-switched lasers with custom-made pulse energy and peak power as well as pulse repetition frequency [1, 2]. Recently, a passively Q-switched Nd-doped mixed gadolinium yttrium vanadate crystal (Nd:GdxY1-xVO4) laser has been shown to produce high energy pulses compared to that achieved with Nd:YVO4 and Nd:GdVO4 [3,4]. In this paper, we present the first demonstration of Q-switched operation of a transversely diode-pumped mixed gadolinium yttrium vanadate (Nd:GdxY1-xVO4) slab laser with a bounce amplifier geometry operating at high repetition frequency of up to 650kHz and pulse duration of less than 50 ns. The average power in Q-switched operation was as much as 17.5W at the pump power of 41W. By changing the mixture ratio x, we also adjusted the pulse repetition frequency region of the stable Q-switching operation of the system without any loss of the average output power and energy extraction efficiency. The output power and repetition frequency of the actively Q-switched operation are the highest values, to the best of our knowledge, obtained in Nd-doped mixed vanadate lasers. 2. Experiments 2.1. CW operation Diode-side-pumped bounce amplifier configuration based on neodymium-doped slabs including Nd:YVO4, Nd:GdVO4 and Nd:YAG ceramic has been successfully demonstrated to produce high powers at ultrahigh efficiency and high beam quality output [5-10]. Figure 1 shows a schematic diagram of the bounce amplifier setup. Four a-cut, 2mm x 5mm x 20mm, 1-at.% Nd-doped vanadate slabs including Nd:YVO4, Nd:GdVO4 (CASIX Co., Ltd) and mixed vanadate Nd:Gd0.4Y0.6VO4 and Nd:Gd0.6Y0.4VO4 (Hortek Crystal Co., Ltd) were used for the experiment. Their end faces were AR coated for 1μm and wedged to the normal of the pump face to prevent self-lasing within the crystal.

Fig. 1. Experimental setup.

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The vanadate slab amplifiers with a bounce geometry were transversely pumped by a continuous-wave (CW) diode array at 808 nm. The diode output was delivered by a 12.7mm cylindrical lens (CLD) to be a line with dimensions of ~0.2mm x 15mm on the pump face of the slabs. The laser cavity was formed by a high reflectivity flat mirror, M, for 1064 nm and a partial reflectivity flat output coupler, OC, also at 1064 nm. Two cylindrical lenses, CL and 1 CL , (f=50mm) in the vertical direction were used to match the laser mode to the small gain 2 region in the vertical dimension. With this setup, the laser mode reflected at an internal incidence angle of 80o with respect to the pump face normal. We investigated the CW output power from the mixed vanadate lasers using output couplers with various reflectivities. The optimum reflectivity of the output coupler used was ~50%. Figure 2 summarises the experimental CW output powers from four vanadate slab lasers with the 50% reflectivity output coupler. Though the mixed vanadates showed slightly higher lasing thresholds in comparison with that of the YVO4 and GdVO4, there was no significant difference between the thresholds for the two mixed vanadates (Nd:Gd0.4Y0.6VO4, Nd:Gd0.6Y0.4VO4). These results are consistent with previous experiments obtained by the mixed vanadate lasers. The experimental slope efficiencies of the mixed vanadate lasers were > 60%.

Output power (W)

30

GdVO4 YVO4 Gd0.6Y0.4VO4 Gd0.4Y0.6VO4

20

10

0

0

10

20

30

40

Pump power (W) Fig. 2. CW output power from vanadate and mixed vanadate slab lasers.

2.2 Q-switching To achieve active Q-switching, an acousto-optic modulator (AOM) was placed between the output coupler and cylindrical lens CL1. The total cavity length was ~26cm. When the Nd:Gd0.4Y0.6VO4 was used, stable laser operation was observed within the PRF range of 200650 kHz. Figure 3 shows a snap shot of the temporal evolution of the Q-switched output from the Nd:Gd0.4Y0.6VO4 laser at the PRF of 400 kHz.

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Intensity (a.u.)

1.0

0.5

0.0

0

100

200

300

400

Time (ns) Fig. 3. Q-switch pulse from Nd:Gd0.4Y0.6VO4 laser at the PRF of 400kHz.

Below 200 kHz PRF of the AOM Q-switch, pre-lasing was seen. Above PRF of 700 kHz, missing pulses were observed, which can be attributed to insufficient energy storage in the amplifier between consecutive Q-switched pulses (Fig. 4). When the Nd:Gd0.6Y0.4VO4 was used, stable laser operation with a single giant pulse output was observed within the PRF range of 100kHz - 400kHz. The average output power of 17.5W was achieved at the maximum pump level (41W).

Intensity (a.u.)

1.0

0.5

0.0 0

2

4

6

8

10

Time (μs) Fig. 4. Missing pulses seen in the Nd:Gd0.4Y0.6VO4 laser output at the PRF of 700 kHz.

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Average Output Power (W)

20

(a)

10

0

0

100

200

300

400

500

600

700

PRF (kHz) 80

(b) Pulse width (ns)

60

40

20

0

0

100

200

300

400

500

600

700

PRF (kHz)

Fig. 5. (a) Average output power and (b) pulse width as a function of PRF. Red and blue squares show experimental plots in Nd:Gd0.6Y0.4VO4 and Gd0.4Y0.6VO4 lasers, respectively. Red and blue solid curves show the simulated plots.

As stated in previous publication [4], Nd:Gd0.4Y0.6VO4 exhibits a relatively large stimulated emission cross-section in comparison with Nd:Gd0.6Y0.4VO4 allowing it to operate at a higher pulse repetition frequency, and this is consistent with our results. The experimental results at the maximum pump level are summarised in Fig. 5. These results show that the use of the mixed vanadates adjusted the pulse repetition frequency region of the stable Qswitching operation without significant loss of the average output power and energy extraction efficiency. The spatial form of the Q-switch output from the mixed vanadate laser was near diffraction-limited in the vertical (y-axis) but was multimode along the horizontal axis (xaxis). Its corresponding beam propagation factors Mx2 and My2 were 1.3 and 5.2, respectively. The My2 value was a little bit lower than that (~6) in CW operation because of the aperture effects of AOM. High repetition Q-switching operation based on pure vanadate (Nd:YVO4, Nd:GdVO4) bounce amplifiers have been previously reported by co-workers. Stable Q-switching operation in Nd:YVO4 laser has been shown to produce 16W of average power with pulse duration of 15-25 ns in the range 100-500kHz pulse repetition frequency [11]. High repetition Q-switched operation in Nd:GdVO4 bounce geometry laser has shown to produce 34-38W of average power in the range 150-600kHz with pulse durations of 19-25ns [12-13]. Maximum pulse repetition frequency for stable Q-switching is expected to be around 1MHz. Our experiments show that the Q-switching performance in the mixed vanadates is comparable to that in pure vanadates. Mixed vanadates are very promising materials for high

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repetition Q-switching (>100kHz) operation and can potentially provide better control over adjustment of pulse repetition region through control of the Y:Gd mixture ratio. 3. Discussion In order to confirm the Q-switching performance in the mixed vanadate lasers, we also simulated numerically the temporal evolution of the Q-switched output on the basis of the conventional rate equations [9]. The inversion population density n and photon density φ can be written by,

dn η pump Ppump ⎛ λ p ⎞ n ⎟ − cσ e nφ − ⎜ = ⎟ ⎜ dt V hc τ ⎠ ⎝ f dφ l φ n = cσ e nφ a − + Ω dt l τc τ f

( 2)

⎞ 2l ⎛ ⎛ 1 ⎞ ⎜ ⎟ ⎜ ln ⎜ ⎟ + Li + ς (t )⎟ c ⎝ ⎝ R⎠ ⎠

(3)

1

τc

=

(1)

where ηpump is the pump quantum efficiency, Ppump is the pump diode power, V is the volume of the pumped region, hc/λp is the photon energy of pump diode, c is the velocity of light, σe is the stimulated emission cross-section, τf is the fluorescence lifetime of the upper level, la is the length of pumped region, l is the cavity length, Ω is the factor accounting for the fraction of spontaneous photons contributing to the lasing, R is the reflectivity of the output mirror, Li is the internal loss of the cavity, and ζ(t) is the variable loss of the AO modulator, respectively. The pulse energy of the Q-switched output was also estimated by using the formula [9],

Ep ≈

U s A ⎛ 1 ⎞ ⎛⎜ ni ln⎜ ⎟ ln 2 ⎝ R ⎠ ⎜⎝ n f

⎞ ⎟ ⎟ ⎠

(4)

where Us is the saturation fluence, A is the laser mode cross-section, ni is the initial inversion population density, and nf is the final inversion population density, respectively. The values given in Table 1 were used to calculate the theoretical temporal evolution of Qswitched pulse. The values of stimulated emission cross-section and fluoresence lifetime of the mixed vanadates were obtained from reference [3]. The laser mode cross-section was difficult to measure accurately and it was optimised by comparing the simulated results with the experimental ones. The average power of the output was obtained by the product of the simulated pulse energy and the PRF. The Q-switched pulse width was defined as the FWHM of the simulated Q-switched pulse.

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Table 1. Physical parameters for numerical simulation.

ηpump Ppump V

λp σe

Parameter, unit Pump quantum efficiency Maximum pump power, W laser volume, cm3 Wavelength of pump diode, cm

Value 1 40 0.006

Stimulated emission cross-section, cm2

6.5x10-19 (Nd:Gd0.4Y0.6VO4)

0.8 x10-4

τ

Spontaneous fluorescence lifetime, μs

l la

Cavity length, cm Active length, cm Spontaneous emission factor

Ω

4.8x10-19 (Nd:Gd0.6Y0.4VO4) 125 (Nd:Gd0.4Y0.6VO4) 138 (Nd:Gd0.6Y0.4VO4) 26 2 1.0×10-5 0.29 (Nd:Gd0.4Y0.6VO4) 0.39 (Nd:Gd0.6Y0.4VO4) 0.5 0.03

2

Us

Saturation fluence (J/cm )

R Li A

Reflectivity of output coupler Internal dissipative optical losses Cross-section of laser mode region, cm2

0.3×10-2

5

Peak Power (kW)

4

3

2

1

0

0

100

200

300

400

500

600

700

PRF (kHz) Fig. 6. Peak power of Q-switched output from mixed vanadate slab lasers. Red and blue squares show experimental results in Nd:Gd0.6Y0.4VO4 and Gd0.4Y0.6VO4 lasers, respectively. Red and blue solid curves show the simulated plots.

Red and blue curves in Fig. 5 show the simulated plots as a function of the PRF. When Nd:Gd0.4Y0.6VO4 was used, the average power of the Q-switched output increases gradually with increasing PRF. Above PRF of 300 kHz, the average power reaches the maximum level. The pulse width of the Q-switched output is shorter than 50ns even at the high PRF of 650kHz. Nd:Gd0.6Y0.4VO4 exhibits a relatively low stimulated emission cross-section in comparison with Nd:Gd0.4Y0.6VO4, and it can operate well at a lower pulse repetition frequency. The maximum average power of the Q-switched output appears within PRF range of 200-400 kHz. There is good agreement between the simulation and experiment. The peak power of the Q-switched output defined by dividing the pulse energy by the pulse width is also shown in Fig. 6. The simulated values show qualitative agreement with the experimental data, though there is a slight mismatch of the absolute value. These results show

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that in the low PRF range (100kHz) of mixed gadolinium yttrium vanadate Nd:GdxY1-xVO4 slab lasers with a bounce amplifier geometry. By changing the mixture ratio x, we can adjust the pulse repetition frequency region of the stable Q-switching operation without any loss of the average output power and energy extraction efficiency. We have also simulated numerically the average power and pulse width of the Q-switched output and found good agreement between experiments and simulations. The bounce lasers based on the mixed vanadate slabs show the potential for producing high average power output with custom-made peak power at high pulse repetition frequencies of over 100 kHz. Acknowledgments The authors acknowledge support from the Joint Research Project of the Japan Society for the Promotion of Science, and Engineering Physical Science Research Council (UK) under grant number GR/T08555/01. T. Omatsu’s e-mail address is [email protected].

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