Picosecond master-oscillator, power-amplifier ... - OSA Publishing

Picosecond master-oscillator, power-amplifier system based on a mixed vanadate phase conjugate bounce amplifier Naoki Shiba1, Yasuhito Morimoto1, Kenji Furuki1, Yuichi Tanaka1, Kouji Nawata1, Masahito Okida1, and Takashige Omatsu1,2 1

Department of Information and Image Sciences, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan 2 PRESTO Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama, Japan [email protected]

Abstract: We demonstrate a high average power ~4 ps output from a phase conjugate laser system based on a diode-side-pumped Nd:Gd0.6Y0.4VO4 bounce amplifier. An average output power of 16.2 W with a peak power of 210 kW was achieved. A corresponding extraction efficiency of 23% was measured. © 2008 Optical Society of America OCIS codes: (190.5040) Phase conjugation; (320.5390) Picosecond phenomena; (140.3580) Lasers, solid-state; (140.3280) Laser amplifiers.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

G. J. Sphler, R. Paschootta, U. Keller, M. Moser, M. J. P. Dymott, D. Kopf, J. Meyer, K. J. Weingarten, J. D. Kmetec, J. Alexander, and G. Truong, “Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam,“ Opt. Lett. 24, 528-530 (1999). U. Keller, K. J. Weingarten, F. X. Kartner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Honninger, N. Matuschek, and J. Aus der Au, IEEE J. Quantum Electron 2, 435 (1996). J. E. Bernard, and A. J. Alcock, “High-efficiency diode-pumped Nd:YVO4 slab laser,” Opt. Lett. 18, 968970 (1993). M. J. Damzen, M. Trew, E. Ross, and G. J. Crofts, “Continuous-wave Nd:YVO4 grazing-incidence laser with 22.5 W output power and 64 % conversion efficiency,“ Opt. Commun. 196, 237-241 (2001). A. Agnesi, L. Carra, F. Pirzio, G. Reali, A. Tomaselli, D. Scarpa, and C. Vacchi, “Amplification of a lowpower picosecond Nd:YVO4 laser by a diode-laser side-pumped grazing-incidence slab amplifier,“ IEEE J. Quantum Electron 42, 772-776 (2006). K. Nawata, Y. Ojima, M. Okida, T. Ogawa, and T. Omastu, “Power scaling of picosecond Nd:YVO4 masteroscillator power amplifier with a phase-conjugate mirror,” Opt. Express 14, 10657-10662 (2006). K. Nawata, M. Okida, K. Furuki, and T. Omastu, “MW ps pulse generation at sub-MHz repetition rates from a phase conjugate Nd:YVO4 bounce amplifier,” Opt. Express 15, 9123-9128 (2007). J. Liu, Z. Wang, X. Meng, Z. Shao, B. Ozygus, A. Ding, and H. Weber, “Improvement of passive Qswitching performance reached with a new Nd-doped mixed vanadate crystal Nd:Gd0.64Y0.36VO4,” Opt. Lett 28, 2330-2332 (2003). H. Zhang, J. Wang, C. Wang, L. Zhu, X. Meng, M. Jiang, and Y. T. Chow, “A comparative study of crystal growth and laser properties of Nd:YVO4, Nd:GdVO4 and Nd:GdxLa1-xVO4 (x = 0.80, 0.60, 0.45) crystals,” Opt. Mater. 23, 449-454 (2003). Y. F. Chen, M. L. Ku, L. Y. Tsai, and Y. C. Chen, “Diode-pumped passively Q-switched picosecond Nd:GDxY1-xVO4 self-stimulated Raman laser,” Opt. Lett. 29, 2279-2281 (2004). J. Liu, X. Meng, Z. Shao, and M. Jiang, “Pulse energy enhancement in passive Q-switching operation with a class of Nd:GdxY1-xVO4 crystals,” Appl. Phys. Lett. 83, 1289 (2003). T. Omatsu, M. Okida, A. Minassian, and M. J. Damzen, “High repetition rate Q-switching performance in transversely diode-pumped Nd doped mixed gadolinium yttrium vanadate bounce laser,” Opt. Express 14, 2727-2734 (2006). T. Omatsu, M. Okida, A. Minassian, and M. J. Damzen, “Passive Q-switching of a diode-side-pumped Nddoped mixed gadolinium yttrium vanadate bounce laser,” Appl. Phys. 90, 445–449 (2008). T. Imaizumi, M. Goto, Y. Ojima, and T. Omatsu, “Characterization of a picosecond phase conjugate Nd:YVO4 laser system,“ Jpn. J. Appl. Phys. 43, 2515-2518 (2004). H. Jiang, H. Zhang, J. Wang, H. Xia, X. Hu, B. Teng, and C. Zhang, “Optical and laser properties of Nd:GdVO4 crystal,” Opt. Commun. 198, 447-452 (2001).

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Received 30 Jul 2008; revised 25 Aug 2008; accepted 25 Aug 2008; published 29 Sep 2008

13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16382

16. J.-C. Diels and W. Rudolph, Ultrashort laser pulse phenomena : Fundamentals, Techniques, and Applications on a Femtosecond Time Scale (Academic, San Diego, Calif., 1996), 32-38. 17. K. Buse, S . Riehemann, S. L. Oheide, H. Hesse, F. Mersch, and E. Kraetzig, “Refractive indices of single domain BaTiO3 for different wavelengths and temperatures,” Phys. Stat. Sol. (a) 135, K87-K89 (1993). 18. Y. Ojima, K. Nawata, and T. Omatsu, “Over 10-watt picosecond diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror,” Opt. Express 13, 8993-8998 (2005)

1. Introduction High average-power picosecond sources have been attracting intense attention for a variety of commercial and scientific applications, such as nonlinear frequency conversion processes, nonlinear microscopy and microfabrication [1, 2]. Nd-doped vanadate bounce amplifiers, wherein an intense inversion population can be produced below the pump face, have been successfully demonstrated to generate high average-power picosecond outputs without a conventional regenerative amplifier configuration at ultra-high efficiencies [3-6]. To date, Nawata et al. have demonstrated highly intense, high-average-power, near-diffraction-limited picosecond output from a Nd-doped yttrium vanadate (Nd:YVO4) bounce amplifier with a photorefractive phase conjugate mirror in combination with a pulse selector formed by a RbTiPO4 (RTP) Pockels cell [7]. A maximum output power of >25 W with a peak power of >5 MW was achieved at a pulse repetition frequency of 0.3-1 MHz. The pulse width of the output from the system was measured to be as much as ~8 ps, and was limited by the finite gain bandwidth of the Nd:YVO4 amplifier. Mixed vanadate technology, in which, for example, a fraction of Gd ions in the gadolinium vanadate (GdVO4) are substituted by Y ions, allows for the customization of the fluorescence spectrum, thereby yielding adjustable laser parameters such as gain bandwidth and stimulated emission cross-section [8-11]. Recently, we successfully demonstrated actively and passively Q-switched Nd-doped mixed gadolinium yttrium vanadate (Nd:GdxY1-xVO4) lasers with custom-made pulse repetition frequency and peak power ranges [12. 13]. The Nddoped mixed vanadates are promising candidates for much shorter picosecond pulse generation, i.e., ~5 ps pulse, at high efficiencies. In this paper, we demonstrate high average-power sub-5 ps (4 ps) output from a diodeside-pumped phase conjugate Nd:Gd0.6Y0.4VO4 amplifier, for the first time. The maximum average power of 16 W was obtained at a pump power of 70 W. The corresponding peak power of the output was 0.2 MW at the pulse repetition frequency of 20 MHz. 2. Experiments 2.1 Experimental setup Figure 1 shows the experimental setup used in this study for the phase-conjugate amplifier system. The amplifiers used for the experiment were a-cut, 2 mm × 5 mm × 20 mm, 1.5at.% Nd-doped Gd-rich mixed vanadate Nd:Gd0.6Y0.4VO4 (Hortek Crystal Co., Ltd) slab and a 1.5at.% Nd-doped GdVO4 slab. The two end faces of the amplifiers were AR coated for a wavelength of 1 μm, and they were wedged to the normal of the pump face to prevent selflasing within the crystal. A continuous-wave (CW) 808 nm pump diode array output was linefocused by a cylindrical lens CLD (f =12.7 mm) on the pump face of the amplifiers. The maximum pump power was 56 W. The master laser used was a commercial 20 mW continuous-wave mode-locked Yb-doped fiber laser, having a pulse width of 5.1 ps and a pulse repetition frequency (PRF) of 20 MHz. Its lasing frequency was 1063.5 nm. A polarizing beam splitter (PBS), a Faraday rotator (FR), and a half-wave plate (HWP1) formed an optical isolator to prevent feedback to the master laser. The master laser beam was focused using cylindrical lenses, HCL1 (f =400 mm) and VCL (f =100 mm), so that the master laser beam spatially matched the ellipsoidal gain volume. The amplified master laser beam was retro-reflected, and it backed to the amplifier by 4f imaging optics formed by a prism mirror and a spherical lens L1 (f =100 mm). The

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external incident angles of the master laser beam and the amplified master laser beam with respect to the pump surface were 15o and 18o, respectively. After passing through the amplifier twice, the amplified beam was collimated by two cylindrical lenses, VCL (f =100 mm) and HCL2 (f =200 mm), and was directed toward a phase-conjugate mirror based on a Rh:BaTiO3 crystal by imaging optics including two spherical lenses L2 (f =150 mm) and L3 (f =75 mm). The polarization of the amplified beam was rotated using a half-wave plate, HWP, and it lay in the extraordinary plane of the Rh:BaTiO3 crystal to maximize the two-wave mixing gain [14]. The phase conjugation of the amplified beam was automatically fed back to the amplifier. After passing through the amplifier twice, it was ejected as an output by a PBS. The 1000-ppm Rh-doped BaTiO3 crystal used was 0º-cut relative to the normal to the caxis with dimensions of 6 mm × 5 mm × 4 mm. The crystal surfaces were AR-coated for 1 μm. The crystal was mounted on a copper block cooled by a water re-circulating chiller, and the temperature of the mount was maintained at ~16 oC. A self-pumped phase conjugate mirror was formed by the Rh:BaTiO3 crystal and an external loop cavity with 4f imaging optics (f =100 mm). The angle of the external loop cavity was 15o and its length was 400 mm. As indicated in our previous publications, a phase conjugate reflectivity of ~50% was typically achieved with this system.

Fig. 1. Experimental setup of the phase-conjugate amplifier system.

2.2 Experimental results Figure 2 shows the average output power in the Nd:Gd0.6Y0.4VO4 and Nd:GdVO4 amplifiers as a function of the pump power. Below the pump power of 30W, the Nd:Gd0.6Y0.4VO4 showed low output power when compared to Nd:GdVO4. Above the pump power of 30W, the Nd:Gd0.6Y0.4VO4 exhibited almost the same slope efficiency as that of Nd:GdVO4. The output power in the Nd:Gd0.6Y0.4VO4 and Nd:GdVO4 amplifiers reached 7.2 W and 8.5 W at the maximum pump level (56 W), respectively. As shown in Fig. 3, the output of the Nd:GdVO4 had a pulse duration of 6.3 ps, while the master laser had a pulse-duration of 5.1 ps. We #99607 - $15.00 USD

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assumed Gaussian-pulse shapes (Fourier transform product = 0.441) for all autocorrelations, and thus, the corresponding Fourier-transform limit was 2.4ps. Pulse broadening was induced by the narrowing of the lasing frequency (Fig. 4) due to the finite gain-band of the Nd:GdVO4 [15]. On the other hand, the output in the Nd:Gd0.6Y0.4VO4 had almost the same spectral bandwidth (0.7 nm) as that of the master laser output, though its center frequency was shifted towards the red so that it matched spectrally the gain-band of the Nd:Gd0.6Y0.4VO4. The pulseduration of the output was estimated to be 4.1 ps.

Average Power (W)

10

Nd:Gd0.6Y0.4VO4 Experimental Nd:GdVO4 Experimental Nd:Gd0.6Y0.4VO4 Simulated Curve Nd:GdVO4 Simulated Curve

8 6 4 2 0

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Pump Power (W) Fig. 2. Average output power in the Nd:GdVO4 and Nd:Gd0.6Y0.4VO4 amplifiers as a function of the pump power.

Master Laser Nd:GdVO4 Nd:Gd0.6Y0.4VO4

Intensity (a.u.)

1.0

0.5

-10

-5

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Time (ps) Fig. 3. The intensity autocorrelation trace of the output.

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Master Laser Nd:GdVO4 Nd:Gd0.6Y0.4VO4

Intensity (a.u.)

1.0

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0.0 1062

1063

1064

1065

Wavelength (nm) Fig. 4. The wavelength trace of the output.

The peak power estimated by using the measured pulse width of the output as a function of pump power is shown in Fig. 5. The maximum peak power of the output in Nd:Gd0.6Y0.4VO4 reached 87 kW, which was 1.2-times higher than that in Nd:GdVO4. The mixed vanadate is capable of generating higher peak power picosecond pulses when compared to conventional Nd:GdVO4.

Peak Pewer (kW)

100 Nd:GdVO4 Nd:Gd0.6Y0.4VO4

80 60 40 20 0

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Pump Power (W) Fig. 5. Estimated peak power with the measured pulse width of the outputs as a function of pump power.

2.3 Power scalability of Nd:Gd0.6Y0.4VO4 amplifier For further power scaling of the system, we replaced the pump diode with a three-bar diode stack. As shown in Fig. 6, the output power was almost proportional to the pump power, and reached a maximum of 16.2 W at a pump power of 70 W, and a corresponding energy extraction efficiency of ~23% was measured. Above this pump level, severe fractional thermal loading of the amplifier impacted the performance of the system, and saturation of the output power was seen. To achieve the efficient output from the system, the amplifier beam must be

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200

Experimental Simulated Curve

16

160

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120 8 80 4 0

40 0

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70

Peak Power (kW)

Average Power (W)

relayed to be a φ1-3mm spot onto the PCM as related in our previous publication [6]. The saturation was mainly due to inadequate recollection of the amplified beam into the PCM by the strong thermal lens effects in the amplifier. The measured pulse width of the output was ~4 ps (Fig. 7), and the maximum peak power of the output was estimated to be ~210 kW.

0

Pump Power (W) Fig. 6. Output power as a function of pump power under three-bar diode stack pumping.

Intensity (a.u.)

1.0

Signal Light Phase Conjugation

0.8 0.6 0.4 0.2 0.0 -10

-5

0

5

10

Time (ps) Fig. 7. Intensity autocorrelation trace of the output under three-bar diode stack pumping.

The output also exhibited a near Gaussian profile, as shown in Fig. 8, and the corresponding beam-propagation factors, M x2 and My2, were