We demonstrated high-repetition-rate (sub-MHz) MW pulse generation by combining a picosecond phase conjugate laser system based on a diode-side-pumped Nd:YVO4 bounce amplifier with a pulse selector based on a RbTiOPO4 electro-optical modulator. Peak output powers in the range of 2.8-6.8 MW at a pulse repetition frequency range of 0.33-1.0 MHz were achieved at an extraction efficiency of 34-35%.
©2007 Optical Society of America
High-power picosecond pulse sources are useful light sources for a variety of commercial and scientific applications, including spectroscopy, nonlinear frequency conversion processes, nonlinear microscopy and microfabrication.
Neodymium-doped yttrium vanadate (Nd:YVO4)  is a promising solution for high-average-power picosecond lasers, since it has a large stimulated-emission cross section as well as a broad emission-band compared with the conventionally used Nd:YAG.
A picosecond master-oscillator power amplifier (MOPA) based on a side-pumped Nd:YVO4 bounce amplifier, which is capable of producing an extremely high single-pass gain (>1,000) [2,3], has been successfully demonstrated to generate high output powers as well as highly efficient outputs without a conventional regenerative amplifier [4, 5]. Agnesi et. al, have demonstrated 8.4W high-quality picosecond output from the Nd:YVO4 bounce amplifier with an optical efficiency of 30%.
In particular, using a phase conjugate mirror (PCM) can enable high output powers from such systems without degrading the beam quality by utilizing a self-aligned multipass amplifier. Recently, Nawata et. al, demonstrated an over-25-W near-diffraction-limited output from a Nd:YVO4 MOPA utilizing a photorefractive PCM formed from rhodium-doped barium titanate (Rh:BaTiO3) . However, the peak output power of this system was limited to ~30 kW.
In this paper, we present, for the first time, how we were able to achieve high-repetitionrate (sub-MHz) MW pulse generation by combining this system with a pulse selector based on a RbTiOPO4 (RTP) electro-optical modulator. Peak output powers in the range of 2.8-6.8 MW at a pulse repetition frequency range of 0.33-1.0 MHz from a pump power of 74 W were demonstrated at an extraction efficiency of ~35%.
2.1 Experimental setup
Figure 1 shows the experimental setup of the phase-conjugate amplifier system used in these experiments. A commercial 200-mW continuous-wave mode-locked Nd:YVO4 laser, having a pulse duration of 6.2 ps and a pulse repetition frequency (PRF) of 100 MHz, was used as the master laser. The amplifier used was a transversely diode-pumped 1at.% Nd:YVO4 slab with dimensions of 20 mm×5 mm×2 mm. A continuous-wave (CW) 808-nm diode array output was line-focused by a cylindrical lens (CLD) (f=20 mm) on the pump face of the amplifiers. The maximum pump power was 74 W.
An external electro-optical modulator (EOM) formed by a RTP crystal  and polarizing beam splitters (PBS) was triggered electrically by clock signals synchronized with the pulses from the master laser, and was used to select output pulses in the frequency region of 0.33-1.0 MHz. A PBS, a Faraday rotator (FR), and a half-wave plate (HWP2) were used to form an optical isolator to prevent feedback to the master laser. The master laser beam was focused by cylindrical lenses, HCL1 (f=500 mm) and VCL (f=75 mm), so that the master laser beam spatially matched the ellipsoidal gain volume. The amplified master laser beam was retroreflected and relayed to the amplifier by 4f imaging optics formed by two mirrors and a spherical lens L (f=100 mm). The external incident angles of the master laser beam and the amplified beam with respect to the pump surface were 12° and 15°, respectively.
After passing through the amplifier twice, the amplified beam was collimated by two cylindrical lenses, VCL (f=75 mm) and HCL2 (f=150 mm), and it was then relayed onto a phase-conjugate mirror based on a Rh:BaTiO3 crystal [9, 10] by imaging optics formed by two cylindrical lenses, HCL3 (f=200 mm) and HCL4 (f=75 mm). The polarization of the amplified beam was rotated using a half-wave plate HWP3 so that it lay in the extraordinary plane of the Rh:BaTiO3 crystal, thereby maximizing the two-wave mixing gain. 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.
A BaTiO3 crystal with 1000-ppm Rh-ion doping was cut at 0° relative to the normal to the c-axis, and its dimensions were 8 mm×7 mm×8 mm. The crystal surfaces were AR-coated for 1 µm. The temperature of the crystal mount was maintained at ~20°C. A self-pumped phase conjugate mirror was formed by the BaTiO3 crystal and an external loop cavity with 4f imaging optics (f=150 mm). The angle of the external loop cavity was 15°, and its length was 600 mm. With this system, phase conjugation built up within a couple of minutes, and it had a typical reflectivity of ~50%. The loop cavity length was much longer than the coherence length of the master laser (~3 mm), thus preventing the formation of reflection and 2k gratings and ensuring that frequency-narrowing effects were negligible .
2.2 Experimental results
The average output power as a function of the pump power at a PRF of 100 MHz is shown in Fig. 2(a). The output power was almost proportional to the pump power, and reached a maximum of 26.3 W at the maximum pump level. The peak power of the output was ~34 kW and the corresponding energy extraction efficiency was ~35%.
When the EOM was turned on, the laser started to operate in the PRF range of 0.33-1.0 MHz. In this case, the master laser power was 1-3 mW. Figure 2(b) shows the average power as a function of PRF at a pump power of 74 W. In this PRF region, no significant change in the output power was observed, and an average output power of >25 W was typically obtained. This implies that the amplifier in this system was saturated, even with a very weak master laser input.
The intensity autocorrelation trace of the output is shown in Fig. 3. The output had a pulse-duration (FWHM) of 7.6 ps for a Gaussian-shaped pulse. The output from the system included undesired amplified prepulses together with the main output pulses. Figure 4 shows that these undesired pulses were rather weak compared with the main pulse, and the contrast ratio of the undesired pulse to the main pulse was measured to be ~0.0017 (1:600) using a PIN photodiode. In these experiments, the peak and average powers of the main pulses without the prepulses were estimated at various PRFs. The peak power of the main pulses was 2.8 MW at a PRF of 1 MHz, and it reached up to 6.8 MW at a PRF of 0.33 MHz (Fig. 5). This value is 200-times higher than that at a PRF of 100 MHz and the average power of the main pulses still exceeded 17 W. The dashed lines in Fig. 5 indicate the numerically simulated values based on the continuous-wave gain saturation formula and the partitioned gain model reported previously . There is good consistency between the experimental results and the values obtained in the simulation.
The spatial profile of the output beam at the maximum pump level is shown Fig. 6. It shows a near-Gaussian profile, and the corresponding beam-propagation factors, M 2 x and M 2 y, were <1.5, and <1.2, respectively. In contrast, the beam propagation factor M 2 x of the incident amplified beam on the phase conjugate mirror was ~4.4. These results demonstrate that the system compensates for thermal distortions inside the amplifier.
Finally, we also demonstrated the microfabrication of a SiO2 glass surface by using the output from the system. The output beam from the system (PRF=1 MHz, average output power ~10 W) was focused using an objective lens (×20, Mitsutoyo Co.) to be a ~ϕ15 µm spot on the surface of a SiO2 glass substrate mounted on a scanning stage. In this case, the intensity (~1 TW/cm2) of the focused output should exceed the laser-breakdown threshold for a solid surface of SiO2 . When the focused output was scanned on the substrate, a ~15 µm-wide groove was fabricated on the SiO2 glass substrate (Fig. 7). The scanning speed (~1 cm/s) was limited by the scanning stage that we used. By using a faster scanning stage with our system, rapid microfabrication with a scanning speed of >1 m/s should be attainable.
We have demonstrated high-repetition-rate (sub-MHz) MW pulse generation by combining a phase-conjugate Nd:YVO4 bounce laser system with a pulse selector based on a RTP electro-optical modulator for the first time. Output pulses with a peak power of 2.8-6.8MW were produced in the pulse repetition frequency range of 0.33-1.0 MHz, and a maximum peak power of ~6.8 MW was achieved at a PRF of 0.33 MHz. The output had a pulse width of 7.6 ps and a near-diffraction-limited profile. Also, the corresponding extraction efficiency was ~35%. Agnesi et. al. demonstrated high-energy picosecond pulse generation from an Nd:YVO4 bounce amplifier . However, their PRF was limited to 100 Hz. Our system, which is capable of producing MW picosecond pulses at high repetition rates of sub-MHz, is a promising solution for ultrahigh-speed microfabrication.
Further power scaling is limited by increases in the undesired prepulse energy due to the poor pulse contrast ratio (~0.0017) of the pulse selector. Further, undesired heat loading due to insufficient energy extraction also limits the further pumping of the amplifier. By refining the pulse selector and the amplifier’s cooling system, further power scaling up to >10 MW should be achievable. Our system can generate high-quality MW pulses in the high repetition frequency range of 0.33-1.0 MHz, and it can be potentially applied to new fields, including laser microfabrication.
The authors acknowledge support from a Scientific Research Grant-in-Aid (19018007, 18360031) from the Ministry of Education, Science and Culture of Japan and the Japan Society for the Promotion of Science.
References and links
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