We have produced a high beam quality pico-second laser based on a continuous-wave diode pumped Nd:YVO4 slab amplifier with a photorefractive phase conjugate mirror. 12.8W diffraction-limited output with a pulse width of 8.7ps was obtained.
©2005 Optical Society of America
High average-power, pico-second lasers have been studied intensely because of their suitability for applications in nonlinear optics, spectroscopy and industrial applications, such as laser machining and laser display .
Neodymium doped yittrium vanadate (Nd:YVO4) is a promising crystal for high average power pico-second lasers, because it has a large stimulated emission cross-section, as well as a broad emission-band, in comparison with conventional Nd:YAG [2–4].
Recently, a side-pumped bounce amplifier based on Nd:YVO4, in which a very high inversion population density occurs in a shallow absorption depth below the pump face, has been successfully demonstrated to produce high powers at high efficiency [5–8].
However, the poor thermal properties Nd:YVO4 induce frequently degradation of beam quality and limitation of power scaling. The use of a phase conjugate mirror (PCM) is one method for correcting thermal issues in laser systems and producing high powers and high quality output. Until now, high quality master oscillator power amplifiers (MOPA) utilizing the PCM have been reported to generate >10W diffraction-limited output in CW or nanosecond regime [9,10]. In recent years, we have demonstrated multi-watt, diffraction-limited pico-second output from MOPA with a PCM based on a photorefractive rhodium doped barium titanate (Rh:BaTiO3) .
In this letter, we present power scaling results of a pico-second phase conjugate MOPA system using a multi-pass amplifier geometry. With this system, a maximum output power of 12.8W was achieved. This value is the highest, to the best of our knowledge, obtained using a phase conjugate MOPA system in the pico-second regime.
2.1 Phase conjugate master-oscillator amplifier
Figure 1 shows the experimental setup of the phase conjugate, double pass amplifier system. The system used a transversely diode-pumped 1.0 at.% a-cut Nd:YVO4 crystal. The crystal had dimensions of 20mm × 5mm × 2mm. The end surfaces of the crystal were AR-coated for 1μm and cut at 5° relative to the normal of the pump face to prevent self-lasing within the crystal. Temperature of a crystal holder was maintained at ~10°C by a water re-circulating chiller.
The pump diode was a CW single-bar diode array, and its wavelength was 809nm. The fast axially-collimated diode output was focused to be a line with dimensions of 0.2mm × 12mm on the crystal. The polarization of the output was parallel to the c-axis of the crystal, thereby yielding maximum absorption. Under these conditions, an experimental small signal gain of over 8,000 was achieved.
A commercial CW mode-locked Nd:YVO4 laser was used as a master laser. It had a pulse duration of 7.4ps and a pulse repetition frequency of 100MHz. A polarizing beam splitter (PBS), a faraday rotator (FR) and a half wave plate (HWP1) formed an optical isolator to prevent optical feedback to the master laser. The master laser beam was line-focused by a cylindrical lens (CL) to obtain a good spatial overlap between the master laser and an ellipsoidal gain volume. The polarization of the amplified beam was rotated using a HWP2 to lie in an extra-ordinary plane of Rh:BaTiO3 crystal, thereby yielding maximum two wave mixing gain. The amplified beam was focused to be a spot with diameter of 2-3 mm onto the BaTiO3 crystal by using lenses L1~L3.
The BaTiO3 crystal with 1000 ppm Rh ion doping was cut at 0° relative to the normal of the c-axis, and it dimensions were 8 mm × 7 mm × 8 mm. The crystal surfaces were AR coated for 1μm. A self-pumped, phase conjugate mirror was formed by the BaTiO3 crystal and an external loop cavity with 4f imaging optics. The loop length was 600mm. The focal length of lenses was 150mm. With this system, the phase conjugate reflectivity was typically ~50%. The phase conjugation of the amplified master beam fed-back automatically to the amplifier again. After passing through the amplifier, the amplified output was extracted by the PBS.
The severe Bragg’s selectivity of reflection and 2k gratings formed inside the crystal can frequently induce frequency-narrowing effects of the phase conjugation. The loop cavity length (~60 cm) was much longer than the coherence length of the master laser (~3mm), and thus, the existence of reflection and 2k gratings was inhibited. Ensuring that, the frequency-narrowing effects were negligible.
Experimental output power as a function of the input master laser power at the pump level of 30 W is shown in Fig. 2. When the input power was above 3mW, saturation of the output power was observed. The maximum output power achieved 8.2 W at the input power of 200mW. The corresponding energy extraction efficiency was 27 % .
2.2. Multi-pass phase conjugate master-oscillator amplifier
We investigated further power scaling of the system by using a multi-pass amplifier geometry. A schematic diagram of the experimental setup is shown in Fig. 3. The amplified master laser was retro-reflected by a prism mirror and was relayed by a spherical lens to the amplifier. After passing through the amplifier twice, the amplified beam was directed toward the phase conjugate mirror. The phase conjugation of the amplified beam passed through the amplifier twice and was extracted off as output by the PBS.
Experimental output power as a function of the input master laser power is shown in Fig. 4. An energy extraction efficiency of 36 % was achieved at an input power of 290mW. This value is 1.3 times larger than that obtained with the previous setup. We also measured output power as a function of pump diode power. The input power was fixed at 290mW. As shown in Fig. 5, the output power was almost proportional to the pump power. With this system, a maximum output power of 12.8W was obtained at a pump power of 38W. This is highest value, to the best of our knowledge, obtained by phase conjugate MOPA in the pico-second region.
The spatial form of the output exhibited a near Gaussian profile. The corresponding beam-propagation factors M2 was <1.5 at the pump level of 30W (Fig. 6). In contrast, the beam propagation factor of the incident amplified beam onto the phase conjugate mirror was 2.6. These results show that the system is capable of compensating for the thermal distortions inside the amplifier.
2.3. Autocorrelation trace
Experimental intensity autocorrelation traces are shown in the Fig. 7. The phase conjugate MOPA output had a pulse duration of 8.7ps for a Gaussian shaped pulse, while the signal pulse exhibited a pulse duration of 7.4ps. As described in our previous paper , pulse broadening is induced by the frequency narrowing effect due to the finite gain-band (~1nm) of the amplifier. Figure 8 shows the lasing spectrum of signal and PC-MOPA output. The spectral width of the signal beam was 0.33nm. By substituting experimental parameters into a numerical approximation (1/∆λoutput=1/∆λgain+1/∆λsignal), we expect the spectral width of the amplified beam to be 0.25nm. This value is consistent with experimental one.
A difficulty arises in the modeling, since the small-signal gain is a function of the angle of the input beams. We partitioned the amplifier into three regions. The model is illustrated schematically in Fig. 9. The region B, in which all beams (the master laser I1 , the retro-reflected amplified beam I2 , the phase conjugation I3 , and the amplified output I4 ) overlap spatially, has a length that is necessarily shorter than the full gain length, and there are additional regions A, C in which the non-overlapping beams separately experience different gains. The B region is approximately 70% of the gain-length seen by the master laser beam. The simulation was performed by using the continuous-wave gain saturation formula. The phase conjugate reflectivity, and the internal loss of the relay optics between the amplifier and the phase conjugate mirror were taken to be 50% and 10%, respectively. As shown in Fig.4, there is good agreement between the simulated curves and the experimental data.
We have demonstrated >10W pico-second diffraction-limited output from a diode-pumped Nd:YVO4 slab amplifier with a photorefractive phase conjugate mirror. A maximum power of 12.8W was achieved by using a multi-pass amplifier geometry. The corresponding extraction efficiency from diode to output was 34%. Further improvement of the output power can be achieved by refinements of the pump diode. The system can also be extended to generate high peak power pulses by using a pulse selector based on an electro-optical modulator.
The authors acknowledge support from a scientific research grant-in-aid (11555010, 15035202) from the Ministry of Education, Science and Culture of Japan and the Japan Society for the Promotion of Science.
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