Single-Stage YAG Amplifier Sets Power Record
A research group in Japan has achieved 132 W of average green power at 1 kHz by frequency doubling an Nd:YAG master oscillator power amplifier system. Both this second-harmonic power level and the 362 W of average fundamental power at 1 kHz from the system are the highest outputs achieved with a single-stage amplifier. An immediate application will be to increase the pump power available for ultrashort-pulse Ti:sapphire lasers.
The laser was designed and built by scientists from the Japan Atomic Energy Research Institute in Kyoto, Hamamatsu Photonics KK in Shizuoka and Osaka University in Osaka. It comprises an Nd:YAG master oscillator that is Q-switched at 1 kHz and a zigzag Nd:YAG slab oscillator that serves as both a preamplifier and a power amplifier.
Pulses emerging from the oscillator make a double pass through the amplifier and are polarization-rotated by two passes through a quarter-wave plate so that they can be separated from incoming pulses with a thin-film polarizer. After being spatially filtered, the pulses are image-relayed to the slab amplifier. The image relaying maintains a uniform beam intensity profile and avoids optical damage from Fresnel diffraction.
A new master oscillator power amplifier system produces 132 W of frequency-doubled green light. The system has an unusually small footprint, thanks to its single-stage amplifier, which serves as both a preamplifier and a power amplifier.
The pulses double-pass the amplifier again, this time through a 45° Faraday rotator, so the effect of thermal birefringence during the first pass is negated during the second. Next, a stimulated Brillouin scattering cell reflects the pulses back for a third double pass of the amplifier. These reflected pulses are the phase conjugates of the incident ones, so phase distortions introduced during the second double pass of the amplifier are reversed during the third. The Brillouin cell also helps compensate for thermal lensing in the slab.
After the third double pass, a thin-film polarizer extracts the pulses and sends them to a KTP doubling crystal. A dichroic mirror then separates the 532-nm and 1.06-µm pulses.
The zigzag Nd:YAG amplifier is water-cooled and pumped from both sides by laser diode arrays at 808 nm. Each array is capable of 9 kW of peak power in 200-µs pulses at 1 kHz. The pulses to be amplified enter the slab at near-normal incidence through antireflection-coated surfaces and zigzag through the medium by total internal reflection from its faces. The Nd:YAG slab is 5 mm thick and 246 mm long, with 32-mm-wide faces for optical pumping, and has 0.85 percent atomic Nd3+ doping. Because of the small size of the amplifier head and the compact design of the optical system, the entire master oscillator power amplifier package has a footprint of only 1 X 1.5 m.
The scientists oriented the 10 X 10 X 10-mm KTP crystal for type II phase matching and maintained it at 80 °C. Although the threshold for photochromic damage in KTP usually is reported to be at least 100 MW/cm2, the group limited power density in the crystal to approximately 70 MW/cm2 to ensure reliable operation.
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