Diode-Pumped Kilowatt Laser Is Brighter, More Powerful

Jennifer L. Morey

With the goal of advancing diode-pumped solid-state laser technology by making lasers brighter and more powerful, TRW Inc. of Redondo Beach, Calif., introduced its newest diode array-pumped kilowatt laser at Photonics West. The company says this short-pulse device shows an order of magnitude increase in brightness and average power over lasers existing at the start of the advancement program in 1991.
The Nd:YAG system, which employs a master oscillator power amplifier architecture, achieved pulse energies of 10 J at 1.06 µm and 5 J at 0.53 µm. The architecture created a device with an average power approaching 1 kW (940 W) at 1.06 µm in a single beam line. Beam quality was as high as 1.253 diffraction- limited.
David W. Mordaunt, deputy manager of TRW's laser research department, said that diode-pumped lasers had up to 100-W average power in 1991, but with low beam quality and many aberrations. Scaling diode-pumped solid-state lasers to the kilowatt range presented the problem of maintaining a high beam quality -- a problem that phase conjugation solved.

Supporting roles
Employing a liquid-based stimulated Brillouin scattering cell, the phase conjugator corrected most of the aberrations in the system, while demonstrating high reflectivity and fidelity. Mordaunt added that without phase conjugation the beam quality wouldn't be as high.
Development of the kilowatt diode also relied heavily on advances in component technology. To store enough energy to extract 10 J per pulse in a single beam line, the company needed slabs of Nd:YAG laser material that were larger than traditional slabs.
They developed a process to optically join and diffusion bond three polished Nd:YAG segments at high temperatures; the resulting slabs had material and optical properties consistent with single Nd:YAG pieces. TRW also required a 50 percent energy conversion for the second harmonic generator. For this purpose, they coupled four KTP crystals into an array.
Funded by the Defense Advanced Research Projects Agency, the advancement project is intended to benefit military applications such as long-range illumination.
In the future, the company hopes to achieve second harmonic generation with up to 500 W average power in the green.