Rotators Stop the Feedback
Brent D. Johnson
High-power laser beams present unique challenges to laser designers. When Positive Light Inc. agreed to build a glass-amplified laser for the Japan Atomic Energy Research Institute, it combined rotators and isolators from Electro-Optics Technology Inc. in a laser system designed to produce two 20-J uncompressed beams. The rotators were employed with large-aperture polarizers to optically isolate the amplifier stages from each other and to eliminate optical feedback into the amplifier chain.
Rotators and isolators such as this are integral components of a glass-amplified laser that is being built for a Japanese institute.
The beams are used to generate x-rays for such applications as stereoscopic microscopy, ultrafast imaging, high-beam-energy diagnostics and ultraprecision machining.
Steve Edstrom, Positive Light's custom laser product manager, described the laser as a chirped pulse amplification system that takes a 200-fs pulse from a continuous-wave mode-locked Ti:sapphire oscillator operating at 80 MHz and extends it spectrally to >1 ns in a grating stretcher. It amplifies the pulse, first in a Ti:sapphire regenerative amplifier and then in a chain of Nd:glass amplifiers.
The pulse reaches 20 J before compression back to 700 fs. The system amplifies the final stretched pulses to 20 J in a 2-in. glass amplifier rod with a beam diameter of ~43 mm. It then relays the beam into a compressor with a 6-in. diameter, where the beam compresses to 10 J with a 6-in. diameter.
Edstrom explained that the large-diameter Faraday rotator allows the beam to propagate unclipped to the final amplifier stages. Magnetic Faraday rotators, which use rare-earth permanent magnets to produce a uniform 45° polarization rotation, prevent optical feedback and parasitic oscillations commonly associated with multiamplifier systems such as the chirped pulse amplifier.
Electro-Optics Technology's Mike Torrance said that the critical issue involved in making very large Faraday rotators concerns radial uniformity. The challenge, he said, is to ensure that the rotation angle at the center of the glass optic is close enough to the rotation angle at its outer edges to keep optical leakage through the rotator low enough to attain 30-dB isolation when placed between crossed polarizers.
For small-aperture rotators, this can be accomplished relatively easily because the center of the optic is not far from its outer edge. However, this is not the case with large-aperture rotators, so attaining high radial uniformity is difficult.
An alternative to the Faraday rotator for protection against optical feedback is a pulse slicer with a fast rise and fall time. The pulse slicer extracts single pulses from the laser and transmits a portion of them, reducing the pulse rate. The requirements for the Japanese institute's system were such that pulse slicers were used in addition to Faraday isolation to meet the pulse contrast ratio specifications.
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