SYMPOSIA PAPER Published: 01 January 1988

Operational Limits for ZnSe/ThF Multilayer Mirrors in the Los Alamos Free-Electron Laser Oscillator


The Los Alamos free-electron oscillator is driven by an rf linear accelerator at an energy of ̃20 MeV. This L-band linac delivers electrons in a 100-μs-long train of micropulses (̃10-ps width) each separated by 46 ns. A semi-confocal resonator (g1g2 = 0.87 to 0.95) is used to maximize the optical intensity within the magnetic undulator by which extraction of energy from the electrons and optical gain are maximized.

For operation in the spectral range of 8 to 12 μm. multilayer dielectric reflectors of ZnSe/ThF4 on ZnSe substrates have been used for the end mirrors. At moderate laser power, these reflectors have performed without evidence of deterioration in the multiple environment of laser photons, gamma rays and neutrons [1]. However, when the time-average optical intensity on the mirrors exceeded the range of 500 to 700 kW/cm2, the resonator losses increased and the output power dropped [2]. Post inspection revealed coating damage in the form of a collection of many elongated pits plus one or two larger (1 × 1 mm2) pits penetrating to the ZnSe substrate. The damage mechanism most consistent with the observed morphology is heating of microscopic absorbing defects leading to film rupture by excessive stress.

Author Information

Newnam, BE
Bender, SC
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Developed by Committee: F01
Pages: 404–404
DOI: 10.1520/STP24448S
ISBN-EB: 978-0-8031-5033-1
ISBN-13: 978-0-8031-4481-1