Laser oscillator by Coherent is at core of LIGO gravitational wave detection

March 29, 2016
Coherent also supplied other lasers for LIGO and polished some of the large-aperture optics.

A laser made by Coherent (Santa Clara, CA), a Mephisto laser oscillator, played a central role in the recent, groundbreaking detection of gravitational waves, which confirmed certain aspects of Einstein's theory of general relativity. Mephisto lasers have higher stability and lower output noise than any other commercial laser, according to Coherent, which made them the choice to act as the first stage oscillator in the world's major gravitational wave detection programs. This includes the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) that made this first observation by comparing synchronous data (recorded on September 14, 2015) at two sites in the United States - Livingston, LA, and Hanford, WA.

Each of the two LIGO sites consists of a L-shaped laser interferometer, approximately 4 km in length. Yet each recorded gravitational wave caused effective changes in the interferometer path length of just 1 part in 1022. In absolute terms, this is less than 1/10,000 the diameter of a proton. Because of this, the Advanced LIGO has been generally referenced as the most sensitive scientific experiment ever constructed.

In terms of the laser, the detected path length changes correspond to less than 10-12 of the 1064 nm laser wavelength. Even using the high-stability Mephisto laser oscillators, to achieve this unprecedented level of sensitivity the LIGO team had to develop complex control systems to further lower the laser frequency noise by orders of magnitude, as required to detect the very faint signal.

This was possible because Mephisto lasers are based on nonplanar ring oscillator (NPRO) technology. Here, the entire intracavity beam path is contained within a small neodymium-doped gain crystal which is impervious to contamination, easy to stabilize, and which lends itself to thermal and piezoelectric fine frequency tuning. External access to these tuning inputs was critical to the stabilization schemes used at LIGO. Other applications of Mephisto lasers include atom trapping, squeezed states research, quantum optics, fiber sensing, and coherent communications research.

In addition to the Mephisto, Coherent supplied other lasers and components to the international teams working at LIGO and European-based VIRGO facilities, including Prometheus, a frequency-doubled laser based on Mephisto, and polishing of some large-aperture optics.

Source: Coherent

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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