Lasers Tweaked to Catch Gravity Waves

Michael D. Wheeler

Researchers at Adelaide University had a bright idea: Develop a laser to boost the sensitivity of gravitational wave detectors.

From Galileo's homemade telescope in the 17th century to the great observatories of the 20th century, astronomers had relied on visible light to probe the mysteries of the universe. In the last half-century or so, however, scientists have made significant breakthroughs in understanding the cosmos by monitoring a larger portion of the electromagnetic spectrum -- notably, radio waves, x-rays and gamma rays.

Despite these advances, limitations remain, largely because most of the sky may be populated by dark matter, which by definition emits virtually no electromagnetic radiation. But Einstein predicted that all matter would produce another sort of wave when objects accelerate or when their gravitational fields interact: disturbances in the fabric of space-time called gravitational waves. Now teams worldwide are building laser interferometers to detect these flexures as they pass.

The instruments face a daunting challenge. Gravitational waves move objects almost imperceptibly, about 1/10,000 of the diameter of a proton, and for only a few milliseconds at a time.

The interferometers are typically built around continuous-wave Nd:YAG lasers, which are favored for their low-noise, single-frequency operation, high power and efficiency. Hopeful gravity-wave astronomers split the laser beam in two and direct the halves to bounce between widely spaced mirrors along the legs of the instrument. When recombined, any gravitational waves that had distorted the length of the legs are detected as an interference pattern in the beam.

The Adelaide team is adding a chain of pump sources to stabilize the output of the Nd:YAG even further, explained Peter Veitch of the university's department of physics. A nonplanar ring oscillator will injection-lock a 5- to 10-W, diode-pumped coplanar folded zigzag slab ring laser that will injection-lock the high-power, diode-side-pumped folded zigzag slab laser.

Veitch said the design has the required power for the interferometric detectors. Next, his team will demonstrate that it also has the needed beam quality and reliability to catch a gravity wave.

"The most exciting aspect of gravitational waves is that they offer a radically new window to the universe," Veitch said. "Astrophysicists expect amazing discoveries in addition to the expected sources of gravitational waves such as supernovae, coalescing binary neutron stars and black holes.