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Deformable Mirrors Raise Sensitivity of Gravitational Wave Detectors

The sensitivity of ground-based gravitational wave detectors, like the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), could now be increased by a new type of deformable mirror recently developed by scientists.

The illustration shows the cross-section of a thermal bimorph mirror and its constituents. Controlling the temperature of the mirror changes the curvature of the reflected wavefront. Overlaid on the cross-section is the simulated radial stress, showing a concentration of stress at the boundary of the two layers, where the adhesive holds the structure together. Image Credit: Huy Tuong Cao, University of Adelaide.

Mild ripples in space-time, known as gravitational waves, are measured by Advanced LIGO. Such gravitational waves are induced by distant events like collisions that take place between neutron stars or black holes.

In addition to improving today’s gravitational wave detectors, these new mirrors will also be useful for increasing sensitivity in next-generation detectors and allow detection of new sources of gravitational waves.

Huy Tuong Cao, Research Team Leader, University of Adelaide Node, Australian Center of Excellence for Gravitational Waves Discovery

Deformable mirrors are used for shaping and controlling laser light. They have a surface composed of minuscule mirrors that can be actuated, or moved, individually to alter the mirror’s overall shape.

As described in Applied Optics, the journal of The Optical Society (OSA), Cao and collaborators have developed the world’s first deformable mirror that is based on the bimetallic effect, where mechanical displacement is achieved by using a temperature variation.

Our new mirror provides a large actuation range with great precision,” added Cao. “The simplicity of the design means it can turn commercially available optics into a deformable mirror without any complicated or expensive equipment. This makes it useful for any system where precise control of beam shape is crucial.”

Cao and Aidan Brooks from LIGO conceptualized this novel technology as part of a visitor program between the LIGO Laboratory and the University of Adelaide, supported by the National Science Foundation and the Australian Research Council.

Building a Better Mirror

Ground-based gravitational wave detectors typically utilize laser light that travels back and forth down the two arms of an interferometer to track the distance between mirrors provided at the end of each arm. Gravitational waves induce a moderate but noticeable change in the distance between these mirrors.

To identify this slight variation, highly accurate laser beam steering and shaping are needed and this is achieved with the help of a deformable mirror.

We are reaching a point where the precision needed to improve the sensitivity of gravitational wave detectors is beyond what can be accomplished with the fabrication techniques used to make deformable mirrors.

Huy Tuong Cao, Research Team Leader, University of Adelaide Node, Australian Center of Excellence for Gravitational Waves Discovery

Thin mirrors were used by a majority of the deformable mirrors to cause a large amount of actuation, but such thin mirrors are known to create unwanted scattering because it is difficult to polish them.

Hence, using the bimetallic effect, the scientists were able to develop a new kind of deformable mirror by connecting a metal piece to a glass mirror. When both these components were heated together, it was observed that the metal expands more than that of the glass and causes the mirror to bend.

The latest design is compact, produces a massive amount of accurate actuation, and needs minimum changes to currently available systems.

The fused silica mirrors, as well as the aluminum plates used for producing the deformable mirror, are available in the market. The scientists attached the two layers by carefully choosing a bonding adhesive that would increase actuation.

Importantly, the new design has fewer optical surfaces for the laser beam to travel through, added Cao. “This reduces light loss caused by scattering or absorption of coatings.”

Precision Characterization

Precision characterization methods are needed to produce a highly accurate mirror. An extremely sensitive Hartmann wavefront sensor was developed and built by the scientists to determine how the shape of laser light is changed by the deformations of the mirror.

This sensor was crucial to our experiment and is also used in gravitational detectors to measure minute changes in the core optics of the interferometer. We used it to characterize the performance of our mirrors and found that the mirrors were highly stable and have a very linear response to changes in temperature.

Huy Tuong Cao, Research Team Leader, University of Adelaide Node, Australian Center of Excellence for Gravitational Waves Discovery

Furthermore, the tests demonstrated that the adhesive is the major restricting factor for the actuation range of the mirrors. The team is now working to resolve the limitation induced by the adhesive and will conduct additional tests to confirm the compatibility before integrating the deformable mirrors into Advanced LIGO.


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