Superstable laser shines in minivan test

Compiled by Photonics Spectra staff

Think of the jounces you sometimes feel when riding in a car or a van. Now imagine taking a laser on the road that is sensitive to even minimal vibration.

In a step toward making the most advanced atomic clocks more portable, scientists have designed and demonstrated a superstable laser that operates in a cramped, vibrating location: a minivan.

The experiment, led by physicists at the National Institute of Standards and Technology (NIST), shows how advanced lasers can be made both stable and transportable enough for field use in geodesy (measurement of the Earth), hydrology, signal generation and space-based tests of fundamental physics.

Conducting brief drive tests – short excursions of 5 m across the grass on the NIST campus – the scientists evaluated the infrared fiber laser’s performance with the vehicle stationary, with the motor alternately off and idling, and when moving over uneven ground at speeds of less than 1 m/s. Their findings appeared in the May 23, 2011, issue of Optics Express.

The laser frequency remained stable enough with the car parked – the most likely situation in the field – to be used in some applications today, said David Leibrandt, a NIST postdoctoral researcher.

David Leibrandt, a NIST researcher, tests the stability of an advanced laser set up in a minivan. The laser and related instruments are inside the box, which measures 2 x 2 x 2.5 ft. The stainless steel cylinder at the lower left contains the optical cavity used to stabilize the laser, which is hidden behind the cylinder. Courtesy of NIST.

The group has been building and using ultrastable lasers for more than 10 years, but their large size and delicate nature have made them difficult to transport, he said. The new design, however, makes the device less sensitive to vibrations and could be made much smaller and easier to transport.

Using a common technique, the investigators stabilized the laser’s frequency, locking it to the extremely consistent length of an optical glass cavity. This sphere, about the size of a small orange, hangs in a customized mount with just the right stiffness. They also designed a system to correct the laser frequency when the vehicle moves. Six accelerometers surrounding the cavity measured its linear and rotational acceleration. The accelerometers’ signals were then routed to a programmable computer chip that could predict and correct the laser frequency in less than 100 µs.

The scientists hope that the laser will make it easier to use advanced atomic clocks for geodesy as well as on moving platforms, perhaps in space-based physics experiments or on Earth generating low-noise signals for radar. Studies have indicated that the laser is about 10 times more resistant to undesirable effects from vibration or acceleration than the best radio-frequency crystal oscillators. Improved mechanical design and higher-bandwidth accelerometers could make it even more stable in the future. The research was supported by the Office of Naval Research, the US Air Force Office of Scientific Research and DARPA.