Stabilized laser measures Earth's movement

25 Apr 2007

A wavelength-stabilized laser diode could soon be heading for the sea-floor to study how the Earth's crust moves.

An interferometric tiltmeter based on a wavelength stabilized laser diode could be ideal for making a range of geophysical measurements, claim researchers at the University of Tokyo in Japan. The ultimate goal is to place the system on the seafloor to observe movements in the Earth's crust. (Review of Scientific Instruments 78 026105)

Conventional tiltmeters are large and have inherent disadvantages such as electrostatic or chemical drift as well as low sensitivity. "In comparison, laser-based tiltmeters are compact, have high sensitivity and long life time with extremely small drift," Teruhito Hori from the University's Department of Advanced Materials Science told optics.org.

Hori explains that the team's initial stabilized source, a He-Ne, was not suitable for this application due to power consumption and lifetime issues. "A tiltmeter needs to be compact, durable, have low power consumption, resistance to mechanical disturbances, have long life and wavelength stability," he said. "We have developed a tiltmeter based on a wavelength stabilized distributed Bragg reflector (DBR) laser diode (LD) to meet these demands."

The two main issues that the team needed to address were long term stability of the DBR-LD and low power consumption to ensure the tiltmeter could be battery-powered and used in the field.

Having selected a DBR-LD operating at 852 nm, Hori and colleagues used the Cs-D₂ line as a reference and used the modulation transfer spectroscopic method to stabilize their source. "The DBR-LD offers single-mode oscillation, a narrow linewidth of several MHz and wavelength tunability," said Hori. "We used a DBR-LD with a total output power of 5 mW (half of this for stabilization and half for output. We have also continuously locked the laser to the Cs-D₂ line for more than a week without mode hopping." The team found that the stabilized laser frequency was better than 1x10^-10 in the long-period region. Hori adds that a long term stability superior to 10^-8 is sufficient for their purposes.

The team chose components such as a ferroelectric liquid crystal shutter for the stabilization and a thermoelectric cooler to minimize power consumption. "We selected low-power elements and suppressed the total power consumption to less than 2 W," commented Hori. "This is sufficient for battery operation."

The final interferometric tiltmeter uses two independent Michelson interferometers attached to a baseplate to detect displacement in two orthogonal directions. "The mirrors within the interferometers are attached to a suspended pendulum mass which is subject to gravity," said Hori. "As subduction of the Earth's crust occurs, the distances between mirrors and baseplate vary and tilt is detected."

According to Hori, other fields that could eventually benefit from this technology are Lidar for environmental measurements; estimation of the quantity of botanical-plankton pigment for ocean observation; and cosmic use in projects such as LISA (Laser Interferometer Space Antenna).

"Our next step is to test observations in a ground-based tunnel prior to seafloor observations in preparation for putting a laser tiltmeter to practical use," concludes Hori. This work was supported by the Earthquake Research Institute, University of Tokyo.