At Keio University in Yokohama, Japan, and at Harvard University in Cambridge, Mass., a team of researchers has fabricated an optical vibration sensor by writing a waveguide in glass using femtosecond pulses from a Ti:sapphire laser. The compact sensor, which is resistant to electromagnetic interference, promises an alternative to electric devices for detecting displacements, acceleration and vibration.

The investigators translated a focused 25-MHz train of 55-fs, 20-nJ pulses from the laser at 20 mm/s across a three-piece assembly of soda-lime and borosilicate glass, altering the refractive index of the materials to create a waveguide (see "Wiring' Light with Femtosecond Laser Pulses," Photonics Spectra, December 2004, page 58). Because the central piece of glass is separate from the other two pieces and is supported by a flexible cantilever, it responds to external forces by becoming displaced with respect to the rest of the assembly, which is detectable as increased optical loss across the waveguide.

Using a 1550-nm laser diode as a source and an InGaAs photodiode as a receiver with the fiber-coupled sensor, the researchers determined that the device is sensitive to vertical accelerations as low as 0.01 m/s² and has a linear response over frequencies from 20 Hz to 2 kHz. They observed a 10 percent variation in the frequency response when they increased the temperature from 293 to 343 K. They attributed this to the properties of the material used to fabricate the suspended beam and suggested that alternatives could be employed to minimize the effect.