Laser-frequency monitoring measures millimeter distances to picometer accuracy

A scientist from the National Institute of Standards and Technology (Gaithersburg, MD) has pioneered a laser-based technique that can measure distances on the order of millimeters with picometer accuracy-equivalent to measuring the distance from New York to Los Angeles with an uncertainty of 1 mm. Unlike typical laser-interferometeric methods that count and interpolate the number of wavelengths of light between two points (enabling nanometer-level accuracies), the new method uses the frequency of light to obtain its high accuracies-important to applications in nanotechnology and semiconductor fabrication.

In the technique, light from a tunable laser is coupled into a resonant cavity formed by two mirrors. Because the laser is locked to a cavity resonance, the laser frequency follows the cavity-mode frequency when the cavity length changes; the cavity displacement is determined from the change in laser frequency. To extend the measurement range-normally small due to the tuning range of the laser-two adjacent cavity modes are interrogated simultaneously. Keeping track of this frequency difference as well as the optical displacement of an individual mode, the mechanical change in the cavity length can be monitored over a 50-mm range with accuracy below 10 pm. Contact John R. Lawall at [email protected].