Angle-scanning an etalon precisely measures its thickness variation

Used in imaging solar spectroscopy, tunable Fabry-Perot interferometers made from lithium niobate wafers must have thickness variations of no more than 1 nm rms (root mean square) across their rather large working apertures (tens of millimeters in diameter). Measuring these small variations is difficult, but a group of researchers at CSIRO Industrial Physics (Lindfield, Australia) has come up with a simple approach that involves rotating the etalon in a collimated, frequency-stabilized He-Ne laser beam and measuring the transmission versus angle.

In a standard laboratory environment, a test etalon with a 37.5-mm clear aperture was rotated on a precise stage with a 0.00008° angular resolution and the transmitted light captured by a digital video camera. The angle of maximum etalon transmission was found for each camera pixel and the data used to determine optical etalon thickness (relative to other pixels) at that point; physical thickness was then easily derived. To check, measurements were taken at different angular wafer orientations and on different days. Measurement repeatability was 0.07 nm rms or better and reproducibility 0.16 nm rms or better; absolute thickness variations were on the order of 1.3 nm. Contact John Arkwright at [email protected].

Correction

In “Gallium nitride has low loss at 1550 nm” (Newsbreaks, December 2005, p. 11) we neglected to note that the work was a joint effort by Lucent Technologies’ Bell Labs and Samsung Advanced Institute of Technology.