The Laser Interferometer Gravitational-Wave Observatory (LIGO), laser gyroscopes, and certain forms of laser spectroscopy all share the need for laser optics with very highly reflective (HR) or antireflective (AR) coatings. However, measuring the transmission and reflectance of such coatings is done using traditional spectrophotometry or laser ratiometric techniques, which have difficulty measuring residual transmittance of HR coatings or residual reflectance of AR coatings to better than 0.01%, or 100 parts per million (ppm)—when sub-ppm accuracy is sometimes needed. Researchers at the University of Electronic Science and Technology of China and the Chinese Academy of Sciences (both in Chengdu, China) have come up with a different approach for measuring reflection (R), transmission (T), and optical loss (L) parameters of high-performance laser optics using a two-channel cavity ring-down (CRD) setup for measurement. The setup allows for mapping as well as single-point measurement of either reflection or transmission (L is determined by subtracting T and R from 1).
For measurement, the optic to be tested is placed in one of the two channels. Two ring-down measurements, one in each channel, are recorded at the same time (any DC offsets of the photodetectors are eliminated by fitting the ring-down signals to an exponential function). For an HR mirror with a 4 ppm transmittance, the measured R, T, and L at a single point were 99.99821 ± 0.00004%, 4.042 ±0.008 ppm, and 13.9 ±0.4 ppm, respectively. For an AR sample, the measured T, R, and L at a single point were 99.99279 ±0.00004%, 50.0 ±0.7 ppm, and 22.0 ±0.4 ppm. The low standard deviations for the measurements show the high accuracy of the technique. Reference: H. Cui et al., Opt. Express (2017); https://doi.org/10.1364/OE.25.005807.