Yanling Han

Simultaneous determination of optical loss, residual reflectance and transmittance of highly anti-reflective coatings with cavity ring down technique

Cavity ring down (CRD) technique was employed to measure optical losses (absorption and scattering losses), residual reflectance and transmittance of anti-reflectively (AR) coated laser components with transmittance higher than 99.9%. By inserting the AR coated laser component with parallel optical surfaces into the ring-down cavity and measuring the ring-down time versus the angle of incidence with respect to the surface normal, the optical loss and residual reflectance of the laser component were determined respectively at normal and out-of-normal incidences with repeatability of part-per-million level. The transmittance was also determined simultaneously. Experimental results demonstrated that CRD is a simple, inexpensive and fast technique for highly accurate measurements of optical loss, residual reflectance, and transmittance of AR coated laser components widely used in high-power laser systems.

Measurement of losses in optical components using filtered optical feedback cavity ring down technique

A filtered optical feedback cavity ring down (FOF-CRD) technique employing a continuous wave Fabry-Perot diode laser is employed to measure the total optical losses, i.e., absorption and scattering in optical components with arbitrary thickness. The FOF from the ring down cavity (RDC) is re-injected into the oscillator cavity of the diode laser, and the coupling efficiency of the laser into the RDC is significantly enhanced due to the FOF effect. An optical component having parallel optical surfaces is inserted exactly normal to the light beam in the RDC. The optical losses of the component are obtained from the change in the ring-down time of the RDC containing the component with respect to that of the empty RDC. The measurement results for different samples are in good agreement with conventional laser calorimetry data. The experimental results have demonstrated that the FOF-CRD technique is simple, inexpensive and fast for measuring optical losses of optical components used in high-power laser system.

Reflectivity measurement with optical feedback cavity ring-down technique employing a multi-longitudinal-mode diode laser

Cavity ring-down (CRD) techniques based on measuring the rate of decay of light intensity inside the optical cavity, are widely used for trace gas analysis and high reflectivity measurement. In this presentation a filtered optical feedback CRD (FOF-CRD) technique employing a multi-longitudinal-mode continuous-wave diode laser is investigated for measuring high reflectivity of high reflective mirrors. The original spectrum of the diode laser without the effect of FOF has two longitudinal modes covering tens of the free spectral ranges (FSR) of the ring down cavity (RDC). Due to the relatively broadband spectrum, the theoretical efficiency of coupling the laser power into the RDC is less than 0.05%. In the FOF-CRD scheme, on the other hand, the FOF induced overall spectrum broadening is experimentally observed, with the diode laser running with several longitudinal modes. However the bandwidth of each longitudinal mode is significantly reduced. The coupling efficiency of the laser power into the RDC is higher than 20% in FOF-CRD technique. The enhancement of the coupling efficiency induced by the FOF effect is nearly three orders of magnitude. High accuracy measurements of high reflectivity are achieved with this simple FOF-CRD scheme.

Combined cavity ring-down and spectrophotometry for measuring reflectance of optical laser components

A combined cavity ring-down (CRD) and spectrophotometry technique is developed to measure with sufficiently high accuracy the reflectance of any practically fabricated optical laser component with reflectance ranging from below 0.01% to over 99.999%. In this combined technique, a CRD configuration is employed to measure reflectance higher than 99%, and a conventional spectrophotometric configuration, which is formed by simply removing the rear cavity mirror from the CRD configuration, is applied to measure reflectance below 99%. Uncertainties below 0.0001% for reflectance over 99.99% and below 0.3% for reflectance below 99% are experimentally achieved with CRD and spectrophotometry configurations, respectively, of one single experimental setup.

Simultaneous mapping of reflectance, transmittance and optical loss of highly reflective and anti-reflective coatings with two-channel cavity ring-down technique

We demonstrate the use of a two-channel cavity ring-down (CRD) technique for simultaneously measuring/mapping the reflectance R, transmittance T and optical loss L (absorption plus scattering losses) of highly reflective (HR) and anti-reflective (AR) laser components. High reflectance/transmittance of HR/AR components is measured with the ring-down time of CRD signals, while the low residual transmittance/ reflectance of HR/AR components is determined by the amplitude ratio of two CRD signals, and the optical loss is then determined via L = 1-R-T. Experiments are performed to measure and map R, T, and L of HR mirrors with different transmittance levels from below 1ppm to about 70 ppm (part-per-million) and of one AR window at 635nm. For a 4 ppm-transmittance HR mirror, the measured R, T, and L at one position are 99.99821 ± 0.00004%, 4.042 ± 0.008 ppm and 13.9 ± 0.4 ppm, respectively. For the AR sample, the measured T, R, and L at one position are 99.99279 ± 0.00004%, 50.0 ± 0.7 ppm and 22.0 ± 0.4 ppm, respectively. The sub-ppm standard deviations for R, T, and L indicate the high accuracy of the two-channel CRD technique for the simultaneous measurements of reflectivity, transmittance and optical loss of HR and AR components. High-resolution mappings of R, T, and L of both HR and AR samples are demonstrated. The simultaneous measurements/mappings of reflectance, transmittance, and optical loss with sub-ppm accuracy are of great importance to the preparation of high-performance laser optics for applications such as gravitational-wave detection and laser gyroscopes.

Extinction measurement with open-path cavity ring-down technique of variable cavity length

Open-path cavity ring down (OPCRD) technique with variable cavity length was developed to measure optical extinction including scattering and absorption of air in laboratory environment at 635 nm wavelength. By moving the rear cavity mirror of the ring-down cavity to change cavity length, ring-down time with different cavity lengths was experimentally obtained and the dependence of total cavity loss on cavity length was determined. The extinction coefficient of air was determined by the slope of linear dependence of total cavity loss on cavity length. The extinction coefficients of air with different particle concentrations at 635 nm wavelength were measured to be from 10.46 to 84.19 Mm-1 (ppm/m) in a normal laboratory environment. This variable-cavity-length OPCRD technique can be used for absolute extinction measurement and real-time environmental monitoring without closed-path sample cells and background measurements.