Zhechao Qu

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.

High-sensitivity testing techniques for laser optics

The absorptance and high reflectance measurements of laser optics are presented. In the absorptance measurement, the laser calorimetry (LC) technique is investigated. A rigorous theoretical model describing the laser irradiation induced temperature rise in a coated sample, in which both the finite thermal conductivity and the finite size of sample are taken into account, is developed to optimize the temperature detection geometry to further improve the accuracy of the absorptance measurement. For the high reflectivity measurement, an optical feedback cavity ring-down (OF-CRD) technique, in which a continuous-wave (CW) Fabry-Perot (FP) diode laser is used as the light source, is employed for high reflectivity measurement. The linear and V-shaped schemes are investigated to measure the reflectivity of cavity mirrors and planar test mirrors, respectively. For cavity mirrors with reflectance larger than 99.99%, the measurement error is less than 1ppm.