Accurate high reflectivity measurement based on a novel optical feedback cavity ring-down technique

Abstract

Highly reflective mirrors have been widely used in high power lasers, laser gyros, and gravitational-wave detection, etc. However, reliable measurement of high reflectivity (R>99.99%) is difficult. In this paper a novel optical feedback cavity ring-down technique (OF-CRD) by re-injecting the strong optical feedback from the ring-down cavity (RDC) into the oscillator cavity of a Fabry-Perot diode laser is developed for the ultra-high reflectivity measurement. The laser line is narrowed and occasionally in resonance with one or more ring-down cavity modes. The amplitude of the RDC output signal is enhanced by a factor of over two orders of magnitude, compared with the conventional phase-shift CRD technique. Four pairs of cavity mirrors with different reflectivity are used to investigate the sensitivity and reproducibility of the OF-CRD technique. The accuracy is greatly enhanced from about 0.003% to 0.00003% as the reflectivity of cavity mirrors increases from about 99.8% to 99.996%. A folded RDC with cavity length of 70cm is constructed by inserting a planar test mirror into the linear RDC and the reflectivity of the test mirror is statistically determined to be 99.9526±0.0004%. The OF-CRD is simple, reliable, highly-sensitive and cost efficient.