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.
When a modulated laser beam irradiates an optical component, the laser-induced surface deformation has both direct
current (DC) and alternating current (AC) portions. Explicit surface deformation and surface thermal lens (STL) theory
models are developed to describe the DC and AC portions of the surface deformation and corresponding STL signals.
Experimentally, a setup combining laser calorimetry (LC) and STL technique is developed to measure the absolute
absorptance and laser-induced surface deformation of optical components. The absorptance measurement is implemented
by LC with excellent stability and repeatability. The surface deformation measurement is realized with STL amplitude
by defining an approximately linear relationship between the AC (or DC) STL amplitude and the maximum AC (or DC)
deformation. As an example, the deformation value of a BK7 substrate coated with a TiO<sub>2</sub>/SiO<sub>2</sub> film stack of absolute
absorptance 1.32×10<sup>-3</sup>, irradiated by a 1064nm laser with 3.8W power is determined to be 34.3 nm with the experimental
STL amplitude, in good agreement with the theoretical value of 35.8 nm calculated by the explicit surface deformation
model. An indirect approach is proposed to determine accurately the irradiation beam radius by fitting the experimental
data of the radial AC intensity change at the detection plane to the explicit STL model. By performing a theoretical fit to
the experimental frequency dependence of the STL amplitude, the thermal properties of the optical component (i.e. the
thermal diffusivity) can also be determined.
A simple and sensitive photothermal technique-photothermal detuning (PTDT), which is based on the absorption-induced shift of reflectance or transmission spectrum of an optical coating, is developed to measure the absorption of coated optical components. A PTDT theory is developed to describe the signal's dependence on the structural parameters of the optical coatings and on the geometric parameters of the experimental configuration. An experiment is performed to measure the PTDT signal of a highly reflective multilayer coating used in 532nm by using a probe beam with a wavelength of 632.8nm. By optimizing the incident angle of the probe beam, the measurement sensitivity is maximized. Good agreements between the theoretical predictions and experimental results are obtained.
A theoretical model for the surface thermal lens (STL) signal with modulated top-hat and Gaussian beam excitations is
developed. For an optical coating sample, distributions of the temperature and surface deformation in both transient and
quasi-steady states are deduced, and the STL amplitudes, corresponding to the alternating current (AC) deformation and
direct current (DC) deformation, respectively, are defined. Numerical simulations and comparison results for the
temperature and deformation demonstrate that there exist large differences in radial distributions between the AC and
DC cases especially in the high modulation frequency range. The pulsed or AC STL amplitude under the top-hat beam
excitation is approximately two times of that under the Gaussian beam excitation at the optimum detection distance in
the high frequency, and correspondingly, the DC STL amplitude in quasi-steady state with top-hat beam excitation is
only ~1.1 times of that with Gaussian beam excitation at the optimum detection distance. Influences of the heating-beam
radius and modulation frequency on the STL amplitudes are also presented and compared. The application of the STL
technique to the deformation measurement of an optical component is discussed.