Highly sensitive absorption measurements in lithium niobate using whispering gallery resonators

The absorption coefficient of undoped, congruently grown lithium niobate (LiNbO3) for ordinarily and extraordinarily polarized light is measured in the wavelength range from 390 to 2600 nm using whispering gallery resonators (WGRs). These monolithic cavities guide light by total internal reflection. Their high Q-factor provides several hundred meters of propagation for the coupled light in millimetre size resonators allowing for the measurement of absorption coefficients below 10-2 cm-1, where standard methods such as Fourier-transform or grating spectroscopy meet their limit. In this work the lowest measured value is 10-4 cm-1 at 1700 nm wavelength. Furthermore, the known OH-overtone at 1470 nm wavelength can be resolved clearly.