High-sensitivity photoacoustic absorption spectroscopy of nonlinear optical materials

Nonlinear optical materials are important to extend the spectral coverage of existing lasers, mainly via frequency doubling and with optical parametric oscillators. The quality of the materials and components, e. g. in terms of residual absorption, is pivotal for the performance of the devices. The paper presents high-sensitivity absorption measurements of nonlinear optical materials. They were performed using a photoacoustic spectrometer which combines high sensitivity with broad spectral coverage. This allows one not only to quantify the level of residual absorption but also to assist in the characterization of the materials in terms of optically relevant impurities and imperfections. The spectrometer covers the wavelength range between 407 and 2600 nm using a pulsed optical parametric oscillator as excitation source. Pulse energies up to 100 mJ allow one to record absorption spectra with a sensitivity down to 10 ppm/cm. The paper presents spectra of lithium niobate and lithium triborate crystals which are important for high-power nonlinear optical applications. The results are discussed with respect to material impurities and the suitability of individual samples for frequency conversion.