Owing to the continuous advancement in laser technology, lithium triborate (LBO) has emerged as a widely used nonlinear frequency-doubling crystal in high-power laser systems. Optical thin films are typically applied to the surfaces of LBO crystals to mitigate energy losses caused by Fresnel reflections. However, the laser-induced damage thresholds of both LBO crystals and films deposited on their surfaces typically limit the maximum output power of laser systems. This paper comprehensively reviews several decades of studies on LBO crystals and films deposited on their surfaces. In particular, it includes an examination of the properties of LBO crystals, the laser-damage thresholds of LBO crystals, and the laser-damage thresholds of thin films deposited on LBO crystals, with a summary of research achievements and recommendations on issues to be further investigated. Additionally, this paper discusses the promising prospects for the widespread application of LBO crystals.
The laser damage characteristics of the thin-film polarizers for the wavelength of 532 nm and AOI of 56° were investigated using a Nd:YAG laser system with a wavelength of 532 nm and a pulse width of 9 ns. The results showed that the damage morphologies induced by nodular defects in the witnesses is significantly different compared to 1064 nm polarizers. The nodular seeds are incompletely ejected after on-shot laser and butterfly-like damage pits are formed, which gradually develop into the typical nodular ejection pits as laser irradiation. Analysis of the internal structure showed that single or two adjacent SiO2 particles formed the nodular seeds, which were mainly located in bottom layers and were tightly bound to the surrounding layers. In addition, the simulated electric field intensity (EFI) distributions and damage morphologies of nodular defects with different structures were compared, and it was found that the formation of the nodular damage is closely related with the EFI distributions in the thin films.
The dichroic mirror is broadly used in the broad-field multi-object spectrometer, which is the key component to separate incident light into several wavelength channels. The design and fabrication of the broad angular spectrum dichroic mirror is investigated in this paper. The global optimization is applied to obtain the low passband ripple and the sharp transition between the transmissive and reflective wavelength range. The dichroic mirror was prepared by ion beam sputtering deposition. The results showed the average reflectance was larger than 99% between 310nm and 550nm and the transmittance (single side, mean-polarization) was larger than 98% between 570nm and 1000nm with the angle of incidence 28°± 5°. The film thickness distribution and film sensitivity of the dichroic film were also analyzed. This research relieves the feasibility of the design and fabrication of the broad angular spectrum dichroic mirror by ion beam sputtering deposition process.
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