Off-surface optic axis birefringent filters for smooth tuning of broadband lasers.

Transition metal-doped gain media, such as Ce:LiCAF, Ti:sapphire, Cr:ZnSe, and Fe:ZnSe, possess wide gain bandwidths that could provide ultra broadly tunable laser output with the usage of adequate intracavity tuning elements. Birefringent filters (BRFs) are a low-cost and easy to use solution for tuning. However, for ultrabroad gain media, regular on-surface optic axis BRFs could not provide smooth tuning of laser wavelength in the whole emission range. Basically, regular BRFs could not accommodate a large enough free spectral range with acceptable modulation depth variation while tuning, due to their slow tuning rates. Motivated by this, in this study, we have numerically investigated the effect of optic axis orientation on filter parameters for magnesium fluoride birefringent tuning plates. We have shown that a magnesium fluoride BRF with an optic axis diving by 30° into the plate could provide smooth tuning of ultra-broad laser gain media. A similar analysis has shown that for broadband tuning applications, the optimum optic axis diving angle lies around 25° for crystal quartz BRFs. The proposed filters have the potential to be useful in tuning of broadband lasers in continuous-wave, long-pulsed, and femtosecond operation regimes.