Optical Characterization of a New Amplifier Media

About this project: The advances made in laser technology must address vast and continuously evolving needs in multiple application areas [1]. Among different types of lasers, the all-solid-state lasers, ideally diode pumped (DPSSL) are best placed to address majority of these needs because of a robust and compact setup, a relatively high efficiency with corre-sponding low thermal cooling requirements, and a long lifetime [2]. Research and de-velopment on nonlinear optics and laser physics have been growing fast over the past twenty years. Behind such an impressive growth is the need for excellent thermal prop-erties and high damage threshold for high power and high energy applications, a large emission spectrum for ultra-short pulse generation, a long lifetime of the excited state for high-energy extraction and the access to new wavelength ranges [1]. Depending on the type of problem to be addressed, these issues demand the development of materials which have a host and a dopant specific in each case. Thus, research on and charac-terization of new laser materials is inevitable to answer to the above mentioned chal-lenges or societal needs of today and tomorrow in general [2].

Efficient use of laser materials requires full characterization of their absorption and emission properties. For most classes of crystals, measurements of these properties can be done along dielectric frame axes which are best described with 3-by-3 second rank linear permittivity tensor. However, in the case of monoclinic crystals, the maximum values of absorption and fluorescence are not along the principal axes of the dielectric frame, but tilted at an angle with respect to one of the axes of the dielectric frame [1]. In general, monoclinic crystals are biaxial crystals such that the imaginary part of refrac-tive index is different than principal refractive indices in the tri-rectangular dielectric frame (x, y, z) [3].

In this master thesis project, we have studied Yb-doped borate type-Yb:Li6(Y/Gd)(BO3)3-monoclinic crystals. The use of these new amplifier media was explored for the devel-opment of new solid-state lasers in different spectral domains: the near-infrared with Ytterbium-doped crystals for short-pulse generation. The project was done in the Short lasers: applications & materials (SLAM) research group which is mainly focusing on pho-tonics at the University of Bordeaux, France. Characterization and tests of new laser materials is one of the themes of the group.  Based on previous experiments, some of the anisotropic properties of highly concentrated Ytterbium-doped monoclinic gain me-dia with hosts from borate family, Yb-doped borate type crystals were characterized. As a continuation of these studies on borate-type crystals [2, 4-6], the relationship between linearly polarized light, absorption and emission of these crystals with the real and imaginary refractive index surfaces was studied with a new and relatively simple approach.

Conclusions: With a consideration of the so called dielectric frame, the relative dielectric permittivity tensor of monoclinic crystals is described by a 3x3 diagonal real matrix plus imaginary matrix with diagonal and two similar off-diagonal elements. The off-diagonal elements depict the relative rotation of the imaginary plane with respect to the principal dielectric plane that are perpendicular to one of the dielectric axes which is parallel to the 2 fold axis or perpendicular to the mirror m. With this concept, from each host matrix, two types of Yb-doped monoclinic crystal samples were prepared and the dependence of the absorption coefficient on the polarization angle for a linearly polarized source at the maximum absorption wavelength of the samples was studied.

From the result of these experiments, the orientation of the dielectric axes were identi-fied with respect to the crystallographic orientations. Consequently, this leads to the full description of the imaginary index surfaces of the LGB:Yb and LYB:Yb monoclinic sam-ples.

However, for laser operations, the “good” absorption axis is not necessarily the “best” emission axis. Therefore, further investigations should be made on the lasing and gain experiments which was already tried during the course of this project [Appendix B, Ap-pendix C]. The absorption experiments should also be improved with a better mechanics for the crystal mounts to minimize angular misalignment of the samples.

Generally, in this master thesis project, we were able to develop rather simple and complete theoretical and experimental approach to characterize the absorption of Yb-doped borate type monoclinic crystals or monoclinic crystals in general. Given the po-tentiality of Yb-doped crystals for future laser performances, it was an amazing journey.