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Femtosecond inscription of waveguide lasers in crystals

Final Activity Report Summary - WGLASER (Femtosecond Inscription of Waveguide Lasers in Crystals)

An monolithic and extremely compact microchip waveguide Q-switch laser based on diffusion bonded YAG:Nd and YAG:Cr crystals was designed and fabricated. Such laser required lower pump emission power than traditional microchip laser because of an efficient coupling of the pump emission from a delivering fibre to the crystal waveguide. Owing to the waveguide design the laser kept its parameters within a wide range of repetition range, which varied from a single pulse up to 10 kHz.

A waveguide saturable absorber was inscribed in the bulk of YAG:Cr4+ crystal. Such a saturable absorber was incorporated in a fibre laser by simple direct butt-coupling to a conventional Yb doped silica fibre. A Q-switched operation of a fibre laser, integrated with the waveguide saturable absorber, was also demonstrated.

Experimental evidence demonstrated that both multi-photon absorption and electron plasma absorptions were responsible for energy deposition and refractive index change in wideband transparent dielectrics. The orders and absorption coefficients of the multi-photon processes and thresholds for permanent refractive index change were measured for numerous series of wideband crystals and glasses at the wavelength of 0.8 µm.

It was found that a beam with elliptical cross-section was a very effective tool for femtosecond inscription because of suppression of self focusing and improved control of energy deposition process. This was a key technological trick for inscription of low loss waveguide in YAG crystals.