Yb:CaGdAlO4 crystal for 47-fs diode-pumped laser

We report the first femtosecond laser operation of Yb³⁺:CaGdAlO₄ crystal. Pulses as short as 47-fs have been obtained.

Laser materials suitable for efficient, simple and compact ultrafast laser sources should provide a broad emission bandwidth, exhibit favorable thermal properties, and offer the possibility to be directly diode-pumped. Ytterbium-doped crystals are very promising in this respect. Nevertheless, the creation of ultrashort (<100 fs) oscillators [1]–[3] and the quest of ideal crystals for high power applications (synonymous of high thermal conductivity) is still a real challenge. Various new ytterbium-doped crystals were grown and tested in the femtosecond regime during the past few years such as Yb: BOYS^[1] , Yb:SYS^[2] that allowed the production of particularly short pulses of 69 fs and 70 fs, respectively. On the other hand, some less-broad-emission-band crystals also allowed the production of very short pulses: 71 fs with Yb:KYW^[3] and very recently 61 fs with Yb:YVO₄^[4] thanks to the use of the Kerr lens mode-locked (KLM) technique. It has the advantage of producing shorter pulses than with SESAM (101 fs with Yb:KYW^[5] and 120 fs with Yb:YVO₄^[6]) but the disadvantage of a lower stability and a difficulty for selfstarting of the mode-locked regime. Nevertheless, up to now the ytterbium crystals never outperformed ytterbium-doped glass, phosphate or silicate, which generated pulses as short as 58 fs and 61 fs^[7] respectively. But, the poor thermal conductivities and weak emission cross-sections of glasses are strong limitations for the development of laser systems, especially towards high average power. So we are still in need of an ytterbium doped crystal exhibiting very broad and smooth absorption and emission cross-sections, with a high thermal conductivity and a higher emission cross-section. The first key point would be to have not only a large emission band, but also a very flat gain. A crystal matching these criteria seems to have been found in the name of the ^[8]. Fig. 1. Absorption and emission cross sections of Yb:CALGO (left graph) and (right graph) polarizations.