Suppression of nonlinear phonon relaxation in Yb:YAG thin disk via zero phonon line pumping

Abstract

A quantitative comparison of conventional absorption line (940 nm) pumping and zero phonon line (ZPL) (969 nm) pumping of a Yb:YAG thin disk laser is reported. Characteristics of an output beam profile, surface temperature, and deformation of a thin disk under the different pump wavelengths are evaluated. We found that a nonlinear phonon relaxation (NPR) of the excited state in Yb:YAG, which induces nonlinear temperature rise and large aspheric deformation, did not appear in the case of a ZPL pumped Yb:YAG thin disk. This means that the advantage of ZPL pumping is not only the reduction of quantum defect but also the suppression of NPR. The latter effect is more important for high power lasers.

Status of the High Average Power Diode-Pumped Solid State Laser Development at HiLASE

“…First, we demonstrated that the pulsed pumping can improve the operation of the 1-kHz regenerative amplifier [33]. Further, we showed the advantages of zero-phonon line pumping, not only with respect to a lower thin disk temperature, a lower thin disk temperature difference between the lasing and non-lasing conditions [31], but also a lower amount of thin disk deformations [34][35][36]. The details will be described later in the text.…”

“…The zero-phonon line pumping reduces the heat generated in the laser medium. On the other hand, the absorption line width is only 2.8 nm, which requires the usage of more expensive wavelength-stabilized pump diodes with a narrow emission line [29][30][31][32].…”

“…In the case of 969-nm pumping, the temperature rise is linear, and the aspheric deformation is weak. Interestingly, the temperature difference between the lasing and non-lasing regimes is small, which simplifies the resonator alignment and is less susceptible to the damage caused by lasing failure [31].…”

Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE

“…Thin disks used in the regenerative amplifiers are diamond-bonded Yb:YAG single crystals (220 µm thick, 7 at. % doping) continuously pumped at the zero-phonon-line wavelength (968.8 nm) [24,25].…”

“…An important requirement for application in lithography is stability of all laser parameters and compactness in the actual application field. Intensive study has been performed at the HiLASE Center to confirm the high beam quality thin-disk laser with good pointing stability [24,25]. Higher conversion efficiency from pump pulse to amplified pulse energy is the key to realizing these characteristics.…”