In this paper, the operation properties of unstable resonators with graded reflectivity mirror (GRM-UR) are studied numerically in a solid-state thin-disk laser in terms of beam quality and power threshold. By comparing to traditional unstable resonator with same output coupling fraction, results show that the GRM-UR is advantageous to suppress ASE effect, but hard to achieve better beam quality when the phase aberrations cannot be well compensated within the cavity.
A eigenmode expansion method (EME) is proposed to solve the laser eigenmode of optical resonator with intra-cavity phase aberration (ICPA) semi-analytically. In this model, the eigen-equation of OR, so called self-reappearance condition is translated to be a linear eigen-value problem, and it is proved that all eigen-modes can be obtained for any resonators. The linear eigen-value problem is solved numerically, and it gives out the transverse distribution and corresponding eigen-value of each eigenmode, which describe the light ﬁeld and diﬀraction loss, respectively. Compared with traditional methods, EME is a semi-analytical method which is unlimited by the order of phase aberration, and it can be solved without numerical iteration. The existing of local modes (LM) in OR with ICPA is proved with EME, which may be the source of local damage on solid medium. And the use of output coupler with transmission, such as graded reﬂectivity mirror (GRM), can prevent the appearance of LM and improve beam quality. Specially, for the ICPA coupled with laser extraction, the linear eigen-value equations become a nonlinear problem, which are numerically solved by the ﬁnite-diﬀerence Jacobian method. The result shows that the optical resonator exhibits transverse modal instability (TMI) with certain cavity parameters.
Criteria of selecting the slab size in high-power end-pumped Nd:YAG zigzag amplifier were
given, the optimized slab thickness is suggested 2~3 mm, with the corresponding slab length less than 15 cm.
Power extraction in the slab was calculated by a smart ray tracing technique, the optimized input flux is 3~4 times
the saturated flux (Is) with a maximum optical-optical (o-o) efficiency of 44.5% for the single-slab single-pass
stage, and 0.1~0.2 Is with a maximum o-o efficiency of 41% for the four-slab angularly double-pass stage.
The chemical oxygen-iodine laser (COIL) is the shortest wavelength and high-power chemical laser demonstrated. To
model the complete COIL lasing interaction, a three-dimensional formulation of the fluid dynamics, species continuity
and radiation transport equations is necessary. The computational effort to calculate the flow field over the entire nozzle
bank with a grid fine enough to resolve the injection holes is so large as to preclude doing the calculation. The approach
to modeling chemical lasers then has been to reduce the complexity of the model to correspond to the available
computational capability, adding details as computing power increased. The modeling of lasing in COIL is proposed,
which is coupling with the effects induced by transverse injection of secondary gases, non-equilibrium chemical
reactions, nozzle tail flow and boundary layer. The coupled steady solutions of the fluid dynamics and optics in a COIL
complex three dimensional cavity flow field are obtained following the proposal. The modeling results show that these
effects have some influence on the lasing properties. A feasible methodology and a theoretical tool are offered to predict
the beam quality for the large scale COIL devices.
Amplified spontaneous emission (ASE) and parasitic oscillations (PO) in disk lasers with unstable resonators
are investigated theoretically. The efficiency loss due to ASE/PO is essentially determined by the product of
disk diameter D and "self-maintain gain" of ASE/PO. Threshold to establish the laser oscillations in presence
of ASE/PO is explored as the self-maintain gain of ASE/PO is higher than threshold gain gt of the resonator.
The criterion to ignore ASE/PO in disk laser is given by gtD < 2.0 as an ASE absorber is clad, gtD < 0.55 as
the disk side is roughened, and gtD <0.8 as the effective ASE path is half reduced. The time evolution of laser
output exhibits a strenuous relaxation oscillation, in which ASE plays a role of strong damping, and consume
the energy before each of sub-pulses.