A new concept for compensation of birefringence in solid-state rod lasers is presented. The basic idea of our approach is the simultaneous use of an in-cavity polarization rotator and an imaging optical system between the two passes through the laser rod. The theoretical analysis based on matrix optics shows that perfect compensation for the birefringence and bifocusing in laser rods is possible for a large pumping range. Experimental results with two Nd:YAG rods are also presented. The measured depolarization loss per transit for a test beam at 0.63 μm is less than 5% for pumping power up to 6kW per rod. This would allow efficient generation of high-power TEM00 mode and linearly polarized multimode output power from Nd:YAG oscillators in rod geometry.

A novel approach for compensation of birefringence in cylindrical Nd: YAG rods

Yttrium aluminum garnet (YAG) has thermal, mechanical, and optical properties that vary strongly with temperature. In this paper, we show that when such properties are taken into account the resulting thermally induced distortions in rod amplifiers cannot be described as quadratically varying with rod radius, and thus cannot be completely corrected using a simple spherical lens. In addition, we show that for operation with a coolant temperature at 77 K, thermally induced distortion is minimal and birefringence is substantially reduced when compared to that during operation with a coolant at room temperature.

Nonlinear thermal distortion in YAG rod amplifiers

We study thermally induced birefringence in crystalline Nd:YAG zigzag slab lasers and the associated depolarization losses. The optimum crystallographic orientation of the zigzag slab within the Nd:YAG boule and photoelastic effects in crystalline Nd:YAG slabs are briefly discussed. The depolarization is evaluated using the temperature and stress distributions, calculated using a finite element model, for realistically pumped and cooled slabs of finite dimensions. Jones matrices are then used to calculate the depolarization of the zigzag laser mode. We compare the predictions with measurements of depolarization, and suggest useful criteria for the design of the gain media for such lasers.

/pdf/thermally-induced-birefringence-in-nd-yag-slab-lasers-cxmlvo1dlp.pdf

Thermally induced birefringence in Nd:YAG slab lasers

A system for the compensation of thermally induced birefringence in laser
rods that consists of two identical Nd:YAG laser rods, a telescope to image
the principal planes of the rods onto each other and a 90° quartz rotator
to change the state of polarization between the rods has been set up, and its
sensitivity to transversal and longitudinal misalignment has been investigated
experimentally. A theoretical model to describe the increase of depolarization
that is due to misalignment has been developed and compared with the
experimental results.

Investigation of the misalignment sensitivity of a birefringence-compensated two-rod Nd:YAG laser system

A thorough analysis of the losses due to thermal-gradient-induced stresses in slab lasers is presented in which depolarization losses due to thermal-gradient-induced stresses are calculated properly by treating the changes in the dielectric impermeability tensor. In previous analyses of this problem, the approach in which the index of refraction is treated as a tensor quantity is only applicable in the limit of very small changes. Issues of how to correctly write the reduced-suffix forms of the tensors are also considered. Our formalism is then applied to an edge-pumped Yb:YAG slab laser. We consider straight-through propagation as an example of the formalism and indicate its application to the case of zigzag propagation.

Calculation of thermal-gradient-induced stress birefringence in slab Lasers-II

The thermally induced stress birefringence in YAG rods and slabs has been analysed theoretically. After carrying out the transformation of the fourth-rank photoelastic and piezo-optic tensors from the crystal lattice coordinate system to the laboratory coordinate system, it is possible to calculate easily the modified indicatrix for any stress distribution. The calculations show that the birefringence in a zigzag slab, in general, depends on the angle under which the slab is cut from the boule.

U. Wittrock

Papers

Photoelastic effects in Nd:YAG rod and slab lasers