Polarisation dependence on aplanarity in the near-planar ring laser

• Main Author: Wells, Peter Vincent
• Language: en
• Published: University of Canterbury. Physics 2011
• Online Access: http://hdl.handle.net/10092/5529

Polarisation of the beam in a near-planar skew-rhombus ring laser is dependent on the aplanarity of the geometrical ray-path and the defects of its cavity mirrors. With reduction in the anisotropy of birefringence in state-of-the-art multilayer dielectric-stack cavity mirrors it is shown that the polarisation measurements for a ring laser employing them would have picoradian sensitivity to the dihedral (fold) angle of the geometrical ray-path. Correction of the residual anisotropy of birefringence in multilayer dielectric-stack mirrors without a substantial increase in the reflectance losses is shown to be possible through reduction in the thickness of the top protective layer. The fundamental limits of the reflectance loss for multilayer mirrors is of the order of 1 part in 10 9 for mirrors employing SiO2 and TiO2 and these limits are shown to be set by the associated losses in the internal materials due to the Urbach tail near the band gap and to the thermodynamic density fluctuations in fabrication. Theoretical expressions have been found which predict the polarisation dependence on the dihedral angle of the geometrical ray-path for three types of near-planar skew-rhombus ring laser. An expression has been found which predicts the tilts required for the mirrors in a symmetric near-planar ring laser to produce a given closed geometrical ray-path. As a special case it is shown that for a near-planar square ring laser which employs pairs of planar and spherical mirrors the dihedral angle is equal to the sum of the tilts of the planar mirrors. A computer program has been prepared which calculates the power reflectances in the s- and p-directions, the coefficients of power transmission into the substrate in the same respective directions, and the anisotropy of birefringence for a multilayer dielectric-stack mirror design. We use the measurements of the amplitude transmission coefficients into the substrate for a given multilayer mirror design and the multilayer mirror program to show that the (real) refractive indices of the layer materials can be determined uniquely and that they can be determined with a fractional uncertainty which is one order of magnitude lower than the greatest fractional uncertainty of the measured amplitude transmission coefficients into the substrate. An experimental 1m x 1m square ring laser which has been constructed and made operational is described. A review is made of polarimeter designs and it is shown that the measurement accuracy can be improved for each by accounting for the anisotropic attenuation of their optical components. A polarimeter which has been constructed is also described.