External-cavity birefringence feedback effects of microchip Nd:YAG laser and its application in angle measurement

External-cavity birefringence feedback effects of the microchip Nd:YAGlaser are presented. When a birefringence element is placed in theexternal feedback cavity of the laser, two orthogonally polarizedlaser beams with a phase difference are output. The phase differenceis twice as large as the phase retardation in the external cavityalong the two orthogonal directions. The variable extra-cavitybirefringence, caused by rotation of the external-cavitybirefringence element, results in tunable phase difference betweenthe two orthogonally polarized beams. This means that the roll angleinformation has been translated to phase difference of two outputlaser beams. A theoretical analysis based on the Fabry--Perot cavityequivalent model and refractive index ellipsoid is presented, whichis in good agreement with the experimental results. This phenomenonhas potential applications for roll angle measurement.