Abstract

The novel resonator features efficient totally polarised fundamental-mode output and unidirectional operation in a compact and scaleable architecture. The anisotropy of the laser crystals enforces unidirectional operation and completely avoids thermally induced depolarisation. The resonator architecture is shown in figure 1 and is an advanced version of the original variable-configuration resonator (VCR).^1,2 We observed and exptein that the original resonator supports unexpected modes with circular polarisation at the linear polariser. Due to this the output of the VCR with Nd:YAG rods was completely un-polarised. The improvement was now achieved by replacing the quarter-wave plates (QP) by Faraday rotators (FR) allowing for the use of birefringent crystals. Starting at crystal 1 with the light linearly polarised at an angle of 45° with respect to the drawing plane the polarisation is first rotated by 45° in FR 1 so that the light at the linear polariser P is reflected towards FR 2. Here the light propagates into crystal 2 where it is reflected at the HR coating C. Due to the doublepass through FR 2, back at the polariser P, the light is transmitted into arm 3 where it makes another double-pass through FR 3 and crystal 3. After this it is reflected at the polariser and propagates to the output coupler OC and back. Because of the double-pass through the QP the polarisation is rotated again by 90° and the light is transmitted through the polariser P and propagates to crystal 1 where the round-trip of this ring resonator starts again. In principle this roundtrip is also possible in the opposite direction. However, if the crystals are properly aligned the light always has extraordinary polarisation for one of the two propagation directions and ordinary orientation if propagating in the other direction. Due to the higher gain for the extraordinary beam, the laser does only operate in the direction with the extraordinary polarisation.

© 2000 IEEE