Abstract: A non-monolithic ring laser cavity comprising: (a) a gain medium (1); (b) a first polarization rotation element arranged to rotate the polarization of light propagating in the cavity with a predetermined handedness irrespective of the direction of propagation of the light; (c) a second polarization rotation element (7) arranged-to rotate the polarization of light propagating in the cavity with a handedness which is dependent upon the direction of propagation of the light; (d) a polarization selection element (5, 6) arranged to cause loss to light propagating in the cavity, the loss being determined by the polarization of light incident upon the polarization selection element; wherein the polarization selection element comprises at least one mirror spaced away from the gain medium (1) and arranged to reflect light at an angle displaced from the normal such that the reflectivity of the at least one mirror is sufficiently polarization dependent that the laser oscillates uni-directionally.