Abstract: A laser system comprising a gain medium configured to amplify incident electromagnetic radiation and a nonlinear optical element configured to convert electromagnetic radiation amplified by the gain medium to a shorter wavelength. The laser system is configured to introduce mode competition and nonlinear effects such that the nonlinear optical element produces output electromagnetic radiation having a frequency spectrum comprising a first peak formed of a first group of frequencies and a second peak formed of a second group of frequencies. A trough separates the first and second peaks. The first and second peaks are the only dominant peaks in the frequency spectrum. The output electromagnetic radiation has a coherence curve comprising a contrast ratio of less than about 0.1 at an optical path difference that is within the inclusive range of about 1.5 mm to about 2.5 mm.

Abstract: An optical apparatus comprising a gain medium exhibiting polarisation dependent absorption along two axes, the gain medium having a weakly absorbing axis and a strongly absorbing axis, an optical pump source arranged to direct pump light towards a first face of the gain medium such that the pump light entering the gain medium has a component of its polarisation parallel to the weakly absorbing axis, a polarisation modifying apparatus and one or more reflectors which are together arranged to modify the polarisation of pump light which exits the gain medium through a second face of the gain medium, and to direct the pump light with modified polarisation back towards said second face of the gain medium.

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.