Abstract: The cooling of the outer surface of the rod or other solid active element of a solid state laser is improved by causing coolant to travel in a plurality of, preferably streamline, helical flows extending around this surface in contact with it. The respective longitudinal components of a pair of the helical flows preferably extend in opposite directions along the surface to improve the uniformity of cooling.

Abstract: For separation of second and higher order second harmonics from a composite laser beam, a substrate is provided with a binary level relief structure that, in the preferred construction, consists of a regular series of parallel substantially rectangular grooves on one or both of its major surfaces. In a transmissive grating, which is traversed only once by the beam, the depth h of the grooves is chosen to be .lambda. divided by n.sub.1 -n.sub.0, where n.sub.0 is the refractive index external to the grating (assumed to be approximately unity for air), n.sub.1 is the refractive index of the substrate, and .lambda. is the wavelength that will pass directly through the grating, e.g. the second harmonic. Wavelengths greater than .lambda., e.g. the fundamental 2.lambda., will be diffracted away from the beam path. When a reflective coating is applied on the surface with the relief structure, the groove depth is chosen to be .lambda. divided by 2n.sub.0 or 2n.sub.

Abstract: To maintain substantially constant the temperature of a diode junction of a pulsed, cooled, diode laser and hence the wavelength of the optical output of the laser, which laser is used for optically pumping a main, pulsed, solid state laser, the diode junction is heated by an external source during inactive intervals between pulses of the diode laser. This heating, which preferably is achieved by passing a heating current through the diode junction, is adjusted to ensure that, per a unit time greater than the length of an inactive interval, the heat energy received by the diode junction from this external heating together with the heat energy generated in the diode junction itself by the pulses is substantially constant independent of the length of the inactive intervals. Preferably the heating current has a constant level except for a period after each pulse when the level is reduced, e.g. to zero.