A housing for protecting monitoring equipment employed to maintain watch over dusty environments. The housing comprises a cylindrical jacket (1) that accommodates the equipment. The front of the jacket is closed except for one or more apertures (12) and extends into the environment being monitored. The housing is provided with a connection (10) for a coolant or flush in the form of air. One or more outlets (15 & 17) communicate with the inside of the housing in the vicinity of the aperture and are oriented such that the air emerging from the outlets forms a cushion between the aperture and the environment being monitored.

A total internal reflection, face pumped laser is provided, in a single head, with two (or more) generally parallel, spaced apart hosts for providing a higher average power output than the typical single host FPL. A pair of flash lamps, one each adjacent the outer surface of each host, acts as the "pumping" means for the laser and operates in effect to produce asymmetrical pumping of the respective hosts. In order to prevent exceeding the thermal stress limit and thermally inducing bending of the hosts, a system of asymmetrical cooling is provided whereby a greater flow of fluid coolant is provided the surfaces of the two hosts which are closer to the "pumping" flash lamps than to the inner host surfaces which are proximate one another.

In a high average power slab laser, heat generated in the laser slab is conducted through a thin layer of thermally conductive gas, such as hydrogen, helium or air, and then through an optically transparent window into a flow of liquid coolant. In a preferred embodiment, a second flow of liquid coolant is employed for cooling of the pumping lamps such lamp liquid coolant flow being partitioned from the slab liquid coolant flow by means of an optically transparent partition through which the pump radiation is directed from the lamps to the slab. In another embodiment, the slab coolant flow is controlled so as to operate the laser slab in a self-annealing temperature regime, whereby thermal stresses are annealed out in use and higher average output beam power is obtained.

In a slab laser, the optical pumping lamps extend transversely to the mean direction of the laser beam with the spatial period of the lamps being harmonically related to the spatial period of the zig-zag laser beam path within the slab, preferably with equal periods. In addition, the lamps are preferably positioned in registration over the lines of intersection of the central ray of the laser beam with respective broad face of the laser slab. A planar flashlamp reflector is employed for economy of fabrication. Directional lamp reflectors are employed for increasing laser efficiency and performance by discriminating against amplified surface waves.

Atoms in a neodymium:glass rod 20 are excited to a substantially spatially uniform metastable state by flashlamps 21. A flowing fluid 26 cools the flashlamps, but not the rod; so that low temperature gradients are maintained in the rod during isothermal laser operation. Automatic control means 22 turn off the electrical power supply 24 when the temperature in the rod reaches a predetermined limit. A flowing fluid 23 then cools the rod, at a rate low enough to avoid thermal stress therein, while it is not lasing. Segments of reflectors 25 focus the pump photons in the rod so as to substantially balance the cylindrical lensing action of the rod against the radial attenuation through it, and thus to provide substantially uniform density of stored energy in the rod.