The present invention has particular utility in laser systems wherein optical energy of a first wavelength is utilized to pump a gaseous laser medium for producing a CW output beam whose light energy is of a second wavelength. 
A large number of molecular gases are known to produce gain in the sub-millimeter frequency spectrum (far infrared or FIR) on rotational transitions of excited vibrational modes at low pressures.  The current technique of exciting these molecules from a ground vibrational-rotational level to a higher vibrational-rotational level is by coherent optically pumping the laser medium by another laser, typically a carbon dioxide (CO.sub.2) laser, inasmuch as the spectrum lines produced thereby overlap with the vibrational-rotational transitions of the gaseous medium and thus provide the necessary selective excitation, whereby resonant absorption pumping takes place.  For achieving continuous (CW) operation of these optically pumped sub-millimeter lasers, the molecular gas pressure must be lowered to such an extent that the rotational thermalization and the vibrational translational energy exchange between the molecules are in a certain balance.  At these pressures, the absorption coefficients of the laser gases are relatively low and accordingly it takes a long interaction length of the pump laser beam with the sub-millimeter laser gas for efficient use of the available pump power.  Since it's practically inconvenient to build sub-millimeter laser resonators having a length in the order of 10 to 30 meters, resonators typically having a length in the order of 1 to 3 meters in length are generally utilized with the pump beam being injected through a hole in the center of one of the end mirrors which then propagates in an ever expanding uncontrolled mode back and forth between the two resonator mirrors. 
In a conventional design of an optically pumped sub-millimeter (FIR) laser resonator an on-axis aperture or injection hole is provided in one of the end mirrors for coupling of the pump laser beam into the resonator.  In such a configuration, a substantial part of the pump power is lost in the walls of the circular laser tube which acts as an over-size waveguide.  Also, at each reflection off the end mirror having the injection hole, a part of the pump laser beam escapes from the resonator and propagates back on the same axis as the input pump beam, providing a feedback to the pump laser.  Since the feedback radiation is arbitrarily in phase with the pump laser field, it operates to pull the cavity mode frequency of the pump laser.  Frequency changes in the pump laser, in turn, cause frequency changes and changes in the absorption coefficient of the laser being pumped.  Thus, the conventional design of optically pumped sub-millimeter lasers inherently provides an unstable source of radiation that fluctuates in power and frequency. 
Thus pump beam feedback and the pump laser resonances constitute a known problem in the design of sub-millimeter lasers.  Both effects are responsible for substantial instabilities and both effects are caused by the uncontrolled propagation of the pump laser beam in the sub-millimeter laser resonator. 
In the past, several approaches have been proposed for overcoming the feedback problem.  One approach well known in the field of optics is to suppress reflections of laser beams by the use of an isolator.  Optically such a device may be implemented by inserting a quarter wave plate and a polarizer in series into the laser beam.  Another approach has been to use a polarizer and a Faraday rotating crystal.  Still another approach uses an acoustic or optical crystal in the path of the laser beam. 
While these attempts have been more or less successful, basic limitations nevertheless still exist since they require the use of elements in the path of the relatively high power pump laser beam and each element introduces losses thereto.  Additionally, since these elements generally rely on positioning in a waist region, i.e. a region of minimum cross section of the pump laser beam, additional focusing elements are generally necessary to produce another beam waist region at the position of the injection hole.  Also, the pump power absorbed by these elements generates heat that reduces their functional performance.