Abstract:
An inert gas ion laser includes a Brewster window having a plurality of alternating layers of high and low refractive index materials for suppression of unwanted laser frequencies to produce a narrow band transmissive Brewster window with a high power output.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas laser having a frequency-selective multiple layer system formed on a Brewster window of the laser discharge tube. 
     2. Description of the Prior Art 
     It is known in the art to provide means for suppressing a single wavelength of the plurality of wavelengths usually emitted by a gas laser. For example, German Pat. OS No. 23 42 911 discloses coating one of two resonator mirrors (in a gas laser) wherein the coating includes a λ/2 wavelength layer and symmetrically disposed on either side thereof a plurality of pairs of alternating optically dense and optically thin λ/4 wavelength layers, where λ is the wavelength of the unwanted light. Such a layer system yields a pronounced reflection minimum. A defined wavelength, or line, can be filtered out from a series of laser lines that are in relatively close proximity. However, suppressing all but one line of a spectrum of lines with such layer system does present problems. 
     A modified coating could be used to obtain suppression of all but one frequency but the reflection maximums of such layer structures as are required for such suppression are temperature and pressure dependent to such an extent as to result in a noticeable loss of power such as, for example, in an Ar +  laser which has relatively close wavelength spacings. By applying dielectric cut-off filters to both mirrors the conditions for wavelength selection are less critical, although greater costs and more complex manufacturing are involved since the soft material of the mirror surface, which is frequently curved, must be polished and coated. 
     It is also known to use etalons, frequency-selective polarizers or dispersing elements for line selection (see German Pat. OS. No. 20 60 525 or GB OS No. 20 75 746), in conjunction with broad band mirrors. Such additional elements in the resonator however, attenuate the output beam and make the structure more complicated. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple and reliable means for suppressing unwanted frequency components of a laser emission by providing a plurality of layers at a Brewster window of a laser discharge tube. The layers are of alternating high refractive index and low refractive index and each has the same optical thickness. No additional parts or elements are required, and a gas laser utilizing the present invention can be fabricated at relatively low cost. 
     The Brewster window of a gas laser is usually composed of a hard, easily polishable material, such as quartz, having planar surfaces, and as such it is easy to bloom and coat. The mutiple layer structure includes only two type of layers. The thickness of the layers can depart from an ideal thickness value by up to a few percent, so long as layers of the same type are of the same thickness. Compared to frequency-selective mirrors, the present invention provides the advantages of approximately the same attenuation values for both useful and unwanted light frequencies and is somewhat less influenced by changes in temperature and pressure. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a gas laser tube, partially in cross section, including a frequency selective dielectric layer system according to the present invention; and 
     FIG. 2 is an enlarged cross-section showing the multiple layer system of the gas laser shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the FIG. 1, a gas laser is shown including a discharged tube 1 provided with discharge electrodes 2 and 3 adjacent respective ends, and the ends of the discharge tube 1 are provided with respective Brewster windows 4 and 5. A resonator is formed by mirrors 6 and 7 disposed beyond the ends of the discharge tube 1. A multiple dielectric layer system 8 is provided at one Brewster window 5 on the side toward the mirror 7. An ideal value for the layers is ##EQU1## where λ is the wavelength of the desired laser emission line and q is a whole number greater than or equal to zero. 
     The present frequency-selective layer system or means is preferably used with inert gas ion lasers, and, for purposes of illustration, is described in conjunction with an argon (Ar + ) laser having a desired emission frequency of 0.488 μm. The layer system provided on the argon-filled discharge tube 1 includes twelve individual layers 9 and 10 which alternately have a refractive index of 2.2 and 1.46, respectively, and a thickness of 71.2 nm and 120.5 nm respectively. The layers are formed of vapor-deposited Ta 2  O 3  and, respectively, SiO 2 . The Brewster windows 4 and 5 are formed of either quartz or silica. 
     For p-polarized light at a 55.5° angle of incidence, the following reflection factors R P  are derived for individual laser lines λ i  with their excitation probabilities A: 
     
         ______________________________________λ.sub.i (nm)           R.sub.p (%)                    A______________________________________454             11       0.12456             12       0.35466             13       0.2473             9.7      0.3476             7.4      0.75488             0.000    1.5496             6.8      0.7501             17.7     0.4514             51       2.0______________________________________ 
    
     For layer thickness errors of ±1 percent and for stray light and absorption losses totaling less than 0.05 percent, the losses for the 0.488 μm wavelength line amount to about 0.2 percent. The remaining laser wavelength lines are suppressed; in particular, the 0.514 μm wavelength line is suppressed, which is greatly excited since its R p  value is especially high precisely in this frequency range. 
     Although an argon laser has been described in conjunction with the present invention, the multiple layer system can also be used for the selection of other Ar +  wavelength lines or for the selection of wavelengths of other inert gas lasers and, in suitable instances, can even be employed for the selection of more than one frequency line. 
     The low-refraction and/or the high-refracting layers can be selected to be thicker to boost frequency selectivity, such as where q is greater than one. Partial application of the multiple layer system to the inside of a Brewster window is also possible, and the provision of layers to both sides of the window provides thermal stability. It is also foreseen to apply the layers by some method other than vapor depositing, such as by means of a sputtering technique. Furthermore, it is inconsequential whether the Brewster windows forms a gas-tight tube closure or is held in a purely mechanical fashion. Thus, there has been shown and described a frequency selective dielectric layer system for a gas laser which is economical to manufacture and efficient in operation. 
     It is apparent from the foregoing specification, the invention is susceptible to being embodied with various alterations and modifications which may differ particularly from those that I have described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent granted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.