Patent Publication Number: US-4922214-A

Title: Apparatus to couple laser radiation and microwave energy using a microwave waveguide

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus to couple laser radiation into a microwave waveguide or other microwave structure. 
     2. Description of the Prior Art 
     For uranium isotope separation, a laser wavelength for isotope-selective excitation is required which cannot be set with the required precision using fixed frequency lasers known in the prior art. 
     The prior art includes electrooptic modulation, a method by means of which the desired wavelengths can be produced utilizing frequency shift principles. If the required modulation frequencies are in the microwave range, it is necessary to incorporate an electrooptic modulator crystal into a microwave waveguide, so that the laser beam and modulating microwave can pass through the electrooptic modulator crystal substantially colinearly. Using this method, it is difficult to couple the laser radiation into the microwave waveguide. 
     Devices known in the prior art couple a laser beam by means of a hole provided in a microwave elbow [See, for example, Appl. Phys. Lett. 34(7), Apr. 1, 1979, Page 452, FIG. 1. This prior art publication is hereby expressly incorporated by reference as if the entire contents thereof were fully set forth herein.] Using this method, this hole may have only small dimensions, as compared to the dimensions of the hollow conductor, in order not to interfere with the microwave mode and in order to prevent undesirable reflection. 
     This means that the laser beam must be focused through the provided coupling hole. With the energies required for an industrial process, this produces problems with breakdowns and arcing in the vicinity of the coupling hole. Moreover, it is difficult to adjust the laser beam to the geometry of the microwave waveguide, in order that the beam will illuminate, as much as possible, the cross sectional area of the electrooptic crystal. This is necessary, however, to obtain a high degree of efficiency with simultaneously high laser power. 
     Examples of issued U.S. patents which generally discuss the modulation of laser radiation with microwave radiation are U.S. Pat. No. 4,208,091, issued June 17, 1980 and entitled &#34;Broadband Microwave Waveguide Modulator for Infrared Lasers&#34; and U.S. Pat. No. 4,118,676, issued Oct. 3, 1978 and entitled &#34;Method and Apparatus for Driving an Optical Waveguide with Coherent Radiation&#34;, both of these issued U.S. patents being hereby expressly incorporated by references as if the entire contents thereof were fully set forth herein. 
     OBJECT OF THE INVENTION 
     One object of the present invention is the provision of an improved coupling device of the type described above, so that a laser beam can be coupled, the cross section of which may be relatively easily adjusted to that of the microwave waveguide. The invention disclosed herein makes possible a blocking of the microwave in the direction of the entry aperture of the laser beam, so that this aperture can have practically the same cross section as the common exit aperture of the microwave waveguide. 
     SUMMARY OF THE INVENTION 
     In general, the invention features a coupling device for coupling laser radiation and microwave radiation, the coupling device including a microwave waveguide apparatus for guiding the microwave radiation along a substantially defined path, and the microwave waveguide apparatus having a microwave entry aperture and a common exit aperture for both the microwave radiation and the laser radiation. A laser entry aperture apparatus is provided in the microwave waveguide apparatus for admitting the laser radiation into the interior of the microwave waveguide apparatus. A bandstop filter apparatus substantially reduces any emission of the microwave radiation via the laser entry aperture apparatus. 
     In a preferred embodiment, the invention features a coupling device for coupling laser radiation and microwave radiation, the coupling device including a microwave waveguide apparatus which has a first hollow tube-shaped member, the microwave waveguide apparatus also having a microwave entry aperture and a common exit aperture for both the microwave radiation and the laser radiation. A laser entry aperture apparatus is provided in the microwave waveguide apparatus for admitting the laser radiation into the interior of the microwave waveguide apparatus. The laser entry apparatus and the common exit aperture for both the microwave radiation and the laser radiation are substantially aligned with the major longitudinal axis of the first hollow tube-shaped member. The microwave entry aperture includes an opening in the first hollow tube-shaped member and a second hollow tube-shaped member interconnecting with the first hollow tube-shaped member and surrounding the opening. The first hollow tube-shaped member has a substantially circular cross sectional profile, and the second hollow tube-shaped member has a substantially rectangular cross sectional profile. The second hollow tube-shaped member extends substantially perpendicular to the major longitudinal axis of the first hollow tube-shaped member. The preferred embodiment of the coupling device also includes a bandstop filter apparatus for substantially reducing any emission of the microwave radiation via the laser entry aperture apparatus, the bandstop filter apparatus including an extension of the first hollow tube-shaped member which has a plurality of outstanding annular ribs spaced from one another in the direction of the major longitudinal axis of the first hollow tube-shaped member, each of the plurality of outstanding annular ribs having a sharp edged U-shaped cross sectional profile. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A preferred embodiment of the invention is illustrated in the sole accompanying FIGURE, and is explained in detail below. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the figure, a microwave waveguide configured according to the invention generally includes an orthomode coupler, e.g. made of copper, comprising a tube 1 with a circular cross section and a tube 2 having a rectangular cross section connected perpendicular to tube 1. Additionally connected to the tube 1, is a tubular bandstop filter 3, also having a circular cross section. The bandstop filter 3 has several corrugations 3a, 3b, 3c pointing radially outward which, when viewed in cross section, have a sharp edged, U-shaped configuration, and which are located at different axial distances from one another. 
     The laser beam 4 enters through an aperture 3d of the bandstop filter 3 and exits the tube 1 of the microwave waveguide through a common exit aperture 1a. The microwave 5 enters through an entry aperture 2a into the rectangular tube 2 of the microwave waveguide, is deflected in the tube 1 and exits the tube 1 through the common exit aperture 1a. In a microwave waveguide which operates at approximately 16 GHz, using a known prior art coupling device of the type described above, the hole for the laser beam may have a maximum diameter of only 6 to 8 mm for a microwave waveguide diameter of approximately 14 mm. In contrast, in an arrangement configured according to the present invention, the entry aperture 3d of the bandstop filter 3 may, under similar conditions, have an open diameter of about 12 mm, therefore allowing the use of higher powered lasers, as well as permitting an easier adjustment of the laser beam 4 to the geometry of the orthomode coupler 1, 2. The microwave 5 is thereby deflected toward the common exit aperture 1a with practically no loss in the coupler. 
     The invention as described hereinabove in the context of a preferred embodiment is not to be taken as limited to all of the provided details thereof, since modifications and variations thereof may be made without departing from the spirit and scope of the invention.