Fiber optic light launching assembly
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     With the increasing emphasis on designing suitable fiber optics, a wide variety of types have evolved which exhibit numerous dissimilar operational parameters. Standardization and categorization of the fibers calls for testing them and evaluating them for a variety of applications. When input excitation is applied to a test fiber, the spot size and numerical aperture (cone convergence solid angle), of the light input is generally fixed by the input optics. Since the multitude of fibers undergoing testing are different in their optical and physical properties to some extent, little uniform control usually is exercised over the light excitation parameters. The input equilibrium conditions generally are not approached and, consequently, measurement errors result. The errors are found to be magnified when short fiber sample lengths are employed. Thus there is a continuing need in the state of the art for an apparatus which permits the accurate control of input excitation conditions for a test optical fiber to permit near-equilibrium launch condition for accurate measurement of fiber parameters 
     SUMMARY OF THE INVENTION 
     The present invention is directed to providing an apparatus for determining the parameters of a test optic fiber. A source of light which has been suitably apertured illuminates a lens arrangement which collimates it and limits it to a beam of given diameter. The light spot is imaged onto the test optic fiber and the numerical aperture is adjusted to near-equilibrium launch conditions for accurate measurement of the fibers&#39; parameters. 
     The prime object of the invention is to provide an apparatus for determining the parameters of fiber optics. 
     Yet another object is to provide an apparatus for adjusting the light spot and numerical aperture of a variety of test optic fibers. 
     Still another object is to provide an apparatus which permits an operator to observe the positioning of a light spot and to make proper adjustments on a test optic fiber. 
     Still another object is to provide the test apparatus which permits the precise adjustment of a test optic fiber with respect to a light spot. 
     Yet another object is to provide an adjustable, observable apparatus capable of accommodating differently diametered test optic fibers to nearly approximate equilibrium launched conditions. 
     These and other objects of the invention will become more readily apparent from the drawings when taken with the ensuing description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing depicts in schematic form the optive interrelationship of the elements of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, a test apparatus 10 for a test optic fiber 11 assures that differently sized optic fibers can be tested for operational parameters quickly and accurately. The apparatus varies the spot size of a light beam emanating from a light source 12 onto an optic fiber undergoing a test as well as changing the excitation numerical aperture over a wide range in a continuous manner. The elements of the invention allow a technician to view the input end of the test optic fiber and examine the condition of the test fiber&#39;s surface and to adjust the spot such that it is properly centered on the optic fiber&#39;s core. 
     A typical apertured source of light can be that light emanating from a plastic clad silica fiber for example, a model PS-05-35 marketed by the ITT Company which is flooded with a filtered light from an incadescent source. Needless to say the light source could be a light emitting diode or any other extended light source capable of providing illumination at pre-established levels. 
     A fixed focal length lens 13 receives the light from the source. The lens chosen for purposes of demonstrating this inventive concept is a Soligor 135 mm f/2 camera lens although a number of other suitable lenses could be substituted. The lens functions to collimate the light emanating from the source and, as such, must be located approximately one lens focal length from the source. An f stop adjustment 14 allows variance of a collimated beam of light which passes to a zoom lens 15. A focussing adjustment 16 on the zoom lens focuses the light spot created by the fixed focal length lens and its f stop adjustment onto the exposed end surface of test optic fiver 11. Precise positioning of the test optic fiber on the fiber&#39;s end is readily accomplished by a three axis micropositioner 17. Thus, the focused light spot is centered on the optical core of the test optic fiber. 
     A beamsplitter 18, for example, a model 23-8916 marketed by Ealing is interposed in the light path between the fixed focal length lens and the zoom lens. The beamsplitter allows an observer to view the end of test optic fiber 11 through a viewing telescope 19. The scope also is manufactured by Ealing and is a 23-0466 model. Because the light rays&#39; impinging upon the beamsplitter are collimated, the telescope is focused to infinity. 
     In operation, a test optic fiber of a given diameter is secured onto an extension of three axis micropositioner 17. The spot size of the collimated light is properly dimensioned by the adjustment 14 on fixed focal length lens 13 so that it properly fills the fiber core of test optic fiber 11. The micropositioner and zoom lens are adjusted so that the test optic fiber numerical aperture is matched and equilibrium conditions at the launch end of the test optic fiber are established. 
     The dimensions of the spot size can be expressed as a function of the focal length of the lens and the diameter is given by: 
     
         S.sub.2 =S.sub.1 (F.sub.2 /F.sub.1) 
    
     Where: 
     S 2  =spot diameter at test optic fiber 11 
     S 1  =diameter of source 12 
     F 2  =focal length of lens 15 
     F 1  =focal length of lens 13 
     In like manner the relationship expressing the numerical aperture of a test optic fiber can be expressed by: 
     
         NA=(F.sub.1 /F.sub.2) sin tan.sup.-1 (1/2f.sub.1) 
    
     Where: 
     NA=excitation numerical aperture 
     f 1  =f number of lens 13 
     From the foregoing both the spot size and the numerical aperture of the excitation light can be focused and continuously adjusted over a wide range of values. For example, for a spot size of 35.2μ meters, the excitation numerical aperture is adjustable from 0.11 to 0.62; for a spot size of 50.0μ meters, the numerical aperture ranges from 0.08 to 0.47; and for a spot size of 96.2μ meters, the numerical aperture is adjusted between 0.04 to 0.24. Naturally, different lens arrangements, light sources and test optic fibers would yield other adjustment ranges. Thus, practically any optical fiber can be matched to a source to assure that input equilibrium conditions are achieved over short lengths, a necessity for accurate measurement of optical attenuation, pulse dispersion (frequency response), and fiber numerical aperture for example. 
     While a single fixed focal length lens and a single zoom lens have been referred to in the foregoing discussion, the lens types can be varied including for example, a pair of zoom lenses or one different zoom lens can be used when the situation so demands. Likewise, more than three degrees of freedom can be employed at the micropositioner, for example, by allowing for angular as well as linear motion. Finally, the arrangement disclosed can be employed to illuminate other things besides optical fibers undergoing test and could find application wherever a variable spot size and a variable light ray solid angle are required. 
     Obviously, many modifications and variations of the present invention are possible in the light of the above teachings, and, it is therefore understood that within the scope of the disclosed inventive concept, the invention may be practiced otherwise than as specifically described.