Abstract:
An apparatus and method for optimizing the collimation of the output of an optical fiber through a collimating lens, comprising placing the beam through a collimating lens, and then comparing a characteristic feature of the image to a calculated reference image. In one embodiment a side lobe, or non-central local maximum is used as the characteristic feature. The invention is ideally suited for use with a few-mode fiber, and may be utilized for a single mode fiber with the addition of an appropriate optical element between the lens and the observing point. The calculated reference image in one embodiment is calculated assuming an ideal lens and optical element, or in another embodiment a measured optical element is utilized. In another embodiment the calculated reference image is adjusted to optimize the performance of the optical assembly for a specific operating criteria or a combination of criteria. Such criteria include optical attenuation, wavelength dependent loss and the extinction ratio of specific unwanted modes.

Description:
[0001]    CROSS-REFERENCE TO RELATED APPLICATIONS  
           [0002]    The present application claims the benefit of the filing date of co-pending U.S. provisional application, S/No. 60/325,187 filed Sep. 28, 2001, entitled “METHOD OF COLLIMATION”.  
         BACKGROUND OF THE INVENTION  
         [0003]    The invention relates generally to the field of fiber optics, and more specifically to a method and apparatus for collimation of a light beam exiting an optical fiber.  
           [0004]    Optical fiber has become increasingly important in many applications involving the transmission of light. When a beam of light is transmitted through an optical fiber, the energy follows a number of paths which are called modes. A mode is a spatially invariant electric field distribution along the length of the fiber. The fundamental mode, also known as the LP 01  mode, is the mode in which light passes substantially along the fiber axis. Modes other than the LP 01  mode, are known as high order modes. Fibers which have been designed to support with minimal loss only one mode, the LP 01  mode, are known as single mode fibers. A multi-mode fiber is a fiber whose design supports multiple modes, and typically supports over 100 modes. A few-mode fiber is a fiber designed to support only a very limited number of modes. For the purpose of this patent, we will define a few mode fiber as a fiber supporting fewer than 20 modes at the operative waveband. Fibers may carry different numbers of modes at different wavelengths, however in telecommunications the typical wavelengths are near 1310 nm and 1550 nm.  
           [0005]    Light transmitted through an optical fiber can also be subject to different types of optical interactions to filter, change the mode, modulate, split, combine, or otherwise act on the light. In most cases two or more fibers are led into an enclosure operating as an optical system. The input light entering the enclosure, usually but not always on one fiber, interacts with some optical device within the enclosure, and the resulting light exits the enclosure via one or more fibers. One example of a two port system is a transverse mode transformer as described in U.S. Pat. No. 6,404,951 whose contents are incorporated by reference.  
           [0006]    In practice the light exiting the fiber is typically collimated by a lens, such as a GRIN lens or an aspherical lens as the first step in the desired optical interaction. The distance between the end of the fiber and the lens is adjusted to arrive at the desired collimation point, following which the fiber location is secured by applying an adhesive or by laser welding. In some applications the desired point is defined as when the beam is focused to a minimum spot size at a predetermined distance, or a combination of beam size and beam divergence at some predetermined distance. In still other applications the beam is collimated in the far field, by adjusting for a minimum spot size. In practice however, a range of distances between the end of the fiber and the lens arrive at a minimum spot size. In some optical systems, most notable a transverse mode transformer, extremely precise collimation is desired.  
           [0007]    In some optical systems a phase element is utilized. A phase element is an optical element which imparts a predetermined phase shift or phase change to a specific segment of the wavefront propagating through the element.  
           [0008]    U.S. Pat. No. 6,168,319 describes a method and apparatus of aligning a collimator assembly requiring only a single-axis adjustment and for which the collimator may be paired with any other similarly aligned collimator. The position of the ferrule/fiber is adjusted within a tube while the size of the resultant beam is measured at a fixed distance from the output of the lens. In practice such a method is ideally suited to a single mode fiber, which contains only the LP 01  mode with a Gaussian shape. The collimation of the output of a few mode fiber, and particularly one which predominantly consists of a single high order mode, can not be optimized with a high degree of accuracy utilizing spot size.  
           [0009]    Thus there is a need for a method and apparatus for aligning a collimator assembly suitable for use with a fiber carrying predominantly a single high order mode.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art methods of aligning a collimator assembly. This is provided in the present invention by utilizing a numerical analysis of the expected intensity pattern, measuring the location of characteristic features of the expected intensity pattern at a pre-determined distance, and comparing the characteristic features of the received beam with the expected pattern.  
           [0011]    In an exemplary embodiment the invention comprises a method of optimizing collimation of the output of an optical fiber comprising the steps of: supplying an end of an optical fiber and a lens in optical communication with the end of the optical fiber; calculating an expected reference image, the expected reference image having at least one characteristic feature not present in the output of single mode fiber; observing the output of the lens and calculating the differential between the location of the characteristic feature in the output and the expected reference image. The end of the optical fiber is then moved in relation to the lens so as to minimize the differential, thus optimizing the collimation.  
           [0012]    In one embodiment the fiber comprises a few mode fiber, while in another embodiment the output is observed at a distance greater than the Fraunhofer zone, while in yet another embodiment the output is observed at a distance less than the Fraunhofer zone. In another embodiment the characteristic feature comprises a side lobe, or a non-central local maximum.  
           [0013]    In one embodiment the reference image is adjusted to achieve a minimal loss for the optical subsystem for which the optical fiber and lens are a part. In another embodiment the reference image is adjusted to achieve a minimal wavelength dependent loss for the optical subsystem for which the optical fiber and lens are a part. In yet another embodiment the reference image is adjusted to achieve a maximal extinction ration for a specific undesired mode.  
           [0014]    In a preferred embodiment an additional optical element is placed in the optical path, and the output of the optical element is observed. Further preferably the optical element comprises a phase element. Still further preferably the fiber comprises a single mode fiber, and the characteristic feature is a function of the optical element. In further embodiment the reference image is a function of the designed values of an optical element, while in another further embodiment the reference image is a function of a measured actual optical element. In another further embodiment the characteristic feature comprises a side lobe, or a non-central local maximum.  
           [0015]    The invention also comprises an apparatus for optimizing collimation at the output of an optical fiber through a lens. The apparatus comprises an end of an optical fiber, a lens in optical communication with the end of the optical fiber; a means of observing the output of the lens and a computer comprising an expected reference image, the expected reference image having at least one characteristic feature not present in the output of single mode fiber. The difference between the location of the characteristic feature in the output and the location of the characteristic feature of the expected reference image is calculated by the computer and the end of the optical fiber is moved in relation to the lens so as to minimize the differential, thus optimizing the collimation.  
           [0016]    In one embodiment the fiber comprises a few mode fiber. In another embodiment the output is observed at a distance greater than the Fraunhofer zone, while in yet another embodiment the output is observed at a distance less than the Fraunhofer zone. In one embodiment the reference image is adjusted to achieve a minimal loss for the optical subsystem for which the optical fiber and lens are a part. In another embodiment the reference image is adjusted to achieve a minimal wavelength dependent loss for the optical subsystem for which the optical fiber and lens are a part. In yet another embodiment the reference image is adjusted to achieve a maximal extinction ration for a specific undesired mode. In another embodiment the characteristic feature comprises a side lobe, or a non-central local maximum.  
           [0017]    In a preferred embodiment the apparatus further comprises an optical element. In a still further embodiment the optical element comprises a phase element. In still another further embodiment the fiber comprises a single mode fiber, and the characteristic feature is a function of the optical element.  
           [0018]    In one further embodiment the reference image is a function of the designed values of an optical element, while in another further embodiment the reference image is a function of a measured actual optical element. In another further embodiment the characteristic feature comprises a side lobe, or a non-central local maximum.  
           [0019]    Additional features and advantages of the invention will become apparent from the following drawings and description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which like numeral designate corresponding elements or sections throughout, and in which:  
         [0021]    [0021]FIG. 1 illustrates a high level block diagram of a setup useful in launching a high order mode into a fiber;  
         [0022]    [0022]FIG. 2 illustrates a high level block diagram of a setup useful in aligning the collimation for a high order mode in accordance with the invention;  
         [0023]    [0023]FIG. 3 illustrates a high level diagram of a releasable ferrule holder as shown in FIG. 1 and FIG. 2;  
         [0024]    [0024]FIG. 4 illustrates a calculation of the expected image of the LP 02  mode in an embodiment of the invention;  
         [0025]    [0025]FIG. 5 illustrates a high level flow chart of a program used to compare a characteristic feature of the image with a characteristic feature of the expected image;  
         [0026]    [0026]FIG. 6 illustrates a plot of ferrule position vs. difference in location of the characteristic feature in an embodiment of the invention;  
         [0027]    [0027]FIG. 7 illustrates an expected image vs. an actual image for an improperly collimated lens;  
         [0028]    [0028]FIG. 8 illustrates an expected image vs. an actual image for a properly collimated lens; and  
         [0029]    [0029]FIG. 9 illustrates a high level block diagram of a collimator for a single mode fiber to which optical elements have been added. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The invention allows for the alignment of a collimator comprising a dominant high order mode using image processing.  
         [0031]    [0031]FIG. 1 illustrates a high level block diagram of an exemplary embodiment of a setup  10  useful for effectively launching a high order mode into a few mode fiber and comprises light source  30 , single mode fiber (SMF) 40 , open mode converter  20 , ferrule  120 , few mode fiber  110  and power meter  140 . Open mode converter  20  comprises SMF  40 , collimator housing  50 , phase elements  60 , tube  70 , collimating lens  90 , holder  80 , stage  100 , few mode fiber  110 , ferrule  120  and releasable holder  130 . The output of light source  30  is connected to one end of SMF  40 , and the other end of SMF  40  enters the input of open mode converter  20  and is terminated in first ferrule  120  (partially shown) secured within collimator housing  50 . First lens  90  is secured within collimator housing  50  so as to collimate the light exiting SMF  40 . Phase elements  60  are secured in tube  70  so as to ensure proper placement and alignment with respect to collimator housing  50  and in particular first lens  90 , and one end of tube  70  is secured to collimator housing  50 . Second lens  90 , contained within a housing (not shown) is secured to the other end of tube  70  and tube  70  is secured by holder  80  to a firm surface, such as an optical table. Second ferrule  120  is secured in releasable holder  130  and releasable holder  130  is secured to a firm surface, such as an optical table, by movable stage  100 . A first end of few mode fiber  110  is terminated by second ferrule  120 , which is secured in releasable holder  130 . Few mode fiber  110  exits open mode converter  20  and its second end is terminated at third ferrule  120 . The output of the second end of few mode fiber  110  is connected as the optical input to power meter  140 .  
         [0032]    In an exemplary embodiment, light source  30  comprises a laser emitting light at the operational bandwidth of the device to be aligned, for example a laser diode emitting at 1550 nm. Further in an exemplary embodiment, the collimator housing  50  is of the type described in U.S. Pat. No. 6,340,248, whose contents are incorporated by reference, however any collimator of sufficient precision may be utilized. Phase elements  60  are designed to selectively alter the phase of a portion of the wavefront so as to accomplish a mode transformation in a manner as described in U.S. Pat. No. 6,404,951.  
         [0033]    In operation, first lens  90  is secured within collimator housing  50 , and is aligned using standards methods known in the prior art. First lens  90  and phase elements  60  act to create a focused image of the desired mode to be launched at the focal point of second lens  90 . The invention will be described in relation to a few-mode fiber, however this is not meant to be limiting in any way. The invention is equally applicable to a single mode fiber, and to a fiber carrying more modes than the modes supported by a few mode fiber. The limiting factor is only that the theoretical mode shape must be known. The ends of few mode fiber  10  are secured within second and third ferrule  120  in a manner known to those skilled in the art, and polished to a predetermined facet angle (for example 8°) to prevent back reflection along the optical axis. Second ferrule  120  is secured in holder  130 , which is designed for easy installation and removal of ferrule  120 , and in one embodiment consists of a threaded connector such as an FC/APC. Holder  130  is secured to a firm surface, such as an optical table, by movable stage  100 , which allows for fine positioning in all axes and angles, for a total of 5 degrees of freedom, of ferrule  120 . The initial placement of ferrule  120  in holder  130  is designed to be at the focal point of second lens  90 . The second end of few mode fiber  110  is secured within third ferrule  120  in a manner known to those skilled in the art, and polished to a predetermined facet angle (for example 8°) to prevent back reflection along the optical axis. Stage  100  is moved in micron steps in all three axes thereby positioning the first end of few mode fiber  110  to arrive at a maximum reading on the power meter, which is indicative of minimum loss. Once a minimum loss point is achieved, a high order mode, such as the LP 02  mode, is being dominantly launched into the first end of few mode fiber  110 . The second end of fiber  110  will thus exhibit primarily the dominant LP 02  mode, however other modes, notably the fundamental or LP 01  mode will also be present to a small extent. The second end of few mode fiber  110  is then disconnected from power meter  140 .  
         [0034]    The system will be described in connection with a specific high order mode, the LP 02  mode. However it is to be understood that any high order mode can be aligned for, including but not limited to the LP 11 , LP 03 , LP 12  and LP 21  modes. In the event that the mode is an odd mode, that is the intensity does not peak at the center, a minimum energy point at the center is substituted for the described maximum found at the center of the even mode. Alignment may also be accomplished in the case of more than one mode, with the limiting factor being the need for a theoretical calculation of the expected image shape.  
         [0035]    [0035]FIG. 2 illustrates a high level block diagram of an exemplary embodiment of a setup  150  useful in aligning the collimator of a few mode fiber, comprising light source  30 , open mode converter  20 , few mode fiber  110 , ferrule  120 , releasable holder  130 , collimator housing  50 , lens  90 , infrared camera  160 , movable stages  100 , data connection  180 , monitor  190  and computer  170 . Light source  30  is connected to the input of open mode converter  20 , and the output of open mode converter  20  comprises a first end of few mode fiber  110 , as described above in relation to FIG. 1. The second end of few mode fiber  110  is secured within a ferrule  120 , which is inserted into a collimator housing  50  and secured by holder  130  to first movable stage  100 . Lens  90  is secured in collimator housing  50 , and the output light from lens  90  is directed over distance d to the optical input of infrared camera  160 . Infrared camera  160  is secured to second movable stage  100 . The electrical output of infrared camera  160  is connected by connection  180  to computer  170 , and monitor  190  is connected to infrared camera  160  and computer  170 .  
         [0036]    In operation light source  30  is connected as in FIG. 1 above to one end of SMF  40  whose other end acts as the input to open mode converter  20 . Open converter  20  acts as described in relation to FIG. 1 above, to launch predominantly a single high order mode into the first end of few mode fiber  110 . Holder  130  is secured to first movable stage  100 , which in one embodiment is adapted to allow only for single axis movement, and first movable stage  100  is secured to a firm surface such as an optical table (not shown). The single axis movement allows for modifying the depth of insertion of the ferrule  120  into the collimator housing  50 . Modifying the depth of insertion of the ferrule  120  modifies the location of the tip of fiber  110  in relation to the collimating lens  90  located in collimator housing  50 .  
         [0037]    Infrared camera  160  is secured to second movable stage  100 , and is secured at a fixed known distance, d, from lens  90  in collimator housing  50 . In one embodiment the distance is 53 centimeters. The precise distance utilized must be known, and is preferably past the beginning of the Fraunhopfer zone, indicating that the field seen by infrared camera  160  is the far field. In another embodiment the near field is utilized, and an additional optical element such as a phase element or a lens (not shown) is required. In an exemplary embodiment, second movable stage  100  is adjustable in the x-axis and y-axes, so as to properly align infrared camera  160  with the output light exiting lens  90  of collimator housing  50 . The distance d, however should not be changed. As a first approximation a hand held infrared sensor card is utilized, and second movable stage  100  is adjusted to approximately center the beam on the camera. In practice, once an initial location for second movable stage  100  is found, any changes in the beam location caused by adjusting the position of ferrule  120  in collimator housing  50  are typically compensated for by software. The distance between the second end of few mode fiber  110  and lens  90  is adjusted by moving first stage  100  in the z-axis, and the image observed by infrared camera  160  is fed to computer  170  by connection  180  which processes the image and compares the image to a reference as will be further described below. In an exemplary embodiment connection  180  comprises a network connection or a direct connection such as an RS-232 port connection. Computer  170  may be a personal computer, workstation or other general computing device such as a microcomputer, microprocessor or mainframe computer, or any other computing platform all without exceeding the scope of the invention.  
         [0038]    The image seen by infrared camera  160  is displayed on monitor  190  and is used to maintain the center of the beam location on the center of infrared camera  160 , and to allow the operator to find the best match between the captured image and a reference image, as will be described further below. In another embodiment, the monitor is not utilized and a computerized algorithm finds the center of the image and finds a best match with a reference image. Once the proper location is found, ferrule  120  is secured in collimator housing  50  utilizing known methods such as laser welding or by applying an adhesive. The completed assembly is thus properly collimated and may be removed from holder  130 .  
         [0039]    [0039]FIG. 3 illustrates a high level diagram of an embodiment of releasable holder  130  comprising stage  100 , fixed portion  200 , first pivot  210 , stationary arm  220 , spring  230 , second pivot  240  and movable arm  250 . Fixed portion  200  is secured to stage  100 , and stationary arm  220  is pivotally secured to fixed portion  200  by first pivot  210 . Movable arm  250  is pivotally connected to stationary arm  220  by second pivot  240 , and is urged towards the closed position by spring  230 . Preferably pivot  210  is a tightened pivot, such that stationary  220  is not easily movable, however it can be pivoted with sufficient force. Ferrule  120  is secured by opening movable arm  250 , placing ferrule  120  in place against stationary arm  200 , and releasing movable arm  220 , which is then urged by spring  230  towards movable arm  220  securing ferrule  120 . Pivot  210  is supplied for ease of loading of ferrule  120 .  
         [0040]    [0040]FIG. 4 illustrates a calculated reference image in which the x-axis represents pixel number and the y-axis represents normalized intensity. The reference image was calculated utilizing an angular spectrum method, known to those skilled in the art, in which the propagation of the theoretical field to the camera is calculated taking into account among other things the pixel size of the camera. Other methods of calculating the reference image may be utilized as known to those skilled in the art, the image of a “gold sample” may be utilized or an image based on a large sampling of “good” units may be utilized without exceeding the scope of the invention. The image of FIG. 4 shows a cross section of an LP 02  mode as calculated based on the actual profile of few mode fiber  110 , after propagation through lens  90  and the distance, d, between lens  90  and the camera  160 . The LP 02  mode exhibits a central peak  260 , and symmetric side peaks  270 . One realization of the present invention is that in an actual image containing the LP 02  mode as the primary mode, where other modes are present, the height and shape of the central peak  260  need not be consistent with the calculated image. However the location of the secondary symmetric peaks  270 , which are based on transmission of the actual desired mode, do not change appreciably because of the existence of other modes.  
         [0041]    As indicated above, the system is being described in connection with a specific high order mode, the LP 02  mode. However it is to be understood that light exiting in any high order mode can be collimated using the invention, including but not limited to the LP 11 , LP 03 , LP 12  and LP 21  modes. For the desired mode a characteristic that is invariant in location due to the presence of other modes is utilized. Preferably the characteristic is substantially at a point where other present modes are negligible. This will typically comprise the location of a specific side lobe or local maximum or minimum point away from the center. Alignment may also be accomplished in the case of more than one mode, with the limiting factor being the need for a theoretical calculation of the expected image shape, and finding a characteristic such as a peak or minima whose location is unchanged despite the presence of multiple modes.  
         [0042]    [0042]FIG. 5 illustrates a high level flow chart of a program suitable to run on computer  170  for finding the optimum collimation point while moving ferrule  120  of FIG. 2. In step  1000  the system is initialized, and in step  1010  the calculated reference image is loaded. Preferably the calculated reference image is a vector of values with its maximum at the center of the expected far field image. In step  1020  the image from infrared camera  160  is captured and stored. In step  1030  the image is scanned to find the center of mass of the pattern. The x-coordinate of the center of mass is defined as:  
               〈     X   com     〉     =         ∑     m   ,   n                f   6          (     x   ,   y     )       *   x         ∑       f   6          (     x   ,   y     )                   Equation                   1a                                 
 
         [0043]    where m,n are the coordinates of the pixels, and f is the value of each pixel. The 6 th  power is utilized so as to arrive at the center of the mass of the center peak, without taking into account the side lobes. In the event the mode does not have a center peak, the center of the mass of the entire object is utilized. Similarly the y coordinate of the center of mass is defined as:  
               〈     Y   com     〉     =         ∑     m   ,   n                f   6          (     x   ,   y     )       *   y         ∑       f   6          (     x   ,   y     )                   Equation                   1b                                 
 
         [0044]    In step  1040  the center is set to the x,y coordinates of the center of mass. It is to be understood that as ferrule  120  of FIG. 2 is moved the image may shift position on infrared camera  160 , and thus the center of the image must be recalculated after each repositioning of the ferrule. In the event that the full image has moved off the camera, second stage  100  is adjusted to bring the image fully onto infrared camera  160 .  
         [0045]    In step  1050  the x-axis across the center of the actual image is viewed, which as mentioned above has the maximum intensity at its center, and the location of the secondary peaks are found. In step  1060  the y-axis of the image is viewed and the location of the secondary peaks are found. In step  1070  the location of the secondary peaks on the reference image are identified. In step  1080  the offset between the secondary peaks on the x-axis is calculated using the formula:  
         Δ=Δ left +Δ right   Equation 2  
         [0046]    where Δ left  represents the distance between the reference peak and the captured peak to the left side of the center, and Δ right  represents the distance between the reference peak and the captured peak to the right side of the center. In step  1090  the offset between the location of the secondary peaks on the y-axis is calculated using equation 2, and in step  1100  the difference between the calculated values of Δx and Δy are compared. If the differential is greater than a predetermined amount, the program proceeds to step  1120  which displays an astigmatism error message indicating that the image is not sufficiently symmetric. In an exemplary embodiment the predetermined amount is two camera pixels. If the program in step  1100  determines that the differential is no more than the predetermined amount the program proceeds to step  1110 , in which the total differential is displayed according to the formula:  
         Δ total =Δ x =Δ y   Equation 3  
         [0047]    The operator moves ferrule  120  from a position which is about 1.5 times larger than the focal distance, towards the lens  90 , until a minimum value of Δ total  is found.  
         [0048]    [0048]FIG. 6 illustrates an embodiment of a typical curve showing Δ total  in which the x-axis represents the position of ferrule  120  in microns, and the y-axis represents Δ total  in pixels. It is to be noted that two zero crossings occur, one at approximately 10 microns, with a second zero crossing at approximately 38 microns. The first minimum in the curve must be bypassed in order to find the second minimum which achieves a flat wavefront. The existence of multiple zero crossings is to be confirmed for each setup by calculating the reference image at multiple ferrule locations. The location of the characteristic feature being utilized is then noted, and if multiple coincident locations of the characteristic are found, the proper zero crossing position giving an ideal wavefront is utilized. In the exemplary embodiment utilizing the LP 02  mode, it is the second zero crossing closest to the lens which represents a flat wavefront.  
         [0049]    [0049]FIG. 7 displays an image of the reference curve  300  against an actual measured curve  310  of an improperly collimated beam. The x-axis indicates the distance in location from the center in microns, and the x-axis indicates the intensity of the beam in arbitrary units. The differential between the location of the secondary peaks is clear, and thus curve  310  is not that of a properly collimated beam.  
         [0050]    [0050]FIG. 8 displays and image of the reference curve  300  against an actual measured curve  310  of a properly collimated beam. The x-axis indicates the distance in location from the center in microns, and the y-axis indicates the intensity of the beam in arbitrary units. The close fit of the secondary peaks indicates that the collimator is properly aligned.  
         [0051]    In another embodiment, the above invention is utilized for collimation of a fundamental mode beam of light which does not contain a secondary peak. An optical element, such as a phase element  60  is added to modify the beam so as to generate a unique pattern, containing secondary peaks as will be described further below in relation to FIG. 9. The position of those secondary peaks are then utilized in the manner described above.  
         [0052]    [0052]FIG. 9 illustrates a collimator assembly to which has been added phase elements, and comprises single mode fiber (SMF)  40 , collimator housing  50 , phase elements  60 , tube  70  and collimating lens  90 . The expected output of the assembly of FIG. 9 can be calculated given the shape of the curves of phase elements  60 , the length of tube  70  and the shapes of the collimating lens  90 . This expected output is utilized as the reference image loaded in step  1010  of the program of FIG. 5. Utilizing such a construct allows for the use of the inventive method herein described with a single mode fiber.  
         [0053]    The above invention has been described in relation to a calculated reference image. In an exemplary embodiment the calculated reference image takes into account the propagating modes from the few mode fiber  110 , an ideal collimating lens  90  and ideal phase elements  60 . In another embodiment, measured phase elements  60  are utilized. In another preferred embodiment the reference image is adjusted based on the desired operational criteria of the subsystem so that collimation is defined as an ideal working point. The operational criteria of the optical subsystem for which the lens and fiber end are utilized comprise a combination of loss, wavelength dependent loss or the amount of optical energy in certain undesired modes.  
         [0054]    The above invention has been described in relation to utilizing an infrared camera the means of observing the output. This in not meant to be limiting in any way, and other means including a visible light camera, a far infrared camera, or a wavefront camera. In an embodiment comprising a wavefront camera the reference image may comprise both an intensity and a phase.  
         [0055]    The above intention has been described in relation to collimation of the output of a single fiber, however this is not meant to be limiting in any way, and is specifically meant to include utilizing a single optical element to optimize the collimation of each fiber in an array of fibers.  
         [0056]    Having described the invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.