Abstract:
An apparatus that efficiently and accurately directs or splits a non-polarized light beam input from a fiber optic into one, two, four or eight output beams wherein the light beam input is passed through a collimator, the apparatus uses non-polarizing beam splitters and the output beams are coupled to fiber optics using a lens matched to divergence of the beam to direct the beam into the fiber optic and uses an iris diaphragm attenuator to adjust the energy of the output beam prior to its entry into the fiber optic.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to beam splitters and more particularly relates to splitting a beam from a fiber optic source to fiber optic receivers. The invention more particularly relates to such beam splitters having input beams that originate as the output from a source fiber optic where the split beams are suitable for being directed to a plurality of secondary fiber optics through fiber optic couplers and the use of beams from the secondary fiber optics for therapeutic treatment, such as photodynamic therapy where an undesirable area on a patient, e.g. a tumor, is exposed to a beam from a secondary fiber optic after absorption by the tumor of a photosensitizing agent such as a porphyrin derivative. More particularly photodynamic therapy is based upon accumulation in tumors of a photosensitizing drug that is activated by visible light to produce a locally cytotoxic agent. For example PHOTOFRIN®, a porphyrin derivative approved for clinical use in the United States, Canada, Europe, and Japan, is activated by 630 nm light. Typically, light emitted from a tunable laser is delivered to a lesion by an optical fiber.  
           [0002]    A problem associated with photodynamic medical procedures, e.g. photodynamic therapy, is that often numerous areas on the same patient in fact require treatment. Time involved in setting up and individually treating each of the numerous areas by a single light beam source can be extensive often exceeding the useful life of injected photosensitizing compound. Further, sequential treatment results in high cost due to time involved for trained personnel and inefficient use of costly equipment as well as significant discomfort on the part of the patient. It is of course possible to provide multiple light beam, e.g. laser, generators so that multiple areas can be simultaneously treated. Unfortunately, however, the cost for providing multiple beam generators for a single patient treatment is prohibitive.  
           [0003]    It is known that laser beams can be split by beam splitters that comprise a partially reflective and partially transparent surface so that an incident laser beam is partially reflected and partially transmitted so that the beam is effectively split into two parts. Unfortunately, there has been no way to practically, consistently or economically commercially manufacture such surfaces so that they all will reflect 50 percent of the beam energy and transmit 50 percent of the beam energy at the same particular incident angle (the angle of the beam to the surface that splits the beam energy in half).  
           [0004]    The manufacture of a beam splitter apparatus for more than two output beams thus would have been very difficult since the manufacturing process would have to take the particular incident angle of each individual beam splitter into consideration which requires the calculation of numerous angles of reflection and resulting various alignments and does not permit the use of any kind of standardized set angle hardware within the apparatus. The assembly of such a multiple beam splitter thus would have been tedious, time consuming and unacceptably expensive.  
           [0005]    It has thus not been possible to easily and inexpensively manufacture a beam splitter to form four or more output beams where the output beam energies are within ten percent of each other and certainly not within five percent or less of each other.  
           [0006]    It has been recently found that a beam splitter for a laser beam source could be made that overcame the above problems, e.g. as described in U.S. Pat. No. 6,084,717; however, such a device was not suitable for splitting a beam from a fiber optic source. Laser beam sources, e.g. as used in the device described in U.S. Pat. No. 6,084,717, are polarized, of small diameter, e.g. less than a few a millimeters, and without significant divergence. Laser beams are however expensive to produce and difficult to direct in that the entire laser source must be moved. By contrast, an input beam from a fiber optic is unpolarized, usually larger in diameter than a laser beam and has much greater divergence. A fiber optic source has definite advantages in that the beam can be easily directed simply by moving the end of the fiber optic without moving the entire light producing apparatus. Unfortunately, since beams from a fiber optic source are unpolarized, polarization thus cannot be used to attenuate individual beams from a fiber optic source. Further, even if fiber optic beam sources were conceived to be possible for use in splitters, due to divergence of beams from fiber optics, lens arrangements for laser beam sources would be completely unsuitable for beams from fiber optic sources. For example, diameter, focal length, and spacing for lenses suitable for laser beams would be different than such parameters for beams from fiber optics. Known devices for splitting beams from a laser source are thus completely unsuitable for splitting light beams from a fiber optic source.  
           [0007]    The beam from a multimode fiber optic source is non-polarized and highly divergent.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows a top view of an embodiment of the invention arranged to provide a single output beam.  
         [0009]    [0009]FIG. 2 shows a top view of an embodiment of the invention arranged to provide two output beams.  
         [0010]    [0010]FIG. 3 shows a top view of an embodiment of the invention arranged to provide four output beams.  
         [0011]    [0011]FIG. 4 shows a top view of an embodiment of the invention arranged to provide eight output beams.  
         [0012]    [0012]FIG. 5 is a perspective view of a preferred embodiment of a beam splitter and mount used in the invention.  
         [0013]    [0013]FIG. 6 is a perspective exploded view of a preferred embodiment of a fiber optic coupler for use in the invention. The figure also has two cross-sectional views relating to the alignment of treatment fibers to the output beams of the apparatus.  
         [0014]    [0014]FIG. 7 shows a side view of the assembled optic coupler of FIG. 6.  
         [0015]    [0015]FIG. 8 shows a cross-sectional view of the coupler taken on lines A-A of FIG. 7.  
         [0016]    [0016]FIG. 9 a  shows a cross-sectional view of an SMA 905 multimode fiber coupler.  
         [0017]    [0017]FIG. 9 b  shows a cross-section of an SMA sleeve holder for use in the invention.  
         [0018]    [0018]FIG. 10 shows a cross-sectional view of an assembled SMA 905 connector.  
         [0019]    [0019]FIG. 11 a  shows a centering plug as assembled into the connector of FIG. 10.  
         [0020]    [0020]FIG. 11 b  shows a cladded and jacketed fiber optic assembled into the connector of FIG. 10.  
         [0021]    [0021]FIG. 12 is a perspective view of a tool used to align the optical elements of the apparatus. 
     
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0022]    In accordance with the invention an apparatus is therefore provided to efficiently and accurately split a light beam from a fiber optic input into eight outputs where the beam powers of the output beams are within ten and preferably within five percent of each other. More particularly, the invention comprises a light beam splitter apparatus capable of forming eight output beams from a single input from a fiber optic, where the apparatus has the following described structure.  
         [0023]    Importantly, the input fiber optic coupler has a lens that collimates the beam from the fiber optic and the couplers to receive split beams have lenses suitable for receiving the split beams, i.e. they have the correct focal length and size. Especially important is that the couplers each have attenuators to adjust the strength of the beam as it enters the coupled fiber optic so that all exit fibers can be adjusted to emit beams of the same or different powers. Since the beams are not polarized, non-polarizing plate beam splitters are required. The Brewster window attenuators of the prior art will not function with unpolarized light. The invention therefore incorporates a variable iris located behind each coupler lens.  
         [0024]    The apparatus includes an arrangement of seven beam splitters each of which splits an incident light beam from a fiber optic into two beams of approximately equal power at an identical incident angle for each splitter. The first splitter is one of the seven that has the ratio of reflected to transmitted power extremely close to unity(R/T=1) with a variance of less than about 0.5% at the common incidence angle. This choice is critical since a power imbalance produced at this splitter is propagated throughout the system. The second and third splitters are two that have R/T ratios very close to unity (a variance of less than about 1%) at the common incidence angle and are arranged so that the reflected and transmitted beams of the first splitter are directed to the second and third splitters. The second and third splitters each provide a reflected and transmitted beam when receiving an incident light beam.  
         [0025]    The last four of the seven splitters can be those with R/T ratios different from unity at the common incidence angle but still close to unity (within a few percent and preferably within 1.5 percent). They are arranged so that the reflected and transmitted beams from the second and third splitters are directed to the fourth, fifth, sixth and seventh splitters at the common incident angle to provide their incident light beams. Each of the fourth, fifth, sixth and seventh splitters provide two light beams of nearly equal power when receiving an incident light beam to provide an apparatus output of at least eight beams.  
         [0026]    An apparatus was constructed according to the above method of selecting beamsplitters. At the design wavelength of 630 nanometers the eight output beams powers had a standard deviation of ±3%. At 665 nanometers (the wavelength of a future photosensitizer) the standard deviation was 15%. This demonstrates the wavelength sensitivity of non-polarizing plate beamsplitters.  
         [0027]    In a preferred embodiment of the invention, the apparatus further includes a second input beam collimator and an adjustable redirecting means for the beam input from the source fiber optic and redirecting it to selected splitters. The redirecting means may, for example, be adjusted to redirect the initial beam input from the source fiber optic away from all splitters to provide an apparatus output of a single beam. The redirecting means may also be adjusted for intercepting the initial beam input to redirect it toward one of the fourth, fifth, sixth and seventh splitters to provide an apparatus output of two beams or may be adjusted for intercepting said initial input to redirect it toward one of the second and third splitters to provide an apparatus output of four beams.  
         [0028]    The redirecting means usually includes a repositionable mirror but may employ a prism rather than a mirror for redirecting the input beam. A mirror is usually preferred because it can usually redirect a beam with less energy loss than a prism. In a preferred embodiment, the redirecting means is a front surface mirror (called a fold mirror) having at least four adjustable positions wherein:  
         [0029]    a) Position one intercepts the initial light beam input from a fiber optic to reflect the beam away from the splitters to provide a single light beam permitting a single apparatus output from an intercepting fiber optic coupler. Because of the flexibility of fiber optics, the fiber optic may simply be moved to a location so that its output impinges upon a mirror to reflect it to a single fiber optic output from the apparatus.  
         [0030]    b) Position two intercepts the initial light beam input from a fiber optic to reflect it toward one of the sixth and seventh splitters to provide two light beams to provide two apparatus outputs.  
         [0031]    c) Position three intercepts the initial light beam from a fiber optic to reflect it toward the third splitter to provide four beams permitting four apparatus outputs, and  
         [0032]    d) position four parks the fold mirror in a location that does not interfere with an eight beam output. Position four permits the input beam to strike the first splitter at the first splitter particular incident angle to provide eight apparatus outputs. Position four directs the input to the first splitter at the first splitter incident angle.  
         [0033]    The energy of the apparatus output light beams is to be directed to and carried by fiber optics, thus fiber optic couplers are provided in paths of the apparatus output beams to receive the energy of such beams and for directing beam energy through a fiber optic. The use of such couplers provide an unexpected advantage in that the couplers provide a back reflection beam having an energy of from about 2 to about 6 percent, usually about 4 percent, of the energy received from an output beam. A means for aligning the output beams with the couplers can therefore be provided by varying the position of the couplers relative to an image formed by their back reflection beams.  
         [0034]    The invention further includes a method for providing light beam treatment to a patient by simultaneously directing a plurality of light beams from fiber optics receiving energy from the apparatus of the invention to areas on a patient in need of light beam treatment. For example, the apparatus of the invention may be used as at least a part of a light delivery system for photodynamic therapy, e.g., in conjunction with fiber optic positioners as disclosed in U.S. Pat. No. 5,671,317.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    “Laser beam” as used herein means a single frequency coherent beam of light. “Light” means any electromagnetic radiation within the ultraviolet, visible and near infrared ranges.  
         [0036]    “Beam” means light energy that travels in an essentially straight line direction in a vacuum subject to divergence.  
         [0037]    “Light beam” as used herein means a beam of light that may be nearly coherent but is not as coherent as a laser beam.  
         [0038]    “Splitter” means a device that divides a beam into two beams that travel in different directions. “Splitters” are usually partially transparent mirrors that reflect a part of the beam energy and transmit a part of the beam energy to form two beams.  
         [0039]    “Incident light beam” means a beam that strikes a splitter at a particular incident angle and is the beam being split.  
         [0040]    “Transmitted light beam” means the beam exiting a splitter.  
         [0041]    “Reflected light beam” means the beam leaving the input surface so that the angle between it and the input beam is twice the angle of incidence.  
         [0042]    “Beam Splitter apparatus” means an apparatus for splitting light beams that includes one or more splitters.  
         [0043]    “Input beam” means a beam to be split that enters a beam splitter apparatus.  
         [0044]    “Coupled beam” mean a beam coupled to a fiber optic.  
         [0045]    “Output beam” means a beam after passing through a optical element such as a beamsplitter or a lens.  
         [0046]    “Particular incident angle” means an angle of incidence to a surface of a beam splitter that will divide a beam striking the splitter at that angle into two beams having essentially equal energy.  
         [0047]    The invention may be illustrated by reference to the following preferred embodiment.  
         [0048]    A beam splitter apparatus or device was designed and constructed to split the output from a fiber optic into up to eight beams of essentially equal power, i.e., within about 10 percent of each other, preferably within about 5 percent of each other and most preferably within about 3 percent of each other. The power from each beam could be independently varied when necessary. The beam splitter device of this preferred embodiment is compact and of simple, low cost construction. Beam splitter mounts, collimators and fiberoptic coupler bodies were designed and fabricated specifically for the apparatus. Other components were obtained commercially and modified as required.  
         [0049]    It has been found that the beam from a fiber optic cable with a 200 micron diameter core and a N.A.(Numerical Aperture) of 0.16 or less (beam divergence half-angle of 9.2 degrees or less) can be collimated, split into multiple beams and the energy in each beam focused into a spot only slightly larger than 200 microns. This energy can be efficiently input to treatment fibers having a core diameter of 400 microns or larger. The design concepts described in U.S. Pat. No. 6,084,717 are employed with such modifications as dictated by the nature of the light source. The beam splitting elements are non-polarizing and wavelength specific. The power of each split beam is set by an adjustable iris located behind the focusing lens. Two input beam collimators are used, one for the 8 beam output and one for the 4, 2, or 1 beam output(s). A modification is made to the method of terminating the 0.16 N.A. fiber optic cable with an SMA 905 connector. An insert aligns the 200 micron core of the source fiber with the longitudinal axis of the connector so that is approximately collinear with the optical axis of the apparatus. (The output of FDA approved lasers for photodynamic therapy is accessed through a SMA 905 connector port).  
         [0050]    The beam splitter apparatus divides a beam from a fiber optic into up to eight beams of essentially equal power with negligible loss in total power. The device also allows four, two or one output(s), each of which may be coupled to an optical fiber for the delivery of therapeutic light to an individual treatment field. Means to control the power of the individual beams are provided, e.g. in the form of iris diaphragm attenuators, in order to equalize the treatment powers when optical fibers of different efficiencies were used, or when the treatment plan required dissimilar power outputs from each fiber. The apparatus is compact which is especially desirable because space is at a premium in clinics and laboratories.  
         [0051]    The apparatus has eight fiber optic couplers and two fiber optic collimators. Each of these ten pieces are identical in construction and have a 12.7 mm diameter, 25.4 mm focal length plano-convex lens, with the exception that the bodies of the two collimators do not have a machined cavity to hold a variable iris.  
         [0052]    The apparatus has seven plate beamsplitters acquired by the following selection process. Since the light source is not polarized, non-polarizing plate beam splitters are required. These are commercially available and designed to achieve at a specific wavelength a 50/50±5% split of the incident beam at a 45 degree angle of incidence with less than 0.5% power absorption. The ±5% tolerance is too great for a system in which each output beam is a result of three successive divisions. However these beamsplitters can be used at other than the designed angle of incidence to achieve a ratio of reflected power to transmitted power of unity (R/T=1) with negligible effect on the polarization and absorption properties. A commercially available 50 mm square by 3 mm thick plate beamsplitter designed for 632.8 nm works well enough at 630 nm. Three of these (preferably from the same manufactured lot) are cut into twelve 25 mm squares and each is measured for the incidence angle that produces a 50/50 split at 630 nm. The angle of incidence chosen for the apparatus is that for which one beamsplitter has an R/T ratio of unity, two have R/T ratios within 1% of unity and four of the remaining nine are selected that are the closest to a 50/50% split at that angle.  
         [0053]    The optical components were mounted on a 14.4-inch by 13.5-inch by ½-inch aluminum jig plate which serves as an optical table  10 . In FIG. 1 a one inch diameter plane mirror  11  is mounted on a mirror holder  12  fastened to a carrier  13  which translates on a microrail  14  parallel to the collimated beam  15 . The mirror is set to the same angle of incidence as the seven splitters. A rod  16  passes through a centering block  17  and is attached to the carrier  13 . A set screw with spring ball  18  in the centering block  17  engages one of four grooves in the rod  16  when the mirror  11  is at the desired location. The collimated input beam  15  is produced when the light from fiber optic source  19  passes through a collimating lens  20  mounted in the collimator  21 . When the 1 st  groove (closest to the handle of rod  16 ) is engaged mirror  11  intercepts the collimated input beam  15  to provide four beam outputs (FIG. 3). When the 2 nd  groove is engaged two outputs are provided (FIG. 2) and when the 3 rd  groove is engaged, one output is provided (FIG. 1). Eight beams (FIG. 4) are provided when the collimated input beam  15  is redirected by moving fiber optic source  19  to a second collimator  22  and the 4 th  groove of rod  16  is engaged (parked position) so that mirror  13  intercepts no light beams.  
         [0054]    As shown in FIG. 1, one output beam may be obtained. Fiber optic source  19  provides light beam  15  through collimating lens  20 . Beam  15  is directed to mirror  11  when the 3 rd  groove in rod  16  is engaged which reflects the beam to coupler  23 . Lens  24  focuses the beam onto output fiber optic  26  through iris attenuator  25 .  
         [0055]    As best seen in FIG. 2, two output beams  27  and  28 , respectively coupled to fiber optics  29  and  30 , may be obtained. Input beam  15 , from fiber optic source  19 , as previously described, is reflected by mirror  11  to splitter  28  when the 2 nd  groove in rod  16  is engaged. About half of the energy from the beam is reflected from splitter  28  to form beam  28   a  and the other half passes through splitter  28  to form beam  28   b.  Beams  28   a  and  28   b  are then coupled to fiber optics  29  and  30  respectively, through couplers  31  and  32  respectively. Lens  33  focuses beam  28   a  onto output fiber optic  29  through iris attenuator  34  and lens  35  focuses beam  28   b  onto output fiber optic  30  through iris attenuator  36 .  
         [0056]    As best seen in FIG. 3, four output beams coupled to fiber optics may be obtained. Beam  15  is reflected by mirror  11  to splitter  37  where it forms beams  38  and  39  when the  1 st groove in rod  16  is engaged. Beam  38  is directed to splitter  40  to form beams  38   a  and  38   b  which are directed to couplers  41  and  42  respectively. Lens  44  focuses beam  38   a  onto output fiber optic  45  through iris attenuator  46  and lens  47  focuses beam  38   b  onto output fiber optic  48  through iris attenuator  49  as previously described. Beam  39  is directed to splitter  50  to form beams  39   a  and  39   b  which are in turn directed to couplers  51  and  52  respectively. Lens  53  focuses beam  39   a  onto output fiber optic  54  through iris attenuator  55  and lens  56  focuses beam  39   b  onto output fiber optic  57  through iris attenuator  58 .  
         [0057]    As best seen in FIG. 4, eight beams can be formed by moving input fiber optic  19  to collimator  59  and mirror  11  to the parked position. Lens  74  in collimator  59  produces a collimated beam  15  which is directed to splitter  60  to form beams  61  and  62 . Beam  61  is then directed to splitter  63  to form beams  64  and  65  each of which are again split by splitters  66  and  28  to form beams  64   a,    64   b,    65   a  and  65   b  which are directed to couplers  68 ,  23 ,  31  and  32  respectively. Similarly beam  62  is directed to splitter  37  to form beams  69  and  70  which are in turn directed to splitters  40  and  50  to form beams  69   a,    69   b,    70   a  and  70   b  which are directed to couplers  41 ,  42 ,  51  and  52  respectively. Lens  71  focuses beam  64   a  onto output fiber optic  72  through iris attenuator  73 . The other seven output beams are coupled to their respective output fiber optics as previously described in FIGS.  1  to  3 .  
         [0058]    The beam splitter apparatus of the preferred embodiment is shown in FIGS.  1 - 12 . A preferred beamsplitter mount  90  is shown in FIG. 5. It is machined from 1-½ inch×1-½ inch×¼ inch thick bronze angle  92 . It has a 1 inch square base  94  and a 1-¼ inch high front  96  into which a 3 millimeter deep shelf  98  is cut to hold a 3 millimeter thick beamsplitter  100 . A {fraction (1/16)} inch radius groove  102  is machined on center at the base of the mount. This groove accepts a ⅛ inch diameter dowel pin  102  which registers the position of the axis of rotation of the mount. When the incident beam is centered on the face of the beamsplitter and the mount is rotated about the dowel pin the reflected beam rotates without a lateral displacement. The mount is held in place with a ¼-20 bolt and bellville spring washer through a {fraction (5/16)} inch hole in the base of the mount.  
         [0059]    The components of the coupler and a side view of the assembled coupler are shown in FIGS. 6 and 7. It consists of a commercial lens holder  75 , a commercial iris  76 , the coupler body  77  machined from brass, the fiber optic holder  78  also machined from brass and a modified commercial SMA to SMA mating sleeve connector  79  (shown in cross-section in FIG. 9 b ). The treatment fiber is threaded onto the mating sleeve. As shown in the cross-sectional view of the sleeve  79  the modification consists of an enlargement of the bore of the end of the sleeve  79  to prevent clipping the marginal rays of the focused beam (shown by the dotted lines with arrows). When the machined surface  80  at the opposite end of the sleeve  79  is in contact with reference surface  81  in an SMA 905 connector  81   a  (FIG. 9 a ) the output or input surface  82   a  of an optical fiber  82  is at a predetermined point within the sleeve. This point can be set in the focal plane of the lens in the collimators and couplers by using the source fiber set screws  83 - 84  at low power. A sharp magnified image of the fiber surface  82  is formed at infinity when the sleeve  79  is threaded into the fiber optic holder  78  so that the surface  82   a  is in the focal plane of the lens  75   a.  This position of the sleeve is secured with a lock nut (not shown). The surface  82   a  can be set on the optical axis of the lens  75   a  by adjusting set screws  83  against the ¾ pound end pressure exerted by a spring ball in the opposed set screws  84 .  
         [0060]    SMA 905 connectors are not designed to produce an on-axis output beam (the hole in the ferrule has a low length to diameter ratio). An on-axis output beam as shown in FIG. 10 can be obtained with the following procedure. FIG. 11 b  shows the internal structure of a preferred silica/silica multimode 0.16 N.A. fiber  82 , having a    3   mm jacket  82   b.  The fiber  82  is prepared for insertion into the SMA connector  81   a  by stripping a section of the 3 mm diameter jacket  82   b  exposing a 600 micron diameter buffered fiber  82   c.  The buffer is stripped from the tip of the fiber so that the 250 micron diameter cladded core  82   d  can fit through the hole in the ferrule of the connector  81   a.  FIG. 11 a  shows a cross-section of a  16  gauge hypodermic tubing  82   e  held in a centering plug  82   f  machined from brass. The hypodermic tube was reduced in diameter to the inside diameter of the ferrule and its length is the internal length of the SMA connector  81   a.  The plug is sized to slip inside the connector and holds the tube on center. The hypodermic tube and the plug are coated with epoxy and inserted into the connector. The stripped length of fiber is also coated with epoxy and inserted into the hypodermic tube as shown in FIG. 10. Standard procedures are followed in securing the connector to the cable jacket and polishing the surface of the fiber. The fiber optic source for the beamsplitter apparatus must have a N.A. of 0.16 or less and be terminated with an SMA connector as described above in order to efficiently transfer the source power to the treatment fibers.  
         [0061]    A ray trace computer program was written in Visual Basic Language which calculated spot diagrams for every optical surface. It showed that all of the light emitted by a 200 micron diameter, 0.16 N.A. optical fiber could be split into eight collimated light beams which could be focused into eight spots of less than 300 micron diameter. The program calculated the position of 18 dowel pins marking the optical axis of the multiple beam segments. Seven pins located the position of the 7 beamsplitter mounts, 8 pins marked the 8 exit beams (these are not seen in FIGS.  1  to  4  as they are located under the output fibers), 1 pin,  85  in FIG. 4, located the center line of that collimated beam which produced 8 output beams and 2 pins,  86  and  87  in FIG. 3, marked the center line of the collimated beam used in the 4, 2, or 1 output beam(s) configuration. The program calculated the coordinates of the mounting holes for the 7 splitters and the 5 optical rails.  
         [0062]    [0062]FIG. 12 is a perspective drawing of a tool  104  used to align the optical elements of the apparatus. A dowel pin fits in a hole  106  (dotted lines) drilled in the base of the tool. Four orthogonal grooves  108   a,    108   b,    108   c  and  108   d  mark the center of a target hole  110  at the height of the optical axis. The hole is smaller than the collimated beam diameter. This makes it easy to visually center the beam on the target.  
         [0063]    Alignment procedure for the beam splitter apparatus begins with all of the output fiber optic couplers and the fold mirror removed. The alignment tool is set on pin  85  (FIG. 4). The fiber optic source  19 , set to emit a trace light beam, is connected to collimator  59  which is mounted on a carriage barely visible in the drawing. The carriage is slid onto a microrail to a position at which the trace beam is centered on the target hole in the tool. The tool is set on the pin that locates the output beam for fiber  72 . The position of the light source in the focal plane of lens  74  is adjusted using set screws  83  and  84  (FIG. 8) so that the collimated beam passes through the unaligned beam splitters  60 ,  63  and  66  and is centered on the target. The target is moved back to pin  85  and the collimator is repositioned on the microrail, if required, to center the beam on the target. This procedure is repeated until the beam is on target at both pin locations. The position of collimator  59  is secured on the rail with a locking nut on the carriage.  
         [0064]    Setting the angle of incidence on beam splitter  60  is the next step in the alignment. The tool is set on the pin that locates the output beam for fiber  57 . The mount of beamsplitter  60  is rotated so that the reflected beam  62  passes through beamsplitters  37  and  50  and is centered on the vertical groove in the tool. Plastic shims in increments 0.0005 inch thickness are placed under the front or back of the mount of beamsplitter  60  as required to center the collimated beam on the horizontal grooves on the target. A ¼-20 bolt through a Bellville spring washer and the oversize hole in the base of the beamsplitter mount fasten the mount to the jig plate. This establishes beam  70   b  since the subsequent small adjustments in the angles of beamsplitters  37  and  50  required to set the reflected beams  69  and  70   a  will not cause an appreciable lateral shift in the transmitted beam  70   b.  Next collimator  52  (minus the treatment fiber) is mounted on a carriage and positioned on the microrail so that beam  70   b  on passing through lens  56 , the open iris  58  and the SMA sleeve falls on the center of the target. Using the same procedure beamsplitters  63  and  37  and collimators  32  and  41  are aligned next with the tool set on the pins locating the output beams for fibers  30  and  45  respectively. Next beamsplitters  66  and  50  and collimators  23  and  51  are aligned using the tool placed on the pins locating the output beams for fibers  26  and  54  respectively. Finally beamsplitters  40  and  28  and collimators  42  and  31  are aligned using the tool placed on the pins locating the output beams for fibers  48  and  29  respectively.  
         [0065]    When a light beam enters a coupler, light is reflected from the input face of the attached fiber and from the distal end of the fiber since neither end of the fiber core is antireflection coated. The light from the distal end travels back through the fiber toward the coupler lens. That light and the light reflected from the input face are collimated by the coupler lens and travel up stream toward the fiber optic source  19 . The power in the reverse beam is reduced by 50% at each of the three beamsplitters encountered in the path toward the source  19 . Each reverse beam forms three images at infinity of the fiber face, some at the left side of FIG. 4 and some at the bottom of the figure. These can be observed on cards placed along the edges. For example the reverse beam from fiber  57  forms a reflected image to the left side at beamsplitter  50 , another reflected image to the same side at beamsplitter  37  and a transmitted image at the bottom side at beamsplitter  60 . When the other seven couplers are blocked by closing their irises the images due to fiber  57  alone are observed. Fiber  57  can be moved about in the focal plane of lens  56  by the set screws  83  (FIG. 6). There is a striking change in the images when the focused beam moves off the metal face of the SMA ferrule onto the quartz core. It appears as though a thin circular film is being drawn across a dimly lit circle. The focused beam is centered on the core of the treatment fiber when the film is centered on this circle. Thus the reverse beams of light are used to align each treatment fiber.  
         [0066]    The final alignment task is setting the fold mirror to the same angle of incidence as the beamsplitters. As shown in FIG. 3 the source fiber  19  is moved to collimator  21 . The collimator is aligned in the same manner as collimator  59 , FIG. 4, using the alignment tool at pins  86  and  87 . The plane mirror  11 , the mirror mount  12  and the carriage  13  are assembled and placed on the microrail  14 . The grooved rod  16  is attached to the carriage. An approximate setting of the angle is made by turning the mount  12  on the carriage  13  and sliding the carriage on the rail so that the parallel edges of a card are in contact with the mirror face and the beamsplitter  37 . The distance from the right edge of carriage  13  to each groove in the attached rod  16  was calculated from data provided by the ray trace computer program. The rod  16  is set in the  4  output beam position. The horizontal and vertical controls  89  on the mirror mount  12  are adjusted so that the two beams  39   b,  the result of two transmissions, and  38   b,  the result of three reflections, are on target when the alignment tool is set on the pins behind couplers  52  and  42 . The apparatus is now aligned.  
         [0067]    The main utility of the beam splitter apparatus of the invention is to convert what would otherwise be a laborious and time consuming procedure into an acceptable and practical treatment using a fiber optic input beam without the complexity of using a laser beam that is difficult to control and adjust. For example, photodynamic therapy for treatment of patients with numerous basal cell tumors of the skin as a result of a genetic defect (nevoid basal cell carcinoma syndrome) is highly effective and desired by these patients because of often superior cosmetic outcomes compared to surgical procedures. Because each tumor or site must be exposed to therapeutic light for approximately 24 min., the sequential treatment of 40-50 lesions (as commonly presented) is completely impractical. The ability to treat 8 such lesions or sites simultaneously by using the beam splitter and specially designed fiber positioners makes this a practical and acceptable procedure for these patients and others with a large number of skin lesions.