Abstract:
An illumination system includes a multimode diode-laser and two optical fibers. Light from the diode-laser is directed into the first optical fiber having a first core diameter. The light exits the first optical fiber and is directed by an optical system into a second optical fiber having a core diameter greater than the first optical fiber and a numerical aperture greater than the numerical aperture of the optical system. A light beam exiting the second optical fiber has an intensity distribution having sharp edges and uniformity better than plus or minus ten percent over a central ninety percent of the beam.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed in general to optical arrangements for projecting light output of lasers. The invention is directed in particular to an optical arrangement for projecting a uniform beam from the output of a multimode diode-laser. 
     DISCUSSION OF BACKGROUND ART 
     Semiconductor diode-lasers are finding application as sources of therapeutic treatment radiation in many types of scientific and medical apparatus. They have advantages in that they are small, efficient, reliable, and can have emission wavelengths selected within a wide range of wavelengths by appropriate selection of the composition of active semiconductor materials of the diode-lasers. 
     Diode-lasers used in medical apparatus in particular are multimode diode-lasers. The multimode operation results from providing a relatively wide active-region in the diode-laser to increase the amount of power generated by the laser. The wide active-region gives rise to multiple transverse lasing modes. A disadvantage of a multimode diode-laser is that light output from an emitting aperture of the laser which may be as much as 100 times as wide as it is high provides a highly elliptical output beam. The light diverges relatively narrowly, for example at about 10 degrees in the width direction of the aperture (the slow axis), and more widely, for example at about 35 degrees in the height direction (fast axis) of the aperture. 
     Light output from a multimode diode-laser is often transported to a point of use by an optical fiber. The diode-laser-light is collected into optical fiber by a lens, for example, a cylindrical lens disposed between the diode-laser and the optical fiber and aligned in the slow axis. The combination of the lens and the optical fiber provides for a certain degree of “circularization” of the diode-laser-light output. Typically, light is output from the optical fiber as a beam having a generally circular cross-section and a numerical aperture (divergence) which is about the average of the numerical aperture of the diode-laser output in the fast and slow axes. A significant problem, however, is that the intensity distribution of light output of the optical fiber in the near field is uneven due to the multiple modes propagated in the optical fiber. This uneven intensity distribution as well as the numerical aperture of the optical fiber output can fluctuate due to temporal changes in the light output of the diode-laser or changes in orientation or bending of the optical fiber. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for delivering a laser beam from a laser via an optical fiber with the laser beam having a substantially uniform near-field intensity distribution on delivery from the optical fiber, for example, with variation less than about ±10 percent of a nominal average. The uniform laser beam may be delivered to an optical apparatus for use therein. In one aspect, apparatus in accordance with the present invention includes a laser delivering a beam of laser-light in one or more modes and first and second multimode optical fibers. 
     An optical arrangement receives the laser-light from the laser and directs the laser-light into the first optical fiber at an entrance end thereof. The first optical fiber is configured such that the laser-light exits the first optical fiber at an exit end thereof in a second number of modes, the second number of modes being greater than the first number of modes. An optical system receives the laser-light exiting the first optical fiber and directs the laser-light into the second optical fiber at an entrance end thereof. The second optical fiber is configured such that the laser-light exits the second optical fiber at an exit end thereof as an output beam in a third number of modes, the third number of modes being greater than the second number of modes. 
     Increasing the number of modes in the laser-light by transporting the laser-light through the second optical fiber combined with mixing of the modes within the second optical fiber provides that the near field uniformity of intensity of the output beam at the exit end of the second optical fiber is better than the near-field uniformity of intensity of the laser-light beam emitted at the exit end of the first optical fiber. 
     In one embodiment of the present invention, a portion of the second optical fiber is retained in the form of one or more bends. Selecting an appropriate number and radius of curvature of the one more bends provides that the near-field intensity distribution across the output beam is substantially constant or uniform. 
     In one particular example of apparatus in accordance with the present invention, the apparatus is directed to providing a beam of substantially uniform intensity distribution and variable size at a focal plane of an ophthalmic slit-lamp microscope for use in treatment of age-related macular degeneration. The laser is a multimode diode-laser emitting at a wavelength of about 689 nanometers (nm). The first optical fiber has a core diameter of 100.0 μm and length of about 1.0 meters (m). The second optical fiber has a core diameter of 200.0 μm and a length of about 3 m. A portion of the second optical fiber is retained in a loop or 360° bend having a diameter of about 30 millimeters (mm). The output beam at the end of the second optical fiber has a uniformity of intensity better than ±10% over a central 90% of the beam. A second optical system receives the output beam and provides a magnified image of the end of the optical fiber at a focal plane of the slit-lamp microscope. The image, of course, has essentially the same uniformity of illumination intensity as the output beam at the end of the optical fiber. The second optical system is a zoom optical system arranged such that the image can be selectively adjusted in diameter between about 0.4 mm and 5.0 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. 
     FIG. 1 schematically illustrates one preferred embodiment of apparatus in accordance with the present invention including a multimode diode-laser, a cylindrical lens directing light from a diode-laser into a first multimode optical fiber, a first optical system directing diode-laser-light received from the first optical fiber into a second multimode optical fiber and a second optical system directing light received from the second optical fiber to a focal plane of a slit-lamp microscope. 
     FIG. 2 is a graph schematically illustrating near-field intensity distribution of light output from the first optical fiber of FIG.  1 . 
     FIG. 3 is a graph schematically illustrating near-field intensity distribution of light output from the second optical fiber of FIG.  1 . 
     FIG. 4 schematically illustrates another preferred embodiment of apparatus in accordance with the present invention similar to the apparatus of FIG. 1 but wherein a 360° bend is formed in the second optical fiber. 
     FIG. 5 is a graph schematically illustrating near-field intensity distribution of light output from the second optical fiber of FIG.  4 . 
     FIG. 6 schematically illustrates details of a retaining arrangement for the 360° optical fiber bend of FIG.  4 . 
     FIG. 7 schematically illustrates details of an alternate bending and retaining arrangement for retaining two 180° bends in the second optical fiber of FIG.  4 . 
     FIGS. 8A and 8B schematically illustrate details of another retaining arrangement for the 360° optical fiber bend of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, one preferred embodiment  20  of apparatus in accordance with the present invention includes a multimode diode-laser  22 , here, illustrated with the slow axis and fast axes thereof respectively perpendicular to and in the plane of the drawing. Diode-laser  22 , here, has an emitting aperture (not visible in FIG. 1) having a height of about 1 μm and a width of about 100 μm. A beam of light  24  output by the diode-laser is collected by a cylindrical lens  26  into a multimode optical fiber  28  having-input and output ends  28 A an  28 B respectively. Those skilled in the art will recognize that more complex optical arrangements for collecting and delivering light to optical fiber  28  may be used without departing from the spirit and scope of the present invention. 
     Optical fiber  28  has a core diameter of 100 μm and preferably has a numerical aperture equal to or greater than the numerical aperture of light output in the slow axis by diode-laser  22 . The core diameter is selected such that the optical fiber can support a number of transmission modes for light  24  greater than the number of modes emitted by the diode-laser. Cylindrical lens  26  reduces the fast axis divergence of diode-laser-light  24  to allow it to be accepted by optical fiber  28 . In this example optical fiber  28  has an intrinsic numerical aperture of 0.22. Diode-laser-light  24  emerges therefrom as a beam  24 A with a numerical aperture of about 0.15. 
     Light  24  includes contributions from multiple transverse operating modes of diode-laser  22 . Each one of these operating modes can be transmitted along optical fiber  28  in a number of modes characteristic of the fiber. Accordingly, if a number N modes is output by diode-laser  22  then a*N (where a&gt;1 and is characteristic of the optical fiber) modes are transmitted along optical fiber  28 . 
     FIG. 2 schematically illustrates the intensity distribution (curve  30 ) of light emitted from output end  28 B (beam  24 A) of optical fiber  28  at a near-field position indicated in FIG. 1 by arrow A. The beam is essentially circularized by transmission along optical fiber  28 . Intensity at the edges of the distribution rises (or falls) sharply. However, intensity distribution in region  32  of curve  30  is very non-uniform or uneven. This results, inter alia, from the mode interaction of transmitted modes and, as noted above, is subject to change with changes in orientation of the optical fiber. The intensity distribution of FIG. 2 is not atypical of prior-art diode-laser-light delivery systems wherein only a single optical fiber is used for the laser-light delivery. An ideal intensity distribution would appear-as depicted in FIG. 2 by dashed curve  34 , i.e., with sharp (vertical) rising and (or falling) sides  36  and a flat (constant intensity) top  38 . This type of intensity distribution is often referred to in the art as a “top-hat” distribution. 
     Continuing now with reference again to FIG. 1, light from beam  24 A emitted from optical fiber  28  is received by an optical system  40  including lenses  42  and  44 . Here it should be noted that while lenses  42  and  44  are depicted schematically in FIG. 1 as single optical elements, these lenses may include two or more such elements. Optical system  40  should also be considered as being exemplary and may include more than two lenses as is known in the art. Optical system  40  focuses light  24 A into an entrance end  46 A of a multimode optical fiber  46 . Lens  42  is preferably arranged to form an intermediate image of exit end  28 B of optical fiber  28  in a position approximately located on lens  44  as depicted in FIG. 1 by arrow B. This minimizes the influence of fluctuations in the far-field output distribution of optical fiber  28  on the angular distribution of light  24 B delivered into optical fiber  46 . 
     It should be noted here that optical fibers  28  and  46  discussed herein are depicted, for simplicity, without cladding or protective sheathing. Those skilled in the art, however, will recognize that cladding and/or sheathing of fibers are usual in such an application. Those skilled in the art will also recognize that these are passive optical fibers which do not provide any amplication of light transported therealong. 
     Optical fiber  46  preferably has a product of core diameter and numerical aperture greater than the product of core diameter and numerical aperture of optical fiber  28 . This enables it to support more transmission modes than optical fiber  28 . In this example, a core diameter of 200 μm and an intrinsic numerical aperture of 0.22 is selected for optical fiber  46 . Such an optical fiber having a pure silica core is readily commercially available. A pure silica core is preferred for its advantageous power handling and transmission properties. Optical system  40  has a numerical aperture of 0.10, i.e., smaller than the intrinsic numerical aperture of optical fiber  46 . Accordingly, optical fiber  46  is not filled by light  24 A directed into it by optical system  40 . Under-filling the numerical aperture of optical fiber  46 , inter alia, provides that light  24 B output by the optical fiber also has a numerical aperture of 0.10. In the example of apparatus  20 , this is required to fit the output light into a slit-lamp microscope described herein below. 
     In apparatus  20 , optical fiber  28  has a length of about 1.0 m and optical fiber  46  has a length of about 3.0. Generally, a combined length of about 0.2 m or greater is preferred to allow adequate mode mixing. More preferably the length of any individual fiber is about 0.2 m or greater. However the length of any individual optical fiber is preferably less than 10.0 m to prevent the optical fiber from being filled by the light input. Preventing filling of the optical fiber by the light input, as discussed above, allows the light output to exit the optical fiber with the same numerical aperture as the numerical aperture of the light input. 
     As noted above, light  24 B emerging from optical fiber  46  includes contributions from a*N modes delivered at input end  46 A of optical fiber  46 . Each of these modes can propagate in optical fiber  46  in a number of modes characteristic of the optical fiber. Accordingly, the number of modes emerging from output end  46 B of optical fiber  46  can be defined as a*b*N (where b is a number greater than 1 and characteristic of optical fiber  46 ). 
     A zoom optical system  45  is arranged to provide, via a fold mirror  48 , a magnified image of output end  46 B of optical fiber  46  at a plane  49  which, in this example, is the focal plane of an ophthalmic, slit-lamp microscope assembly  50 . The image size is selectively adjustable between 0.4 millimeters (mm) and 5.0 mm in diameter. Optical system  45  is depicted here, for simplicity, as including only two lenses  43  and  47 . Those skilled in the art will recognize that a well-corrected zoom optical system typically includes more than two lens elements. A preferred arrangement for optical system  45  includes four lens elements, two thereof fixed and two thereof movable to allow variable spacing between the elements. 
     A patient&#39;s eye  52  receives light  24 B for treatment. In this example eye  52  is being treated by photodynamic therapy (PDT) for age-related macular degeneration (AMD). Light  24  is the treatment light. Fold mirror  48  is coated to provide reflection at the wavelength of light  24 , here 689 nm, and transmission at shorter (visible light) wavelengths. The eye  52  is illuminated by a slit-lamp  54  via a fold mirror  56  which is partially reflective and partially transmissive for visible light. The treatment is observed (eye  57 ) through a microscope  58  having binocular eyepieces  60  (only one depicted in FIG.  1 ). It should be noted here that slit-lamp microscope  50  is depicted simply and schematically in FIG. 1 merely for illustrating a preferred application of apparatus in accordance with the present invention. Slit ophthalmic microscopes are well known to those skilled in the art. A detailed description and depiction of such a microscope is not necessary for understanding principles of the present invention and, accordingly, is not presented herein 
     FIG. 3 schematically illustrates the intensity distribution (curve  70 ) of light emitted from output end  46 B of optical fiber  46  at a near-field position indicated in FIG. 1 by arrow C. It can be seen in region  72  of curve  50  that unevenness of intensity distribution is greatly reduced compared with the distribution of FIG.  2 . There is a greater number of ripples or a greater modulation frequency, however, the depth of the ripples, or modulation, (indicated by arrows M) is significantly less than the fluctuations in region  32  of curve  30  (see FIG.  2 ). This results from a mode mixing effect provided by the increase in the number of modes resulting from the above-discussed mode-multiplying effect of optical fiber  46 . 
     The reduction of ripple or modulation depth minimizes the fluctuation of energy at the site of delivery, in this example the retina of a patients eye. Fluctuations in the distribution are generally within the ripple or modulation depth. It can also be seen that intensity at edges  74  of the distribution rises (or falls) sharply, closely approximating edges of a top-hat distribution. Curve  70  departs from the ideal top-hat distribution, however, inasmuch as highest intensity is located toward the center of the distribution curve. An arrangement for “flattening” the intensity distribution of light emerging from optical fiber  46  is described below with reference to FIG.  4 . 
     In FIG. 4, another preferred embodiment  80  of apparatus in accordance with the present invention is similar to apparatus  20  of FIG. 1 with an exception that optical fiber  46  has a loop  82  or bend formed therein. It has been found that if loop  82  has a sufficiently small diameter, the intensity distribution of light at the output end  46 B can be “flattened” to generally equalize intensity at the center and edges of the near field distribution without increasing the numerical aperture of the far-field distribution. 
     FIG. 5 schematically illustrates the intensity distribution (curve  90 ) of light  24 B emitted from output end  46 B of optical fiber  46  including loop  82  at a near-field position indicated in FIG. 4 by arrow D. It can be seen that in region  92  of curve  90  the modulation depth (M) of the intensity distribution is about the same as in region  72  of FIG. 3, and the steeply rising or falling edges of the distribution of FIG. 3 are preserved. Generally, however, the intensity is about the same at the center of the distribution as at the edges thereby providing a close approximation to the ideal, top-hat or substantially constant intensity distribution. 
     Regarding loop  82 , of optical fiber  46 , a single such loop or bend has been found to provide an adequate approximation to the top hat distribution. In the above described example where optical fiber  46  has a core diameter of about 200 μm and an intrinsic numerical aperture of 0.22, a diameter of about 30.0 millimeters (mm), i.e., a radius of curvature of about 15.0 mm for loop  82  was found effective. It was found that if the diameter (radius of curvature) of loop  82  was made too small, the output numerical aperture of light emerging from optical fiber  46  could exceed the numerical aperture of optical system  45  thereby reducing the general intensity level in an image projected thereby. In this arrangement, a near-field (at exit end  46 B of optical fiber  46 ) uniformity of intensity (including low and high frequency variation) of less than ±10% of a nominal average value was obtained over a central 90% of the light output beam. 
     It was also determined that, at an optimum diameter for a single loop  82 , there was no significant improvement in the distribution curve if a second such loop were formed in the optical fiber. However, the possibility that two or more loops of less than optimum diameter for a single loop may provide a close approximation to top-hat intensity distribution is not precluded. 
     Some advantageous “distribution-flattening” effect of the loop may also be achieved by forming in fiber  46  a single bend of less than 360 degrees, i.e., less than a complete loop, or by forming the fiber into a succession of such bends in the same or opposite directions. The number and radius of curvature of the bends must be selected, as discussed above, to achieve the distribution-flattening effect. 
     Whatever bending arrangement is selected, it is important that some means be provided to retain the fiber in the selected bent condition. This is required because variations in the fiber bending could result in variations in output beam-uniformity. Further, in the case of relatively tight bends such as the 15 mm radius bend discussed above, retention is necessary to overcome a tendency of the optical fiber to straighten itself under its own spring action. One convenient means of retaining the optical fiber in the 360° bend or loop  82  of FIG. 4 is schematically illustrated in FIG.  6 . Here, a retaining board or card  100  has apertures  102 ,  104  and  106  extending therethrough. Optical fiber  46  is passed successively through apertures  102 ,  104  and  106  to form the 360° loop or bend  82 . 
     Another bending and retaining arrangement for optical fiber  46  is schematically illustrated in FIG.  7 . Here, a retaining board or card  110  has apertures  112 ,  114  and  116  extending therethrough. Optical fiber  46  is passed successively through apertures  112 ,  114  and  116  and retained in a serpentine form including two 180° bends  118  and  119  in opposite directions. 
     Yet another, non-exhaustive, bending and retaining arrangement is schematically depicted in FIGS. 8A and 8B. This arrangement is particularly suitable for maintaining a loop  82  in a sheathed optical fiber  46 . In this arrangement, a machined plastic component  120 A includes longitudinal grooves  122  and  124  intersecting a circular groove  126 . Optical fiber  46  is laid first into longitudinal groove  122 , then laid into circular groove  126  and longitudinal groove  124  to form loop  82 . Once the optical fiber is thus laid into the grooves, component  120 A is covered by a mating component  120 B including mating grooves (not shown). Components  120 A and  120 B are then secured together by screws  128  via apertures  130  in the mating components. 
     The present invention is described above with reference to delivering light from a multimode diode-laser into an ophthalmic slit-lamp microscope assembly for providing PDT treatment for age-related macular degeneration. The apparatus and method of the present invention, however, is not limited to the exemplified use nor to delivering light from multimode diode-lasers. The invention is applicable to delivering light from any other laser, either single mode or multimode, emitting radiation which can be transmitted along a multimode optical fiber. From the description of the present invention provided herein, those skilled in the art to which the invention pertains may devise other embodiments and uses thereof without departing from the spirit and scope of the invention as defined by the claims appended hereto.