Abstract:
A system for aligning a laser beam with the end of an optical fiber includes optics for focusing the laser beam toward the end of the optical fiber. A plurality of light receptors are positioned around the end of the optical fiber and, as intended for the present invention, each light receptor generates a light signal which is indicative of the light intensity from the laser beam that is incident on it. Connected with this plurality of light sensors is a comparator which creates an error signal that is proportional to a difference between selected light signals from the light receptors. The laser beam can then be moved relative to the end of the optical fiber in response to the error signal to align the laser beam with the end of the optical fiber. When alignment is achieved, the light signals will be substantially equal to each other and the error signal will be a null.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to light beam alignment systems. More particularly, the present invention pertains to systems for focusing a laser beam to a predetermined specific point in space. The present invention is particularly, but not exclusively, useful for aligning the focal point of a laser beam with the end of an optical fiber. 
     BACKGROUND OF THE INVENTION 
     As is well known, an optical fiber is an elongated ultra-pure glass fiber that includes a central core and has an outer cladding which completely surrounds the core along the length of the fiber. Importantly, the central core has a higher refractive index than its outer cladding and the optical fiber is, therefore, capable of conducting modulated light signals from one end of the fiber to the other by total internal reflection. 
     Optical fibers can be generally categorized as either single mode or multimode fibers. More specifically, for single mode optical fibers, the diameter of the inner core is comparable with the wavelength of the light that is being propagated. Consequently, there is only one mode of light propagation through the fiber. Multimode fibers, on the other hand, have a core diameter sufficiently larger than the wavelength of light to allow propagation of light energy in a large number of different modes. Still, in order to avoid mode dispersion and the resultant distortion of signals, the core size of an optical fiber has dimensional limitations. The consequence of all this is that optical fibers, when used for communications purposes, will typically have relatively small diameters, e.g. approximately fifty microns (50 μm). While such small diameters may be advantageous for many applications, small diameters also raise issues about how best to direct light from a light source into the optical fiber for subsequent transmission through the fiber. 
     In order to effectively use optical fibers in communications systems it is essential there be some ability to optically interconnect the optical fiber with the light source. Heretofore, this has typically been accomplished by mechanical means. Specifically, by establishing a mechanical link between the light source and the optical fiber, it has been possible to be reasonably assured that light from the light source will enter the optical fiber. The situation changes, however, when it is necessary to establish an optical link between a light source and an optical fiber across free space. Further, the difficulty in establishing such a free space link is compounded as the distance between the light source and the optical fiber is increased. Nevertheless, there are applications wherein it may be desirable to establish a laser (light) communications link across a free space distance that may be as much as five hundred meters, or more. 
     In light of the above, it is an object of the present invention to provide a system for aligning a laser beam with the end of an optical fiber which is capable of directing a focused laser beam onto the inner core of an optical fiber. Another object of the present invention is to provide a system for aligning a laser beam with the end of an optical fiber which will effectively establish a laser communications link across free space through a distance in excess of several hundred meters. Yet another object of the present invention is to provide a system for aligning a laser beam with the end of an optical fiber that is simple to use, relatively easy to implement, and comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     A system for aligning a laser beam with the target end of an optical fiber includes optics which will focus or direct the laser beam toward the fiber&#39;s target end. A plurality of three or more light receptors are positioned to surround the end of the optical fiber and they are, preferably, coplanar with the end of the optical fiber. Importantly, each of the light receptors is capable of generating a light signal that is indicative of the intensity of the portion of the laser beam that is incident on the particular light receptor. For the purposes of the present invention, the light receptors can be tracking optical fibers which are juxtaposed with the target optical fiber. 
     For the system of the present invention, a comparator is electronically connected with each of the light sensors and is used for creating an error signal. Specifically, the error signal that is generated by the comparator is proportional to a difference between selected light signals as they are generated by the light receptors. Using standard feedback control techniques, this error signal can then be used for moving the laser beam relative to the end of the optical fiber in response to the error signal. With this controlled movement, the system is able to align the laser beam with the end of the optical fiber. 
     By way of example, consider a system using four light receptors which are arranged around the target end of the optical fiber in diametrically opposed pairs. Within this arrangement, all four of the light receptors and the end of the optical fiber will be located in an x-y plane. Specifically, one pair of receptors will be aligned along an x-axis, while the other pair is aligned along a y-axis. The optics of the system can then be used to locate the focal point of the laser beam in this x-y plane (i.e. the laser beam is focused onto the x-y plane in three dimensional space so that z=0). With the laser beam focal point in the x-y plane, one pair of light receptors can then be used by the comparator to position the end of the optical fiber relative to the laser beam in the x direction (i.e. x=0), while the other pair of light receptors can be used to position the end of the optical fiber relative to the laser beam in the y direction (i.e. y=0). As indicated above, this is accomplished using the error signal. More specifically, when the light signals from the pair of light receptors on the x-axis are equal, the laser beam will be centered in the x direction. Likewise, when the light signals from the pair of light receptors on the y-axis are equal, the laser beam will be centered in the y direction. Stated differently, the error signal will be a null when all of the light signals from the respective light receptors are substantially equal to each other. 
     Movement of the target end of the optical fiber relative to the laser beam, in order to obtain a null error signal, can be accomplished in any of several ways. First, the optical fiber itself can be moved relative to the optical system focusing the laser beam. Second, the optical fiber and the focusing optics can be mounted on a base, and the base can be moved. Third, the system&#39;s focusing optics can be mounted on the base along with the optical fiber and the optics can include a plurality of mirrors, of which one is a secondary mirror. For this configuration the secondary mirror can be moved until the error signal is null. 
     In a refinement for the system of the present invention, the optics used for directing the laser beam toward the end of the target optical fiber can be configured to profile the laser beam. For the purposes of the present invention, the laser beam needs to be profiled with a high-intensity region and a low-intensity region. Specifically, the high-intensity region will be centrally located in the laser beam and the low-intensity region will be peripherally located in the laser beam to surround the high-intensity region. Importantly, the low-intensity region of the profiled laser beam must have sufficient intensity to generate the light signals that are required from the light receptors for generating the error signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is an elevational cross section view of a system in accordance with the present invention; 
     FIG. 2 is a perspective view of a target optical fiber juxtaposed in an arrangement with tracking optical fibers (light receptors) for receiving an incoming laser beam; 
     FIG. 3 is a diagram of a closed loop feedback control system that is indicative of the control system which is used for the present invention; 
     FIG. 4 is a front elevational view of the arrangement of the target optical fiber and tracking optical fibers as seen from the line  4 — 4  in FIG. 1; 
     FIG. 5 is a schematic diagram showing the generation of an error signal for the arrangement shown in FIG. 4; and 
     FIG. 6 is a schematic diagram of a light intensity profile for a laser beam useful for the purposes of the present invention as would be seen in both the x and y directions of a plane transverse to the laser beam path as identified by the line  4 — 4  in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a system in accordance with the present invention is shown and is generally designated  10 . As shown, the system  10  includes a base  12 , and a reflector  14  which is mounted on the base  12 . Also, a secondary mirror  16  and a receiver  18  are mounted on the base  12 . Further, a comparator/computer  20  is provided for the system  10 . 
     As intended for the system  10 , the comparator/computer  20  can be any type device or electronic circuit which is capable of differentiating between light intensity signals and, in turn, generating a signal that is proportional to such differences. In accordance with subsequent disclosure, a purpose of the present invention is to correctly position components of system  10 . For this purpose, the comparator/computer  20  can be alternatively connected with the base  12  via a control line  22 , or with the secondary mirror  16  via a control line  24 , or with the receiver  18  via a control line  26 . If desired, however, all three of these connections can be implemented simultaneously. In any case, it is intended for the comparator/computer  20  to provide positional control over the base  12 , the secondary mirror  16 , or the receiver  18  via respective control lines  22 ,  24  or  26  for purposes of aligning the receiver  18  with an incoming laser beam  28 . As will be appreciated by the skilled artisan, the actual movement of the base  12 , the secondary mirror  16  or the receiver  18  in response to signals from the comparator/computer  20  can be accomplished by mechanisms (not shown) which are well known in the pertinent art. 
     FIG. 1 shows that the system  10  is intended to receive the incoming laser beam  28  and to then focus the laser beam  28  onto an end  30  of the receiver  18 , by using various optical elements. More specifically, the optics for the particular embodiment of the system  10  shown in FIG. 1 allow the laser beam  28  to enter the housing of the base  12  through an aperture  31  and be incident on the reflector  14 . The laser beam  28  is then focused by the reflector  14  onto the secondary mirror  16  which, in turn, focuses the laser beam  28  onto the end  30  of the receiver  18 . For the purposes of the present invention, the aperture  31  can be covered with glass, or with some other light transparent material, and the secondary mirror  16  and receiver  18  can be moveably or fixedly mounted on the base  12  in any manner well known in the pertinent art. The particular configuration for the system  10  shown in FIG. 1, (i.e. one which includes the reflector  14 , secondary mirror  16  and receiver  18 ) is only exemplary. In fact, as contemplated for the present invention, components corresponding to the base  12 , reflector  14  and secondary mirror  16  are optional. In sum, it is only necessary that the laser beam  28  somehow be focused by optics onto the end  30  of receiver  18 . 
     As shown in FIG. 2, the end  30  of receiver  18  exposes a target optical fiber  32  which includes a core  34  and a cladding  36 . For purposes of orientation and subsequent discussion, the end  38  of optical fiber  32  is shown located in the x-y plane of an x-y-z Cartesian coordinate system. Thus, within this orientation, the laser beam  28  will be directed toward the end  38  of the target optical fiber  32  along the z-axis. As also shown in FIG. 2, a plurality of light receptors  40  (of which the light receptors  40   a ,  40   b ,  40   c  and  40   d  are exemplary) are arranged around the target optical fiber  32 . For the present invention, these light receptors  40  can be tracking optical fibers with respectively associated detectors, and can be of the same type optical fiber as is used for the target optical fiber  32 . For the purposes of the present invention the detectors can be of any type well known in the pertinent art and, preferably, are included as components of the comparator/computer  20 . Further, although four light receptors  40  are shown for the preferred embodiment of the receiver  18  in FIG. 2, it will be appreciated by the skilled artisan that, in line with subsequent disclosure, and because any three points can define an x-y plane, as few as three tracking optical fibers (light receptors  40 ) will suffice for the purposes of the system  10 . Nevertheless, for the preferred embodiment of the system  10 , four light sensors  40  are to be used and arranged as diametrically opposed pairs substantially as shown in FIG.  2 . More specifically, the light sensors  40   a  and  40   c  are aligned on the y-axis and respectively positioned on either side of the target optical fiber  32 , while the light sensors  40   b  and  40   d  are similarly aligned on the x-axis. 
     FIG. 3 shows a generalized diagram for the closed loop feedback control that is used by the system  10  to position the end  38  of the target optical fiber  32  relative to the incoming laser beam  28 . To do this, the control elements  42  of the system  10  react to an error signal, e, that is generated by the difference between a reference input  44  (the desired position of end  38 ) and a feedback signal  46  (the actual position of end  38 ). The resultant error signal, e, is then used to obtain a desired output  48  (i.e. movement of the end  38  from its actual position to its desired position). For the system  10 , the control elements  42  will include components of the system  10  that position and move either the base  12 , the secondary mirror  16 , or the receiver  18 . These movements then constitute the desired output  48 , and this output  48  continues to be generated until the receiver  18  is properly positioned relative to the laser beam  28 . More specifically, an output  48  is generated until the laser beam  28  is effectively directed to enter the core  34  of target optical fiber  32  for further transmission through the fiber  32 . 
     An error signal, e, which can be used by the system  10  to move the optical fiber  32  into alignment with the laser beam  28 , will be best appreciated by reference to both FIG.  4  and FIG.  5 . First, it is to be understood that the laser beam  28  can be focused by the optics of system  10  such that the focal point of the beam  28  will lie in the x-y plane. Once the focal point of beam  28  is in the x-y plane, however, there is still the problem of moving the focal point of the beam  28  in the x-y plane onto the optical fiber  32  so that as much light as possible from the laser beam  28  is able to enter the optical fiber  32 . With this in mind, consider the situation indicated in FIG. 4 wherein the laser beam  28 ′ is not focused directly onto the optical fiber  32  but, instead, is slightly off center. 
     For the condition shown in FIG. 4, the laser beam  28 ′ is actually incident on only a portion of the optical fiber  32 . Thus, an optimal use of the laser beam  28 ′ is not possible. To obtain this optimal use the laser beam  28 ′ needs to somehow be steered to the position of laser beam  28  as shown in FIG.  4 . Thus, note that portions of the laser beam  28 ′ are incident on the light sensors  40   b  and  40   c , but that no portion of the laser beam  28 ′ (or at least a relatively small portion of the laser beam  28 ′) is incident on either of the light sensors  40   a  or  40   d . Thus, because no portion (or a very small portion) of light beam  28 ′ is incident on the light sensor  40   a , the light signal  50   a  from light sensor  40   a  will have substantially a zero value (see FIG.  5 ). On the other hand, because a portion of the light beam  28 ′ is incident on the light sensor  40   c , the light sensor  40   c  will generate a light signal  50   c  that has some absolute value. The difference between these light signals  50   a  and  50   c  will then cause an error signal, e, to be generated which will indicate that the laser beam  28  needs to move relative to the optical fiber  32  through a distance, Δy. The important point here is that there is a difference in value between the light signals  50   a  and  50   c . Further, depending on the relative magnitudes of the light signals  50   a  and  50   c , the error signal, e, will also indicate whether Δy is positive (light signal  50   a &lt;light signal  50   c ) or negative (light signal  50   a &lt;light signal  50   c ). Similarly, because laser beam  28 ′ is incident on light sensor  40   b  (i.e. light signal  50   b  has value), but it is not on light sensor  40   d  (i.e. light signal  50   d  at least has a different value than light signal  50   b ), there is a difference between the light signals  50   b  and  50   d . Thus, the error signal, e, will indicate the need to move the optical fiber  32  relative to the laser beam  28  through a distance, Δx. Again, a positive or negative direction for the distance Δx will be determined by the relative magnitudes of the light signals  50   b  and  50   d.    
     As long as there is a difference between light signal  50   a  and  50   c , the error signal, e, will indicate a need to move some distance Δy. Also, as long as there is a difference between light signal  50   b  and  50   d , the error signal, e, will indicated a need to move some distance Δx. Using conventional feedback control techniques, this will continue as long as there is an error signal, e. When the error signal, e, is a null, however, the laser beam  28  will be centered on the target optical fiber  32  as desired (see FIG.  4 ). 
     The operation of system  10  may be enhanced to some extent by profiling the laser beam  28 . Specifically, as shown in FIG. 6, a profiled laser beam  28  will include a high-intensity region  52 , which is located in the central portion of the laser beam  28 . The profiled laser beam  28  will also include a low-intensity region  54  which is located peripherally in the laser beam  28  and which substantially surrounds the high-intensity region  52 . As shown in FIG. 6 for a laser beam  28  having an intensity, I, a profiled laser beam  28  can be configured such that most of the light in the beam  28  is in the high-intensity region  52 . It is important, however, that the intensity of light in the low-intensity region  54  of the laser beam  28  be sufficient for generating appropriate light signals  50 . 
     The actual profiling of the laser beam  28  can be accomplished in any of several ways known in the pertinent art, such as by using compound refractive surfaces. For example, the refractive surface  56  shown on secondary mirror  16  in FIG. 1 can be specifically contoured to generate the high-intensity region  52 , while the remainder of the secondary mirror  16  is left to generate the low-intensity region  54 . Alternately, a diffractive element can be placed in the beam to create the appropriate contours. 
     In summary, during the operation of the system  10 , the light receptors  40   a-d  (tracking optical fibers) will be used as described above to generate an error signal, e. The comparator/computer  20  then, in response to the error signal, causes positional adjustments to be made by either the base  12  (via line  22 ), the secondary mirror  16  (via line  24 ), or the receiver  18  (via line  26 ). With these adjustments, the laser beam  28  is caused to be focused onto the target optical fiber  32 . As shown in FIG. 1, the optical fiber  32  is connected with communications equipment  58 . Thus, upon proper alignment of the system  10 , any communication signals that are carried on the laser beam  28  will be transmitted to the communications equipment  58  for subsequent use. 
     While the particular Systems and Methods for Aligning a Laser Beam With an Optical Fiber as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.