Abstract:
A coupling device has a first focusing element positioned on a first optical axis. The first focusing element is couplable to receive output light beams from a plurality of optical fibers, and has a first focusing power selected to direct the light beams to intersect the first optical axis at a first intersection position. A second focusing element is spaced apart from the first focusing element by a first separation distance along the first optical axis and is positioned to receive the light beams from the first focusing element. The second focusing element has a second focusing power and the first separation distance is selected to parallelize the light beams received from the first focusing element. A system for providing access to light beams propagating through a plurality of fibers uses two of the coupling devices. The two devices are relatively oriented to have opposing second focusing elements so that a beam path of at least one of the parallellized beams from the first device lies coincident and antiparallel to a beam path of at least one of the parallelized beams from the second device.

Description:
BACKGROUND 
     The present invention is directed generally to a fiber optic device, and more particularly to a device for producing parallelized output beams from a multiplicity of fibers. 
     Optical fibers find many uses for directing beams of light between two points. Optical fibers have been developed to have low loss, low dispersion, polarization maintaining properties and can also act as amplifiers. As a result, optical fiber systems find widespread use, for example in optical communication applications. 
     However, one of the important advantages of fiber optic beam transport, that of enclosing the optical beam to guide it between terminal points, is also a limitation. There are several optical components, important for use in fiber systems or in fiber system development, that are not implemented in a fiber-based form where the optical beam is guided in a waveguide. Instead, these optical components are implemented in a bulk form that light must freely propagate through. Examples of such components include, but are not limited to, isolators, circulators, polarizers, switches and shutters. Consequently, the inclusion of a bulk component in an optical fiber system necessitates that the optical fiber system have a section where the beam path propagates freely in space, rather than being guided within a fiber. 
     Free space propagation typically requires that the beam from each fiber be collimated and directed along the axis of the bulk component being used in the free-space propagation section. Usually, this necessitates that a collimating lens be positioned at the input fiber to collimate the incoming light and a focusing lens be positioned at the output fiber to focus the freely propagating light into the output fiber. The free-space propagation section lies between the two lenses. The introduction of a free-space propagation section requires that the collimating lens and the focusing lens are each aligned to their respective fibers and also that the focusing lens is correctly aligned relative to the collimated beam path from the collimating lens. The alignment of the collimating and focusing lens remains critical, irrespective of the number of fibers. Accordingly, the alignment process becomes more complex and time consuming when multiple fibers require the alignment of multiple collimating and focusing lenses. 
     In addition, each collimating and focusing lens and each fiber has to be supported transversely. The provision of transverse support increases the total cross-section required by each fiber/lens assembly, thus resulting in a large system. 
     Accordingly, there is a need for an improved approach to introducing a free-space propagation section into fiber optic systems that is simpler to align and is more compact. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention relates to a device that produces parallel optical beams from a plurality of optical fibers, and to a system that uses two such devices to produce a free-space propagation region within a fiber optic network. 
     One embodiment of the invention is a device that permits two-way coupling between a free-space optical component and a plurality of fibers. The device includes an assembly couplable to the fibers receive light. The assembly includes a first focusing element positioned on a first optical axis to receive output light beams from the optical fibers, where the first focusing element has a first focusing power selected to direct the light beams to intersect the first optical axis at a first intersection position. A second focusing element is spaced apart from the first focusing element by a first separation distance along the first optical axis and positioned to receive the light beams from the first focusing element, the second focusing element having a second focusing power, the first separation distance being selected to parallelize the light beams received from the first focusing element. The assembly may be provided with pig-tailed fibers. 
     Another embodiment of the invention is a system for providing access to light beams propagating through a plurality of fibers. The system includes first and second sets of optical fibers and two coupling modules coupled to a respective set of optical fibers. Each coupling module includes a first focusing element positioned on a module optical axis that is coupled to receive light beams from output ends of the respective set of optical fibers. The first focusing element has a first focusing power selected to direct the light beams to intersect the module optical axis. The coupling module also has a second focusing element spaced apart from the first focusing element by an interelement separation distance along the module optical axis and positioned to receive the light beams from the first focusing element. The second focusing element has a second focusing power and the interelement separation distance is selected to parallelize the light beams received from the first focusing element. The first and second coupling modules are relatively oriented to have opposing second focusing elements so that a beam path of at least one of the parallellized beams from the first coupling module lies coincident and antiparallel to a beam path of at least one of the parallelized beams from the second coupling module. 
     Another embodiment of the invention is a method of producing a set of parallel light beams from outputs from a first set of optical fibers. The method includes arranging output faces of the optical fibers relative to a first focusing element and directing, with the first focusing element, output light beams from the optical fibers to intersect a first optical axis. The method also includes parallelizing, with a second focusing element, the output light beams so that the light beams intersecting the first optical axis propagate in essentially parallel directions. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIGS. 1A and 1B illustrate different embodiments of a multiple beam coupling module according to the present invention; 
     FIG. 2A illustrates a pair of multiple beam coupling modules used in conjunction with multiple fibers to produce a free-space propagation region, according to an embodiment of the present invention; 
     FIG. 2B illustrates a practical embodiment of the example illustrated in FIG. 2A; 
     FIGS. 3A-3D illustrate different configurations of coupling modules to accommodate bulk optical components having different optical geometries; and 
     FIG. 4 illustrates a configuration of a single coupling module. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is applicable to optical fiber systems, and is believed to be particularly suited to introducing a free-space propagation section into an optical fiber system. The approach presented here may be simpler to align than conventional systems and be more compact. 
     The present system typically includes use of a coupling module that receives the input from a number of input fibers and generates a set of freely propagating, parallel optical beams. This is termed a parallelizing operation. The coupling module is simple to align and includes only two lenses, irrespective of the number of input fibers. The coupling module may be used to couple the light from the fibers to a free-space device, for example a detector array. 
     The coupling module can also be used in a reverse manner, to receive a number of parallel, freely propagating beams and to focus these beams into a number of output fibers. This is termed a deparallelizing operation. 
     Since each coupling module can be used for both parallelizing and deparallelizing operations, a free-space coupling unit can be constructed having a region for free-space propagation between two coupling modules. The first coupling module parallelizes light from one set of fibers to generate parallel beams propagating through the free-space propagation region. The second coupling module deparallelizes the light into the second set of fibers. Likewise, for light travelling in the opposite direction through the fiber system, the second coupling module parallelizes light received from the second set of fibers to propagate freely along parallel beam paths in the free-space propagation section. The first coupling module deparallelizes the light into the first set of fibers. 
     A schematic of a coupling module  100  is illustrated in FIG. 1A, showing the optical paths followed by beams from two input fibers,  102  and  104 . No limitation on the number of input fibers is suggested by the illustration of only two input fibers. Two fibers are employed in the illustration for the purposes of clarity and simplicity of the following explanation. 
     The coupling module  100  includes two lenses, a first lens  106  and a second lens  108 , positioned on the optical axis  110 . The light paths  112  and  114 , from fibers  102  and  104  respectively, pass through the first lens  106  and are directed to cross the optical axis  110  at the position marked C. Where the outputs of the fibers  102  and  104  are aligned parallel to the optical axis  110 , the position C is separated from the first lens  106  by a distance equal to the focal length, f 1 , of the first lens  106 . After crossing the axis  110 , the beam paths  112  and  114  propagate to the second lens  108  which is positioned at a separation “d” from the first lens  106 . Where the second lens has a focal length f 2 , the separation d is approximately equal to f 1 +f 2 . Following transmission through the second lens  108 , the beam paths  112  and  114  propagate parallel to the optical axis  110 . 
     Although the beam paths  112  and  114  are illustrated to be collimated between the two lenses  106  and  108 , this is not a necessary condition. Collimation of the beam paths  112  and  114  between the first and second lenses  106  and  108  depends on the divergence of the optical beams passing out of the optical fibers  102  and  104 , the separation between the first lens  106  and the fibers  102  and  104 , and the focal length of the first lens. 
     After transmission through the second lens  108 , the beam paths  112  and  114  converge to produce beam waists  116  and  118  respectively, where a beam waist is the narrowest width of the beam, found at a focus. The beam waists  116  and  118  are located in the plane BB, designated by a dashed line. The separation distance between the plane BB and the second lens  108  depends on the divergence of the light beams entering the first lens  106 , and the separation distance between the output faces of the fiber  102  and  104 . The separation distance d is set to be equal to f 1 +f 2  in order to maintain a parallel output from the second lens  108  and is not available as an adjustment. 
     Each beam waist  116  and  118  forms an image of the output face of the respective fiber  102  and  104 . It is an important feature of the invention that the coupling module  100  relay an image of the output faces of the fibers  102  and  104 , lying on the input plane AA designated by a dashed line, to the image plane BB. The image formed at plane BB may be a magnified image. 
     The first and second lenses may be different types of lens, for example, they may be spherical or aspheric, and may be bi-convex, plano-convex or meniscus. The selection of lens type is dependent on the particular system and the level of acceptable optical aberration, which translates to optical loss. 
     Another embodiment of a coupling module  150  is illustrated in FIG.  1 B. The coupling module  150  receives the output from two input fibers  152  and  154 . The coupling module  150  includes first and second lenses  156  and  158  aligned along an optical axis  160 . The first lens  156  is a gradient index (GRIN) lens, a type of lens commonly used in conjunction with optical fibers due to the barrel shape and the flat optical surfaces that are perpendicular to the lens axis. The GRIN lens  156  may be of any suitable pitch that diverts the beam paths  162  and  164  to cross the axis  160 . If the GRIN lens is a quarter pitch lens, the output face  157  of the GRIN lens is positioned at the crossing point C. Where the GRIN lens  156  has a pitch of less than 0.25, for example in the range 0.18 to 0.23, the crossing point C lies beyond the output face  157 . Similarly, where the pitch of the GRIN lens  156  is more than 0.25, then the crossing point C lies within the GRIN lens  156 . 
     The second lens  158  may be, for example, a plano-convex aspheric lens, oriented with the planar surface  159  oriented towards the crossing point C to reduce aberration effects. 
     The coupling module  150  relays an image of the input plane to the image plane BB. Where the fibers  152  and  154  are butted up against the GRIN lens  156 , the input plane is coincident with the input face  155  of the GRIN lens  156 . The image plane BB may also be referred to as the conjugate plane, because the beam waists  166  and  168  may be regarded as conjugate images of the output faces of the fibers  152  and  154 . 
     The coupling modules  100  and  150  may be used in applications where inputs are received from a number of fibers to be delivered to a non-fiber component or system. For example, the coupling modules  100  and  150  may be used to couple the outputs from fibers in a fiber array to corresponding detectors in a detector array. The coupling modules  100  and  150  may also be used for coupling a free space input of multiple beams to an array of fibers. For example, in one approach to demultiplexing dense wavelength division multiplexed (DWDM) signals, a single, multiplexed, optical beam is diffracted from a curved diffraction grating. The components at different wavelengths, separated by the grating, may be coupled by the coupling module into a number of fibers, each fiber corresponding to one of the wavelength components. 
     FIG. 4 illustrates another application of a single coupling module  400  used with free space components. Several fibers  402  are coupled to the coupling module  400  to produce corresponding parallel, free-propagating beams  404 . The free-propagating beams  404  pass through a Faraday rotator  406  that rotates polarization of the incoming beams through 45°. A reflector  408 , which, for example, may be a reflective coating on the rear surface of the Faraday rotator  406 , retroreflects the beams  404  back through the Faraday rotator for a further 45° rotation. The reflector  408  is positioned at the conjugate plane, or that plane containing the focus of each beam  404  after passing through the second focusing element of the module  400 , so that each beam  404  is coupled back into its corresponding fiber  402 . Such an arrangement results in a polarization rotated beam propagating in a backwards direction through each fiber  402 . This may permit the compensation of unwanted polarization effects within the fibers  402 . The reflector  408  may also be provided as a separate element spaced apart from the Faraday rotator, and need not be a reflective coating on the Faraday rotator. 
     FIG. 2A illustrates a free-space device that uses two opposing coupling modules to create a region of free-space propagation within an optical fiber system. The two coupling modules  200  and  220  are arranged along the same optical axis  210 , although this need not be the case, as is discussed below. 
     The first coupling module  200  receives input light from input fibers  202  and  204 . The first coupling module  200  has a first GRIN lens  206  and a second aspherical lens  208 . As discussed above, other types of lenses may also be used. The beam paths  212  and  214  are directed to cross the optical axis  210  by the first lens  206  and are parallelized by the second lens  208  to be parallel with the optical axis  210 . In addition, the beam paths  212  and  214  converge to beam waists  216  and  218  at the image plane BB. In other words, the first coupling module  200  relays an image of the input plane, the plane upon which the exit faces of the input fibers  202  and  204  are located, to the image plane at BB. 
     The second coupling module  220  is coupled to fibers  222  and  224 . The second coupling module  220  has a first GRIN lens  226  and a second aspherical lens  228 . As discussed above, other types of lens may also be used. The beam paths  232  and  234  are directed to cross the optical axis  210  by the first lens  226  and are parallelized by the second lens  228  to be parallel with the optical axis  210 . In addition, the beam paths  232  and  234  converge to beam waists  236  and  238  at the image plane BB. In other words, the coupling module  220  relays an image of its input plane, the plane upon which the exit faces of the input fibers  222  and  224  are located, to the image plane at BB. 
     When the beam waists  216  and  236  from the two first fibers  202  and  222  are collocated at the plane BB then, by reciprocity, the image of the exit face of the fiber  202  is focused to the exit face of the corresponding fiber  222 , and the image of the exit face of the fiber  222  is focused to the exit face of the fiber  202 . Likewise, the image of the exit face of fiber  204  is focused to the exit face of the fiber  224 , and vice versa. 
     Using this system, light coupled out of each fiber is propagated through the free-space region  240  between the two coupling modules  200  and  220 , and is redirected into corresponding fibers on the other side of the free-space region. A bulk optical component  242  may be placed between the two coupling modules  200  and  220  to operate on the optical beams propagating through the free-space region  240 . As previously described, the bulk optical component is a component that is not implemented in an optical fiber form, and may be an optical switch or array of optical switches, a spatial light modulator, an isolator, a circulator, a filter or some other bulk optical component. The separation between the coupling modules  200  and  220  may be adjusted to compensate for the optical path length traveled through the bulk optical component  242 , so that the conjugate planes of each coupling module  200  and  220  remain coincident. 
     In one particular embodiment, the coupling modules  200  and  220  are made to be identical. In other words, the first lenses  206  and  226  have the same focal length, f 1  (or pitch in the case of a GRIN lens), the second lenses  208  and  228  have the same focal length, f 2 , and the interelement separation between the first and second lenses within each coupling module, d, is the same. An advantage provided when the first and second coupling modules are the same is that the size of the beams focused into the second set of fibers is the same as the size of the beams emitted by the first set of fibers and vice versa. Another advantage provided by this symmetrical arrangement is that the fabrication and assembly process is simplified. 
     Where the images formed by each coupling module  200  and  220  are not coincident on the same image plane BB, the optical coupling efficiency from one set of fibers to the other set of fibers may be reduced. Further, it will be appreciated that for efficient transfer of optical power from the first set of fibers to the second set of fibers, and vice versa, the geometrical arrangement of each set of fibers should correspond with the other. For example, where the coupling modules  200  and  220  are identical and produce symmetrical imaging from one fiber set to the other, it is important that the lateral displacement and azimuthal position of one fiber, e.g. fiber  202 , relative to the optical axis is the same as that for its corresponding, e.g. fiber  222 . However, there is no requirement that the coupling modules  200  and  220  be identical. 
     It is not necessary that the fibers be coupled to the coupling module in a one-dimensional pattern. The fibers may also be coupled in a two-dimensional pattern. The separation between different fibers may be regular, as in an array, or may be irregular. Generally, corresponding fibers on either side of the system are positioned relative to the optical axis to mutually transmit and receive light. Thus, where the set of fibers associated with one coupling module is arranged in, for example, a 4×4 array, the set of corresponding fibers associated with the other coupling module is also in a 4×4 array. The spacing between fibers in each array may be different, depending on the optical properties of each coupling module  200  and  220 . It will be appreciated that, although the fibers in one fiber set may advantageously be arranged in a symmetrical manner around the optical axis, a symmetrical arrangement is not a necessary condition, and the fibers may be arranged in an asymmetrical arrangement about the axis. Further, there is no requirement that there be a one-to-one correspondence between the fibers on either side of the system. Accordingly, there may be coupling of only just one beam from one coupling module to the other coupling module, even though each coupling module is provided with multiple beam paths. 
     Another embodiment of a free-space device is illustrated in FIG.  2 B. The optical coupling modules are the same as those illustrated in FIG. 2A, but four optical fibers are coupled to each side of the device, rather than two. Additionally, the optical path between fiber sets is traced out for simplicity, rather than illustrating the width of the optical beam. Each fiber may be regarded as a port enabled for input and output to the device. It can be seen that port  202 A on the left side of the device has an optical path coupling to port  222 A on the right side of the device. Likewise, ports  202 B,  202 C, and  202 D on the left side of the device have optical paths coupling to corresponding  222 B,  222 C,  222 D on the right side of the device. The device illustrated in this figure also includes mounting components for holding the optical components in position relative to each other. Although the illustrated mounting components may be cylindrical, this is not intended to be a limitation of the invention, and the mounting components may have a non-circular cross-section, for example square. 
     The fibers at ports  202 A to  202 D are butted against the input face of the  205  of the GRIN lens  206 . The fibers are held by a chuck  252  that is mounted within first mounting ring  254 . The fibers at ports  202 A to  202 D pass through apertures  256  through the chuck  252  and may be held in place within the chuck by, for example, an epoxy or other suitable adhesive. The chuck  252  and the GRIN lens  206  may be held in place within the first mounting ring  254  by epoxy or other suitable adhesive. The ends of the fibers at ports  202 A to  202 D and the input face of the GRIN lens  206  may be polished at a small angle, for example 8°, and be anti-reflection coated to reduce return reflections. 
     The coupling module  200  is formed with the first mounting ring  254  and the second lens  208  each mounted within a module ring  258 . The first mounting ring  254  and the second lens  208  may also be epoxied in place, or mounted using any other suitable method, such as another adhesive or soldering. In assembly, the second lens  208  is mounted within the module ring  258  and then the first mounting ring is positioned within the module ring  258 . The separation between the GRIN lens  206  and the second lens  208  is adjusted until the beam paths beyond the second lens are parallel. One method of ensuring that the beam paths are parallel is to measure the amount of light retroreflected into each fiber by a mirror placed behind the second lens while adjusting the interelement separation between the GRIN lens  206  and the second lens  208 . The free space beams are deemed to be parallel when the level of retroreflected light in each fiber is optimized at the same interelement separation. The level of retroreflected light may further be optimized when the mirror is positioned at the conjugate plane of the coupling module. The first mounting ring  254  is then fixed at the position that is identified as producing parallel beams, using epoxy, adhesive, soldering, or some other suitable method. 
     Two identical modules  200  and  220  are then positioned within an outer sleeve  260 , separated by the bulk optical component  242 . The relative orientation between the modules  200  and  220 , and the intermodule separation are set so as to achieve maximum optical coupling between the modules  200  and  220 . The optimum intermodule separation is achieved when the image plane of the first module  200  coincides with the image plane of the second module  220 , as discussed above. The modules  200  and  220  are then fixed in position within the outer sleeve  260  at the optimum relative orientation and intermodule separation. The modules  200  and  220  may be fixed using epoxy, adhesive, soldering, or any other suitable method. 
     The bulk optical component  242  may be positioned within the outer sleeve  260  as illustrated, or may be mounted on one of the modules prior to that module being inserted into the outer sleeve  260 . 
     It is common for a component such as the assembly shown, to be provided to the user with fiber pig-tails for coupling to the fibers of a fiber optic system, for example by fusion splicing, using a connector, or in some other appropriate manner. Accordingly, the fibers  202 A to  202 D and  222 A to  222 D may be fiber pig-tails that are rigidly attached to whole assembly  270 . However, this is not a limitation of the device, and the fibers of the fiber optic system may be directly coupled to the first focusing elements of the free-space coupling device  270 . 
     Such an assembly may be very compact. In some embodiments of the invention, the GRIN lenses  206  and  226  may have a length of a few millimeters, while the second lens may have a focal length in the range of around 2-10 mm. Accordingly, the overall length of the device, between GRIN lenses, may be in the range of approximately 8 to 40 mm, although larger or smaller devices may also be formed. 
     It will be appreciated that other methods of assembling coupling modules and of mounting modules to produce a free-space coupling device may be employed, and the invention is not limited to those methods illustrated here. For example, the coupling modules may be mounted separately on a bench top with adjustable mounts to provide the necessary degrees of freedom for alignment of the parallel beam paths of each coupling module. 
     Different embodiments of free-space coupling device are illustrated in FIGS. 3A to  3 C. In the embodiments illustrated in FIGS. 3A and 3B, the optical axis of each coupling module is not coincident with the optical axis of the other coupling module. An embodiment in which the optical axis of one coupling module is translated relative to the other is illustrated in FIG.  3 A. Each coupling module  300  and  320  is shown in schematic form only. The first coupling module  300  has two input fibers  302  and  304 , and produces two output beams  312  and  314  parallel to the optical axis  310  of the first module  300 . Likewise, the second coupling module  320  has two input fibers  322  and  324 , and beam paths  332  and  334  that are parallel to the optical axis  330  of the second module  320 . 
     The bulk optical component  342  is positioned in the free-space  340  between the two coupling modules  300  and  320 . The bulk optical component  342  translates optical beams passing therethrough, but does not change the direction of propagation. Thus, for example, the beam that is input to the bulk optical component  342  along beam path  312 , is output along path  334 , and vice versa. Also, the beam that is input along path  314  is output along path  334 , and vice versa. Therefore, a bulk optical element  342  that offsets optical beams passing therethrough may be accommodated by the free-space device where the offset between the modules&#39; optical axes  310  and  330  is equal to the amount by which the bulk optical component spatially translates passing optical beams in a transverse direction. 
     In the embodiment illustrated in FIG. 3B, the coupling modules  300  and  320  are the same as those illustrated in FIG.  3 A. However, in this case the bulk optical element  392  deviates a beams passing therethrough by an amount θ, for example by reflecting the beams off a mirror  391 . In order to accommodate this, the optical axes  310  and  330  are set at a relative angle of θ. Therefore, after the beam path  312  from the first coupling module  300  has been redirected by the bulk optical element  392 , its path lies coincident, but antiparallel, with the beam path  332  from the second coupling module  320 . 
     In the embodiment illustrated in the FIG. 3C, the bulk optical element  380  includes a partially reflecting surface  382 , which may partially reflect the beams passing therethrough, or may totally reflect only some of the beams passing there through. The reflector  382  is illustrated as partially reflecting all of the beams passing through. A first coupling module  300  has input fibers  302  and  304 , and produces respective parallelized beams  312  and  314 . A portion of beam  312  is transmitted by the reflector  382  as beam  332  and is coupled into the second coupling module  320 . The reflected portion of beam  312  is directed into the third coupling module  360  by the reflector  382  as beam  372 . Likewise, a portion of beam  314  is transmitted by the reflector  382  as beam  334  and is coupled into the second coupling module  320 . The reflected portion of beam  314  is directed into the third coupling module  360  by the reflector  382  as beam  374 . The beams  332 , 334 ,  372  and  374  are then coupled to respective fibers  322 ,  324 ,  362  and  364  within the coupling modules  320  and  360 . It will be appreciated that light may be coupled in a reverse direction into the first coupling module  300  from the second and third coupling modules  320  and  360 . It is preferable in this embodiment that the conjugate planes of all three coupling modules  300 ,  320  and  360  are coincident, so as to preserve efficient coupling from one module to another. 
     It will be appreciated that other configurations may also be employed, for example by adding a fourth coupling module to the T-configuration of the embodiment illustrated in FIG. 3C to create a X-configuration coupler. 
     Furthermore, additional coupling modules may be cascaded using a number of partial reflectors so that light from a single module can be coupled into a number of other modules. This is illustrated in FIG. 3D, which shows a system similar to the one illustrated in FIG. 3C, except that a second bulk optical element  380   a  follows the first bulk optical element  380   a , and a fourth coupling module  360   a  receives light from a partial reflecting surface  382   a  in the second bulk optical element  380 . The second coupling module  320  receives light that has been transmitted through both of the bulk optical elements  380  and  380   a . To increase the optical coupling efficiency from the first coupling module  300  to the other coupling modules  320 ,  360  and  360   a , the optical path length between the first coupling module  300  and each of the other coupling modules  320 ,  360  and  360   a  is approximately equal to the sum of the image distances of the first coupling module  300  and the respective modules  320 ,  360  and  360   a . For example, where the image distances of all the coupling modules  300 ,  320 ,  360  and  360   a  are the same value, d 1 , then the optical path length from the first coupling module  300  to each of the other coupling modules  320 ,  360  and  360   a  is set at approximately two times d 1  for high optical coupling efficiency. Hence, the third coupling module  360  is displaced downwards in the figure relative to the fourth coupling module  360   a  in order to maintain a similar optical path length to the first coupling module  300 . 
     As noted above, the present invention is applicable to fiber optic systems and is believed to be particularly useful in producing a free-space propagation region suitable to receive bulk optical components that require the free propagation of light, rather than guided wave propagation. A single coupling module may be useful in coupling between a number of fibers and a free space component in either or both the forward and reverse directions. Two coupling modules in a back-to-back arrangement permit coupling from one set of fibers to a free-space optical component and then into a second set of fibers. Although there may be one-to-one correspondence between the first and second sets of fibers, this is not a necessary condition, and there may be coupling only between one fiber of the first set and one fiber of the second set. 
     Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.