Abstract:
A system for coupling a transmit fiber and a receive fiber to a light source and a photodetector, respectively, includes a unitary clear plastic member having lenses and total internal reflection (TIR) mirrors which cooperate to provide propagation along perpendicular X, Y and Z axes. The light source, photodetector and unitary optical member are surface mounted to a printed circuit board having drive circuitry for the light source and processing circuitry for the photodetector. By enabling surface mounting of the detector and light source, the cost of fabrication is significantly reduced.

Description:
TECHNICAL FIELD 
     The invention relates generally to systems for coupling signals using optical fiber equipment and more particularly to the coupling of signals between optical fibers and devices that generate or detect optical signals. 
     BACKGROUND ART 
     In communication applications, high-density packaging is an important issue. This is true whether the communications are based upon transmissions of electrical signals or optical signals. A related consideration is the cost of fabrication. The density of fiber optic modules that can be positioned along a printed circuit board (PCB) of a fiber optic-based Local Area Network (LAN) hub or similar system significantly impacts the per channel cost of the system. 
     Small-form fiber optic modules allow a high density of transmit and receive channels. Such modules include fiber optic transceivers which enable a light source to be coupled to a transmit fiber and allow a detector to be coupled to a receive fiber. FIG. 1 illustrates one known arrangement for coupling optical fibers  10  and  12  to a pair of solid state device packages  14  and  16  that are secured to a PCB, not shown. The optical fibers are embedded within a connector  18 . For a standardized MT-RJ fiber pair, the center-to-center distance of the two fibers is 0.75 mm. For example, the connector may be a Mini-Mechanically-transferrable Push-On (Mini-MPO) connector type. Typically, the density of the transmitter and receiver channels is determined by the size requirements of the device packages. The solid state devices must be packaged to provide both optical and electrical isolation of the transmit and receive signals, as well as environmental protection. In the example shown in FIG. 1, the packages of the devices are shown as conventional Transistor Outline (TO) style cans. Each of the cans includes a pair of leads  20  and  22  that are soldered to the printed circuit board, which includes the drive and processing electronics for a transmitter chip  24  and a detector chip  26 . The transmitter chip may be a light emitting diode (LED) or a laser diode. The detector chip  26  may be a photodiode. 
     The TO cans  14  and  16  of FIG. 1 are positioned such that when the connector  18  is inserted into a female connector seated on the same PCB as the TO cans, the exchange of optical signals is along a single plane. With regard to the transmit channel, the transmitter chip  24  generates an optical signal that is directed to a lens  28  of the TO can  14 . The can lens  28  is cooperative with a collimating lens  30  to produce a collimated beam that impinges the mirror  32 . The first mirror redirects the light path to a second mirror  34 , which again redirects the light path for alignment with the transmit fiber  10 . A lens  36  focuses the signal onto the aligned fiber  10 . 
     The receive channel follows a path similar to the transmit channel, but in the opposite direction. Light from the fiber  12  is collimated by a lens  38  and impinges a third mirror  40 . The redirected optical signal is again redirected by a fourth mirror  42 . The optical signal is then operated upon by a lens  44  and a can lens  46  to focus the received signals onto the detector chip  26 . 
     The arrangement of FIG. 1 provides beam translation along two axes, i.e., the X axis and the Z axis indicated in FIG.  1 . This allows the spacing between the two channels to be increased from the 0.75 mm spacing of the connector  18  to a greater spacing between the two TO cans  14  and  16 , e.g., a spacing of 6.2 mm. 
     Other optical couplers for connecting optical fibers to TO cans are known. U.S. Pat. No. 4,701,010 to Roberts describes a molded body having reflecting surfaces for connecting a fiber to a detector TO package or an emitter TO package. A slot extends into the body to allow insertion of a filter, such as a dichroic mirror, or a beam splitter, depending upon the desired application. 
     While the prior art systems operate well for their intended purposes, what is needed is a system that provides further reductions in the fabrication cost, without a sacrifice in optical performance. 
     SUMMARY OF THE INVENTION 
     A system for coupling optical fibers to optical devices includes lenses and mirrors which cooperate to provide propagation in perpendicular X, Y and Z directions, so that the devices can have a greater center-to-center spacing than the fibers and can be mounted to have axes perpendicular to the axes of the fibers. In the preferred embodiment, the optical devices include a light detector and a light transmitter that are surface mounted to a printed circuit board, while the lenses and mirrors are formed in a unitary optical member that is also mounted to the surface of the printed circuit board. By enabling surface mounting of the detector and transmitter, the cost of fabrication is reduced. Moreover, the surface mounting potentially reduces the circuit board real estate required for positioning the detector and the transmitter. 
     The optical member is made of a clear plastic material and includes a pair of light propagation paths that are mirror images of each other, but are oppositely directed. The term “clear” is defined herein as being characterized by low losses in the transmission of an optical signal. A first pair of input/output lenses is positioned on one surface of the optical member. Preferably, the input/output lenses are hyperbolic, so as to provide beam collimation. It is this first pair of input/output lenses that is aligned with the transmit optical fiber and the receive optical fiber. The lenses have axes that are aligned with a first pair of mirrors. The mirrors are total internal reflection (TIR) surfaces for reflecting optical signals. In the preferred embodiment, the TIR surfaces are at 45° angles to the axes of the associated lenses, so that right angle bending occurs. Each of the mirrors is aligned with a second mirror that provides right angle bending to a second pair of input/output lenses on a surface perpendicular to the surface having the first pair of input/output lenses. Again, the input/output lenses are preferably hyperbolic. 
     At the first surface, the optical member includes alignment pins or equivalent structure for ensuring precise registration between closely spaced duplex fibers and the first pair of lenses. At the opposite ends of the propagation paths, the second pair of lenses is aligned with a photodetector and a light source. For example, the photodetector may be a photodiode and the light source may be an LED or a laser diode. The photodetector and the light source are surface mounted on a PCB. 
     For the transmitter path, signals are generated by the light source, which is aligned with one of the lenses. The beam from the light source is collimated by the lens for propagation to a first TIR surface. The distance between the first lens and the first TIR surface defines a substratedirected segment of the transmit propagation path. The first TIR surface redirects the beam to a second TIR surface. The intermediate path segment between the two TIR surfaces is perpendicular to the substrate-directed segment and is parallel to the PCB on which the light source is mounted. The second TIR surface reflects the beam along a fiber-aligned segment to the hyperbolic lens which focuses the beam onto the transmit fiber. 
     For the receiver channel, the receive fiber directs optical signals to the hyperbolic lens having an axis that defines a fiber-aligned segment of a receiver propagation path. The beam is collimated by the lens and is reflected by a TIR surface to an intermediate segment of the path. The beam is again reflected to a substrate-directed segment aligned with the hyperbolic lens which focuses the beam onto the photodetector. 
     An advantage of the invention is that the photodetector and the light source are mounted directly to the PCB. The two devices may be mechanically and electrically mounted using a silver or gold epoxy to a gold-plated PCB that includes the processing electronics for the photodetector and the drive electronics for the light source. An advantage of the invention is that the mounting of the photodetector and light source is less expensive than the packaging associated with conventional TO cans. Moreover, the surface mounting may enable the devices to be more closely spaced than when the devices are mounted within separate TO cans. For the optical member formed in accordance with the invention, the spacing between the substratedirected hyperbolic lenses is determined by the intermediate segments of the propagation paths through the optical member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a prior art approach to coupling optical fibers to light generating and light detecting devices. 
     FIG. 2 is a side view of a surface mounted optical member and surface mounted electronic devices for providing optical coupling to fibers in accordance with the invention. 
     FIG. 3 is a front view of the optical member of FIG.  2 . 
     FIG. 4 is a top view of the optical member of FIG.  3 . 
     FIG. 5 is a top sectional view of the optical member of FIG. 3, taken along lines  5 — 5  of FIG.  3 . 
     FIG. 6 is a top view of the optical member of FIG. 5 connected to a fiber-bearing ferrule. 
     FIG. 7 is a bottom view of the optical member of FIG.  2 . 
     FIG. 8 is a back view of the optical member of FIG.  7 . 
     FIG. 9 is a back view of the optical member, taken along lines  9 — 9  in FIG.  7 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 2, an optical member  48  is shown as being mounted to a surface of a printed circuit board (PCB)  50 . The PCB also includes a surface mounted light source and a surface mounted photo-detector. Only one of the surface mounted electronic devices  52  is shown in the side view of FIG.  2 . Integrated circuit chips  54  and  56  represent the drive circuitry for the light source and the processing circuitry for the photodetector. Such circuitry is well known in the art. 
     The optical member  48  is a fiber optic module optical system in which transmit and receive propagation paths have three segments, each segment being perpendicular to the other two segments. For the transmit propagation path, the beam from a light source (e.g., the device  52 ) is directed upwardly along the Y axis, inwardly along the Z axis, and then forwardly along the X axis. On the other hand, the receive propagation path is directed rearwardly along the X axis, outwardly along the Z axis, and then downwardly along the Y axis to the photodetector (e.g., the device  52 ). 
     Each of the propagation paths includes entrance and exit hyperbolic collimating and focusing lenses. Each path also includes a pair of TIR surfaces that provide 45° mirrors and right angle bending. The length of the intermediate path segments between the two mirrors determines the center-to-center spacing between the light source and the photodetector. That is, the lengths of the intermediate path segments determine the spacing between substrate-aligned path segments of the propagation paths. The fiber-aligned path segments are directed along the X axis. The distance between the fiber-aligned path segments is determined by the fiber-to-fiber distance of the mating fiber optic connector. In the preferred embodiment, the optical member  48  is designed for mating with a standardized MT-RJ fiber pair. According to the MT-RJ standard, the spacing is 0.75 mm. On the other hand, the separation of the light source and photodetector may be 6.2 mm. Therefore, in this embodiment, the intermediate path segments are each 2.725 mm in length (i.e., (6.2 mm-0.75 mm)/2). 
     The light source is an optical transmitter that may be an LED, a laser diode, or the like. The light source is pulsed to generate data signals for communication. The photodetector may be a photodiode or any other device that converts optical signals to corresponding electrical signals. 
     In the preferred embodiment, the optical member  48  is a unitary molded member that is configured to include four surface lenses and four TIR surfaces. While the mirrors will be described as being “total internal reflection” surfaces, some undesired scattering loss can occur at each surface. 
     FIG. 3 is a front view of the optical member  48 . The molded plastic member includes two alignment pins  58  and  60  that are equidistantly spaced from the center. The alignment pins can directly contact a fiberbearing ferrule to ensure repeatable interfacing with the ferrule or can be utilized to properly position a precision metal plate (plate  62  in FIG.  6 ), which is used to ensure repeatable accurate interfacing with a fiber-bearing ferrule. A lens  64  is an exit port for a transmitter channel, while a separate lens  66  is an entrance port for the receiver channel. In the preferred embodiment, the lenses are hyperbolic lenses. For the transmitter channel, the hyperbolic lens  64  focuses an exit beam onto the aligned optical fiber. At the receiver channel, the hyperbolic lens  66  collimates the beam that is introduced into the optical member  48  from the aligned receive optical fiber. 
     FIG. 4 is a top view of the optical member  48 , while FIG. 5 is a top view taken along section lines  5 — 5  in FIG.  3 . The optical member includes a planar upper region  68  and sloping regions  70  and  72  that are at 45° angles relative to the upper region. The configurations of the sloping regions can be seen more clearly in the rear views of FIGS. 8 and 9. As will be explained below, the sloping regions function as the TIR surfaces that reflect the transmit beam and receive beam between the X direction and the Z direction. 
     As best seen in the top sectional view of FIG. 5, the solid portion of the optical member  48  between the two sloping regions  70  and  72  includes a V-shaped cutaway that forms two TIR surfaces  74  and  76 . These surfaces reflect the transmit and receive beams between propagation in the Z direction and propagation in the X direction. Also shown in FIG. 5 are a transmit fiber  78  and a receive fiber  80 . The axis of the transmit fiber  78  is aligned with a hyperbolic focusing lens  82 . The receive fiber  80  has an optical axis that is aligned with a hyperbolic collimating lens  84 . A light signal that exits from the receive fiber  80  enters the optical member  48  at the hyperbolic focusing lens  82 . The beam is reflected from the Z direction to the X direction by the TIR surface  76 . The beam is then reflected from the X direction to the Y direction by the TIR surface that is formed by the sloping region  72 . 
     The propagation path of an optical signal to the transmit fiber  78  follows an opposite path that is otherwise the mirror image of the receive path. That is, an upwardly directed optical signal along the Y axis is reflected to the X direction by the TIR surface formed by the sloping region  70 . The redirected beam is again redirected to the Z direction by the TIR surface  74 . The beam exits via the hyperbolic focusing lens  82 , thereby being focused onto the transmit fiber  78 . 
     Portions of the transmit and receive propagation paths are shown in FIG.  6 . As noted briefly above, the alignment pins  58  and  60  of the optical member  48  are used to position a precision metal plate  62  having pin-receiving bores  85  and  87 . The plate  62  includes a second pair of alignment pins  89  and  91 . The transmit and receive fibers  78  and  80  are embedded in a ferrule  86  having bores  88  and  90  with a center-to-center spacing that matches the spacing between the two alignment pins  89  and  91  of the precision metal plate. When the ferrule is mated to the plate, the bores  88  and  90  receive the alignment pins  89  and  91 , ensuring proper registration of the fibers  78  and  80  to the axes of the hyperbolic lenses  82  and  84 . As indicated in FIG. 6, when a beam exits from the receive fiber  80 , the beam diverges as it approaches the collimating lens  84 . The lens collimates the light energy, which is reflected from the TIR surface  76  to the TIR surface of the sloping region  72 . The arc  92  does not represent a structural feature. Instead, the arc represents the portion of the sloping region  72  that shows the impingement of the receive beam onto the TIR surface that redirects the beam to the Y direction. A similar arc  94  is shown for the transmit beam. The transmit beam is reflected by the TIR surface of the sloping region  70  from an upward direction to an inward direction and is reflected by the TIR surface  74  from the inward direction to the Z direction. The beam is focused by the hyperbolic lens  82  onto the transmit fiber  78 . 
     In FIG. 6, the segments of the propagation paths between the hyperbolic lenses  82  and  84  and the TIR surfaces  74  and  76  will be referred to herein as “fiber-aligned segments.” These segments extend in the Z direction. The segments that extend from the TIR surfaces  74  and  76  to the TIR surfaces of the sloping regions  70  and  72  will be referred to as the “intermediate segments.” These segments propagate in the X direction and determine the center-to-center distance between the light source and the photodetector at the PCB level. The segment from the sloping regions  70  and  72  to the photodetector and light generator will be referred to as the “substrate-directed segments.” These segments extend in the Y direction. 
     Referring now to FIG. 7, the bottom view of the optical member  48  exposes the hyperbolic lenses  96  and  98  that are aligned with the light generator and the photodetector. As noted above, the center-to-center distance between the lenses  96  and  98  may be 6.2 mm. The lens  96  is the collimating lens for introducing optical signals from the light generator to the interior of the optical member. The lens  98  is the focusing lens for optical signals which exit from the optical member. 
     The molded member includes a wall  100  that separates the compartments for the light source and the photodetector. By providing separate compartments  102  and  104  for the electronic devices, the optical and electrical isolations of the transmitter and receiver operations are enhanced. The two compartments  102  and  104  may be seen more clearly in FIG.  9 . FIG. 9 is a sectional view taken along lines  9 — 9  of FIG. 7, but with the optical member  48  being shown in an upright condition and being mounted to the PCB  50 . Also mounted to the PCB are a light source  106  and a photodetector  108 . 
     The sectioning lines through the optical member  48  in FIG. 9 provide a clearer illustration of the V-shaped “inlet” that defines the 45° TIR surfaces  74  and  76 . (The entire “inlet” is shown in FIG. 8.) In operation, optical signals generated by the light source  106  will undergo collimation upon reaching the hyperbolic collimating lens  96 . The collimated beam will propagate in the Y direction until it impinges the TIR surface of the sloping region  70 . The beam is then reflected at a right angle for propagation in the X direction. Reflection from the TIR surface  74  aligns beam propagation with the hyperbolic focusing lens  82  of FIG.  6 . The lens  82  focuses the beam onto the transmit fiber  78 . 
     Still referring to FIGS. 6 and 9, the optical signals from the receive fiber  80  direct a beam toward the hyperbolic collimating lens  84 . The collimated beam is reflected from the Z direction to the X direction by the TIR surface  76 . The beam is again redirected by the TIR surface of the sloping region  72 . This second reflection positions the beam for downward propagation in the Y direction to the hyperbolic focusing lens  98 . The beam is focused by the lens  98  onto the photodetector  108 . 
     As previously noted, an advantage of the invention is that the photodetector  108  and the light source  106  are mounted directly to the PCB  50 . These devices may be mechanically and electrically surface mounted using a silver epoxy to gold plating on the PCB. The result is that a more cost efficient arrangement is provided than achieved using the conventional TO cans to house the photodetector and light source.