Patent Publication Number: US-6905257-B2

Title: Optical interface for 4-channel opto-electronic transmitter-receiver

Description:
This application is a Continuation-In-Part of U.S. Ser. No. 09/790,246, Filed Feb. 20, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to the field of optical fiber data transmission and communication and more particularly concerns an optical interface in the form of an optical interface for interconnecting a 4-channel optoelectronic transmitter-receiver module and an 8-fiber optical fiber transmission ribbon. 
   2. Background of the Invention 
   Optoelectronic transmitter-receiver or transceiver modules serve to convert electronic signal to optical signals for transmission via optical fibers and also to convert optical signals received via optical fibers to electronic signals. The signals processed by the optoelectronic transmitter-receiver modules typically consist of digital signals exchanged between two electronic data processing systems or subsystems, for example, in high speed digital data processing systems such as digital telephone switchboards and digital computers. Each system or subsystem is provided with such a module and the two modules are interconnected by an optical fiber transmission cable. 
   The optoelectronic transmitter-receiver module generally consists of a housing, a number of integrated circuit chips mounted in the housing, a laser diode array driven for emitting pulses of light into the optical fiber cable and a photo-detector diode array for detecting light pulses received from the optical fiber cable. The laser diode array operates as the optical transmitter while the photodetector diode array serves as the optical receiver. The transmitter and the receiver are contained in a common housing or package. Electrical drive signals applied to the laser diodes are converted to light pulses, while light signals received by the photodetector diodes are converted to electrical signals for processing by appropriate electronic circuits in the transceiver module. Each laser diode/photodetector diode supports one channel of communication, and multiple laser diode/photodetector diode arrays support multi-channel communication. Multi-channel optoelectronic transceivers require multiple optical fiber interconnections, a need which is frequently met by use of flat ribbons made up of parallel optical fibers. The optical fiber ribbons are commercially available in different standard widths, most commonly 4-fiber, 8-fiber and 12-fiber ribbons. The ribbons are terminated at opposite ends with corresponding 4-fiber, 8-fiber or 12-fiber optical ribbon connectors, such as industry standard MPO or MTP® optical fiber connectors. The fiber ends of the ribbon lie in a common plane and form a linear array of evenly spaced fiber ends. The termination of optical fiber ribbons represents a significant cost in the manufacture of these systems because the fiber ends in the ribbon must be polished very accurately. For this reason, it is generally desirable to minimize the number of ribbon terminations in a given system. 
   For example, 4-channel optoelectronic transceivers call for bi-directional 4-channel optical fiber interconnections, that is for an 8-fiberribbon with 4 optical fibers in each direction of the cable. A difficulty arises in the construction of such multi-channel optoelectronic transceivers in that the laser diode transmitter arrays and the photodetector diode receiver arrays consist of separate semiconductor chips which necessarily are mounted at a small distance apart from each other in the transceiver package. The laser and photodetector diodes cannot be manufactured on a single chip in order to maintain even spacing between all diodes and thus match the even spacing between adjacent fibers across the width of the optical fiber ribbon. If the laser diode and the photodiode are mounted adjacently, then the light from the laser enters the adjacent photodiode as a result of reflection or the like, and crosstalk may arise. In a four channel transceiver the array of four transmitter diodes and the array of four receiver diodes are separated by a gap and for this reason the individual diodes cannot be aligned with the eight more closely spaced fibers ends of an 8-fiber optical ribbon termination. It is possible to provide two separate 4-fiber ribbons each with its own 4-fiber connector aligned with one of the 4-diode arrays, one ribbon for connecting the 4-diode transmitter array of one transceiver to the 4-diode receiver array of the opposite transceiver; and another ribbon for connecting the 4-diode receiver array of the one transceiver to the 4-diode transmitter array of the opposite transceiver. However, this solution requires four ribbon terminations on the two separate ribbons. It is more desirable both from a viewpoint of economics as well as simplicity of construction and maintenance to provide a single 8-fiber ribbon for interconnecting two 4-channel transceivers. The temperature of heat resistance of the fiber ribbon is about 100° C. If a high temperature treatment such as the solder reflow for the module is applied, the ribbon may be damaged. 
   SUMMARY OF THE INVENTION 
   This invention provides three alternative interconnections between 4-channel opto-electronic transmitter-receivers, making it possible to use a single 8-fiber ribbon or a 12-fiber ribbon for interconnecting two 4-channel transceivers having separate, spaced apart transmitter and receiver diode arrays. 
   This invention also makes it possible to manufacture an optical interface (including a module) in which the crosstalk scarcely occurs and a high temperature treatment can be performed. 
   This invention provides three alternative interconnections between 4-channel opto-electronic transmitter-receivers, two of which make possible the use of a single 8-fiber ribbon for interconnecting two 4-channel transceivers having separate, spaced aparttransmitter and receiver diode arrays. 
   More particularly, the invention in a first one of its embodiments is an optical interface for a 4-channel opto-electronic transmitter-receiver module, the module having a module housing including at least one housing wall, an opening in the housing wall, a transmitter chip comprising a 4-element, i.e. 4-diode, laser diode array and a receiver chip comprising a 4-element, or 4-diode, photodetector array, the transmitter chip and the receiver chip being mounted in the wall opening. The optical interface has an optical adapter comprising a bent fiber optical head having an optical head body with a proximal face and a distal face, eight optical fibers extending through the head body, each of the fibers having a fiber end on each face, the fiber ends on the proximal face being grouped in two fiber end arrays each comprising four fiber ends evenly spaced from each other, the fiber end arrays being spaced apart from each other by a distance greater than the spacing between adjacent fibers in the arrays, the fiber ends on the distal face being evenly spaced from each other in a single array of eight fiber ends, the head body being secured to the module housing with the proximal face towards the housing wall and with the fiber end arrays in optical alignment with a corresponding one of the 4-diode laser diode array and the 4-diode photodetector array. As a result, an 8-fiber optical ribbon connector such as an optical fiber connector terminating an optical fiber transmission ribbon of eight evenly spaced fibers can be mechanically coupled and optically interfaced to the single array of eight fiber ends on the distal face of the optical head body for optical communication of the transmitter-receiver module via the optical fiber transmission ribbon. It is desirable that the optical head body be configured for supporting each of the eight optical fibers with a minimum radius of curvature between the distal and the proximal faces no lesser than recommended by the manufacturer of the optical fibers. 
   Four of the optical fibers extending through the optical head body comprise a transmitter fiber group and the other four of the optical fibers extending through the optical head body comprise a receiver fiber group, the fibers in the transmitter group being single mode optical fibers, the fibers in the receiver group being multimode optical fibers, whereby coupling losses between the fibers of the receiver fiber group and the fibers of a single mode fiber transmission ribbon are reduced and alignment of the receiver diode array to the bent fiber optical head is facilitated. 
   In a second one of its embodiments the invention is an optical interface which includes a straight fiber optical head and an adapter cable. The straight fiber optical head has a head body having a proximal face and a distal face, eight optical fibers extending through the head body, each of the fibers having a fiber end on each face, the fiber ends on each of the proximal face and the distal face being grouped in two fiber end arrays, each array comprising four fiber ends evenly spaced from each other, the fiber end arrays being spaced apart from each other by a distance greater than the spacing between adjacent fibers in the arrays. The optical head body is secured to the module housing with the proximal face towards the housing wall and with each of the fiber end arrays in optical alignment with a corresponding one of the 4-diode laser diode array and the 4-diode photodetector array. The adapter cable has a 12-fiber optical ribbon proximal connector, an 8-fiber optical ribbon distal connector, and an 8-fiber adapter ribbon between the proximal and the distal connectors, the ribbon being divided at one end thereof into two 4-fiber widths, each of the widths being terminated at an outer four of twelve fiber terminations of the proximal connector, the 12-fiber proximal connector being optically and mechanically mateable to the straight fiber optical head for optically interfacing each of the two 4-fiber widths to a corresponding one of the fiber end arrays of the optical head, and an opposite end of the adapter ribbon being terminated by the 8-fiber optical ribbon distal connector; whereby the 8-fiber distal connector can be mated to another 8-fiber optical fiber connector terminating an 8-fiber optical fiber transmission ribbon so as to interface the transceiver module for optical communication via the optical fiber transmission ribbon. The eight straight optical fibers extending through the head body may be mutually parallel between the proximal and the distal faces. 
   The four of the eight optical fibers extending through the optical head body comprise a transmitter fiber group and the other four of the eight optical fibers extending through the optical head body comprise a receiver fiber group, the fibers in the transmitter group being single mode optical fibers, the fibers in the receiver group being multimode optical fibers, and all eight fibers in the adapter cable being single mode optical fibers, whereby coupling losses between the fibers of the adapter cable and the fibers of the receiver fiber group of the optical head are reduced and alignment of the receiver diode array to the bent fiber optical head is facilitated. 
   In a third one of its embodiments the invention is an optical interface which includes a straight fiber optical head as described in connection with the second embodiment. However, in lieu of the adapter cable, a 12-fiber optical transmission ribbon is terminated at each of its opposite ends with a 12-fiber optical connector, and one 12-fiber optical connector is optically coupled to the two 4-fiber end arrays on the distal face of the optical head, such that four optical fibers of the 12-fiber transmission ribbon remain unused. 
   The invention may be also summarized in more general form as an optical interface for a 4-channel opto-electronic transmitter-receiver module, the module having a module housing including at least one housing wall, an opening in the housing wall, a transmitter chip comprising a 4-diode laser diode array and a receiver chip comprising a 4-diode photodetector array, the transmitter chip and the receiver chip mounted in the wall opening, an adapter unit having eight optical fibers each with a proximal fiber end and an opposite fiber end, the proximal fiber ends being grouped in two fiber end arrays each comprising four fiber ends evenly spaced from each other, the fiber end arrays being spaced apart from each other by a distance greater than the spacing between adjacent fibers in the arrays, each of the fiber end arrays being supported in optical alignment with a corresponding one of the 4-diode laser diode array and the 4-diode photodetector array, the opposite fiber ends being evenly spaced from each other in a single array of eight fiber ends adapted for interfacing with an 8-fiber optical fiber connector terminating an optical fiber transmission ribbon of eight evenly spaced fibers for optical communication of said transmitter-receiver module via the optical fiber ribbon. 
   The general summary of the preceding paragraph covers the two more specific forms of the invention earlier summarized as the first embodiment and the second embodiment in earlier paragraphs. 
   In the first embodiment the adapter unit includes an optical head body having a proximal face and a distal face, the eight optical fibers extending through the head body, each of the fibers having a fiber end on each face, the proximal fiber ends being on the proximal face, the opposite fiber ends being on the distal face, the optical head body being secured to the module housing with the proximal face towards the housing wall for supporting the fiber end arrays in the optical alignment with a corresponding one of the 4-diode laser diode array and the 4-diode photodetector array. 
   In the second embodiment the adapter unit includes an optical head having a head body with a proximal face and a distal face, the eight optical fibers extending straight through the head body, the fibers having the proximal fiber ends on the proximal face and distal fiber ends on the distal face, the proximal fiber ends and the distal fiber ends being grouped in two fiber end arrays, each array comprising four fiber ends evenly spaced from each other, the fiber end arrays on each face being spaced apart from each other by a distance greater than the spacing between adjacent fibers in the arrays, the optical head body secured to the module housing with the proximal face towards the housing wall and with each of the fiber end arrays on the proximal face in optical alignment with a corresponding one of the 4-diode laser diode array and the 4-diode photodetector array; and an adapter cable comprising a 12-fiber optical fiber proximal connector, an 8-fiber optical fiber distal connector, an 8-fiber ribbon between the proximal and the distal connector, the ribbon being divided at one end thereof into two 4-fiber widths, each of the widths being terminated at an outer four of twelve fiber terminations of the proximal connector, the 12-fiber optical fiber proximal connector being mateable to the optical fiber receptacle for optically interfacing each of the two 4-fiber widths to a corresponding one of the fiber end arrays of the optical head, the opposite fiber ends being at an opposite end of the ribbon and terminated at the 8-fiber optical fiber distal connector for interfacing to a transmission ribbon. 
   In a third embodiment of the invention the transceiver modules is equipped with a straight fiber optical head body as in the second embodiment. However, in lieu of an adapter cable a 12-fiber optical transmission ribbon is terminated at each of its opposite ends with a 12-fiber optical connector, one of the 12-fiber optical connectors being optically coupled to the two fiber end arrays on the rear, exposed face of the optical head body, such that four optical fibers of said transmission ribbon remain unused. 
   In the second or third embodiment described above, the optical head may have a guide hole for positioning the optical fiber connector. Accordingly, the optical fiber connector such as an optical fiber connector of the MPO type can be directly attached and detached. 
   In a fourth one of its embodiments, the invention is an optical interface which includes a straight fiber optical head as described in the second or third embodiment. However, an optical head has a heat sink and a block of crystallized glass. For example, a plurality of v-grooves are formed in the block made of crystallized glass. Optical fibers are arranged along the plurality of v-grooves, respectively. An optical fiber array is constructed by the plurality of optical fibers. Each of the laser diode array and the photodetector array is mounted in a sub-mount having the heat sink for releasing the heat generated by operation. 
   In the fourth embodiment, in lieu of the adapter cable, a 12-fiber optical transmission ribbon is used. Twelve optical fibers of the optical transmission ribbon are terminated with one end of a 12-fiber optical connector, and the other end of the 12-fiber optical connector is optically coupled to the fiber end array of the optical fiber array on the distal face on which the surface of the optical head body is exposed. 
   When the optical fiber array is constructed by eight optical fibers, eight optical fibers (each four optical fibers on both sides) of the twelve optical fibers of the 12-fiber optical connector are optically coupled to the fiber end array of the optical fiber array. In this case, the central four optical fibers of the twelve optical fibers are not used. 
   When the optical fiber array is constructed by twelve optical fibers, eight optical fibers (each four optical fibers on both sides) of the twelve optical fibers of the 12-fiber optical connector are optically coupled to the fiber end array for four fibers on both sides of the optical fiber array. Also in this case, the central four optical fibers of the twelve optical fibers are not used. 
   These and other improvements, features and advantages of this invention will be better understood by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top perspective view of a typical 4-channel fiberoptic transceiver module equipped with the optical interface in its first embodiment, shown coupled to an eight fiber optical ribbonconnector, the bent fiber optical head and the optical fiber connector being shown in section along the plane of the optical fibers; 
       FIG. 2  is an end view of the proximal face of the bent fiber optical head body in  FIG. 1 ; 
       FIG. 3  is an end view of the distal face of the bent optical head body in  FIG. 1 ; 
       FIG. 4  is a perspective view of an 8-fiber transmission cable consisting of an 8-fiber transmission ribbon terminated at each end with an 8-fiber connector, showing the special keying of the two connectors; 
       FIG. 5  is a top plan view of a 4-channel optical fiber communications system including two 4-channel transceiver modules interconnected by an 8-fiber transmission cable such as shown in  FIG. 4 , each module interfaced to the transmission cable by a bent fiber optical head according to the first embodiment of this invention; 
       FIG. 6  is a perspective view of the straight fiber optical head according to the second embodiment of this invention; 
       FIG. 7  is an end view of the head body of  FIG. 6 ; 
       FIG. 8  is a top plan view of a 4-channel optical fiber communications system including two 4-channel transceiver modules interconnected by an 8-fiber transmission cable such as shown in  FIG. 4 , each module interfaced to the transmission cable by an adapter cable and a straight fiber head body according to the second embodiment of this invention; 
       FIG. 9  is a plan view illustrating a third embodiment of the optical interface of this invention; 
       FIG. 10  is a sectional view illustrating another arrangement of an optical head of the third embodiment of the optical interface of this invention; and 
       FIG. 11  is a perspective view illustrating an arrangement of an optical head of a fourth embodiment of the optical interface of this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the accompanying drawings wherein like elements are designated by like numerals,  FIG. 1  shows a 4-channel opto-electronic transceiver module generally designated by numeral  10 . The module  10  includes a module housing  12  with four housing walls  14  including a wall  14  apertured by wall opening  16 . A top surface  15  of substrate  18  in the housing supports a number of electronic integrated circuit (IC) chips  20  connected by means of printed circuit conductors  22  to external leads  24 . The transceiver module also includes a transmitter chip  26  comprising a 4-diode laser diode array and a receiver chip  28  comprising a 4-diode photodetector array. The leads  24  will normally be connected to an electronic system or subsystem which communicate with another electronic system or subsystem through a 4-channel optical fiber communications system including two 4-channel transceiver modules  10  interconnected by an optical fiber transmission cable, as will be explained in greater detail below. 
   The transmitter chip  26  and the receiver chip  28  are mounted in the wall opening  16  facing the exterior of the module housing  12 . IC chips  20  include driver circuits which deliver electrical drive currents to the 4-diode laser diode array of the transmitter chip  26  thereby causing the diodes to emit light pulses which carry in optical form the information delivered by the electrical drive currents, thereby converting electronic signals for transmission in optical form via optical fibers, as will be described below. The four photodetector diodes of receiver chip  28  are each illuminated by a corresponding optical fiber and convert the optical signals carried by the optical fibers into electrical signals which are amplified and further processed by corresponding electronic circuits on IC chips  20 . The package leads  24  accept electrical input signals for optical transmission by the module  10  and deliver electrical output signals corresponding to optical signals received by the module  10 . 
   It will be seen in  FIG. 1  that the transmitter chip  26  is mounted on a transmitter substrate  30  while the receiver chip  28  is mounted on a separate receiver substrate  32 . The 4-diode photodetector array of the receiver chip and the 4-diode laser array of the transmitter chip are linear arrays arranged along a common line parallel to the top surface  15  of the substrate  18 . The diodes in each array are evenly spaced in relation to each other, and the diode spacing is the same in each array, to match the even spacing between the centers of adjacent optical fibers in standard commercial optical fiber ribbon terminations. However, the photodetector and laser diodes adjacent to each other at the inner ends of the two arrays are spaced from each other by a distance greater than the spacing of the diodes in each of the arrays and also greater than the spacing between the centers of adjacent optical fiber ends in the fiber ribbon terminations. This is because as a practical matter, the diodes on each of the receiver and transmitter substrates cannot be placed at the very edge of the substrates in order to achieve the same close spacing as between the adjacent diodes in each of the two arrays, and because the two different types of laser and photodetector diodes cannot be manufactured on one common substrate either. The result is that some form of optical interface is needed to transition from the wider 4+4 spacing of the two diode arrays of module  10  to the even fiber end spacing of an 8-fiber optical ribbon termination. 
   Two alternate forms or embodiments of the optical interface according to this invention are described herein. The first embodiment is illustrated in  FIGS. 1 ,  2 ,  3  and  5 .  FIGS. 6 ,  7  and  8  relate to the second embodiment.  FIG. 4  shows a transmission cable which is used in both embodiments. 
   Turning to  FIGS. 1 ,  2  and  3  the optical interface in its first embodiment includes an optical head body  40  which is a rectangular block comprised of an lower half  42   a  and an upper half  42   b . Between the two halves are contained eight optical fiber segments  48  in fiber channels  46 . The two halves may be ceramic substrates fully metal plated on all their exterior surfaces. The channels may be v-grooves defined on one substrate and covered by a plane interior surface of the other substrate. The diode elements on photodiode receiver chip  28  and laser diode transmitter chip  26  are optically aligned to the fiber ends of the head body  40  and the chips  26 ,  28  are fixed to the head body  40  with resin (adhesive or the like). When the optical head body  40  and the optical element array (laser diode array and photodetector array) are formed on the heat sink, for example, it is preferable to perform spot welding by laser. The assembly comprising the head body  40  and the diode array chips  26 ,  28  form an optical head assembly which is mounted to the transceiver housing  12  with resin (for example, adhesive) on the proximal surface  51  to the apertured wall  14  of the housing such that the chips  26 ,  28  are supported through the wall opening  16  within the housing where the diode array chips are electrically interconnected to other appropriate electronic transmitter and receiver circuits. When the optical head body  40  is formed of, for example, crystallized glass which is resistant to high temperature, the optical head body  40  can be soldered to the transceiver housing. In this case, when the optical head body  40  is mounted so that the housing  12  is lidded therewith, it is possible to provide an air-tight enclosed structure of the housing  12 . The optical head is also contained in a fiber ribbon connector receptacle  56  fastened to the module housing  12 . The purpose of receptacle  56  is to receive and mechanically retain an optical ribbon connector in optical coupling with the fiber ends on the rear or distal surface of the optical head. The optical head body has a proximal surface  51  seen in  FIG. 2  which faces the wall opening  16  and each fiber segment  48  has a proximal fiber end  50  on the proximal surface  51  aligned in facing relationship with a corresponding diode on one of the two diode arrays on transmitter chip  26  and receiver chip  28 . The proximal fiber ends are grouped in two fiber end arrays  48   a ,  48   b  of four fiber ends each. For example, if the proximal fiber end  50  is slightly separated from each of the opposed diodes, crosstalk may occur, in which the light from the laser diode array enters the photodetector array. However, when the proximal fiber ends are grouped in the two fiber end arrays  48   a ,  48   b , and the laser diode array and the photodetector array are mounted to be aligned in facing relationship with the fiber end arrays  48   a ,  48   b  as described above, then it is possible to increase the spacing between the laser diode array and the photodetector array as compared with the arrangement pitch of the optical fibers, and it is possible to avoid the crosstalk. One fiber end array  48   a  is in optical alignment with the 4-channel transmitter chip  26  while the other fiber end array  48   b  is in optical alignment with the 4-channel receiver chip  28 . The optical head also has an opposite or distal face  53  seen in  FIG. 3  which faces away from the module housing and on which are arranged the distal fiber ends  54  of fiber segments in a linear array with even center to center spacing of the individual fiber ends  54  such as to match the center to center spacing of the individual fibers of a standard 8 fiber ribbon optical fiber termination. As seen in  FIG. 1  the individual fiber segments  48  of the optical head  40  are bent internally to the head body along their length so as to converge from a wider spacing on the proximal face  51  to a closer fiber spacing on the distal face  53 , and thereby provide the transition from the spaced apart diode arrays of the transceiver module  10  to the close and regular fiber spacing of standard optical fiber ribbon terminations. The bend curvature or radius of the fiber segments  48  between the proximal and distal fiber ends is no smaller than recommended by the fiber manufacturer to avoid degradation of optical signal transmission through the fiber segments. The upper and lower halves  42   a ,  42   b  of the optical head are secured together as by soldering or may be molded so as to contain and enclose the fiber segments. 
   The transceiver module is connected to an optical fiber transmission cable such as shown in FIG.  4  and generally designated by numeral  60 . The cable  60  has a transmission ribbon  62  which is a commercially available standard 8-fiber ribbon terminated at each of its opposite ends with an industry standard 8-fiber optical ribbon connector  64 . The evenly spaced ends  66  of the eight fibers are arranged in a linear array  68 , and each connector has a pair of connector pins  71  which mate into corresponding pin holes  58  of optical head  40  as shown in  FIG. 1  to precisely position and optically align the fiber end array  68  of the transmission cable  60  with the array of distal fiber ends  54  on the distal face  53  of the optical head. An optical fiber connector  64  is retained in optical fiber receptacle  56  by retainer arms  55  of the receptacle to hold the optical fiber connector  64  in optical coupling with the optical head  40 . The optical fiber connector  64  at the opposite end of transmission ribbon  62 , not seen in  FIG. 1 , is similarly coupled to a second transceiver module  10  also equipped with an optical head  40  arrangement similar to that shown in FIG.  1 . The interconnection of two transceiver modules optically interfaced in this fashion to an optical ribbon transmission cable  60  is illustrated in FIG.  5 . As shown in  FIG. 4  the connectors  64  have key tabs  69  which limit the orientation of the connector when inserted in connector receptacle  56 , in that the connector can only be mated to the receptacle with the key tab point upwards in FIG.  1 . In conventional ribbon cables the connectors at opposite ends of the optical ribbon have the key tabs pointing in opposite directions, thereby to force a 180 degree twist along the length of the ribbon so as to preserve same channel numbering on the transmitter and receiver sides. When connecting two identical transceiver modules  10 , however a “straight” ribbon cable is needed to guarantee that the transmit side of one be connected to the receive side of the other module. 
   The optical head body  40  with bent optical fiber segments is, as a practical matter, challenging to manufacture economically on a commercial scale. For this reason, an alternate embodiment of this invention has been devised which is more readily assembled with commercially available components. 
   Turning to  FIGS. 6 ,  7  and  8  the second embodiment of the invention will be described. The second embodiment of the optical interface includes a transition adapter  70  made up of two assemblies: an optical head body  72  and an adapter cable  74 . The head body as shown in  FIG. 7  is comprised of a top and bottom substrate  72   a ,  72   b  similarly to the previously described head body  40  of the first embodiment. The head body  72  has a proximal face and a distal face and contains eight straight optical fibers  76  extending between the two faces, each fiber having a fiber end  78  on each face as shown in FIG.  7 . The eight fibers  76  are mutually parallel but are grouped into two spaced apart fiber groups  76   a ,  76   b  of four evenly spaced fibers  76  each. The fiber ends  78  of the eight fibers are grouped into two fiber end arrays  76   a ′,  76   b ′ of 4 fiber ends each, both fiber end arrays extending along a common line on each face of the head body  72  but spaced from each other by a distance greater than the spacing between individual adjacent fibers  76  in each of the arrays. The spacing between the two fiber groups  76   a ,  76   b  matches the spacing between the receiver and transmitter diode arrays. The head body  72  is assembled to the receiver and transmitter chips  26 ,  28  as described in connection with head body  40  of the first embodiment to form an optical head assembly, to hold the head body  72  to the module housing with each fiber end array  76   a ′ and  76   b ′ on the proximal face  73  of the head body in optical alignment with the 4-diode transmitter and receiver diode arrays on transmitter chip  26  and receiver chip  28 , respectively. The head body  72  has an opposite or rear face (not shown in the drawings) which is similar to the proximal face  73  shown in FIG.  7 . The head body  72  is contained in an optical fiber connector receptacle, such as receptacle  56  in FIG.  1 . 
   The adapter cable  74  consists of a length  82  of commercially available eight fiber optical ribbon which is divided at one end into two 4-fiber widths  82   a ,  82   b . Each 4-fiber width is terminated at a corresponding set of outer four fiber positions of a conventional industry standard 12-fiber optical fiber connector  84 , leaving empty the middle four fiber positions of connector  84 . The spacing between the two groups of fiber ends on the connector  84  matches the spacing between the fiber end arrays  76   a ′,  76   b ′ on head body  72 . The 12-fiber connector  84  is optically coupled to the rear face of the head body, the face opposite to proximal face  73  and facing away from module  10 , by inserting the connector into retentive engagement with the optical fiber receptacle mentioned in the preceding paragraph. When so coupled the fiber ends on optical fiber connector  84  are optically aligned with the fiber ends  78  of the two subarrays of fiber ends  78  of the head body  72  on the outward or rear face of the head body. The opposite end of ribbon  82  is terminated with a standard 8-fiber optical fiber connector  86  which has distal fiber ends of the ribbon  82  and which mechanically and optically couples with an end connector  64  of transmission ribbon  62  of cable  60 , so as to interface the transmitter-receiver module  10  for optical communication via the optical fiber ribbon  62 . Two transceiver modules  10  are each optically interfaced in this manner for two-way communication with each other in FIG.  8 . 
   The optical interface  70  has a greater number of optical connections than interface of the first embodiment described in connection with  FIGS. 1 and 5 . This introduces greater coupling losses in the transmission of light signals through the interface  70 , a tradeoff for the lower cost and easier manufacture of the latter. 
   Coupling losses in both embodiments of the optical interface are reduced by substituting multi-mode optical fibers for single mode optical fibers in the receiving subgroup of fibers of optical head  40  and also in head body  72 . The receiving group of fibers in the first embodiment described above is fiber group  48   b  in  FIG. 1 , while the receiving group of fibers in the second embodiment is fiber group  76   b  in  FIGS. 6 and 7 . Likewise, the multimode fibers also reduce coupling losses of light signals received from the transmission cable  60  at the coupling with the distal fiber ends  54  of optical head  40 , and coupling losses of signals received from the adapter cable  74  into fiber group  76   b  of head body  72 . 
   In a third embodiment of the optical interface of this invention, as shown in  FIG. 9 , an optical head includes a head body  72  as just described, but does not make use of an adapter cable  70  (see FIG.  8 ). Instead, a 12-fiber optical transmission ribbon  102  is used which is terminated at each of its opposite ends with a conventional 12-fiber optical connector  100 . The 12-fiber optical connector  100  is optically coupled to the two fiber end arrays on the distal face of the optical head body  72 , such that each of the two fiber end arrays is optically coupled to an outer four fibers of the 12-fiber ribbon and the middle four optical fibers of the transmission ribbon remain unused. 
   Alternatively, guide holes  106  into which guide pins  104  for the 12-fiber optical connector  100  are inserted are provided on the distal face of the optical head body  72 . That is, the optical head body  72  has a receptacle structure in which the optical connector such as an optical fiber connector of the MPO type can be directly attached and detached. 
   Therefore, in the case of this optical head, the optical connector  100  can be fixed to the head body  72  with ease only by inserting the connector portion of the optical connector  100  without using any clamper such as a plate spring. Further, the optical connector  100  can be detached from the head body  72  with ease by separating the connector portion. 
   Alternatively, as shown in  FIG. 10 , another form is also available, in which the head body  72  has twelve optical fibers  76 , and four channels on both ends each are used. When a conventional or commercially available 12-channel fiber ribbon is used, the 12-channel optical head is easily assembled and inexpensive. Therefore, such an arrangement does not result in increase in cost. In this case, the central four channels are not used. 
   Next, an optical head according to a fourth embodiment of the optical interface of this invention will be explained with reference to FIG.  11 . 
   As shown in  FIG. 11 , the optical head according to the fourth embodiment may comprise an optical fiber block  112  which has an optical fiber array  110  including a plurality of (for example, eight) optical fibers  76  aligned therein, a laser diode block  116  which has a laser diode array  114 , and a photodetector block  120  which has a photodetector array  118 . 
   The laser diode array  114  includes four laser diodes which are aligned on a first semiconductor substrate  122 . The photodetector array  118  includes four light-receiving elements which are aligned on a second semiconductor substrate  124 . 
   The optical fiber block  112  has a lower block section  130  which is composed of a heat sink member, and it has an upper block section  132  which is composed of crystallized glass. The upper block section  132  comprises two members (first and second members) which are stuck to one another. A plurality of (for example, twelve) unillustrated v-grooves are formed on the upper surface of the first member disposed at the lower position in order to retain the plurality of optical fibers  76  in parallel to one another. The lower surface (surface opposed to the v-grooves of the first member) of the second member disposed at the upper position is a flat surface. 
   The optical fibers  76  are arranged in the twelve v-grooves respectively, the first and second members are stuck to one another, for example, with adhesive, and the twelve optical fibers  76  are retained and fixed in the v-grooves. Thus, the optical fiber array  110 , in which the twelve optical fibers  76  are aligned in parallel to one another, is constructed. The upper block section  132  and the lower block section  130  are also stuck to one another, for example, with adhesive. An optical fiber ribbon  102  is coupled via an optical connector  100  to the back surface of the optical fiber block  112  (surface disposed on the side opposite to the front surface on which the laser diode block  116  and the photodetector block  120  are stuck). 
   The laser diode block  116  has a lower block section  140  which is composed of a heat sink member, and it has an upper block section  142  which is composed of, for example, an aluminum nitride substrate (AlN substrate). The laser diode array  114  is secured to the upper surface of the upper block section  140 , for example, with adhesive. 
   The photodetector block  120  has a lower block section  144  which is composed of a heat sink member, and it has an upper block section  146  which is composed of, for example, an AlN substrate. The photodetector array  118  is secured to the side surface of the upper block section  142 , for example, with adhesive. 
   As shown in  FIG. 11 , the laser diode block  116  and the photodetector block  120  are stuck to the front surface of the optical fiber block  112 , for example, with adhesive. Further, the lower block section  130  of the optical fiber block  112  and the lower block section  140  of the laser diode block  116  are subjected to spot welding (welding point  150 ), for example, with YAG laser. Further, the lower block section  130  of the optical fiber block  112  and the lower block section  144  of the photodetector block  120  are subjected to spot welding (welding point  152 ). Thus, the optical head is constructed. 
   In this arrangement, the optical fiber ribbon  102  which is coupled via the optical connector  100  is a 12-fiber ribbon in which twelve optical fibers are aligned. The twelve v-grooves are formed in the upper block section  132  of the optical fiber block  112 . The optical fibers  76  are aligned in the v-grooves respectively. 
   Respective first ends of the twelve optical fibers aligned in the upper block section  132  are optically coupled to the respective optical fibers of the 12-fiber ribbon  102  via the optical connector  100  on the back surface of the optical fiber block  112 . 
   Further, for example, the four fiber ends of the respective second ends of the twelve optical fibers  76  aligned in the upper block section  132  which are arranged on the left are opposed to the four laser diodes of the laser diode block  116 . The four fiber ends which are arranged on the right are opposed to the four light-receiving elements of the photodetector block  120 . The central four fibers of the twelve optical fibers  76  aligned in the upper block section  132  are not used. 
   In this arrangement, the optical fiber block  112  is made of crystallized glass which can endure high temperature. Therefore, the optical fiber block  112  can be attached to the transceiver housing  12 , for example, by soldering the lower block section  130  to the transceiver housing  12  shown in FIG.  1 . In this case, the optical fiber block  112  is attached to the transceiver housing  12  so that the opening of the transceiver housing  12  is lidded with the optical fiber block  112 . Thus, it is possible to provide the air-tight enclosed structure of the transceiver housing  12 . 
   The laser diode block  116  and the light-receiving element block  120  can be mounted separately. Therefore, the spacing between the laser diode array  114  and the photodetector array  118  can be made larger than the arrangement pitch of the optical fibers  76 . Thus, it is possible to avoid the crosstalk. 
   The welding points  150 ,  152  can be provided when the spot welding is performed, because the laser diode block  116  and the photodetector block  120  are mounted separately. Especially, in the case of the structure shown in  FIG. 11 , the size of the lower block section  140  of the laser diode block  116  is different from the size of the lower block section  144  of the photodetector block  120 . A stepped positional relationship is adopted, for example, for the lower end position of the lower block section  140  of the laser diode block  116  and the lower end position of the lower block section  144  of the photodetector block  120 . Such a positional relationship makes it possible to sufficiently provide the welding points  150 ,  152 . 
   In the optical head according to the fourth embodiment described above, the twelve optical fibers  76  are arranged to construct the optical fiber array  110 . Alternatively, the optical fiber array  110  may be constructed by arranging eight optical fibers. In this case, the eight optical fibers are grouped into four optical fibers, and the two groups are separated from each other. The four optical fibers of the first group are opposed to the laser diode array  114 . The four optical fibers of the second group are opposed to the photodetector array  118 . 
   Further, the fiber end array of the first group of the optical fiber array  110  is optically coupled, for example, to the four optical fibers on the left of the optical connector  100 . The fiber end array of the second group is optically coupled to the four optical fibers on the right of the optical connector  100 . Therefore, also in this case, the central four optical fibers of the twelve optical fibers of the 12-fiber ribbon  102  connected to the optical connector  100  are not used. 
   While certain presently preferred embodiments of the invention have been described and illustrated for purposes of clarity and example, various changes, modifications and substitutions will be apparent to those having only ordinary skill in the art without thereby departing from the invention as claimed below.