Patent Publication Number: US-2005129370-A1

Title: Ferrule for connecting optical fibers

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      An object of the present invention is a connection ferrule for optical fibers. It is designed to simplify the use of optical fibers which are an item of increasing utility.  
      An optical fiber is used essentially as a means to convey information in the form of light signals that are normally digitized. This means of transportation has the advantage of efficiently resisting noise, especially electromagnetic noise, and furthermore enabling very high data bit rates. However, since processing in present-day computer devices is of the electronic type, it is important to carry out an optoelectronic conversion of the light signals to be processed at input and output of the optical fiber. Various solutions have been devised for these problems of conversion.  
      2. Description of the Prior Art  
      Certain solutions have entailed the idea of making harnesses. In these harnesses, an optical fiber or a bundle of optical fibers is provided, fixedly at one of its two ends (or at least at one of its ends), with an optoelectronic conversion device. In this case, the optical fiber delivers electrical signals or electronic signals at one or both ends while it can deliver optical signals at another end. The drawback of this type of solution is, firstly, the cost generated by this integration of means. Secondly, the ease with which the fiber can be handled is thereby greatly reduced. Indeed, it will easily be understood that the length of the fiber cannot be adjusted as easily as desired, especially if it is provided on either side with electronic conversion circuits crimped to the ends of the fibers. In this case, it is not at all possible to lengthen or shorten the fiber. All that can be done is to exchange it for another differently sized harness, which however will also be a high-cost harness. Besides, the presence of the electronic conversion circuit leads to the making of a joining piece at the end of the optical fiber. The bulkiness of this joining piece is inconvenient if the fiber has to be threaded into narrow holes to conduct the signals from one place to another.  
      In other solutions, especially disclosed in the document WO 00/55665, an intermediate ferrule has been devised. This ferrule is designed to enable optical connection and is furthermore provided with integrated optoelectronic conversion means. However, owing to the chosen technique of transmission and the mechanical architecture used to make the device, an optical reflection mirror has to be prepared between the exit of the optical fibers and an optoelectronic detector or an optoelectronic emitter responsible for making the conversion. Mirror-based approaches of this kind can also be found in the following documents: U.S. Pat. No. 5,168,537, U.S. Pat. No. 6,132,107, and U.S. Pat. No. 6,161,965. The presence of such mirrors however raises optical and technological problems that impair the efficiency of the optoelectronic conversion undertaken. Indeed, these mirrors imply a specific manufacturing technology, need to be aligned and may be the cause of optical transmission losses.  
      At this stage, we are therefore either faced with solutions in which a bundle is present, as described for example in the document U.S. Pat. No. 5,416,872, or obliged to resolve the problems of reflection referred to here above.  
      In the invention, it is planned to overcome these drawbacks by proposing a ferrule capable of receiving detachable ends of optical fibers (normally presented in a standardized joining piece) and capable of also carrying out optoelectronic conversion, without furthermore having to deflect the light rays coming from or sent to the optical fibers. The receiving of detachable joining pieces in optical port averts the problem of the bundles. It is enough to have a set of optical fiber sections with variable sizes. On the one hand, the joining pieces cost little to make, and on the other hand their compactness allows them to be threaded anywhere. The deflection of the light rays is prevented by placing the useful part of the optoelectronic conversion circuit so that it directly faces a rectilinear optical path coming from the optical port.  
      The ferrule of the invention then has the overall shape of a parallelepiped, of which one of the faces, containing the optical port, is used to receive the detachable ends of the optical fibers, while a face opposite to this receiving face bears an optoelectronic detection and/or emission circuit as well as a control circuit. Preferably, on a face contiguous to these two faces, the package of the ferrule bears contacts enabling the connection of this ferrule to an electronic circuit, especially a printed circuit.  
      Furthermore, given the difficulties of alignment during the positioning of the optoelectronic detection and/or emission circuit facing the optical paths thus made (and in which no optical correction is necessary in principle), a precise positioning is planned using a technique for the reflow soldering of solder beads. This technique has the advantage of providing for positioning with a precision of about one micrometer. Furthermore, by then preferably making the package of the ferrule out of plastic, a notable reduction in the cost of the conversion ferrule is achieved.  
     SUMMARY OF THE INVENTION  
      An object of the invention therefore is a ferrule for the connection of optical fibers comprising an optical port on an input face to detachably receive one or more terminations of optical fibers, optoelectronic circuits for the conversion of optical signals into electrical signals and/or vice versa, placed on an output face opposite the input face and an electrical port providing connection to an electronic circuit, wherein the ferrule has an optical path leading firstly directly onto the optical port, and secondly directly onto a detection or emission part of the conversion circuits and wherein the electrical port is placed on a connection face contiguous to the input and output faces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be understood more clearly from the following description and the accompanying figures. These figures are given purely by way of an indication and in no way restrict the scope of the invention. Of these figures:  
       FIG. 1  is a view in perspective, seen from underneath, of a connection ferrule according to the invention;  
       FIG. 2  is a diagrammatic side view of the ferrule of  FIG. 1 ;  
       FIG. 3  shows a part of the optical fiber connection ferrule of the invention, before the installation of the optoelectronic conversion circuits;  
       FIG. 4  is a diagrammatic view of the preferred mounting of an optoelectronic integrated circuit in the ferrule of the invention;  
       FIG. 5  shows dimensions of the ferrule of the invention and presents improvements of use;  
       FIG. 6  shows a particularly useful installation of a heat sink to cool the optoelectronic conversion circuits. 
    
    
     MORE DETAILED DESCRIPTION  
       FIG. 1  shows ferrule  1  for connecting optical fibers according to the invention. This ferrule  1  has an optical port  2  to detachably receive one or more optical fiber terminations. The optical fibers received are, for example, optical fibers such as  3  provided at their ends with a joining piece  4  that is preferably standardized. The number of fibers  3  may preferably be an even number, with one fiber serving for transmission in one direction, and another for transmission in another direction. The fibers mounted in a flexible sheet may relate to any unspecified number of transmission channels, ideally but not solely, four to-and-fro transmission channels. The joining pieces  4  are used to obtain a preset distance between the different terminations of the optical fibers of a sheet.  
      The joining piece  4  thus has a face  5  designed to abut a face  6  of the ferrule  1 . The face  6  is the one comprising the optical port  2 . In order to provide for the precise positioning, to within about one micrometer, of the ends of the optical fibers  3  in the optical port  2 , the joining piece  4  is provided with pins  8  that get engaged in reserved positions made to match in the face  6 , also in a very precise manner. The pins  8  are used to guide the terminations in the optical port. A package  7  of the ferrule  1  is made of insulating material. Preferably, the package  7  is molded. Preferably it is made of plastic, for example PBT, LCP or polyimide which stands up well to temperature, or any other technical plastic material that stands up to cycles for mounting components by reflow soldering. In the example, the package  7  is furthermore metallized so as to carry electrical tracks.  
      The ferrule  1  also has optoelectronic circuits  9  for the conversion of optical signals into electrical signals and/or vice versa. In the invention, the optoelectronic conversion circuits  9 , at least detection and/or emission circuits of these conversion circuits, are placed on a face  10  of the package  7  that is opposite the face  6  by which the optical fibers have been received. The package  1  has yet another electrical port  11  represented herein by a series of pads forming elevated features on one face  12  of the package  1 . The face  12  is contiguous firstly to the face  10  and secondly to the face  2 .  
      According to an essential characteristic of the invention, shown in  FIG. 2 , the optical signals coming from the optical fibers  3  travel through a preferably rectilinear optical path  13  inside the package  7 . They travel between the optical port  2 , and hence the immediate output of the fiber  3 , and the conversion circuits  9  at which they produce a direct impact or from which they come out directly, in both cases without reflection. The optical path  13  is given shape, in the package  7  by a material that is solid, liquid or gaseous and transparent to light rays. To simplify the explanation, it may be assumed that the package  7  is thus provided with grooves  13  whose orientation is preferably parallel to the pins  8  and is therefore substantially perpendicular to an output face of the joining piece  4  of the optical fibers  3 . These grooves  13  are aligned so that, at their other end  14 , they are placed directly facing and perpendicular to a detection face  15  of the optoelectronic circuits  9 . This mode of action makes it clear that it is possible to do without a reflection circuit whose drawbacks moreover are known.  
      Thus, in the event of the use of optical fibers supported in the package  7  and serving as an interface between the input face of the package and the output face of the package to convey optical signals between the optical port  2  and the optoelectronic components, the holding means constituted by the grooves  13  may be rectilinear. In the case of a use of optical waveguides directly made in the package  7 , the waveguides replacing the interface fibers may be curved, recombined or separated as a function of a desired application.  
      To make the ferrules of  FIGS. 1 and 2 , several solutions are possible. These solutions must furthermore comply with certain constraints. As can be seen in  FIG. 2 , the optoelectronic detection or emission and signal-reshaping integrated circuit  9  is, on the whole, mounted edgewise, perpendicularly to a printed circuit  16  designed to come into contact with the electrical port  11 . The elevation of the pads  17  so that they are in relief with respect to the electrical port  11  furthermore makes it possible to leave space for a blade of air curtain  18 , or for any other material, between the integrated circuit  9  and printed circuit  16  so as to ensure installation and guarantee the reliability of the mounting of the component. As a variant, the contacts  17  of the electrical port may also be fixedly joined and electrically connected to a connector element, one counterpart element of which is fixedly joined to the printed circuit  16  receiving the ferrule.  
      For its electrical connection to the printed circuit  16 , the integrated circuit  9  is connected to metallized pins  19  placed on the face  10  of the package  2 . It is connected to them by solder beads such as  20 . The solder beads  20  are furthermore connected to connection pins  21  of the integrated circuit  9  itself.  
      The technique of setting up an electrical connection of the integrated circuit  9  by solder beads is a technique known as the flip-chip technique, in which a reflow of the solder beads is produced. In practice, during manufacture, the integrated circuit  9  is placed horizontally above the package  7  after the positioning of the solder beads  20 . In this phase, the package  7  is raised vertically with its face  10  on top. Then the entire piece is taken to a reflow temperature of over  260  degrees. Then the solder beads  20  achieve firstly the electrical soldering of the pins  19  to the pins  21 . Secondly, through the surface tensions that develop in the solder, they provide for an exact positioning of these pins  21  relative to the pins  19 . Consequently, if by construction of the integrated circuit  9 , the pins  21  are positioned precisely relative to the detection or emission ports  15  of the electronic circuits  9 , and furthermore the pins  19  are placed, by construction, precisely relative to the output hole  14  of the rectilinear path  13  in the package  7 , then the positioning of the electronic circuit  9  is obtained quite naturally and with high precision, in practice with a precision of about one micrometer. We then have a configuration in which the alignment is perfect, with a well-mastered technology and hence a low-cost result. At the same time, the assembly could be done otherwise, for example by using a precise positioning machine.  
       FIG. 3  shows the making of electrical tracks  22  by which the pins  19  of the package can be connected to the pads  17  of the electrical port  11 . While the package  7  is preferably made of plastic, the metallized tracks  22  may be obtained in different ways. For example, the totality of the package is metallized and the tracks  22  are etched thereon, on all its faces, by wet etching or by dry etching (by laser). As a variant, it is possible to carry out a selective etching of the surface of the package  7 , at the position of the tracks  22 , so as to chemically activate the material of the service of the package at the position of these tracks  7 . Then the package is subjected to a chemical metallization, with the metal particles adhering to the zones that have been activated.  
      It is thus possible to make tracks  22  that spread out not only on one face  12  of the package containing the pads  17  but also on one or more other contiguous faces of the package. Furthermore, at the position where there is a change of face, the tracks show electrical continuity. If need be, the ridges  23  between two contiguous faces  10  and  12  may be rounded to foster the making of this electrical continuity. As can be seen in  FIGS. 2 and 3 , the electrical tracks may be of different lengths according to the remoteness of the pad  17  that they connect to the face  10 .  
      In the invention, it is noted that the electronic circuit  9  must be powered electrically, must receive control or signaling signals, and must transmit signals to be electro-optically converted or that have been electro-optically converted. It will then be chosen to reserve tracks such as  24  and  25 , which have the longest route in the package  7 , for carrying electricity. Tracks  26  of intermediate length will be used for the transmission of the control or signaling signals, while the shortest tracks  22  will serve for the transmission of the signals detected or to be transmitted. In practice, the signals to be transmitted or the converted signals available on the track  22  are very rapidly variable signals. Their variation depends on the bit rate which may be equal to about several gigabits per second. The signals conveyed by the connections  26  are less rapidly variable, for example about one MHz, while the signals on the connections  24  and  25  are for their part direct current signals. The tracks  22  and  24  to  26  are preferably made on the external faces of the package  7 .  
       FIG. 4  is a diagrammatic sectional view of the package  7  as well as the electronic circuit  9 . It furthermore shows that the package  7  is formed by two blocks  27  and  28  joined together. For example, the two blocks  27  and  28  are parallelepiped-shaped, like the package  7 , and have a height, measured perpendicularly to the printed circuit  16 , that is half the height  29  of the entire package  7 . The two blocks  27  and  28  possess means to form rectilinear optical paths at the position  30  at which they meet. In one example, these means are formed by the presence of V-shaped or U-shaped grooves made in at least one of the two blocks  27  or  28 , the other block being possibly devoid of grooves and being flat. If desired, these grooves can be used for the positioning of optical fiber sections therein or for the deposition therein of a polymer resin playing the role of an optical waveguide so as to make the package  7  transparent to light at their position. When optical fiber sections or polymer waveguides are thus placed in the meeting zone  30 , a thrust feature  31  on the face  10  of the package  7  enables the ends of the sections to be polished without damage to the metallized tracks.  
      As a variant, the package is a unique single-piece unit. It is then pierced with rectilinear holes in which the optical fiber sections or waveguide are placed or not placed.  
      The mode of manufacture of the package  7  in two blocks  27  and  28  is preferred because it enables a simpler making of the rectilinear optical paths. The precision of the making of a groove is greater than the precision of the making of a hole, as the former can be far more rectilinear than the latter. Furthermore, the making of the package in two blocks permits the making of paths  13  in the form of a material molded in the grooves before the blocks are attached together.  
      Consequently, the metallized tracks such as  24  and  25  each made partly on each of the blocs are joined, after the two blocks  27  and  28  are attached to each other by electrical bridges such as  32 . The electrical bridges are either simple solders, or used to positioning complementary circuits, especially electrical decoupling circuits, to prevent the transmission of parasitic electronic signals. The two blocks  27  and  28  are joined to each other by bonding or by ultrasonic soldering or by laser, without or without the presence of optical fibers.  
      If necessary, at the position of the port  2  and of the optical output  14 , optical lenses may be placed. Or quite simply, the optical fiber sections placed in the holes or in the grooves have rounded shapes at their ends giving a similar lens effect.  
       FIG. 4  also shows that the detection or emission and conversion integrated circuit  9  can preferably be made in the form of two integrated circuits stacked one on the other. For example, the integrated circuit  9  has the detection (or emission) circuit proper  33 . The circuit  33  is based on VCSEL type diodes. The circuit  9  also has an integrated analog-digital conversion integrated circuit  34 . The integrated circuit  34  converts analog electrical signals produced by the detector  33  into digital electrical signals or vice versa if the circuit  33  is an emitter. Preferably, the integrated circuit  33  is connected, by pins not shown, to the integrated circuit  34  by the reflow of solder beads  35 , of the same type as the solder beads  20  so as to ensure a precise positioning of this integrated circuit  33  relative to the integrated circuit  34 . Since the integrated circuit  34  has itself being placed precisely by beads  20  relative to the output  14  of the package  7 , the result obtained is that the circuit  33  is placed precisely relative to the package  7 . Given the distances, the solder beads  35  will be far smaller than the solder beads  20  so that the integrated circuit  33  can find a place in a gap  36  made between the face  10  of the package  7  and the integrated circuit  34 . Typically, the space between the surface  15  and the face  10  is 100 micrometers.  
       FIG. 5  shows the overall dimensions of the unit formed by the package  7  and the optoelectronic integrated circuit  9 . In practice, a ferrule module according to the invention will have the following dimensions, plus or minus 10%: a length of 5 mm, a width of 7 mm and a height of 2 mm. It will be noted that this height of 2 mm is quite compatible with assembly on a printed circuit  16 , and permits the attachment of several printed circuit boards  16  mounted edgewise and placed against one another.  FIG. 5  also shows that it is possible to use an upper face  37  of the package  7 , opposite the face  12  bearing the electrical port  11 , to position other integrated circuits such as  38  in a position of interconnection between or on electrical linking tracks. The circuit  38  will preferably be a passive type circuit, mounted according to an SMC (surface-mounted component) type of technology.  
       FIG. 6  gives a diagrammatic view of the package  7  connected to an electronic circuit  9 . The electronic circuit  9  has a flat conversion circuit  34  whose surface is substantially parallel to the output face  10  of the package  7 . This construction then permits the positioning of a sink  39  placed flat against the back of the integrated circuit  34 , for example by means of a thermal transmission bonder  40 . Indeed, it can be estimated that an optoelectronic conversion circuit working at very high speed to ensure the bit rate transmitted by the optical fiber is an element that produces a substantial quantity of heat. The fact of having placed the integrated circuit  34  edgewise, perpendicularly to a printed circuit  16  (not shown) then makes it possible to place the sink  39  usefully with its thermal connector plate perpendicular to the printed circuit  16 .  
      In a commercially distributed version, this set is placed in a holding case  41 . The holding case  41  possesses, firstly, the optical port  2  and, secondly, the optical port  11 , both being placed on faces that are perpendicular to the package  7 .  
      It is possible to install a certain number of emitter/receiver pairs made in one or more integrated circuit such as  9  mounted on the face  10  and connect them to the pins such as  19 .  
      The large number of pads such as  17  enables the package to be held on the circuit  16 . If need be, some of them are not functional for making electrical links.