Abstract:
A monolthically integrated VCSEL and photodetector, and a method of manufacturing same, are disclosed for applications where the VCSEL and photodetector require separate operation such as duplex serial data communications applications. A first embodiment integrates a VCSEL with an MSM photodetector on a semi-insulating substrate. A second embodiment builds the layers of a p-i-n photodiode on top of layers forming a VCSEL using a standard VCSEL process. The p-i-n layers are etched away in areas where VCSELs are to be formed and left where the photodetectors are to be formed. The VCSELs underlying the photodetectors are inoperable, and serve to recirculate photons back into the photodetector not initially absorbed. The transmit and receive pairs are packaged in a single package for interface to multifiber ferrules. The distance between the devices is precisely defined photolithographically, thereby making alignment easier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional patent application of U.S. patent application Ser. No. 08/803,891, filed Feb. 21, 1997 now U.S. Pat. No. 6,001,664, of which is a continuation of U.S. patent application Ser. No. 08/593,117, filed Feb. 1, 1996, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to vertical cavity surface emitting lasers (VCSELs) and photodetectors, and more particularly to the application of such optoelectronic devices where they must operate independently but where it is also desirable to have a transmitter and a receiver closely-spaced. 
     2. Art Background 
     There are a number of data communications applications which make use of optoelectronic sending and receiving devices (i.e. light emitters and photodetectors). For fiber optic data communication applications requiring less than 200 MBits/sec., light emitting diodes (LEDs) are the light emitters of choice because they are relatively inexpensive to manufacture. For applications requiring higher speeds, lasers are used as the light emitters. 
     Until recently, most high speed data communications applications employed edge emitting lasers in a serial (single channel) format. With the advent of Vertical Cavity Surface Emitting Lasers (VCSELs), many such applications are now implemented using VCSEL arrays which can be interfaced to ferrules carrying multiple fibers to transmit several bits of data in parallel. At the receiving end, an array of photodetectors is coupled to the multiple fibers. It is the ability to manufacture VCSELs in arrays (an advantage of LEDs) combined with their high speed of operation (an advantage of lasers) which makes VCSELs desirable in such applications. 
     For high-speed serial duplex data communications applications, however, separately packaged light emitters (usually edge emitting lasers) and photodetectors are still employed. For long-haul applications (typically having distances greater than 1 kilometer), wavelength division multiplexing is often employed to transmit and receive data for a duplex channel over the same fiber. Because the primary cost of a long-haul duplex serial data channel resides in the fiber and its installation, complex beam-splitting techniques can be justified at the ends of the channel to separate the transmit and receive data streams from the single fiber. 
     For short-haul or “premises” applications, however, the cost of fiber and fiber installation is relatively less important than the cost of the many transmit and receive functions. Thus, it is the cost of the data transmit and receive components, and particularly the optoelectronic devices and their packaging, which drives cost considerations for short-haul applications. Typical short-haul implementations of a high-speed serial fiber optics data communications channel operating in full duplex still employ two multimode fibers; each one to connect an individually packaged transmitting light emitter to an individually packaged receiving photodetector. This is because the cost of complex beam-splitting components can not be justified. 
     FIGS. 1 a  and  1   b  illustrate the components comprising a typical implementation of a transmit or receive link for a short-haul high-speed duplex data communications application. FIG. 1 a  illustrates a fiber assembly  12 . A round ferrule  26  houses an optical fiber  28  which is located precisely in the center of ferrule  26 . A typical diameter for ferrule  26  is approximately 2.5 mm. Ferrule  26  comes with a latching mechanism  30  which is used to clamp and secure the ferrule to a barrel  32  of an optical sub-assembly  10  which is depicted in FIG. 1 b . Barrel  32  houses optoelectronic device  14  typically in a TO can package  16  which is centrally located in the barrel as shown. Optoelectronic device  14  is typicaly located at an appropriate point within can  16  by a standoff  2 . Driver or amplifier circuitry is coupled to optoelectronic device  14  through leads  22 . A window  18  is provided in the top of the can package to allow transmitted light out or received light in, depending upon whether the optoelectronic device is a light emitter or a photodetector. The TO package is aligned with fiber  28  and epoxied using epoxy  24  to fix the position of the optoelectronic device with respect to the ferrule  26  and hence fiber  28 . Optical elements such as lens  20  are typically provided to focus the light for optimal optical efficiency, particularly where the light emitter is an edge emitting laser. Barrel  32  is designed to mate with latching mechanism  30  of fiber assembly  12 . 
     Both fiber assembly  12  and barrel  10  are precision manufactured for precise mating. Active alignment TO package  16  and optoelectronic device must be performed in the x, y and z axes. First, the optoelectronic device must be precisely aligned within the package  16 . Second, the package  16  must be precisely aligned within barrel  10 . Finally, optical element  20  must be precisely aligned with respect to its distance from the optoelectronic device  14  to achieve proper optical operation. Because a separate package is required for both the transmit side and the receive side of the duplex data channel, a total of twelve active alignments must be performed for each channel and each channel includes the cost of eight precision-manufactured coupling parts. 
     FIGS. 1 c  and  1   d  provide a schematic illustration of the fiber assembly  12  and optoelectronics subassembly  10  of FIGS. 1 a  and  1   b  respectively. 
     FIG. 2 illustrates a typical duplex serial data communications module  40 , which has mounted to it an optical subassembly  52  containing a light emitting device  13  disposed in a TO can package  9  having a window  17 , which is to be mated with an optical fiber assembly  46  and which is dedicated to data transmission. Module  40  also has an optical subassembly  50  mounted to it containing a photodetector  15  disposed in TO can package  11  and which is to be mated with optical fiber assembly  48  and dedicated to receiving data from a remote module not shown. Because of the differing optical requirements of the transmit and receive devices, the modules must often be mounted in a staggered fashion as shown. Moreover, the transmit devices are located at an optically appropriate point in their can packages by standoffs  4  and  6  respectively. 
     Because of the cost of the precision components and the large number of alignments required for implementing duplex serial modules  40 , it is highly desirable to integrate the transmit and receive optoelectronic devices (i.e. light emitter and photodetector) into one package. The integration of the two devices into a single package is not, however, an easily achieved solution. The prior art implementations as illustrated in FIGS. 1 a-d  and  2  cannot be readily adapted to multifiber ferrules currently available for unidirectional data transmission using VCSEL arrays. These multifiber ferrules have fiber spacings which are typically about 250 microns and can be less. The diameter of the TO can package  14  commonly used in present implementations is itself 5600 microns in diameter. Thus, the standard ferrule and barrel would have to grow substantially in diameter to accommodate two fibers having the spacing dictated by the TO cans housing the optoelectronic devices. 
     Even if a substantially larger barrel could be created to integrate the light emitter and photodetector as commonly packaged to receive both a transmit and a receive fiber, it is not clear that the resulting package could provide the necessary separation of incoming and scattered outgoing light beams to prevent crosstalk between the transmit and receive signals (at least not without complex optics and possibly some form of isolation). Although solutions have been disclosed to stack a light emitter (typically an LED) on top of a photodetector to transmit and receive wavelength division multiplexed signals (the light emitter is transparent to the received wavelength), beam-splitting must still be employed at the opposite end. 
     Thus, there is room in the art for an improvement in the area of optoelectronic device fabrication which facilitates the integration of one or more pairs of transmit and receive devices for interface with a single ferrule carrying one or more pairs of fibers having spacings as little as 250 microns or less, to substantially reduce the cost and complexity of implementing high-speed serial duplex data communications channels. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a VCSEL device sufficiently close to a photodetector device to permit the use of commercially available multifiber ferrules having fibers spaced on the order of 750 microns to 250 microns or less. 
     It is a further objective of the present invention to provide the closely-spaced but independently operable optoelectronic devices by monolithically integrating the two devices on the same substrate. 
     It is still further an objective of the invention to provide a process by which multiple pairs of VCSELs and photodetectors can be arrayed on the same substrate. 
     It is still further an objective of the invention to provide a process by which the closely-spaced but independently operable optoelectronic devices can be packaged using known lead-frame or ceramic packaging technologies. 
     It is still further an objective of the invention to integrate any requisite optics with either the semiconductor manufacturing technology or the packaging technology. 
     It is still further an objective of the present invention to simplify significantly the alignment of the fibers to the closely-spaced optoelectronic devices by taking advantage of the photolithographic nature of monolithic semiconductor processing to precisely define the separation between the optoelectronic devices. 
     It is still further an objective of the invention to provide closely-spaced but independently operable VCSEL and photodetector pairs capable of near-field operation requiring no optics and which permit butt coupling between a package containing the optoelectronic devices and a flat faced multifiber ferrule. 
     In a first preferred embodiment of the invention, one or more VCSELs are formed using a known process for manufacturing such devices. The one or more VCSELs comprise an n-type GaAs substrate and a first mirror formed on the substrate, which is a well-known distributed Bragg reflector (DBR) a first spacer or cladding layer which is formed on top of mirror. This first mirror is also doped n-type. An active region is then formed on top of the first mirror, the active region comprising at least one quantum well layer or bulk layer. A second spacer or cladding layer is formed on the active region, with a second DBR being formed on the second spacer layer and doped to have p-type conductivity. 
     On top of the VCSEL layers is grown an etch-stop layer of AlGaAs having about 90% or greater Al content. An extended p-type layer of AlGaAs having more typical alloy proportions is then grown on top of the etch-stop layer. On top of this p-type layer is grown an intrinsic layer (i) which is undoped GaAs. On top of the intrinsic layer is grown an n-type region of AlGaAs. An etching process is then performed to etch away the extended p, i, and n layers where the one or more VCSELs are to be formed. The etching process uses the etch-stop layer to mark the end of the etching process so that the VCSEL area has exposed the top surface of an appropriately designed mirror. A proton implant region is created which separates the one or more VCSELs and the photodetectors formed by the unetched p, i, and n layers. Anode contacts are formed over the non-implanted p regions to form apertures for the VCSELs. A VCSEL cathode contact is formed on the substrate. Anode contacts are also formed on the p region of the p-i-n photodiode and a cathode contact is made to the n region of the p-i-n photodiode. 
     Thus, in this preferred embodiment, a VCSEL circuit can be isolated from a p-i-n photodiode using a proton implant isolation region which is commonly used to isolate VCSELs formed in arrays. The anode contacts to the p region of the p-i-n photodiode may be coupled to ground so that the VCSEL structure which lies underneath the p-i-n photodiode is never turned on and other bipolar parasitic effects are avoided. The width of the proton implant isolation region is typically between 50 and 100 microns. Thus, the VCSEL and the p-i-n photodiode can be separated by an accurately known distance, significantly less than 250 microns if desired. Moreover, the difference in thickness between the VCSEL and the p-i-n photodiode is small, thereby permitting near-field coupling of the optoelectronic devices to fibers. 
     One significant advantage of the first embodiment of the invention is that it requires very few additional steps to an otherwise typical VCSEL manufacturing process. A second advantage is that, when an anti-reflection coating of silicon nitride is applied to the photo-receiving n region of the photodiode, in conjunction with the p-type mirror which underlies the p-i-n photodiode, a high degree of efficiency is achieved. The to silicon nitride anti-reflection coating increases transmission of incoming light into the surface of the p-i-n photodiode. Additionally, any light which is not absorbed by the intrinsic layer of the photodiode on its way through will be reflected from the underlying p-type mirror back into the intrinsic layer and will then have a second opportunity to be absorbed. 
     A second preferred embodiment of the invention employs a VCSEL with an MSM photodiode. The VCSEL is manufactured on a semi-insulating substrate. Because the MSM photodetector employs the semi-insulating layer as its common cathode, the two optoelectronic devices are virtually isolated from one another electronically as a result. A photolithographically defined minimum spacing of 250 microns or less can also be achieved using the second preferred embodiment of the invention. An anti-reflection coating is also preferably employed over the MSM photodetector to increase efficiency. To further enhance electrical isolation between the two devices, an isolation region can also be formed, preferably by implantation. Another advantage of using an MSM photodiode is that the two anode terminals can be used to drive a differential amplifier, thereby permitting common-mode rejection of noise. 
     Either of the two preferred embodiments can be integrated with optically transmissive materials which can be formed into lenses on the surface of the semiconductor. Either embodiment can also be implemented within standard precision manufactured barrels to be aligned with circular ferrules containing multiple fibers. Finally, either embodiment can be encapsulated using known lead-frame or ceramic packaging technology to permit near-field flat coupling between a flat package having an optically transmissive surface and a commercially available flat rectangular ferrule containing multiple fibers. 
     The foregoing objectives and the features of the preferred embodiments will be understood by those of skilled in the art with reference to the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  (prior art) illustrates a fiber assembly of a typical implementation of a high-speed data transmission link using a single optoelectronic device for each half of a duplex channel. 
     FIG. 1 b  (prior art) illustrates an optical assembly of a typical implementation of a high-speed data transmission link using a single optoelectronic device for each half of a duplex channel. 
     FIG. 1 c  (prior art) illustrates a schematical view of the fiber assembly of FIG. 1 a  and the optical assembly of FIG. 1 b.    
     FIG. 1 d  (prior art) illustrates the high-speed duplex data communications module using known precision manufactured parts to implement the transmit and receive links of a high-speed duplex data communications channel of FIG.  2 . 
     FIG. 2 (prior art) illustrates a high-speed duplex data communications module using known precision manufactured parts to implement the transmit and receive links of a high-speed duplex data communications channel. 
     FIG. 3 illustrates a cross-section of an integrated VCSEL and an MSM photodetector in accordance with the present invention. 
     FIG. 4 illustrates a monolithic plan view of a preferred embodiment of the invention. 
     FIG. 5 a  illustrates a cross-section of a VCSEL and a p-i-n photodiode in accordance with the present invention. 
     FIGS. 5 b  and  5   c  respectively illustrate the orientation of the VCSEL laser diode and the p-i-n photodiode as illustrated by the cross-section in FIG. 5 a.    
     FIG. 6 a  (prior art) illustrates a commercially available dual fiber version of a round fiber ferrule. 
     FIG. 6 b  (prior art) illustrates a commercially available rectangular multifiber ferrule. 
     FIG. 7 a  illustrates how the present invention can be packaged using conventional lead-frame technology to facilitate interface to a rectangular multifiber ferrule. 
     FIG. 7 b  illustrates how optical lenses can be integrated with the present invention. 
     FIG. 7 c  illustrates how the present invention can be implemented using butt coupling technology. 
     FIG. 8 illustrates how a transmit and receive pair made in accordance with the present invention can be packaged to interface with a round multifiber ferrule such as the one illustrated in FIG. 6 a.    
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a monolithic semiconductor device having a VCSEL integrated with a photodetector on the same substrate, the VCSEL and photodetector to be operated independently as a transmit and receive device respectively. The VCSEL and photodetector are physically situated in close enough proximity to permit packaging of one or more pairs of the VCSEL and photodetector such that they may be coupled to multifiber ferrules having fiber spacings on the order of 250 microns or less. The present invention also includes a method of manufacturing the independently operated VCSEL and photodetector, as well as the packaging and coupling of one or more pairs of the integrated VCSEL and photodetector and their coupling to multifiber ferrules. 
     A first preferred embodiment of the invention is now discussed in detail in conjunction with FIGS. 3 and 4. As shown in FIG. 3, a VCSEL and a metal-semiconductor-metal (MSM) photodetector are integrated on the same semi-insulating GaAs substrate. The conductance of the semi-insulating substrate is typically between about 10 −12  and 10 −5  ohm-cm. The VCSEL is built on top of the substrate beginning with an n− layer  68 , upon which an n+ layer  70  is grown which forms the cathode of the VCSEL. 
     A first mirror is formed on layer  70 , which is preferably an epitaxially formed distributed Bragg reflector (DBR) which comprises a plurality of alternating semiconductor layers having high and low indices of refraction, with each layer having a thickness of λ/4n, where λ is the wavelength of the optical radiation emitted from the laser and n is the index of refraction of the layer. The semiconductor layers are doped to achieve n-type conductivity. A quantum well (QW) active region  74  is formed between a first spacer  73  and a second spacer  75 , with first spacer  73  formed on the top layer of the first mirror. Active region  74  has at least one QW layer. 
     A second mirror  76  is formed on second spacer  75  and is preferably an epitaxially grown DBR which is comprised of a plurality of alternating semiconductor layers having high and low indices of refraction, with each layer having a thickness of λ/4n, where λ is the wavelength of the optical radiation emitted from the laser and n is the index of refraction of the layer. The second mirror  76  is doped to achieve a p-type conductivity. An isolation implant  80  is formed around the second mirror  76 , and preferably extends to a depth just inside spacer  75 . A mesa region is then etched around the outside of the VCSEL  89  to a depth which exposes the cathode layer  70 . A cathode contact  72  is then formed on the exposed surface of cathode layer  70 , and an anode contact&#39;s formed which overlaps the surface of isolation implant region  80  and the top-most layer of second mirror  76 , and which further defines an aperture  88  which comprises a portion of the surface of the topmost layer of second mirror  76 . Radiation  84  is emitted through aperture  88 . 
     The MSM photodetector  62  is formed on the surface of the semi-insulating substrate  60  as two non-electrically coupled metal patterns  62  and  64 , each having fingers which are interdigitated with one another. When one or both of the patterns is biased to some voltage, carriers generated by received light are swept to the anodes of the two diodes by the applied electric field. Because the MSM operates without conducting any current through the substrate  60 , there is virtually no electrical crosstalk or leakage between the VCSEL  89  and the MSM photodetector  62 . Thus, the VCSEL  89  can emit radiation  84  from aperture  88  based on digital data to be transmitted while MSM photodetector  62  can receive radiation  86  in which is encoded digital data received from a remote data source. To achieve even better isolation, an isolation region  80  can be formed preferably by proton implant between VCSEL  89  and MSM photodetector  62 . 
     FIG. 4 illustrates a plan view of the device which is shown as a cross-section in FIG.  3 . For clarity, corresponding structures will be indicated by identical index numbers. The cathode  70  and its contact  72  of the VCSEL are extended to the boundary of substrate  60  which is furthest away from MSM photodetector  86 . Bond wire  71  can then be used to connect-cathode contact  72  to a bond pad of, for example, a lead frame. The VCSEL anode contact  82  is brought to the same substrate boundary by bond wire  77 , metal extender  79  and bond wire  69 . Metal patterns  66  and  64 , which form the anode and anode terminals of MSM photodetector  62 , are also bonded to the leads of whatever form of packaging is used. One of the metal patterns is typically coupled to a bias voltage while the other is coupled to ground or a different bias voltage. An anti-reflection coating can be employed on the MSM to increase optical efficiency. 
     A second preferred embodiment is disclosed in FIG. 5 a . For convenience and clarity, like structures will be denoted by the same index numbers as in previous figures.. This particular embodiment is preferred because it can be implemented using more standard VCSEL manufacturing processes. A first mirror  78  is formed on a standard semiconductor GaAs substrate  79 . The first mirror is preferably a semiconductor DBR comprising twenty to thirty periods of AlAs/AlGaAs layers. Each of the layers has a thickness of λ/(4n) and is doped to have n-type conductivity. A first spacer or cladding layer  73  is then formed on first mirror  78 , which is either undoped or very lightly doped. An active region  74  is then formed on the first spacer  73  which comprises at least one GaAs QW layer. A second mirror  76  is then formed on top of a second undoped or very lightly doped spacer or cladding layer  75 . The second mirror  76  again preferably consists of alternating layers of AlAs/AlGaAs layers, each being λ/(4n) thick. Second mirror  76  is doped to have p-type conductivity. On top of second mirror  76  is formed a thin etch-stop layer  93 , which has a significantly higher ratio of Al to Ga, about 9 to 1 or greater. On top of the etch-stop layer  93 , an extended p-type layer  100  of AlGaAs is formed. On top of p-type layer  100  is formed an intrinsic layer (i)  102  of undoped GaAs. Finally, an n-type layer  104  is formed on top of intrinsic layer  102 . 
     The structure is then etched in those areas where a VCSEL is to be formed, and not etched where a p-i-n photodiode is to be formed. The etch strips away the n-type layer  104  and intrinsic layer  102  and continues into p-type layer  100  until the etch-stop layer  93  is detected. The etching process is terminated so that the etch-stop layer is etched away and an appropriately thick top layer of second mirror  76  is exposed. Those of skill in the art will recognize that there are other well-known techniques by which the endpoint of an etching process may be detected to end the etching process at the appropriate time and which are intended to be within the scope of the present invention. A proton isolation implant is performed to create isolation region  80  between VCSEL  92  and p-i-n photodiode  90 . The implant region  80  typically achieves a depth which extends just inside spacer layer  75  and has a width preferably between about 50 and 100 microns. A circular metal contact  82  is then formed on the top of mirror  76  and which overlaps slightly implant region  80 . Contact  82  provides access to the anode of VCSEL  92 . A contact  81  is then formed on the back side of substrate  79  and serves as the cathode terminal of VCSEL  92 . Contacts  94  are preferably formed on both sides of p-i-n photodiode  90  which provide electrical access to the anode of p-i-n photodiode  90  as well as to the anode of the VCSEL  91  which underlies p-i-n photodiode  90 . Finally, contact  96  is formed on n-type, layer  104  to form the cathode of p-i-n photodiode  90 . An anti-reflection coating preferably having a thickness of about one quarter wavelength is applied to photo-receiving surface  101 . 
     A simplified schematic of the structure of FIG. 5 a  is shown in FIG. 5 b . VCSEL  92  is operated with forward bias between anode terminal  82  and cathode terminal  98  to produce radiation  84  having a wavelength of λ, p-i-n photodiode  90  is operated with reverse bias between cathode contact  96  and anode contacts  94 . Moreover, anode contacts  94  are shorted to substrate contact  98  to ensure that VCSEL  91  will not become forward biased and emit light. Thus, VCSEL  92  can be operated to emit light encoded with data to be transmitted to a remote receiver employing a similar structure, and p-i-n photodiode  90  can operate to receive radiation  86  which is encoded with data received from the same remote terminal. 
     Those of skill in the art will recognize that the exact order in which the process steps take place, as well as the particular material system used, are irrelevant to the patentability of the present invention. For example, one material system might include a GaAs substrate, GaAs quantum wells, DBR layers of AlAs and AlGaAs, and an intrinsic layer of GaAs. The p and n layers of the p-i-n would also be made of AlGaAs. Other known material systems may be used to produce different wavelengths of emitted radiation and the particular dimensions of the integrated devices may be changed to suit particular transmission modes or packaging requirements. Moreover, although it is desirable that the photolithographically defined spacings between the transmit and receive pairs are preferably small, of course larger spacings can be easily accomodated by the present invention. 
     Those of skill in the art will recognize many advantages of the second preferred embodiment of FIG. 5 a  is that a typical process used to create arrays of VCSELs, including the isolation implant commonly used to separate the individual VCSELs of the array, can be used to create arrays of VCSEL/p-i-n photodiode pairs. The additional steps required to build the p-i-n photodiode on top of the VCSEL process are negligible in cost. Moreover, the difference in the thickness of the two devices is also negligible for purposes of facilitating near-field coupling of the devices to fibers to eliminate the need for optics. Additionally, due to the underlying second mirror of inoperable VCSEL  91 , any light not absorbed by the intrinsic layer  102  of p-i-n photodiode  90  will be reflected back into intrinsic layer  102 , thus having a second chance to be absorbed. Finally, the thicker the intrinsic layer, the lower the capacitance of the p-i-n diode (the faster its operation) and the better its optical efficiency. 
     FIG. 6 a  illustrates how the commonly used single fiber round ferrule can be implemented using two or more fibers. Such fibers are now currently available from Siecor as prototypes. The cylindrical ferrule  110  has the same dimensions (i.e., 2.5 mm) as those ferrules commonly used with only one fiber. Thus, one fiber  112  can be used for transmitting data as coupled to a VCSEL while fiber  114  can be used to receive data from a remote transmitter as coupled to a photodetector. 
     FIG. 6 b  illustrates a commonly available rectangular ferrule which can have eight or more fibers  122 , and which has guides  120  for receiving alignment pins. Rectangular ferrule  116  typically has a polished face  118  for coupling to an array of transmitting VCSELs. This rectangular ferrule  116  can be easily adapted to devices made in accordance with the present invention, such that each pair of fibers  122  can be aligned with a pair of integrated VCSEL/photodetectors. 
     FIG. 7 a  illustrates how a single VCSEL/photodetector pair could be packaged using standard lead-frame technology to be interfaced to a rectangular multifiber ferrule such as the one illustrated in FIG. 6 b . Integrated transmit/receive chip  130  can be epoxied to lead frame  128  and then bonded to bond pads  141  via bond wires  143 . If optics are required, lenses  138  and  136  can be formed over VCSEL  89 ,  92  and photodetector  62 , 90  respectively, either using materials which are formed over chip  130  during the manufacturing of chip  130  or such optics can be integrated within the surface of the plastic encapsulation formed by the package. Lead frame  128  can also have guide pins  140  to be used in conjunction with a rectangular ferrule such as the one shown in FIG. 6 b . FIG. 7 b  shows a side view of FIG. 7 a  to illustrate the use of optics over photodetector  62 ,  90  and VCSEL  89 ,  92 . 
     FIG. 7 c  illustrates how lead frame  128  can be butt coupled to a rectangular ferrule  150  containing two fibers  124  and  126 . If distance  160  is fairly precisely known, and distance  147  between fibers  124  and  126  is fairly precise, a fairly accurate alignment can be achieved between fibers  124  and  126  and VCSEL  89 ,  92  and photodetector  62 ,  90 , because the distance between VCSEL  89 ,  92  and photodetector  62 ,  90  are fairly precise based on the photo-optical alignment process used in manufacturing the integrated semiconductor  130 . Thus, a fairly accurate positioning of the chip  130  with respect to the lead frame  128  during packaging will provide a reasonably accurate passive alignment. Of course, fine alignment can be achieved using well-known active alignment techniques. A further advantage of the coupling technique shown in FIG. 8 is that no optics must be interposed between package  146  and ferrule  150  if the coupling distance  152  is close enough. Of course, a flat transmissive surface  148  can be easily achieved on package  146 . 
     FIG. 8 illustrates a lead-frame package which can be used to interface with a round multifiber ferrule such as the two-fiber ferrule of FIG. 6 a . Barrel  127  is designed to precisely mate with the round ferrule of FIG. 6 b.