The invention relates to a bi-directional optical link submodule based on silicon waferboard technology.

With the advent of communications at optical frequencies, a great effort has been made to provide optical transceiver units that have greater bandwidth capabilities to allow the transmission and reception of audio, digital communication and interactive video signals. Furthermore, because the end user of such modes of communication is in the home or office, there is a requirement that a large number of devices be deliverable that are reliable and yet cost competitive. While the electronics have evolved to achieve the higher data rates and reliability, up until recently the thrust of the product market has focused on bidirectional links that incorporate TO style laser and detector cans. An example of such a device is as disclosed in U.S. Patent 5,127,075 to Althaus, et al, the disclosure of which is specifically incorporated herein by reference. A drawback to a bidirectional link as is disclosed in Althaus, et al. is the required active alignment of the laser/LED and the detector with the focusing and beamsplitting elements in the optical package. This labor intensive alignment results in a bidirectional link that is expensive. Accordingly, passive alignment has become almost required to achieve the cost

requirements while maintaining the reliability and data rate requirements.

One technology that has been employed to effect passive alignment between optical components, active devices and fibers is amorphous or monocrystalline silicon selectively etched to provide grooves, pedestals and standoffs to achieve alignment. This technology is known as silicon waferboard technology to the artisan of ordinary skill. There are a variety of alignment techniques using silicon waferboard that have evolved, and are employed in the present invention. To this end, the present invention makes use of etched surfaces in defined orientations, kinematic mounting using microspheres and standoffs or pedestals to effect the passive alignment of the various optical elements.

The first technique employed is the use of defined surfaces and orientations. This allows for the selective etching of grooves and wells for placement of components and reflective surfaces. The etching can be effected on either monocrystalline material where the defined crystalline planes of the material give known orientations for passive alignment or by reactive ion etching of amorphous material to reveal planes in well defined orientations. The former technique is taught for example in U.S. Patent 4,210,923 to North et al. The latter technique is disclosed in U.S. Patent Application 08/251,061 to Boudreau, et al .

The second above referenced technique to effect the passive alignment is the use of microspheres mounted in etched wells in the silicon, known as kinematic mounting. This technique allows, among other capabilities, the ability to passively align coplanar substrates, and accordingly is employed in the present invention. Alignment by use of alignment spheres is disclosed in U.S. Patent application number 08/362,625 entitled "Kinematic Mounting of Optical and Optoelectronic Elements on Silicon Waferboard" , to Boudreau, et al.

Additionally, to properly passively align the active of the submodule, alignment pedestals are employed. These pedestals are generally formed by well known etching techniques, and the techniques and their utility in the alignment of devices is taught in U.S. Patent 5,182,782 to Tabasky et al .

Another technique employed to achieve accurate, passively aligned light focusing is by the use of holographic elements. In particular, holograms are used for directing light in a wavelength selective manner, for example in multiplexing and demultiplexing. Furthermore, the incorporation of holographic optical elements (HOE's) on silicon is a very efficient manufacturing approach. To this end, the fabrication of the holograms on a silicon substrate for wavelength selective direction of the light to and from selected elements of the optical system incorporates the accuracy

of the optical elements with the ready fabrication and passively alignment of the elements on a silicon substrate. Holographic elements fabricated on silicon is as disclosed in U.S. Patent 5,420,953 and U.S. Patent application 08/269,304 to Boudreau, et al .

One example of the use of a silicon substrate as the optical bench can be found in U.S. Patent 4,904,036 to Blonder. The Blonder patent discloses a transceiver unit in which the discrete elements are mounted on the substrate surface and the waveguides of the unit are mounted in v-grooves etched in the substrate surface. However, the reference to Blonder requires a relatively complicated system for effecting the coupling of the light from the laser diode source to the fiber as well as to the monitor detector and for effecting the coupling of the incoming light from the fiber to the detector. To this end, the Blonder reference uses selectively placed silica waveguides to effect the link between the laser diode and the photodetectors through standard waveguide propagation as well as evanescent coupling.

One particular drawback of bidirectional links up to now, even those using silicon waferboard, has been compactness of the package. For example in the application to Boudreau, et al. above referenced, the separation of wavelengths via the diffractive properties of holograms requires a rather large distance to effect the separated beams.

Accordingly, what is needed is a passively aligned bidirectional link that isolates the transmit and receive wavelengths from one another, and achieves reliable optical signal transmission in compact physical dimensions.

Summary of the Invention

The present invention is a bidirectional link that features separate minimization of electrical and optical crosstalk. The link has passively aligned bidirectional optics and devices mounted on a silicon substrate having selectively etched grooves to effect the alignment of an optical fiber to the optical elements of the link, as well as to effect mounting of the optoelectronic devices in proper optical alignment.

The bidirectional link of a first embodiment uses a first wavelength for the transmission mode and a second wavelength for the reception mode to minimize optical crosstalk between the reception and transmission signals which would result in signal distortion. Accordingly, the use of wavelength dependent elements such as holograms and dichroic materials is required. To achieve this result in a relatively small volume of space, a holographic plate is disposed above the silicon waferboard having holograms and a dichroic element fabricated thereon. The holograms effect the wavelength selective directing of the transmit and receive modes to and from the respective elements as well as the

necessary focusing. The dichroic element provides wavelength dependent filtering for the detector.

Finally, this substrate is then mounted on a module that has the required electronic circuitry to achieve optical transception. The module has a novel cover made of preferably metal or having metal coatings to isolate the drive and receive electronics to minimize electrical cross-talk.

In a second embodiment, the submodule has a silicon or other suitable substrate which is selectively etched for placement of an optical fiber as well as the required elements of the optical link. To this end, an optically transparent plate is etched to hold a half- protruding fiber, and the plate and the fiber are selectively cut at a desired angle to reduce cross-talk and provide the beamsplitting function. The monitor detector and laser are passively aligned to a silicon waferboard substrate having an empty fiber v-groove .

The beamsplitter is then mounted to the Si waferboard by keying the half-protruding fiber from the plate to the empty v-groove on the substrate. The plate/fiber/beamsplitter is placed on top of the silicon substrate with the fiber placed in the groove etched in the substrate. The beamsplitter is placed such that the monitor detector, laser and receiver detector effect optical communication with the fiber via the beamsplitter.

Figure 1 is an exploded view of the module of the present invention showing the cover thereof.

Figure 2 is a view of the submodule of one embodiment of the present invention having the plate not shown.

Figure 3 is a top view of the submodule of the present invention having the plate disposed thereon with the hologram integrally formed thereon.

Figure 4 is an enlarged view of a portion of said submodule shown in Figure 3.

Figure 5 is a cross sectional view of the functioning elements of the submodule of Figure 3.

Figure 6 is an exploded view of the submodule of another embodiment of the present invention.

Figure 7 is a side view of the submodule of Figure 6.

Figure 8 is a top view of the submodule of the invention shown in Figure 7.

Figure 9 shows a cross sectional view of the photodetector of the present invention.

Figure 10 is a top view of the contact area of the photodetector of the present invention.

Figure 1 shows the overall bidirectional transceiver having a cover 104 member that separates the transmit and receive electronics from each other. The cover also provides a hermetic seal between the cover and the multi-chip module 100 by using solder, glass or ceramic as the sealing material. Finally, for purposes of discussion, the transceiver having the circuitry and the silicon chicklet is known as an hybrid integrated optical multi-chip module (MCM) 100. Mounted on the MCM is the silicon chicklet 101 having an optical fiber 102 mounted in a v-groove as well as the mounting structure 103 for the detector.

Turning to Figure 2, the silicon waferboard chicklet of the first embodiment is shown having certain elements missing to ease in explanation at this point. The chicklet 201 has a v-groove 202 for reception of the optical fiber 203, as well as passive alignment wells 204 etched by standard technique. To this end, assuming the substrate of the chicklet has a top surface 203 in the (100) plane, the etched surfaces of the v-groove are well defined in the (111) family of planes. It is the well defined and thus predictable orientation and readily determined dimensions of the etched surfaces that results in accurate passive alignment of the elements of the chicklet. A complete understanding of the etching of monocrystalline silicon can be found in U.S. Patent 4,210,923 to North, et al . Furthermore, reactive ion etching can be employed on an amorphous substrate to effect the etched surfaces. To this end, a

dielectric substrate is made of silica, and the crystallography of the silica is not utilized. Rather, a reactive ion etching process or a wet chemical etch is employed to create the grooves in the silica.

The laser 204 is disposed on top of the silicon substrate 201, and is located using alignment pedestals and standoffs (not shown) that are etched by techniques known to the artisan of ordinary skill. Examples of such standoffs and pedestals are disclosed in U.S.

Patent 5,182,782 to Tabasky et al . , the disclosure of which is specifically incorporated herein by reference. Finally, the microreflector 205 is disposed on the surface of the substrate and is made separately an then mounted, and is located by butting an end against to selectively placed pedestals that are etched to protrude from the surface of the substrate. This reflector or micromirror is etched silicon having a planar reflective surface in the (111) plane that is the same plane and is a continuation of the planar surface 206. The placement of the micromirror 205 is done so as to provide a large reflective surface for the laser. That is, the surface 206 and the mirror 205 give a large combined reflective surface.

Turning now to Figures 3 and 4, we see a top view of the chicklet having disposed thereon the holographic plate 303,403 mounted on the top surface of the substrate 300,400. The bidirectional link module is preferably fabricated from co-fired ceramic preferably

aluminum oxide, aluminum nitride or a glass material. The metal cover may also be Kovar which is then metal coated. The module is fabricated to receive the submodule of the present invention as shown as well as the required communications electronics for the link.

The cover serves to achieve electrical isolation between the receive and transmit circuitry as well as to provide a hermetic seal about the module. The hermiticity is effected by fastening the cover to the module with epoxy solder, glass or ceramics. This cover is made preferably of metal, however other materials that will serve to isolate electrical signals will work. The cover is necessary due to the small dimensions of the overall device that is achieved by virtue of the present invention. The holographic plate is made preferably of monocrystalline silicon, however other materials such as glass are possible. The true requirements of the plate is that it be transparent to light of the transmit and receive wavelengths as well as be readily adaptable to having holograms and dichroic elements integrally formed thereon. Furthermore, the photodetector is mounted on top of the holographic plate, and the passive alignment microspheres 405 are shown clearly in Figure 4.

Figures 3 and 4 are top views of the link in pertinent part to the transmission and reception function. For light being transmitted from the laser 401, 501 mounted on the top surface of the silicon substrate, the light emitted is reflected off the micromirror surface 402,502 and etched reflective

surface 405. This light is reflected off the hologram 306,406 located on the lower surface of the holographic plate. This elliptical hologram is chosen preferably to be elliptical to match in geometry and shape the cross section of the light emitted from the laser 301,401, and then focus the light in circular cross section in order to most efficiently coupled to the fiber. The light is then focused on the far surface of the plate at a dichroic element (not shown in Figures 3 and 4) disposed on the upper surface of the plate and beneath the detector hologram 307, 407. The dichroic element is fabricated to reflect one wavelength (transmit) but pass the other wavelength (receive) . The laser light is reflected from the dichroic element to the fiber hologram 308,408 disposed on the lower surface of the holographic plate. This hologram 308,408 focuses light on the reflective surface etched on the silicon substrate, which then directs the light to the fiber 309, 409 via the reflective surface 310,410.

In a receiving mode, light of a second wavelength different from that of the transmission wavelength is effectively coupled to a photodetector disposed on the top surface of the holographic plate. In the reception mode, light from the fiber 309,409 is reflected off the etched reflective surface 310, 410 at the end of the fiber v-groove, is directed upward to the fiber hologram 408,508 located on the lower surface of the holographic plate. This hologram focuses the light on the dichroic element located on the top surface of the holographic

plate. The light at the reception wavelength is of a chosen wavelength so that it is transmitted undeflected through the dichroic element to the photodiode. The photodetector hologram 307,407 on top of the holographic plate then focuses the light to a small spot in or on the signal receiving detector disposed on top of the holographic plate. The preferred transmit and receive wavelengths are 1.3 microns and 1.55 microns for telecommunications, however other wavelengths are possible. To effect the system the dichroic and the holograms are tailored for the chosen wavelengths.

The light ray paths of the transmit mode are as shown in side view in Figure 5. The dichroic element is shown at 501. This element and the holograms are fabricated as follows. The holograms of the present are formed on a substrate of silicon by techniques disclosed in U.S. Patent 5,420,943 above referenced and U.S. Patent Application No. 08/269,304. The dichroic element 501 is grown on the top surface of the holographic plate by deposition of alternating layers of dielectric, preferably silicon and silicon dioxide. Thereafter, a layer of material 502, preferably glass, is grown. This layer buries the dichroic and serves as the substrate for the fabrication of the detector hologram. To reiterate one of the advantages of the present invention, the dichroic element allows for the isolation of the wavelengths of the transmission and reception modes of the bidirectional link in a very compact area when compared with other methods of wavelength, for

example as disclosed in U.S. Patent Application No. 08/269,304 above referenced. This benefit when coupled with the precise focusing and directing capabilities of the holograms allows for a precise link with good isolation between the transmit and receive modes.

In a second embodiment, another type of chicklet is used. This embodiment uses a fiber beamsplitter to effect the bidirectional link. Turning to Figure 6, the bidirectional link is effected by the use of a beamsplitter fabricated out of the fiber which is further disposed in a glass plate 604, preferably Pyrex TM glass or other optically transparent materials which are expansion matched to Si. With the fiber captured between the glass plate 604 and the substrate 601, the bidirectional optical system is as follows. The photodetector 605 for the reception of light is disposed on the top surface of the glass plate 604. This photodetector can be a PIN detector or a metal semiconductor metal photodetector. The detector 605 functions as the receiver detector for the incoming signal . The source for signal transmission is a semiconductor laser 606. Finally, the monitor detector 607 functions to provide continuous monitoring of the laser in operation to assure constant power from the laser. To facilitate the coupling between the rear facet of the laser and the monitor detector, a relatively short v-groove or well is etched between the rear facet and the monitor to provide a good coupling therebetween. The fiber 602 has an integrally formed

lens 608 disposed thereon to provide a high coupling efficiency between the laser and the fiber. The lens 608 is fabricated as follows. The fiber is etched in a HF to reduce its diameter, and then it is flashed heated with an electric arc to form the curved surface of the lens. Finally, the beamsplitter 701 functions to direct light received from the fiber to the detector 605 while reducing cross talk between light from the laser 606 and the detector 605. To this end, incoming light from the fiber 602 is partially reflected by the beamsplitter to the detector 605, while light from the laser is partially transmitted straight down the fiber. Optical cross talk for the unwanted light directed down to the substrate 601 from the laser or light from the outside source received into the laser in the design of the present invention by proper selection of the angle of the beamsplitter. The beamsplitter has an angle different than 45 degrees, preferably approximately 50 degrees. By selection of an angle other than 45 degrees for the beamsplitter, light is not reflected to the detector 605 from the laser 606. Reduction of cross talk from incoming light impingent on the laser is by using the beamsplitter to reduce light intensity on the laser, and to operate the link at low speed where the signal noise generated from the incoming light is negligible. For high speed operations, a dichroic coating is applied to the fiber to effect separation of light between two different wavelengths where on wavelength is the transmit wavelength and one is the receive wavelength. An example is a transmit wavelength

of 1.55 microns and a receive wavelength of 1.3 microns. Details of the fabrication of a dichroic are discussed above.

The subassembly is fabricated as follows . The lensed fiber is mounted in a U shaped groove etched into the glass plate. The fiber is bonded to the plate by use of a suitable adhesive, preferably a UV curable adhesive. The fiber and the plate are then cut at an angle together by using a diamond followed by a polish to preserve the rotational alignment of the fiber in the glass plate. The face 609 is then coated with a partially reflective coating by placing the section 610 in a coating machine or dichroic coating if the bidirectional link is based on different transmit and receive wavelengths. Once coated, the two sections 610 and 611 are then reassembled and aligned to the laser mounted on the Si substrate by use of the v-groove etched in the Si substrate surface 603. The v-groove etched in the substrate as described provides a high degree of precision in the passive alignment of the two sections 610 and 611 of the beamsplitter. Finally, the tip of the fiber having the integral lens 608 can be located at the proper distance from the laser 606 using microscope inspection before the glass plate sections 610 and 611 are bonded to the substrate 601. After wirebonding of the chips of the subassembly is complete, the subassembly can be mounted in the bidirectional module.

In operation half duplex or full duplex communication can be effected by the bidirectional link

as follows. Light received from the fiber is partially reflected at the beamsplitter 701 to the detector 605, which is demodulated by the receiver electronics 104. Light that is transmitted from the laser 606 is modulated by the driver electronics 103 and is transmitted out through the fiber 602, with negligible coupling of the outgoing light to the detector 605 by the beamsplitter as explained above. The laser output is monitored by the monitor detector 607. In an alternative embodiment places a reflective surface on the reverse side or lower surface 612 of the substrate so outgoing light is reflected to a known location where a monitor detector is placed. Thereby a rear facet laser is not necessary. This alternative embodiment allows for monitoring of the front facet of the laser as well as the ability to couple more power out from the laser as the rear facet can be coated with a highly reflective coating.

Finally, another aspect of the present invention relates to a particular detector assembly using a reduced capacitance detector having elliptical cross section. To this end, by virtue of its design, the capacitance of the detector is reduced by matching as closely as possible the area and shape of the optical beam cross-section to the photosensitive region of the detector. In this case the detector has a substantially elliptical p-metallization with the attendant advantages in the bonding process as disclosed in the Wilson reference. In the particular configuration considered

here, the optical beam axis makes an angle of approximately 19.5 degrees with respect to the normal axis of the PIN chip surface.

The preferred embodiment having been described, it is understood that modifications of the bidirectional link having elements passively aligned by use of silicon waferboard as well as the integral formation of focusing and light directing elements on a silicon substrate are considered within the purview of the artisan of ordinary skill.