|Publication number||USRE40416 E1|
|Application number||US 10/729,582|
|Publication date||Jul 1, 2008|
|Filing date||Dec 6, 2003|
|Priority date||Jun 8, 1998|
|Also published as||US6328482, US6527455, US20020054737|
|Publication number||10729582, 729582, US RE40416 E1, US RE40416E1, US-E1-RE40416, USRE40416 E1, USRE40416E1|
|Original Assignee||Benjamin Jian|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (27), Non-Patent Citations (21), Referenced by (6), Classifications (14), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Priority is hereby claimed to U.S. Provisional Application No. 60/088,374, filed Jun. 8, 1998 entitled LOW COST OPTICAL FIBER TRANSMITTER AND RECEIVER and U.S. Provisional Application No. 60/098,932, filed Sep. 3, 1998 entitled LOW COST OPTICAL FIBER COMPONENTS.
1. Field of the Invention
The present invention generally relates to couplers for coupling optical radiation into and out of an optical fiber.
2. Description of Related Art
Optical fibers have by far the greatest transmission bandwidth of any conventional transmission medium, and therefore optical fibers provide an excellent transmission medium. An optical fiber is a thin filament of drawn or extruded glass or plastic having a central core and a surrounding cladding of lower index material to promote internal reflection. Optical radiation (i.e. light) is coupled (i.e. launched) into the end face of an optical fiber by focusing the light onto the core. For effective coupling, light must be directed within a cone of acceptance angle and inside the core of an optical fiber; however, any light incident upon the surrounding cladding or outside of the acceptance angle will not be effectively coupled into the optical fiber.
It is a difficult task to couple light into the central core of an optical fiber due to its small size and acceptance angle, particularly if the optical fiber is a single mode optical fiber. A typical single mode fiber has a core diameter of only 10 microns and an acceptance angle of only 10°. Single-mode fibers, which are designed to transmit only single-mode optical radiation, are widely utilized for telecommunications applications. Multimode optical fibers have a larger cross-section and a larger acceptance angle than single-mode fibers. For example, a typical multimode fiber has a core diameter of 50 microns and an acceptance angle of 23°. Because any optical radiation outside the core or acceptance angle will not be effectively coupled into the optical fiber, it is important to precisely align the core with an external source of optical radiation.
One conventional practice for making a fiber-pigtailed transmitter is to assemble an edge-emitting laser diode, an electronics circuit, a focusing lens, and a length of optical fiber and then manually align each individual transmitter. To align the transmitter, the diode is turned on and the optical fiber is manually adjusted until the coupled light inside the fiber reaches a predetermined level. Then, the optical fiber is permanently affixed by procedures such as UV-setting epoxy or laser welding. This manual assembly procedure is time consuming, labor intensive, and expensive. Up to 80% of the manufacturing cost of a fiber-pigtailed module can be due to the fiber alignment step. The high cost of aligning optical fiber presents a large technological barrier to cost reduction and widespread deployment of optical fiber modules.
One single-mode fiber has a cylindrical glass core of about 10 microns in diameter surrounded by a glass cladding with a circular outer diameter of about 125 microns. In some connections, slight variations in dimensions can drastically affect coupling efficiency, and therefore some optical fiber manufacturers carefully control the fiber's tolerances. For example, in a splice connection between two optical fibers, a large loss in the transmitted signal can occur if the two inner cores fail to align precisely with each other. For example, if the cores of two 10 micron single-mode fibers are offset by only 1 micron, the loss of transmitted power through a splice is about 5%. Therefore, to reduce coupling losses, manufacturers maintain cladding diameter tolerances within the micron to sub-micron range. For example, Corning Inc. specifies the tolerance of its optical fibers as 125±1 micron.
In order to provide passive alignment of optical fibers, various alignment techniques have been reported based on precisely etched holes on a wafer. For example, in Matsuda et al. “A Surface-Emitting Laser Array with Backside Guiding Holes for Passive Alignment to Parallel Optical Fibers”, IEEE Photonics Technology Letters, Vol. 8 No. 4, (1996) pp. 494-495, a research group at Matsushita in Japan performed an experiment in which a shallow guiding hole on the backside of a back-emitting vertical cavity surface emitting laser (VCSEL) wafer is etched to a depth of 10 to 15 microns and a diameter of 130 microns. A multi-mode fiber stem 125 microns in diameter is inserted into the guiding hole with a drop of epoxy for passive alignment to the VCSEL. This group reported an average 35% coupling efficiency at 980 nanometers. The large core diameter of multi-mode fibers (e.g. 50 microns) allows this approach to be suitable for coupling light into multi-mode fibers; however the lack of a light-focusing mechanism prevents use of this method with single-mode fibers.
U.S. Pat. No. 5,346,583 to Basavanhally discloses a substrate having at least one lens formed on a first surface. An optical fiber guide is etched on a second surface of the same substrate, opposite the first surface. The optical fiber guide is used to mount an optical fiber on the second surface such that the central axis of the optical fiber is substantially coincident with the central axis of the lens, thereby giving the desired alignment. Fused silica and silicon are two common substrate materials. If the substrate material is fused silica (or glass), the fiber guide etch rate is very slow (typically 0.3 micron per minute or less) and as a result it is impossible to obtain fiber guides of sufficient etch depth, which is necessary to obtain precise angular alignment to single mode fibers. According to the method described in the patent, etching is to stop before it reaches the final surface where the lens resides. At the bottom of the etched fiber guide, the surface is typically neither smooth nor flat, which could causescattering and reflection loss if the refractive index of the substrate material is different than that of the optical fiber core (approximately 1.5).
U.S. Pat. No. 5,195,150 to Stegmueller et al. discloses an optoelectronic device that includes a substrate that has a recess for receiving a plano-convex lens and a recess on the other surface of the substrate aligned with the lens to receive an end of an optical fiber. The device disclosed by Stegmueller is subject to the same problems as the device disclosed in the Basavanhally patent.
In order to overcome the limitations of prior art optical fiber couplers, the present invention provides a multilayer optical fiber coupler for coupling optical radiation between an optical device and an optical fiber, including a first layer that has a fiber socket formed by photolithographic masking and etching to extend through said first layer, and a second layer bonded to the first layer. A multilayer optical fiber coupler is described that has a vertical through hole (a “fiber socket”) in a first layer that precisely aligns an optical fiber with an optical focusing element formed in the second layer. A method for forming the fiber couplers is described herein that can advantageously utilize semiconductor processing techniques including photolithography and dry etching to fabricate the couplers. The precision of the fiber socket structure allows single mode optical fibers to be passively aligned, and is also useful for aligning multimode optical fibers.
In one embodiment, a first layer, typically comprising substantially single-crystal silicon, is deep-etched using a suitable process such as silicon Deep Reactive Ion Etching (DRIE) to form an array of fiber sockets that extend through the first layer. A second layer is formed to provide a corresponding array of optical focusing elements. The first and second layers are aligned using alignment fiducials and permanently bonded together, so that the fiber socket in the first layer precisely aligns the core of the optical fiber with the optical focusing element in the second layer. The bonded structure is then diced to form a plurality of separate couplers or arrays of couplers. An optical fiber is affixed into each fiber socket by any suitable means, such as an optical epoxy.
In order to provide precise, passive alignment of the optical fiber within the fiber socket, the fiber socket is formed to be only slightly larger than the fiber diameter. Single-crystal silicon is particularly useful to form the fiber sockets because silicon DRIE techniques have been developed recently as a result of advances in microelectromechanical system (MEMS) research, which allow vertical holes to be etched at high speeds (up to 10 micron/minute at present) with less than 1 micron vertical variation in hole diameter (i.e. ±0.5 micron). In one embodiment, the deep-etching process uses high definition photolithography and an appropriate high etch selectivity mask to create precisely-dimensioned fiber sockets. These fiber sockets then receive precisely-dimensioned optical fibers, thereby accurately aligning the optical fibers within the fiber socket. The fibers are held in place by epoxy or another suitable adhesive.
In one embodiment the second layer comprises borosilicate glass such as PYREX, which is advantageous for several reasons. The glass can be strongly and conveniently bonded to silicon by anodic bonding, which is a dry bonding process. The thermal expansion coefficient of borosilicate glass matches well with that of silicon, which provides a durable and reliable structure. Furthermore, the index of refraction of borosilicate glass approximately matches the index of refraction of the core of the optical fiber, which is the light transmitting section of the fiber, and therefore an optical epoxy can be used that also approximately matches the index of refraction of the optical fiber. In such an embodiment, the glass, epoxy, and optical fiber form a natural index-matched system, eliminating the need for polishing and anti-reflection coating the end face of the optical fiber which are current fiber optical industry practices, and resulting in further cost savings. Due to the index matching in some embodiments, optical radiation advantageously propagates substantially loss-free through the fiber end face, epoxy, and the adjacent surface of the second layer.
Due to the fiber sockets formed to extend through the silicon layer, a large number of single mode optical fiber couplers can be made on the wafer level with very low cost. One cost advantage is attributed to the batch microfabrication process and the elimination of the need to actively align the fiber. For example, assuming a 4-inch integrated wafer and a 1 mm×1 mm die size, about 7800 fully-integrated chips can be obtained by dicing the wafer. This approach allows optical couplers as well as other devices disclosed herein to be manufactured with the same kind of economies of scale as the silicon electronics industry, since the cost of the processing steps are shared by all the individual chips.
The optical fiber couplers are rugged and compact, and can be used in a variety of applications. The fiber couplers can be implemented in a wide variety of embodiments; for example the optical couplers may be incorporated with other devices such as VCSELs.
For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, wherein:
This invention is described in the following description with reference to the Figures, in which like numbers represent the same or similar elements.
As discussed in the background section, some single-mode fibers are constructed with very close tolerances. The highly precise diameter of the optical fiber is useful when a precision etched hole is designed to match it, as described herein.
The optical fiber 100 includes a core 160 and a cladding 162, and in one embodiment the optical fiber is a single mode fiber. The optical fiber has an approximately flat end face 163 adjacent to the second layer. A core section 164 on the end face 163 is approximately aligned with the optical focusing element; for example, in one embodiment the core is approximately aligned with the focal point 152 of the microlens. The optical fiber may be a single mode fiber, which has a small core relative to multimode fibers. It may be noticed that the epoxy 110 is deposited throughout the fiber socket, and fills in the gaps between the end face 163 and adjacent opposing surface 141 of the second layer. In one embodiment the epoxy has an index of refraction that approximately matches the optical fiber, and therefore the end face 163 is not required to be flat, nor is it required to be polished or coated.
An optical device 170 is arranged with respect to the optical focusing element 150 and the optical fiber 100 to provide the desired optical coupling with the core of the optical fiber. The focusing power of the optical element 150 varies dependent upon the utilization of the coupler, the optical device, and the thickness of the second layer; for example some optical devices will require collimation, other optical devices require focusing, others will require no significant focusing power. The optical device 170 can be a source or receiver of optical radiation. An example of a laser source is a laser diode emitter such as a VCSEL (vertical cavity surface emitting laser), and an example of a receiver is a photodetector. If the optical device is a laser source, the optical device 170 is arranged so that optical radiation emitted by it will be coupled into the optical fiber, or conversely if the optical device 170 is a receiver, it is arranged so that optical radiation emitted from the optical fiber will be received. In some embodiments such as shown in
In the embodiment of
Reference is now made to
In step 210, the first layer 130, comprising a silicon wafer, is processed by a dry etching process to create an array of fiber sockets 120 that extend completely through the silicon wafer.
The silicon wafer 130 has a crystalline structure and thickness suitable for the deep etching process that forms the sockets; in one embodiment the silicon crystalline structure is single-crystal although other embodiments may comprise polycrystalline structures. In one embodiment the silicon wafer has a uniform thickness of about 0.4 mm which is sufficient to provide structural support for the optical fiber and within the limits of current deep-etch technology. In other embodiments the thickness of the silicon wafer could range between 0.1 mm and 3.0 mm, for example. Currently available silicon wafers typically have a thin disk configuration that varies from 2 to 8 in diameter. Preferably the silicon wafer is double-polished; i.e. it is polished on each side.
The first layer is etched using any suitable deep-etching process to create an array of fiber sockets 120 at predetermined locations. A suitable deep etching process for silicon is disclosed in U.S. Pat. No. 5,501,893 to Laermer, for example. Commercial etchers are available from vendors such as Plasma-Therm in St. Petersburg, Fla. Suitable etch masks include photoresist and silicon dioxide, for example. A photoresist mask gives about 80-to-1 etch selectivity, and an etch rate of about 2 micron per minute with smaller mask undercut. An oxide mask gives a 150-to-1 etch selectivity with higher etch rate and a greater mask undercut. A photoresist thickness of about 6-7 microns provides through wafer etching of a silicon wafer with a thickness of 400 microns.
A high selectivity etch mask is used to etch gaps 310 (
Next, as shown in
In step 220, a second layer is formed to create an array of optical focusing elements on the outer surface 142. The array configuration in the second layer corresponds with the configuration of the fiber socket array in the first layer, such that each optical element will be precisely registered with a fiber socket when the first and second layers are properly aligned with each other. For example,
The second layer comprises any suitable material, such as fused silica, silicon, or an optical glass such as borosilicate glass. The material of the second layer is selected to be substantially transmissive at the wavelengths of interest. In order to minimize unwanted reflection, in some embodiments the second layer has an index of refraction approximately equal to the optical fiber, i.e. approximately 1.5. In other embodiments in which the index of the second layer does not approximately match the optical fiber, an anti-reflection coating may be formed on the opposing surface of the second layer to reduce optical losses, such as disclosed with reference to FIG. 5. In such cases optical loss at the interface with the second layer is almost completely eliminated. In other embodiments, the opposing surface of the second layer may be coated with another type of coating, such as a beam splitter coating.
Alignment fiducials, such as crosses 425 shown in
In one embodiment the second layer comprises borosilicate glass having a thickness of about 300-400 microns that is etched to provide a refractive microlens array. In other embodiments the optical focusing elements may comprise a diffractive microlens array etched onto the surface of the second layer. In still other embodiments the optical focusing elements comprise gradient-index microlenses that are formed by diffusing ions that vary the index of refraction in a defined manner.
As illustrated in
In step 230, the first and second layers are aligned using the alignment fiducials formed thereon, shown at 415 and 425 in
Examples of bonding methods include anodic bonding, epoxy bonding, metal bonding, glass-frit bonding, wafer direct bonding, and polyimide bonding. If epoxy bonding is utilized, then it may be useful to deposit a thin layer of epoxy, let it begin curing, and then bond the two layers, which would reduce unwanted upwelling of epoxy into the fiber sockets.
In embodiments in which the second layer is glass, anodic bonding is a useful technology for bonding the silicon layer to the glass layer. Many manufacturers use anodic bonding, for example in the manufacturing of the ink-jet printer nozzle. In one embodiment borosilicate glass and silicon are stacked together and heated to 180-500° C. while a voltage of 200-1000 Volts is applied between the two plates for about 10 minutes. The thermal expansion coefficients of the silicon and borosilicate are approximately matched. Borosilicate is highly transparent from 500 nm to over 2000 nm, so it can be used for all the important telecommunication wavelength bands (850, 1300, and 1550 nm). The bonding strength of an anodic bond is so high that for most practical purposes the bonded wafer can be considered as a single wafer.
FIG. 31 3I shows the first and second layer bonded together. In addition, FIG. 31 3I shows the AR coating 154 formed on the upper surface of the second layer 140 at the air interface.
In step 240, the composite wafer that includes the bonded first and second layers is diced into a plurality of separate chips, each comprising one or more optical fiber couplers. In one process, the composite wafer is attached to a wafer carrier and diced through by a diamond saw. In some processes, it may be useful to cut partially through the composite wafer, leaving a narrow section that can be easily broken apart. For example, it may be advantageous to cut through about 90% to 95% of the thickness of the composite wafer, then insert the optical fibers into the fiber sockets, and then break them into individual chips.
In step 250, an optical fiber is provided that has an end face formed therein. In some embodiments it may be useful to polish the end face; however in embodiments in which the index of refraction of the epoxy matches that of the fiber core, polishing is unnecessary.
A suitable adhesive is applied to the end of an optical fiber and/or into a fiber socket. In one embodiment an index-matching epoxy such as Epotech 301, 302, or 353ND, available from Epoxy Technologies, Inc. of Billerica, Mass. is used in order to approximately match the index of the optical fiber and the second layer. The epoxy is selected to be substantially transparent at the intended wavelength.
In step 260, the end sections (fiber tips) of the optical fibers are inserted into the fiber sockets in any suitable manner. In one process, the optical fibers are inserted individually by hand, using a stereo microscope to aid in positioning. It has been observed in some embodiments that the optical fibers can be easily inserted into the fiber sockets with insertion rates of above one fiber per minute. However, if difficulties arise in insertion, a number of solutions are possible. For example, the fiber socket can be made slightly larger in diameter. Grooves can be created on the walls of the fiber socket to allow the epoxy to flow. Also, the cladding on the tip of the fiber can be made to a rounded shape to facilitate insertion, since only the fiber core is important for optical coupling.
Using the method described herein, optical fiber couplers can be implemented in many different embodiments.
In this embodiment, due to the difference in refractive indexes between the second layer and the optical fiber, it is useful to coat the inner surface 141 of the second layer 140 with an AR coating 510 before bonding it to the first layer 130, in order to substantially reduce optical loss due to reflection at the inner surface 141.
Until the present invention, alignment of optical devices with optical fibers and particularly single mode fibers, has been a difficult task. Using the techniques set forth herein to simplify alignment and reduce its cost, many different types of devices can be integrated with the optical coupler on the wafer level at significantly reduced per-unit cost. In addition, integrating an optical device with the optical coupler can provide the advantages of ruggedness and compactness. One particular example to be described is an integrated VCSEL transmitter. In other embodiments, other optical device could be utilized; for example the VCSEL could be replaced with a photodetector to provide an integrated receiver.
Reference is now made to
The embodiment of
In one embodiment the layer 130 is bonded to layer 140 using anodic bonding, and the second layer 140 is bonded to VCSEL layer 603 using optical epoxy 650. The large index difference between a typical VCSEL wafer (refractive index about 3.6) and an optical epoxy (refractive index about 1.5) ensures that the microlens functions properly although the microlens space is filled with an optical epoxy 650 whose index matches that of the glass layer 140. One advantage of this design is that the electrical contacts 630 and 640 are exposed, thereby allowing easy electrical signal connection.
Any reflection from the microlens or any other surface in the optical path back to the VCSEL 600 can be a problem, since such reflection could stop the VCSEL from lasing. Therefore it is useful to form a high quality AR coating 612 with 0.1% residual reflectivity on the microlens surface.
In one embodiment the thickness of the integrated chip shown in
Thermal expansion mismatch among the three layers can be reduced by the choice of borosilicate glass, and by the epoxy bonding process, which can be done at room temperature.
Reference is now made to
This structure will now be compared with that disclosed in Matsuda et al. “A Surface-Emitting Laser Array with Backside Guiding Holes for Passive Alignment to Parallel Optical Fibers”, IEEE Photonics Technology Letters, Vol. 8 No. 4, (1996) pp. 494-495. Matsuda discloses a shallow hole etched on the back of a back-emitting VCSEL wafer. The shallow hole is coated with an anti-reflection coating before a multi-mode fiber is inserted and affixed using optical epoxy. An average of 35% coupling efficiency is achieved in the prior art. According to Matsuda, the main reason for the high optical loss is attributed to the rough surface on the bottom of the shallow hole despite the anti-reflection coating. Matsuda concluded by saying that by improving the surface quality of the bottom, coupling efficiency near unity can be achieved. Compared to the prior art, the bottom of the fiber socket is supported by the AR coated back surface 708 of the VCSEL wafer which should be optically smooth by suitable polishing before wafer bonding. Therefore, it is believed that nearly 100% coupling efficiency can be obtained for the embodiment shown in FIG. 7.
Bonding the VCSEL wafer 703 to the fiber socket wafer 130 may be accomplished using epoxy bonding or metal bonding. The fiber socket structure described herein provides a much stronger support to the fiber than the shallow hole disclosed by Matsuda as discussed above, and it is believed that this support will significantly improve the reliability of the device.
One advantage of the top contact, bottom-emitting VCSEL embodiment shown in
In the embodiment of
The thickness of the integrated chip is about 500 μm assuming thicknesses of 400 micron and 100 micron for the silicon and VCSEL wafers, respectively. The size of each chip can be about 1 mm or smaller.
It is advantageous for the wavelength of the VCSEL to be matched with other optical devices in the system. For example, silicon detectors are common, low-cost photodetectors. However, the lasing wavelength of an InGaAs VCSEL is typically 950-980 nanometers, which is beyond the detection range of low-cost silicon detectors. Currently, 850-nanometer VCSELs are available in GaAs, which can be used with silicon detector; however such VCSELs are available only in a top-emitting configuration. To integrate such a top-emitting VCSEL with the fiber socket wafer, the VCSEL laser must be situated on the VCSEL wafer surface adjacent to the fiber socket wafer 130. In such a case, the electrical contact pads are sandwiched between the VCSEL wafer and fiber socket wafer. In order to provide electrical connections to the accessible, outward-facing surfaces of such top-emitting VCSEL, through wafer via holes filled with metal can be formed in the VCSEL wafer to connect the contact pads to the outer surface, using the teachings disclosed in “Future Manufacturing Techniques for Stacked MCM Interconnections” by Carson et al., Journal of Metal, June 1994, pages 51-55, for example.
It will be appreciated by those skilled in the art, in view of these teachings, that alternative embodiments may be implemented without deviating from the spirit or scope of the invention. This invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.
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|U.S. Classification||385/88, 385/33, 385/89, 385/34, 385/93|
|International Classification||G02B6/42, G02B6/00, G02B6/36|
|Cooperative Classification||G02B6/4239, G02B6/423, G02B6/4204, G02B6/4206, G02B6/4224|
|May 12, 2009||FPAY||Fee payment|
Year of fee payment: 8
|Feb 21, 2013||FPAY||Fee payment|
Year of fee payment: 12