Channeled substrates for integrated optical devices employing optical fibers

A channeled substrate for forming integrated optical devices that employ optical fibers and at least one active optical component is disclosed. The channeled substrate includes a substrate member having an upper surface one or more grooves formed therein, and a transparent sheet. The transparent sheet, which is preferably made of thin glass, is fixed to the substrate member upper surface to define, in combination with the one or more grooves, one or more channels. The channels are each sized to accommodate an optical fiber to allow for optical communication through the transparent sheet between the active optical component and the optical fibers. Channeled substrates formed by molding and by drawing are also presented. Integrated optical devices that employ the channeled substrate are also disclosed.

FIELD

The present disclosure relates to integrated optical devices, and in particular to substrates used to form integrated optical devices that employ optical fibers.

DESCRIPTION OF RELATED ART

Certain types of integrated optical devices combine active optical components, active electrical components, and passive waveguides in the form of optical fibers. Examples of such integrated optical devices include optical transceivers and active cable assemblies (ACAs).

While active alignment of the optical fibers to the active optical components ensures optimum performance of the integrated optical device, it is preferred that the alignment be passive to reduce cost and complexity. Further, it is preferred that standard packaging techniques known in the art be used to form the integrated optical device.

Practical integrated optical devices are fabricated using standard packaging techniques to minimize cost. In general, active optical components may be attached active side up on the substrate and electrically interconnected with wirebonds. In this case, their optical paths are, in certain embodiments, directed upward away from the substrate. In another approach, the active optical components are flip-chip mounted on the substrate so that their optical paths are oriented downward into the substrate or upward through a transparent medium such as a glass window.

Of these two options, the flip-chip mounting approach has several advantages, such as the interface between the active optical components and the optical fibers being protected from the surrounding environment. Also, the optical interface is mechanically stabilized by the carrier substrate, and the electrical links to other devices are short, enabling high-frequency operation. However, a major challenge with the flip-chip mounting approach involves the need to passively align optical fibers beneath the active optical components, and to provide low-loss coupling to the optical fibers.

SUMMARY

An aspect of the disclosure is a channeled substrate for forming integrated optical devices that employ one or more optical fibers and at least one active optical component. The channeled substrate includes a substrate member having an upper surface with one or more grooves formed therein. The substrate also includes a transparent sheet fixed to the substrate member upper surface and that defines, in combination with the one or more grooves, one or more channels each sized to accommodate one of the one or more optical fibers to allow for optical communication through the transparent sheet between the active optical component and the one or more optical fibers.

Another aspect of the disclosure is a channeled substrate for forming integrated optical devices that employ one or more optical fibers and an active optical component. The channeled substrate includes a substrate member having a planar upper surface. The channeled substrate also includes a transparent sheet having one or more grooves formed therein and fixed to the substrate upper surface to define, in combination with the planar substrate upper surface, one or more channels each sized to accommodate one of the one or more optical fibers to allow for optical communication through the transparent sheet between the active optical component and the one or more optical fibers.

Another aspect of the disclosure is a method of forming a channeled substrate for forming integrated optical devices that employ one or more optical fibers and an active optical component. The method includes providing a substrate member having an upper surface, and providing a transparent sheet having opposing surfaces. The method also includes forming one or more grooves in one of the transparent sheet surfaces. The method further includes interfacing the grooved transparent sheet surface with the substrate member upper surface to define one or more channels each sized to accommodate one of the one or more optical fibers to allow for optical communication through the transparent sheet between the active optical component and the one or more optical fibers.

Another aspect of the disclosure is a method of forming a channeled substrate for integrated optical devices that employ optical fibers and an active optical component. The method includes forming a cylindrical glass preform having a substantially rectangular-shaped cross-section and a plurality of channels formed therein. The method also includes drawing the preform to form a cylindrical rod portion smaller than the preform and having substantially the same relative dimensions as the preform, wherein the rod portion channels are sized to accommodate the optical fibers. The method further includes cutting a section of the rod portion to obtain the channeled substrate.

These and other advantages of the disclosure will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.

DETAILED DESCRIPTION

Reference is now made in detail to the present preferred embodiments of the disclosure, exemplary embodiments of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts.

Examples of the present disclosure are directed to various substrates, and methods of using the substrates to passively align arrays of optical fibers with active optical components in integrated optical devices such as low-cost optoelectronic transceivers and ACAs. Example integrated optical devices that employ the substrates of the present disclosure are also described.

The active optical components may be in the form of light sources and include, for example, commercially available arrayed semiconductor laser sources such as edge-emitting light sources (e.g., Fabry-Perot, distributed feedback, ring lasers, etc.) or surface-emitting light sources such as vertical-cavity surface-emitting lasers (VCSELs). Active optical components that emit light are referred to below as “transmitter active optical components.” Active optical components may also be in the form of active optical modulators and include, for example, spatial light modulators, optical phase modulators, electro-absorption modulators, injection-locked optical modulators, optical transistors, free carrier absorption modulators, liquid crystal modulators and semiconductor optical amplifiers, and are generally referred to below as “optical modulators.” Active optical components may also be in the form of detectors or receivers and include, for example, detector arrays such as PIN photodiodes, and are generally referred to below as “receiver active optical components.”

In the discussion below, “wafer scale” means a size sufficient to make multiple members, devices or assemblies on a single structure (such as a semiconductor wafer), which is then diced to recover the individual members, devices or assemblies.

FIG. 1is a perspective view of an example substrate member10having an upper surface12, a lower surface14, and front and back ends16and18. Cartesian coordinates are shown for the sake of reference, with the X- and Y-axes being in the plane of substrate member10and the Z-axis being out of the plane. Substrate member10may be made of at least one of number of materials, such as silicon, InP and its alloys, GaAs and its alloys, GaN and its alloys, GaP and its alloys, quartz, sapphire, transparent conductive oxides including oxides of zinc, tin and indium, glass, ceramic, plastic, metal and any other dimensionally stable material that is suitable for use in optoelectronic and integrated optical device applications.

Substrate member10includes one or more grooves30formed in upper surface12. Grooves30have a bottom31and sidewalls32. In an example embodiment, grooves30run from front end16to back end18, as shown. Grooves30are intended to serve as guides for optical fibers and so are formed to have a width and depth that accommodates (e.g., are slightly larger than) the diameter of an optical fiber. Grooves30may be formed by a variety of processes, including precision sawing, isotropic or anisotropic etching (e.g., reactive ion etching, chemical etching or photo-chemical etching) or molding processes. The cross-sectional profile of grooves30(or grooves30′ discussed below) may be rectangular-shaped, V-shaped, or U-shaped, or some other profile formed by a combination of these and/or other profiles. Besides extending along the entire length of substrate member10as shown, grooves30may also terminate at some location within the substrate member, as discussed below.

After substrate groove fabrication is completed, then with reference toFIG. 2andFIG. 3, a thin transparent sheet50with an upper surface52, a lower surface54, front and back ends56and58and a thickness T50is disposed on grooved top surface12of substrate member10. In an example embodiment, transparent sheet50is made of glass. Transparent sheet50is fixed to top surface12using one of a variety of techniques, such as fixing material62, e.g., a bonding agent, an adhesive or an epoxy. For example, fixing material62may be applied to selected regions of transparent substrate lower surface54using screen printing processes, or to portions of substrate member upper surface12prior to placing transparent sheet50atop the substrate member upper surface. If substrate member10is made of silicon, then in an example embodiment anodic bonding is used to fix transparent sheet50to substrate member upper surface12.

In the case where substrate member10has a coefficient of thermal expansion (CTE) closely matched to that of transparent sheet50, and the substrate can sustain high temperatures (e.g., >850° C.), then in another example, the transparent sheet is fusion bonded to the substrate. Further in this example, the fusion bond is carried out under additional downward pressure applied to transparent sheet50. In another example embodiment, transparent sheet50is laser fused to substrate member10at selected locations.

An exemplary material for transparent substrate50is glass because it is optically transparent over a wide range of wavelengths, can support conductive contacts as well as transparent conductive oxides, is compatible with packaging and assembly processes, and can be fabricated in thin sheets at low cost and with controlled thicknesses. Further, glass is a low-CTE material that can be compositionally engineered to match the CTE of silicon or III-V optoelectronic semiconductor materials used in integrated optical devices, such as GasAs, InP, GaN, GaP and their alloys. In addition, glass has a refractive index that can be made relatively high to limit the diffraction of beam propagation within the glass. This extends the allowable spacing between active optical components and the optical fiber array, as described below. The refractive index can also be made relatively low to reduce back reflections at glass optical interfaces. Alternative suitable materials for transparent sheet50include sapphire and quartz for their various optical, thermal, chemical, and mechanical properties.

An example range of thickness T50for transparent sheet50is between about 75 μm and about 125 μm for low-loss coupling of light of wavelength λ=1.55 μm into multimode optical fibers. In an example embodiment, transparent sheet50includes at least one anti-reflection coating (not shown) on one or both of upper and lower surfaces52and54. The at least one anti-reflection coating may be applied either before or after transparent sheet50is joined to substrate member upper surface12.

Transparent sheet50and substrate member upper surface12form an assembly where grooves30and the transparent sheet define channels66, and may be treated as a single unitary channeled substrate through subsequent processing. The combination of transparent sheet50and grooved substrate10is therefore referred to hereinbelow as “channeled substrate”70. Grooved substrate member10serves as a robust mechanical support for transparent sheet50, which in some cases is too flexible and/or fragile to withstand subsequent processing steps on its own.

With reference toFIG. 4, conductive contacts80are added to upper surface52of transparent sheet50. Conductive contacts80are applied, for example, using screen printing, electrochemical plating, photochemical etch processing or other known processes. During the contact formation process, conductive contacts80are aligned to substrate grooves30so that the subsequently mounted active optical components are aligned with the centers of grooves30.

Conductive contacts80provide electrical contact to the active components. One exemplary type of conductive contacts80have a patterned configuration while another exemplary conductive contact is or includes contact pads. The conductive contacts80ofFIG. 4are shown as having two wider pad sections connected by a relatively narrow wire section. In one example embodiment, conductive pads80are made of a metal such as gold, chrome, tin, titanium, silver, and indium either alone or in combination. In another example embodiment useful for when it is desired to propagate optical beams through conductive contacts80, the conductive contacts are formed from a transparent conductive material, such as oxides of zinc, oxides of indium and oxides of tin. In some embodiments, substrate member10is made of a thick film or sheet of transparent conductive material, in which case both the substrate member and conductive contacts80are combined as one. This configuration is useful, for example, when it is desired to terminate all the common leads of active components to the same electrical ground plane.

With reference now toFIG. 5and toFIG. 6, solder balls82are applied to select conductive contacts80. Active optical components100(e.g., light sources such as VCSELs, photodetectors such as broad-area array detectors, optical modulators such as free carrier optical modulators, etc.) and active electronic components102(e.g., driver chips, receiver chips, etc.) are then flip-chip mounted onto solder balls82. WhileFIG. 5andFIG. 6show one active optical component100mounted over the array of channels66, in general one or more active optical components may be arranged on the same channeled substrate70. In one example embodiment, a transmitter active optical component100in the form of a VCSEL array is positioned over one set of channels66, while a receiver active optical component in the form of a detector array is positioned over a different set of channels.

In the case where active component100is in the form of an optical modulator, and in particular is of the kind that are themselves optically transmissive rather than optically reflective in nature, it is preferable to provide for a substantially transparent path for the light to continue through the active optical component where further use can be made of that portion of the optical beam. Further in an example embodiment, an active electronic component102in the form of an electronic driver or receiver chip is also operably arranged on channeled substrate70.FIG. 6shows channeled substrate70after flip-chip attachment of an active optical component100and an active electronic component102. The configuration ofFIG. 6is also referred to as an “integrated optical device”202.

Note that active optical components100include devices such as edge emitting lasers, planar waveguides and fiber arrays that emit or receive light in a direction parallel to the plane of channeled substrate70. For such active optical components100, an optical right-angle-bend structure or element (not shown) is provided in or adjacent these devices. For example, the edge of a planar lightwave circuit (PLC) may be beveled at an angle at or near 45° to direct light nominally perpendicular to the PLC substrate. Alternatively, a separate right-angle mirror structure may be either be added adjacent the active optical component or formed in some portion of the transparent layer via sawing or other surface profiling method.

Active optical components100may also be oriented perpendicular to channeled substrate70(specifically, perpendicular to upper surface52of transparent sheet50) so that light follows a perpendicular emission path through the transparent sheet in a manner similar to a VCSEL light source.

With reference now toFIG. 7, in the next processing step, integrated optical device202ofFIG. 6is flipped over and attached to a larger substrate120, such as a printed circuit board (PCB) (hereinafter, “PCB substrate”120). PCB substrate120has an upper surface122, which optionally has a hole or recess126formed therein that prevents mechanical interference with the flip-chip mounted active optical and electrical components100and102on channeled substrate70when channeled substrate is mounted in or on the PCB substrate. PCB substrate upper surface122includes conductive contacts80and solder balls82around the perimeter hole or recess126that provide the various electrical connections required between the PCB substrate and integrated optical device202. In an example embodiment, the attachment process may also involve flip-chip mounting of auxiliary or support chips150on PCB substrate120. The result is a PCB assembly204as shown inFIG. 8.

With reference now toFIG. 9, an optical fiber array220having a jacketed portion221and constituted by optical fibers222each having an outer surface223, a core224, and a cleaved (e.g., laser-cleaved) fiber end226, is provided and is inserted into channels66in channeled substrate70. In an example embodiment, optical fiber array220constitutes an optical fiber cable such a ribbon fiber cable. In an example embodiment, fiber ends226are angled and thus have an angled end face228, wherein the end-face angle may be formed at or near 45°, or at other angles relative to optical fiber axis A1that provide improved optical performance (e.g., reduced back reflection, increased bandwidth in multimode fibers, etc.). The pointed shapes of angled fiber ends226facilitate insertion of fibers222into channels66. In an example embodiment, the exposed (open) ends of channels66are flared to further facilitate the insertion and alignment of fiber array220within channels66.

FIG. 10shows PCB assembly204after fiber array220is incorporated therein by being inserted into channels66. Fiber array220is inserted into channels66until angled fiber ends226are aligned to active optical components100. For a transmitter active optical component100in the form of a VCSEL or other light source, in an exemplary method active optical feedback is employed to align fiber array220. This alignment method involves activating active optical component100and adjusting the position of optical fiber array220within channels66until power is maximized at the remote end of the optical fiber array. For a receiver active optical component100, light is launched into the remote end of optical fiber array220, and the receiver power is actively monitored as the position of the fiber array is adjusted within channels66. In another example embodiment, a vision system (not shown) configured to recognize components or fiducials as viewed through channeled substrate70allows for aligning optical fiber array220with active optical component100without the added difficulty of having to power up the active optical component.

Back reflections and etalon effects may occur at the interface between optical fiber outer surface223and sidewalls32and/or transparent sheet lower surface54that defines channels66. In an example embodiment, such effects are reduced using anti-reflection coatings on at least one of optical fiber outer surface223, channel walls32and transparent sheet lower surface54. Alternatively, an index-matched epoxy or index-matched fluid (not shown) is applied at this interface. The index-matched epoxy also serves to compensate for minor variations in the fiber outside diameter in the interface region. Back reflections may also be reduced by appropriate selection of the index of refraction of transparent sheet50at the given wavelength of operation.

As a final assembly step, it may be necessary to mechanically restrain fiber array220so that optical fibers222do not move within channels66.FIG. 11is a schematic diagram similar toFIG. 10but that illustrates an example embodiment where fixing material62is applied (e.g., as a glob of bonding material) over fiber array220at substrate end18. Other example solutions include crimping the fiber array protective jacket221to PCB substrate120so that optical fibers222are immobilized relative to PCB assembly204.

For low-loss coupling of light from a transmitter active optical component100mounted on transparent sheet upper surface52and an optical fiber222mounted near transparent sheet lower surface54, it is best that thickness T50of transparent sheet50not be too thick. For example, with reference toFIG. 12, a light beam100L launched from transmitter active optical component100passes through transparent sheet50. Because of diffraction effects, light beam100increases in diameter as it propagates. If transparent sheet50is too thick, the diameter of light-beam100L will be substantially larger than the diameter of optical fiber core224, resulting in low-efficiency coupling of the light into optical fiber222. Assessment of beam divergence in a transparent sheet50made of glass with an index of refraction of n=1.45 using a VCSEL light source with about an 8 μm aperture and coupling into a multimode optical fiber222having a core224with a diameter of 30 μm indicates that transparent sheet50should have a thickness T50no greater than about 250 μm for λ=850 nm, and no greater than about 130 μm for λ=1.55 μm.

In addition, measurements of link bandwidth for short multimode optical fiber links of less than 300 m as a function of distance between a VCSEL transmitter active optical component100and fiber end226show that optimal performance is best when the separation distance in air is between about 80 um to about 100 μm. This separation distance corresponds to a glass thickness T50of about 150 μm for λ=1.55 μm. Low-cost glass fabrication techniques that use a fusion draw process allow for forming glass transparent sheets50of precise thickness (e.g., ±1 μm) down to thickness T50of at least 100 μm. Thin glass transparent sheets50having a thickness T50of at least 100 μm are sufficiently stiff and relatively easy to handle. Thus, an example range for thickness T50for a glass transparent sheet50is between about 200 μm and about 250 μm when using active optical components100in the form of VCSELs operating λ=850 nm, and is between about 100 μm and about 150 μm for VCSELs operating λ=1.55 μm.

Wafer-Scale Fabrication of Channeled Substrate Assemblies

The process for fabricating channeled substrate70is scalable up to larger (e.g., “wafer scale”) substrate members10and transparent sheets50, thereby allowing many individual channeled substrate70and integrated optical devices202to be fabricated on the same substrate member and then later separated (diced) from each other. The use of larger substrate members10reduces fabrication costs because the various processing steps (e.g., sawing, conductive contact formation, chip attachment and dicing of individual substrates) can be carried out in parallel on a single structure. The shape and thickness of channeled substrate70can be selected to mimic existing standard size wafers and transparent sheets. This allows the larger substrate members10to utilize existing processing equipment without modification, even though the substrate properties (e.g., glass with channel structures) may be very different from the substrates normally handled by the processing equipment.

FIG. 13throughFIG. 18show various example processing steps for forming channeled substrate assemblies70as well as integrated optical devices202and PCB assemblies204when scaled up to larger size substrate members10.FIG. 13is a perspective view of an example method of forming grooves30in substrate member10using a saw300having a saw blade302. Precision sawing operations form grooves30with well-controlled geometries. For example, precision V-groove sawing operations in ceramic and glass materials can produce grooves30where the pitch and depth are controlled to within about 1 μm. Other less-precise sawing operations may still provide more relaxed but acceptable geometrical control of groove structures (e.g., to within a tolerance of about 3 μm to 5 μm) at more economical cost. Such tolerances are generally acceptable for alignment of active optical components100to multimode optical fibers220.

FIG. 14is a perspective view of transparent sheet50being disposed on grooved substrate member upper surface12, andFIG. 15is a similar view showing the transparent sheet in place to form channeled substrate70. At this stage, in an example embodiment the overall size and shape of channeled substrate70is trimmed to match a desired shape.

FIG. 16is a perspective view of the channeled substrate70ofFIG. 15, but with conductive contacts80applied at multiple locations. The process (e.g., pad metallization) for forming conductive contacts80is carried out, for example, using a single masking and patterning operation for the entire channeled substrate70, or by using multiple operations on different portions of the channeled substrate. Pad metallization or patterned transparent conductive oxide (or PTCO) may be applied before or after grooves30are formed. In either case, a precision alignment is made between the mask and the underlying channels66to ensure that when optical fibers222are placed in the channels they are correctly aligned with active optical devices100that are flip-chip mounted on conductive contacts80. The transparency of transparent sheet50facilitates this alignment process. Transparent sheet50also makes it easier to ensure that the surface and buried features of PCB assembly204align to micrometer-scale tolerances.

After conductive contacts80are formed, solder balls82are placed on the conductive contacts. Then, with reference toFIG. 17, active optical components100and the related active electrical components102are flip-chip mounted on channeled substrate70on the different pad metallization (or PTCO) locations. This approach leads to significant cost reduction since solder ball application, flip-chip mounting and solder reflow processes are carried out at wafer scale.

After the flip-chip mounting step, with reference toFIG. 18a dicing operation is carried out to separate out the individual integrated optical fiber devices202. Integrated optical fiber devices202are then optionally mounted into a larger optical sub-assembly (not shown) such as a PCB substrate. Fiber arrays220are then inserted into channels66of respective channeled substrates70to complete the integrated optical devices202or PCB assemblies204as discussed above (see, e.g.,FIG. 7).

Molded Channel Substrates

Precision grooves30may also be formed in various substrate materials using molding operations. In one example embodiment, V-grooves30with features that have micrometer-scale positional accuracy are formed in a molded plastic substrate member10. In another example embodiment, hot pressing of glass sheets into molds is used to created grooves30that vary in size and position with a tolerance of between about 3 μm to about 5 μm. These techniques may be used to create grooved substrate members10.

FIG. 19is a perspective view of a substrate member10similar to that ofFIG. 1, but showing grooves30formed via a molding operation. Unlike sawed grooves30, molded grooves can be readily made to have an end wall34within the substrate member rather than extending from front to back ends16and18. In an example embodiment, end walls34are established by a molded stop that terminates grooves30at 90°. End walls34thus serve as channel end walls and act as a fiber stop when optical fibers are inserted into channels66. In another example embodiment, the molding operation creates molded fiducials350at various locations on substrate member upper surface12, wherein the fiducials are precision aligned with respect to molded grooves30. Fiducials350are used, for example, to aid in aligning transparent sheet50and the subsequent conductive contacts80to grooves30.

FIG. 20is a cross-section view of an integrated optical device202that employs a channeled substrate70having a molded substrate member10. Optical fiber222has an angled fiber end226that guides light in or out of the optical fiber in a direction roughly perpendicular to the fiber axis A1, i.e., upward to active optical component100. During the formation of integrated optical device202, fiber array220is inserted into channels66as described above. However, tapered fiber ends226contact the 90° end wall34and stop at a position where the angled fiber end is aligned with active optical component100. The 90° end wall34does not come into contact with the angled fiber end face228, ensuring that light guided by optical fiber222and that is incident upon the angled fiber end face228undergoes total internal reflection (TIR) and is reflected upward toward active optical component100. Optimum alignment corresponds to optimum optical coupling between optical component100and optical fiber(s)222.

The angle of channel end walls34can be adjusted to support angles less than 90°, such as shown inFIG. 21. In this regard, two general configurations are considered for the angled molded wall end34. In the first configuration, the angle of end wall34is slightly larger than the angled face228at fiber end226, such as shown inFIG. 21. When optical fiber222is fully inserted into channel66, the tip at fiber end226contacts end wall34at its upper portion. In the second configuration shown inFIG. 22, the angle of end wall34is slightly smaller than the angled face228at fiber end226. When optical fiber222is fully inserted into channel66, the tip at fiber end226contacts end wall34at its lower portion.

In both cases, angled channel end wall34serves to align optical fiber end226directly below active optical component100, without directly contacting angled face228at optical fiber end226. Angled end wall34also serves to force optical fiber end236upward into firm contact with lower surface54of transparent sheet50. This ensures that a controlled distance is maintained between active optical component100and optical fiber222. The upward wedging function of angled channel end wall34also allows the depth of groove30to be slightly larger than the diameter of optical fiber222residing therein, which simplifies the process of incorporating optical fiber array220into channeled substrate70. In another embodiment, end wall34is angled such that it is approximately parallel to fiber end face228. In this case, end wall34provides a slight recess at a location that corresponds to fiber core224. This recess allows total internal reflection of light within fiber222by providing a small air gap at one location on fiber end face228. In an example embodiment, other portions of end wall34are configured to contact the fiber end228and force the fiber upward into contact with transparent sheet50.

Channeled Substrates Formed by Drawing

In an example embodiment, channeled substrate70is formed by drawing a suitably configured preform.FIG. 23is a perspective view of a glass preform380having a cylindrical, rectangular-cross-sectioned body382within which is formed a number of channels384. Preform380has opposite ends386and388. Preform380is formed, for example, by machining a block of glass to select dimensions, and drilling holes in the glass block at precise locations at one of ends386and388. Preform380is essentially a scaled-up version of channeled substrate70.

The typical glass preform is fabricated in a geometry that is closely matches the shape of the final product but is many times (e.g., ten to one-thousand times) larger. A preform is generally suspended from one end and heated until the glass softens. Gravity and/or controlled tension applied to the free end of the preform causes the glass to be stretched into a narrow strand or body. The cross-section of this body generally preserves the geometry of the original preform, but with a much smaller size. Using this approach, feature sizes of the drawn article can be controlled down to sub-micrometer resolution.

Thus, with reference now toFIG. 24, in forming a channeled substrate70using glass preform380, preform end386is held and at least a portion390of the glass preform at the free end388is placed in a cylindrical heater400and heated until the glass in portion390softens. Glass in softened preform portion390is stretched (“drawn”) downward so that the preform shape and all internal channels384are tapered down to a smaller geometry. The relative sizes and positions of channels384and other preform dimensions are substantially preserved during the drawing process, resulting in the formation of a reduced-size channeled rod410. Channeled rod410is then cut into a number of channeled substrate70having channels66, as shown inFIG. 25. Because channeled substrate70ofFIG. 25is made of glass, it is transparent at the wavelengths of interest (e.g., the telecommunications wavelengths of 880 nm and 1550 nm) and thus allows for optical communication between the substrate surface and the interior channels. The upper surface of the unitary channeled substrate70ofFIG. 25is denoted52and lower surface is denoted14to keep the reference numbers consistent with the embodiment of the channeled substrate formed from substrate member10and transparent sheet50described above.

Subsequent processing steps on channeled substrate70ofFIG. 25form conductive contacts80on channeled substrate lower surface14as shown inFIG. 26. Flip-chip attachment of active optical components100and active electrical components102is then carried out, with the resulting formation of the corresponding integrated optical device202being shown inFIG. 27. As described above and as shown inFIG. 28, fiber arrays220with fibers222having angled fiber ends230are then inserted into channels66and aligned with active optical components100.

In an alternative assembly sequence, the channeled rod410is cut into longer lengths so that many channeled substrates70may be fabricated from a single, long rod much in the manner of the “wafer scale” fabrication methods discussed above. Following the assembly process described above, pad metallization, solder ball attachment and flip-chip attachment of components are carried out on different locations on the larger “channeled substrate”70to form multiple integrated optical devices202using channeled rod410. Following flip-chip attachment, channeled rod410is then diced to form individual integrated optical devices202, which can then optionally be integrated into a larger optical sub-assembly as described above. Other operations, such as precision polishing of one or more surfaces on the channeled rod410may also be carried out.

Transparent Channeled Substrates with Active Optical Components Mounted on Both Sides

In some applications, active optical and electrical components100and102are mounted on opposite sides of channeled substrate member70. For example, for a transmitter active optical component100such as a VCSEL, optical output power may need to be monitored with a receiver active optical component over the lifetime of the transmitter. In an example embodiment, an optically transparent channeled substrate70is used to flip-chip mount a receiver active optical component100roughly opposite to a transmitter active optical component.

FIG. 29is a close-up view of an optical fiber222and its fiber end226, which has an angled face228having upper and lower cleaved facets228U and228L that serve to split light from light beam100L between optical fiber core224and a nearby photodetector active optical component100. The lower facet228L is formed at a relatively steep angle (relative to horizontal inFIG. 29) so that light100L from VCSEL active optical component100arranged below optical fiber end226is totally internally reflected into fiber core224. Upper facet228U is formed at a more shallow angle so that a portion of light100L refracts through this facet and onto receiver active optical component100arranged above optical fiber end226.

In an example embodiment illustrated inFIG. 30, an array220of dual-faceted fibers222is inserted into channels66of channeled substrate70. The fiber ends226of optical fibers222in optical fiber array220are positioned directly over a corresponding array of transmitter active optical components100and beneath a second array of receiver active optical components. Light100L from the array of transmitter active optical components100is directed into respective fiber ends226, with a portion of this light being reflected into optical fiber core224and a portion of the light being refracted upward toward receiver active optical component100.

Other applications for channeled substrates70are contemplated herein, such as where all active optical components100are flip-chip mounted on one side of the channeled substrate. For example, in one such application, a portion of light100L emitted from a transmitter active optical component100propagates through a portion of channeled substrate70and then reflects off one or more interior or exterior surfaces (or machined facets) before being directed to a receiver active optical component mounted adjacent to a transmitter active optical component device.

Optical Attenuation

For certain transmitter active optical components100, such as light sources in the form of VCSEL-based transmitters, it is often desirable to operate the component at high optical output power levels. Since eye safety requirements place a limit on the maximum optical power carried in an optical link, it is sometimes necessary to attenuate the optical power launched into optical fibers222. Control of the optical power is accomplished in one example by providing a known optical attenuation between active optical component100and optical fiber222, or by positioning (i.e., selectively aligning) the optical fiber so that it only captures a fraction of the light outputted by the active optical component.

While substrate member10, transparent sheet50and preform380may all be made from low-loss optical glass, these items may also be made of doped glass, wherein the dopants are added to the glass in quantities that alter the optical absorption characteristics of the glass while preferably not substantially altering other relevant glass properties such as CTE, thermal conductivity and electrical conductivity. In an example embodiment, substrate member10, transparent sheet50and/or preform380are fabricated using one of a variety of doped glasses to achieve the desired optical attenuation for a given application.

In an example embodiment, transparent sheets50having a select thickness T50that has a corresponding desired attenuation that allows for introducing controlled amounts of insertion loss between optical fibers222and active optical components100. One concern with this approach is whether there will be substantial optical crosstalk with adjacent optical fibers222as light beam100L diverges while traveling through transparent sheet50. Arrays220of optical fibers222are commonly arranged on pitches of 127 μm or 250 μm, so it is best that divergent light beam100L from one active optical component100not spread laterally any more than about half this value. Taking a 62.5 μm lateral spread (i.e., 125 μm beam diameter) as a conservative value, then for light beam100L of wavelength λ=850 nm and generated by an 8 μm diameter VCSEL, the light beam will spread to 125 μm after propagating through about 1.4 mm of glass. The estimated insertion loss associated with coupling into a 30 μm diameter multimode optical fiber222at a 1.4 mm glass thickness is about 10 dB.

For a wavelength λ=1.55 μm and 8 μm diameter VCSEL, light beam100L will spread to 125 μm after propagating through about 0.75 mm of glass. The estimated insertion loss associated with coupling into a 30 μm diameter multimode fiber222at a 0.75 mm glass thickness is also about 10 dB.

Thus, for the commonly used wavelengths of λ=1.55 μm and λ=850 nm, it is possible to introduce significant signal attenuation without introducing significant signal crosstalk between optical fibers222. If signal crosstalk is still a concern, the pitch of substrate channels66can be increased to larger values such as 250 μm or 500 μm. To extinguish or reduce unwanted crosstalk from additional reflections within the channeled substrate70, in an example embodiment either antireflection coatings, absorptive coatings or scattering surface treatments are applied to the grooved substrate (when a grooved substrate is opaque) or the channeled substrate opposite face (when the grooved substrate or drawn channeled substrate is transparent).

Channels and Other Features on the Thin Glass Sheet

In another examplary embodiment for fabricating channeled substrate70, a slightly thicker transparent sheet50(e.g., 0.7 mm to about 1.1 mm) is used to increase the mechanical stability of the transparent sheet during processing. In addition, transparent sheet50is processed in the same or like manner as described above in connection with substrate member10in order to form an array of parallel grooves30′ in upper surface52, as shown inFIG. 31. In one approach, grooves30′ are formed in transparent sheet50by precision sawing, such as used for forming grooves30in substrate10as described above. The formation of grooves30′ results in the formation of a thinned region17between groove bottom31′ and lower surface54. In the present example embodiment, substrate member10is ungrooved so that grooves30′ and substrate upper surface12define channels66′. Example embodiments where substrate member10is also grooved are described below.

After channeled transparent sheet50is combined with unchanneled substrate member10to form channeled substrate70, loss coupling is required between active optical components100, which are mounted on transparent sheet lower surface54, and optical fiber ends226positioned in channels66′. The thickness of thinned region17should be no more than about 150 μm to 250 μm, depending ostensibly on the wavelength λ and other optical properties of active optical components100, as well as on the diameter of (multimode) optical fiber core224. The resulting transparent sheet50is prone to breaking at thinned regions17even under slight mechanical deformation. For example, it is generally difficult to remove grooved transparent sheet50from a pressure-sensitive backing sheet (not shown) commonly used in dicing without breakage of the transparent sheet.

To mitigate the propensity for transparent sheet50to break at thinned regions17, in an example embodiment, the thin regions are reinforced by applying a small amount of support material450, such as an epoxy, boding material or an adhesive, into selected portions of grooves30′, as illustrated inFIG. 32. Support material450is preferably thin enough to flow down into grooves30′ when pressure is applied from transparent sheet upper surface, but thick enough to resist flowing along the grooves. In an example embodiment, support material450comprises a filled CTE-matched thermal cure epoxy from the Corning MCA-xx family, which is available from Corning, Inc., Corning, N.Y.

In an example embodiment, support material450is applied to upper (grooved) surface52of transparent sheet50while still attached to a pressure sensitive adhesive backing (not shown). This allows transparent sheet50to be sufficiently reinforced to survive backing removal and handling during subsequent processing steps. The flow of support material450is preferably limited to specific groove locations using screen printing, automated syringe dispensing or other masking or dispensing methods. This approach allows for many parts to be fabricated on the same transparent sheet50, thereby reducing total part cost. In an example embodiment, an additional adhesive polishing and/or lapping step is used to planarize support material450so it does not rise above transparent sheet upper surface52, as shown inFIG. 33.

Dicing lines460are also shown inFIG. 33to indicate an example of how the larger grooved transparent sheet50is divided into individual transparent sheets. The dicing operation can be carried out immediately on transparent sheet50, or delayed until after substrate member10is attached to the transparent sheet to form channeled substrate70. When substrate member10is attached to transparent sheet50, it provides mechanical reinforcement to thin regions17. The dicing operation can therefore be carried out in a way that exposes the channels after dicing, such as along dicing lines460illustrated inFIG. 34. This enables subsequent processing similar to the steps shown inFIG. 7throughFIG. 11to arrive at an embodiment of integrated optical device202.

If attaching substrate member10to grooved transparent sheet50prior to dicing provides sufficient reinforcement, then the groove reinforcement steps described above can be eliminated. However, attention to thin regions17is required during subsequent pad metallization and flip-chip attachment to ensure that these regions are not excessively mechanically loaded.

In another example embodiment, grooves30′ do not extend between the front and back ends56and58of transparent sheet50. Rather, shorter grooves30′ are formed, for example, by sawing into transparent sheet upper surface52at one or more locations, such as near the center of the transparent sheet, as shown inFIG. 35. This approach is preferably carried out on the aforementioned thicker transparent sheet50having a thickness of about 0.7 mm to 1.1 mm so that the portions of the transparent sheet not removed by the saw provide sufficient mechanical support to the transparent sheet during subsequent processing.

FIG. 36is a schematic, to-scale cross-section view of a 10 mm wide, 0.7 mm thick transparent sheet50that has been plunge-sawed using a 50 mm diameter saw blade302. Saw blade302is lowered until thin region17is about 0.2 mm thick. At this depth, saw blade302just touches top surface52of transparent sheet50at front and back ends56and58.

In an example embodiment, molded substrate member10includes ridges470configured to fit into grooves30′ formed by the plunge-sawing operation and that serve to further define channels66for guiding optical fiber array220to the proper location, as illustrated inFIG. 37. Ridges470includes an angled fiber stop472and an enlarged (e.g., flared) fiber inlet474to simplify the insertion of optical fiber array220into channels66.

FIG. 38provides a view of the bottom side of the molded substrate member10ofFIG. 37, showing the raised ridges470that fit into the glass substrate plunge-sawed grooves30′. An assembled cross-section view of molded substrate member10and transparent sheet50prior to fiber array insertion is shown inFIG. 39. After molded substrate member10and transparent sheet50are joined (e.g., using an adhesive), optical fiber array220is inserted into channels66, as illustrated inFIG. 40. Angled fiber stop472ensures that angled fiber end226is positioned in the correct location and forced into contact with transparent sheet50.

Many of the fabrication techniques discussed above involve removal of material from either substrate member10or transparent sheet50. In other example embodiments, material is removed from both substrate10and transparent sheet50to form channels66defined by both grooves30and30′.

FIG. 41throughFIG. 44show exemplary configurations for substrate member10and transparent sheet50.FIG. 41shows a channel66formed by removal of material only from substrate10.FIG. 42shows a channel66formed by removal of material only from transparent sheet50in combination with a molded substrate member10that has ridges11that fit within the transparent sheet grooves.FIG. 43shows a channel66formed by removal of material from both substrate10and transparent sheet50.FIG. 44is similar toFIG. 43, except that the groove30′ is V-shaped.

V-shaped channels66are an attractive option for optical fiber alignment because the vertex of the “V” allows for accurate positioning. However, a V-shaped channel66also has implications for efficient coupling of light in and out of the angled end236of optical fiber220.FIG. 45andFIG. 46show two different cross-sectional views of two different V-shaped configurations for channel66, wherein the V-shaped channel66is defined by a V-shaped groove30′ and the flat upper surface12of substrate member10by way of example. The “V” portion of groove30′ has a vertex angle θ. A transmitter active optical component100is positioned directly over optical fiber222adjacent lower surface54of transparent sheet50(note that now the lower surface54is on top of channeled substrate70because the upper surface has been grooved). Light from transmitter active optical component100propagates downward through thin region17and strikes the angled V-groove sidewalls32directly above optical fiber222.

The orientation of V-groove sidewalls32causes light beam100L from transmitter active optical component100to diverge as it passes through the V-groove. This divergence laterally broadens light beam100L, causing a portion of the light beam to miss optical fiber core224. The amount of divergence of light beam100L depends on vertex angle θ. InFIG. 45, V-groove30′ has a relatively small vertex angle θ so that light beam100L strikes the V-groove sidewalls32at a relatively high incidence angle. This leads to refraction at an even higher incidence angle via Snell's law. Since the V-groove vertex angle θ is small, the distance between the portion of the V-groove sidewall32where light is incident and optical fiber outer surface223is large. The longer distance means light beam100L has more opportunity to diverge, which increases the likelihood that at least a portion of the light beam will not enter optical fiber core224.

InFIG. 46, V-groove30′ has a vertex angle θ that is larger than that shown inFIG. 44. This reduces the incidence angle of light passing through the V-groove sidewall32, thereby reducing the rate of light beam divergence compared with the configuration ofFIG. 44. Since the V-groove vertex angle θ is larger than shown inFIG. 44, the distance between the portion of the V-groove sidewall32where light beam100L is incident and optical fiber outer surface223is reduced. This smaller distance means that light beam100L has less opportunity to diverge, which reduces the likelihood that at least a portion of the light beam will not enter optical fiber core224.

A similar light beam divergence situation occurs when light beam100L is launched upward from angled fiber end226into a receiver active optical component100mounted adjacent lower surface54of transparent substrate50. The V-groove vertex angle θ can be used to modify the beam divergence, and larger V-groove vertex angles θ are preferable for minimizing beam divergence for this geometry.

In an example embodiment, divergence of light beam100L is controlled to maximize link bandwidth in multimode optical fiber links. Modal dispersion in multimode optical fibers can be reduced by preferentially exciting limited sets of guided modes or guided-mode groups. In particular, excitation of higher-order guided modes can be achieved by launching light into multimode optical fiber core224at a high angle relative to the optical fiber axis A1. As long as the launch angle is less than the optical fiber's cut-off angle, light is guided within the optical fiber in a limited number of high order modes, resulting in reduced modal dispersion and increased link bandwidth for longer links (e.g., >200 m).

In one example, light from light beam100L is preferentially launched into optical fiber core224at higher angles using the V-groove shaped groove30′ shown inFIG. 43. The V-groove vertex angle θ is adjusted to enhance the high-angle launch conditions. In some cases, it may be necessary to reduce the transparent sheet thickness T50, since highly divergent portions of light beam100L are more likely to fall outside of optical fiber core224.

In an example embodiment, the V-configuration of groove30′ is used at the receiver end of an optical link, where the divergence of light beam100L associated with V-groove30′ serves as an angular filter and only allows for high-order mode light to propagate and reach the photodetector (not shown). Low-order-mode light, which propagates roughly parallel to optical fiber axis A1prior to reaching angled face228tends to be directed to locations other than a flip-chip mounted receiver active optical component100by the angled V-groove sidewall surfaces.

In another example embodiment, light from light beam100L is launched into higher-order modes of optical fiber222by using a scattering surface interposed between the transmitter active optical component100and optical fiber222. When groove30′ is formed using a saw, its bottom surface31′ has some degree of roughness due to variations in saw blade302.FIG. 47andFIG. 48show an exaggerated bottom rough surface31′. When light beam100L is launched by transmitter active optical element100, it strikes roughened surface31′ and scatters over a range of scattering angles φ relative to the initial light beam propagation direction. If a large portion of light beam100L is scattered at a moderate scattering angle φ, the scattered portion will tend to excite higher-order modes of optical fiber222, yielding improved link bandwidth performance.

In an example embodiment, the roughness of groove bottom surface31′ is modified by changing the material for saw blade302and treating or dressing the blade surface prior to cutting. In an example embodiment, complex saw blade profiles, such as convex or concave profiles, are used to further enhance the launch conditions and scattering angles φ.

The example embodiments described above are extendable to two-dimensional channeled substrate configurations.FIG. 49is a cross-sectional view of an example PCB assembly204that includes two-dimensional (“stacked”) integrated optical devices202configured in combination with transmitter active optical components100, which are part of a transmitter assembly512. Transmitter assembly512includes conductive contacts80and solder balls82that provide electrical connection to the adjacent channeled substrate70, which also includes conductive contacts.

In an example embodiment, multiple grooved transparent sheets50(or channeled substrates70) are stacked and used to align two-dimensional optical fiber arrays220to two-dimensional arrays of active optical components100on one more flip-chip mounted substrate assemblies. Grooves30may be formed on one or both sides of the grooved substrate70, and stacked and covered with thin glass sheets50where appropriate. Lenses such as lens520are optionally included along the optical path to compensate for increased beam diffraction in the fiber-to-active device interconnection.FIG. 49shows a lens520associated with the uppermost channeled substrate70. Lenses520may also be used to couple light100L into optical fiber222of lower channeled substrate70. Lenses520may be formed via micro-molding or laser forming Lenses520may also include diffractive grating structures. The focal length of lenses520at various stacked fiber array planes are preferably customized to optimize optical coupling performance for individual fiber arrays.

It will be apparent to those skilled in the art that various modifications to the preferred embodiment of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, it is intended that the present disclosure covers the modifications and variations of this disclosure provided they come within the scope of the appended claims and the equivalents thereto.