Abstract:
Methods and structures are disclosed demultiplexing optical signals transmitted over an optical fiber into a silicon substrate and to multiple detectors. The silicon substrate has two spaced-apart surfaces and a diffractive element disposed adjacent to one of the surfaces. Each of the optical signals corresponds to one of multiple wavelengths. The optical signals are directed into the silicon substrate along a path through the first surface to be incident on the diffractive element. The path is oriented generally normal with the first surface and/or with the diffractive element, which angularly separates the optical signals such that each of the wavelengths traverses through the substrate in a wavelength dependent direction to the first surface. Each optical signal is steered from the first surface towards the second surface to be incident on different optical elements that direct them generally normal to the first surface to be incident on one of the detectors.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/771,760, entitled “METHOD AND APPARATUS FOR DEMULTIPLEXING OPTICAL SIGNALS IN A PASSIVE OPTICAL NETWORK,” filed Jun. 29, 2007, which is a nonprovisional of, and claims the benefit of the filing date of, U.S. Provisional Patent Application Ser. No. 60/806,348, entitled “MULTIPLEXED OPTICAL SUBASSEMBLY,” filed Jun. 30, 2006, the entire disclosures of which are incorporated herein by reference for all purposes. 
         [0002]    This application is related to commonly assigned U.S. patent application Ser. No. 11/771,716, entitled “OPTICAL ASSEMBLIES AND METHODS FOR FABRICATION OF OPTICAL ASSEMBLIES,” filed Jun. 29, 2007, the entire disclosure of which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    This application relates generally to optical routing. More specifically, this application relates to passive optical networks. 
         [0004]    Fiber-optic telecommunications systems are currently deploying wavelength division multiplexing (“WDM”) as a mechanism for expanding the capacity of new and existing optical fiber systems. In WDM, multiple wavelengths of radiation simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. 
         [0005]    A passive optical network (“PON”) is a communications architecture in which each single optical fiber transmits multiple channels of information in this way as distinct wavelengths of radiation. The network is termed “passive” because the wavelengths are combined (multiplexed) and separated (demultiplexed) using wavelength-selective splitters or couplers rather than active switching or variable routing components. One example of a PON is the distribution of video (television, cable TV, and/or video downloads), analog or digital telephone (local, long-distance, and/or VoIP), and digital computer data (Internet or network connectivity) from a service provider to a multiplicity of subscribers via optical fiber-to-the-premises (“FTTP” or “FTTx”) connections. At or near each subscriber/user location along such a PON, it is generally necessary to receive and demultiplex two or three incoming optical signals from a single optical fiber. This may be done at relatively wide wavelength spacing compared with the dense wavelength division multiplexing that is used for long-haul transmissions. At least on optical signal is multiplexed and transmitted back into the optical fiber at a distinct wavelength. 
         [0006]    For example, in one implementation, downstream traffic is provided on a 1490-nm channel and upstream traffic is provided on a 1310-nm channel. In another implementation, an additional 1550-nm band is allocated for downstream rf video traffic. 
         [0007]    There are a number of different PON standards in existence. The ITU-T G.983 standard defines standards for ATM PONs (“APON”) and broadband PONs (“BPON”). These represent the earliest PON standards, with BPON building on the APON standard to provide support for wavelength division multiplexing as well as better upstream bandwidth allocation. There has been more recent interest in the ITU-T G.984 standard, which supports higher rates and enhanced security as a further evolution of the BPON standard. In addition, the IEEE 802.3ah standard provides a standard for using Ethernet for packet data. Emerging approaches for providing customers with the “triple-play” of services described above over a common system or network require data-transfer rates in the gigabit/second (Gb/s) range or higher, which can be most easily achieved using optical rather than electronic (“copper”) transmission lines, leading to the nomenclature “GPON” for gigabit passive optical networks. 
         [0008]    In implementing any of these various PONs, there is a general need for structures to manage the transmission of the different signals, both downstream and upstream. Current implementations suffer from a number of deficiencies. For example, persistent concerns with structures that are used exist in the isolation of the different wavelengths, the need to control the absorption of stray radiation, and the cost of fabrication. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the invention provide methods and structures for directing radiation. Embodiments of the invention simultaneously address multiple problems that persist with PON structures currently used. This is achieved in embodiments of the invention by using low-cost silicon fabrication methods that provide high precision to produce a structure that effectively manages wavelength isolation and stray radiation to provide efficient multiplexing and demultiplexing operations. The use of low-cost small-silicon-chip processes permits the use of small beams in tightly controlled free space instead of the use of waveguides, providing improved insertion loss and cost. 
         [0010]    The structure makes use of grating in reflective mode for efficiency and may be produced with a low etch depth for fabrication. A focal spot may then be used that is large enough to operate the PON efficiently, but small enough to separate two beams with dispersion from the grating. The structure provides high absorption of radiation and the size of the grating coupling discriminates the wavelengths, thereby allowing the use of larger aperture detectors for improved tolerance in placement. Beams may be coupled in and out of the chip at near normal incidence for improved optical performance, especially for polarization and placement. 
         [0011]    The use of silicon fabrication methods allows the parallel surfaces of the silicon wafers to be used to provide predictable beam angles. Standard chip-assembly techniques using low-cost passive pick-and-place tools greatly improve the fabrication efficiency over other techniques. 
         [0012]    The result is a solution to the issue of producing PONs that uses small beams in tightly controlled free space instead of waveguides to provide improved insertion loss and cost. A wavelength separation of 60-180 nm can be well achieved with a 100-μm-diameter spacing, with about a 1-μm pitch. Merely by way of example, a focal spot diameter of 25 μm has a focus range of about 1 mm in silicon, allowing spots to be well discriminated within the geometry of the structure. 
         [0013]    These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  provides a schematic illustration of an optical assembly in accordance with embodiments of the invention; 
           [0015]      FIG. 2  provides a schematic illustration of a structure that may be used for an optical coupler in some embodiments; 
           [0016]      FIG. 3  is a flow diagram that summarizes methods of directing radiation in accordance with embodiments of the invention; 
           [0017]      FIG. 4  is a flow diagram that summarizes methods of fabricating an optical subassembly in accordance with embodiments of the invention; 
           [0018]      FIG. 5  is a diagram illustrating intermediate stages in the fabrication of a diffraction grating integral with a surface of an optical subassembly; 
           [0019]      FIG. 6  is a diagram defining different orders of reflective and transmissive diffraction from a diffraction grating of the type produced using the technique illustrated in  FIG. 5 ; and 
           [0020]      FIG. 7  provides graphs demonstrating a comparison of diffraction efficiencies for different diffraction-grating characteristics. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Embodiments of the invention provide optical assemblies that may be used in directing radiation. Such optical assemblies find particular application in applications in which the radiation is directed through a PON, although they may be amenable to uses in other applications also. Rather than adopt a conventional approach to providing such an optical assembly, the inventors have applied their experience with semiconductor-processing techniques to develop a structure that uses a semiconductor substrate to provide a quasi-free-space optical platform. Structural elements of an optical subassembly are fabricated using semiconductor lithographic techniques, providing a structure that may be produced at a cost significantly lower than structures produced using more conventional techniques. 
         [0022]    In certain embodiments, the fabrication techniques advantageously exploit the availability of pick-and-place methods for accurate fabrication of structures. The use of semiconductor-processing techniques in this way results in a structure that has good strength, optical stiffness and humidity control. 
         [0023]    A typical structure in one embodiment is shown with the schematic diagram of  FIG. 1 . In this illustration, the optical assembly is identified by reference number  100  and comprises an optical subassembly  160 , which may be produced using the semiconductor lithographic techniques described in greater detail below. The optical assembly  100  includes a number of additional elements in addition to the optical subassembly  160  that are used principally to provide radiation to and receive radiation from the subassembly  160 , with the subassembly  160  performing the optical routing functions. 
         [0024]    Radiation may be provided to the optical subassembly from two sources, corresponding to sources for downstream and upstream traffic. Downstream traffic  112  is provided by an optical fiber  104  that is coupled to an optical coupler  108 , which expands the size of the beam  112  on which the traffic is carried and collimates it for delivery to the subassembly  160 . The optical coupler  108  may be mounted in different positions in different embodiments, with it being surface mounted in some embodiments. The radiation is directed through a deflector  120  into the subassembly  160  through a first surface  166  and propagates to one of a plurality of diffraction gratings  184  formed integrally with a second surface  170  of the subassembly  160 . The second surface  170  is provided opposite the first surface  166  and may be substantially parallel to the first surface  166 . 
         [0025]    Radiation  112  from the optical coupler  108  is directed substantially normally to the first surface  166  and undergoes angular separation by its encounter with the first diffraction grating. Different wavelength components of the radiation accordingly propagate differently within the subassembly  160 . For instance, a first path  168  may be followed by a first wavelength component, being directed to another of the diffraction gratings  184 - 3 . This diffraction grating  184 - 3  has the effect of reversing the angular redirection of the single wavelength component so that this portion of the signal is now again directed substantially perpendicular to the first surface to a first detector  136 - 1 . Such propagation is again substantially normal to the first surface  166  and may proceed through a first aperture  144 - 1  and deflector  140 - 1 . 
         [0026]    Similarly, a second path  172  may be followed by a second wavelength component to another of the diffraction gratings  184 - 4 . This diffraction grating  184 - 4  also has the effect of removing the angular separation to direct this wavelength component substantially normally to the first surface  166  to a second detector  136 - 2 . Again, the propagation may proceed through a second aperture  144 - 2  and a second deflector  140 - 2  before encountering the detector  136 - 2 . 
         [0027]    Other radiation separated angularly by the first diffraction grating  184 - 1  may propagate along a further path  176  in still a similar fashion, with its angular separation being removed by still another diffraction grating  184 - 5 . This radiation is collected by an optical absorber  148  after propagating substantially normally through the first surface  166 , perhaps after encountering a further aperture  156  and deflector  152 . Collection of this additional radiation by the absorber acts to avoid interference with the other signals by preventing stray radiation from being propagated within the subassembly to be received by either of the wavelength detectors  136 . 
         [0028]    As shown in the drawing, each of the paths  168 ,  172 , and  176  followed by the separated radiation undergoes at least one reflection from the first surface  166  within the subassembly  160 . In the illustrated embodiment, each path  168 ,  172 , and  176  undergoes a single reflection, but other embodiments may provide for multiple reflections from the first surface  166 . To improve the efficiency of coupling the radiation from the downstream beam to the different detectors  136 , the first surface  166  may be covered with a highly reflective coating  180  such as a metal. The coating  180  ensures that losses due to the reflection are minimized and is formed in regions of the first surface  166  where reflections of the signals are to occur according to the geometry of the subassembly and the optical characteristics of the diffraction gratings  184 . 
         [0029]    In converse fashion, the diffraction gratings may be patterned for high reflection, such as by including materials in their fabrication that increase reflectivity. The second surface  170  may also be covered between the diffraction gratings with an absorber material  174 . This advantageously minimizes the level of crosstalk between the different wavelength channels by absorbing any stray radiation from a particular channel before it can follow a path to an unintended detector  136 . Specifically, the different angular direction of propagation of the different wavelength channels permits the detectors to be positioned so that the Gaussian overlay of the different signals is minimized. For instance, with two downstream channels at 1490 nm and 1550 nm, the configuration of the structure causes the input of the 1490-nm beam detected by the 1550-nm detector to be minimized and for the input of the 1550-nm beam detected by the 1490-nm detector to be minimized. The additional inclusion of the absorber  174  further reduces the potential for crosstalk. 
         [0030]    The upstream traffic operates in a generally similar manner but propagates in the opposite direction. The upstream radiation source is shown in the drawing as a laser  124 , although other types of sources may be used in alternative embodiments. Radiation is directed from the laser  124  into the subassembly  160  substantially normally to the first surface  166  towards one of the diffraction gratings  184 - 2 . In some instances, the radiation may pass through a deflector  128  and aperture  132 . The radiation is angularly redirected by the diffraction grating  184 - 2  along path  164  to the first diffraction grating  184 - 1 , which introduces a complementary angular redirection to propagate the radiation upstream into the optical coupler  108  where it is integrated as an upstream signal  116  in the optical fiber  104 . 
         [0031]    There are a number of aspects of the structure shown in  FIG. 1 . The optical path is folded, making the overall size of the chip smaller. As a consequence, thermal expansion of the system does not significantly affect the coupling. Coupling efficiency is also effectively accommodated with the expansion of the beam size by the optical coupler  108 . The desired coupling efficiency can still be achieved when the beam is expanded within tolerances that are 5-25 times as large as for an unexpanded beam. 
         [0032]    In some embodiments, the subassembly is fabricated with a silicon substrate, although other materials may be used in other embodiments. It is generally desirable that the substrate material be substantially transparent at the wavelengths used by the various optical signals. For example, when downstream wavelengths are 1490 nm and/or 1550 nm and the upstream wavelength is 1310 nm, the substrate is preferably substantially transparent at infrared wavelengths. Other examples of materials that may be used include sapphire and glass. Notably, silicon has a relatively large refractive index, being n=3.5 as compared with n=1.5 for glass. The higher refractive index makes the radiation travel more slowly within the material so that the gratings may be made smaller. 
         [0033]      FIG. 2  provides an illustration of a structure that may be used for the optical coupler  108  in some embodiments. This configuration is especially suitable for embodiments in which the optical coupler comprises a surface-mounted optical coupler. In such an embodiment, the optical coupler  108  comprises an integrally fabricated structure that includes a collimating lens  24 , a mirrored surface  26  angled 45° relative to the optical axis  28  of the collimating lens  24 , a short rod-shaped extension  30  that has a flat optical surface  32  at its exposed end, and a cylindrically shaped support or tube extension  34  that supports and aligns the optical coupler against a chip. Butted against the flat surface of the extension  30  is an optical fiber pigtail  38  that characteristically includes a fiber core  40  surrounded by fiber cladding  42 , which is in turn surrounded by a fiber buffer layer  44  that protects the inner structure. A polymer boot  46  surrounds the end of the optical fiber pigtail  38  and part of extension  30  and serves both to provided a mechanism of anchoring the assembly to an extension arm of a chip carrier to protect the optical fiber from being damaged during handling. When the optical coupler  108  is mounted on the backside of a chip, it aligns its collimating lens  24  with a focusing lens and maintains them at a predetermined distance from each other with an intermediate air gap separating them. 
         [0034]      FIG. 3  provides a flow diagram that summarizes methods of directing radiation in accordance with embodiments of the invention. At block  204 , a radiation beam is transmitted downstream from an optical fiber to an optical coupler. The expansion of the beam size and its collimation with the optical coupler are indicated at block  208 . The collimated and expanded beam is transmitted at block  212  to the subassembly chip substantially normally to the first surface  166 . Angular separation of the wavelength components within the subassembly chip occurs at block  216 , permitting the first wavelength component to be directed substantially normally to the first surface  166  after reflection within the subassembly chip at block  220 . When the radiation beam comprises a plurality of wavelength components, the second wavelength component is similarly directed substantially normally to the first surface  166  after reflection within the chip at block  224 . 
         [0035]    The absorption of radiation to avoid spurious transmission to the wavelength detectors is noted at block  228 , with separation being maintained between the first and second wavelength components at block  232  to avoid crosstalk. While blocks  204 - 232  generally indicate how the downstream transmission of signals is effected, upstream transmission of laser radiation is indicated at blocks  236 - 248 . The laser radiation is transmitted into the subassembly chip at block  236 , again substantially normally to the first surface  166 . After reflection within the chip, it is directed substantially normally at block  240  to the optical coupler. Coupling of the radiation into an optical fiber at block  244  allows the upstream transmission of the signal. 
         [0036]    There are a variety of different ways in which the optical subassembly may be manufactured in different embodiments.  FIG. 4  provides a flow diagram that illustrates one such process.  FIG. 4  is organized by separately illustrating steps performed at different locations, as might be done when a portion of the fabrication process is performed at a CMOS foundry and a remainder of the process is performed at a thin-film house. The left column in the drawing corresponds to those steps advantageously performed at the CMOS foundry and the right column corresponds to those steps advantageously performed at the thin-film house. This organization of the steps is not intended to be limiting. In other embodiments, all of the steps might be performed at a single location or the number of locations at which the process is completed might be greater than two. 
         [0037]    The method in this embodiment begins at block  304  with a silicon wafer, whose backside is nitride passivated at block  308 . For instance, the wafer might comprise a wafer having an 8-inch diameter with a thickness of 725±10 μm, polished on both sides. The nitride passivation may involve deposition of a thickness of approximately 1000 Å in a particular implementation. The wafer is prepared to have a defined thickness at block  312 . This may be accomplished by measuring the wafer thickness and using a combination of grinding and polishing to achieve the desired thickness. In one specific embodiment, the wafer is prepared with a thickness of 700±2 μm. Particles may be removed by a subsequent cleaning and degreasing of the thinned wafer. At block  316 , the backside nitride may be removed. This may be followed by nitride passivation of the frontside of the wafer at block  320 . Alignment marks may be provided to the wafer backside using dry-etch techniques at block  324 . 
         [0038]    Such preliminary preparations permit the lithographic formation of the diffraction gratings on the backside at block  328 . This may be accomplished, for example, by using a combination of depositing photoresist, and producing trenches by application of photolithographic and etching techniques as known in the art. A plurality of gratings may be formed in this way, the actual number perhaps depending on the specific application for the subassembly. 
         [0039]    This prepatory work may be followed by removing the frontside nitride at block  332 , followed by a silica passivation deposition on the backside at block  336 . The deposition may proceed to produce a silica thickness in a particular embodiment of about 1000 Å. Alignment marks are transferred to the wafer frontside at block  340 . This is followed by the formation of Ti silicide on the frontside at block  344 . A blanket of SiO 2  may be deposited on the frontside, reaching a thickness of about 5000 Å in one embodiment. The SiO 2  is patterned for the silicide pattern and etched, using the silicon substrate as a stop. Titanium is blanket sputtered on the front side, with thermal annealing being used to produce the Ti silicide. A few minutes of annealing at a temperature of about 700° C. is sufficient. Other annealing techniques may be used in certain alternative embodiments. A wet etch may be used to remove residual titanium metal that is not in contact with the silicon substrate, followed by deposition of SiO 2  on the frontside. This additional deposition provides protection for the chuck from titanium and silicide contamination. 
         [0040]    Backside dielectric protection may be removed at block  348 , with blanket deposition of reflector metal then taking place on the backside at block  352 . This completes the partial preparation at the CMOS foundry so that the wafers may be packaged for transport at block  356 . 
         [0041]    Subsequent processing the wafers at the thin-film house may begin at block  360  with receipt of a wafer from the CMOS foundry. Using the specific example discussed above, such an incoming wafer has a diameter of about eight inches and a thickness of 700±2 μm. The frontside is protective coated with an oxide or nitride over optical-quality silicon polished windows. The backside is coated with a reflector metal. 
         [0042]    At block  364 , the processing of such a wafer continues by removing the protective film from the frontside. An antireflective stack is blanket deposited on the frontside at block  368 . For instance, a broadband antireflective coating that covers an infrared region between 1310 and 1550 nm may be deposited, followed by a SiO 2  top layer. at block  372 , trace and mirror metallization are formed lithographically. A multilayer metallization may be used in one embodiment, such as when a Ti/Ni/Au structure is used. Formation of the metallization layer may use photoresist and contact-lithography techniques, coupled with an acetone lift-off pattern. The metallization is compatible for wire bonding. at block  376 , solder metallization is formed lithographically using a similar technique. Again, a multilayer structure may be used, such as by using a Cr/NiV/AuSn structure in one embodiment. A suitable solder stack thickness in an embodiment is about 0.5 μm. A gold oxidation protection cap may sometimes be applied, perhaps depending on the thickness of the solder stack. 
         [0043]    The production of the subassembly chips is essentially complete at this stage. The chips are accordingly singulated from the wafer at block  380 . When fabrication takes place in two facilities, there is generally no crossover of wafer processing between the facilities, i.e. no resist deposit and processing crossover, although this is not a requirement of the invention. In such instances, all processes and quality control are the sole ownership of either the CMOS foundry or the thin-film house. In addition, wafers generally do not re-enter the CMOS foundry after processing in the thin-film house. This alleviates issues related to wafer cleaning of particulates and materials contamination at the CMOS foundry. This accommodates the fact that the thin-film house may have lower cleanroom controls than the CMOS foundry. It is generally the case that thin-film houses do not have automated wafer handling and processing capabilities. Implementation of automated wafer handling is thus limited by small production volumes rather than by process compatibility. 
         [0044]    There are a variety of different structures that may be used for the diffraction gratings. In one embodiment, the gratings are produced using a binary optics technique illustrated in  FIG. 5 . This technique forms initial trenches at the pitch of the final grating and progressively introduces additional features, as may be understood more clearly with reference to the drawing. An initially flat substrate may be formed into a two-level structure  404  by etching trenches  428  at the grating pitch. This process is repeated at half the pitch as indicated with structure  408 . Trenches  428  at half the pitch are formed by using lithographic techniques and are etched to a depth half that of the initial trenches. This produces a four-level structure  412  having profile  432 . The process is repeated again at a quarter of the pitch as indicated with structure  416 . Trenches  436  at a quarter the pitch are formed by using lithographic techniques and are etched to a depth a quarter that of the initial trenches. This produces an eight-level structure  420  having profile  440 . 
         [0045]    The result of this type of processing produces a diffraction-grating structure having the same initial pitch as the first structure  404 , but having a total depth that is the sum of the depths of the trenches etched at each processing level. In the example of an eight-level structure, the total depth is 
         [0000]    
       
         
           
             
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         [0000]    of the initial trench depth. Even with such a general type of grating structure, there are thus at least three parameters that may potentially by varied to alter the optical characteristics of the structure: the number of levels, the total depth, and the pitch (or some surrogate for these three independent parameters). 
         [0046]    In determining preferred values for these parameters, one optical characteristic that is relevant is the polarization-dependent loss of the grating. This is a characteristic that is frequently of concern in evaluating grating performance, for a variety of different grating designs. The inventors have accordingly performed simulations to evaluate the polarization characteristics of different gratings in which these parameters are varied.  FIG. 6  shows schematically the effect of a grating  508  on an incident beam  504 . A portion of the incident radiation  504  is reflected by the grating  508  and a portion is transmitted. The reflected and transmitted radiation are shown in terms of a spectral decomposition into different transmitted (“T”) and reflected (“R”) orders. In certain embodiments, an eight-level grating is used as a cost-effective midpoint between fewer numbers of levels that provide workable results and higher numbers of levels that may significantly increase production costs. But gratings with different levels may be used effectively in a number of embodiments. 
         [0047]    One set of results is shown in  FIG. 7  for an eight-level diffraction grating having a 0.8 μm pitch when radiation having a wavelength of 1490 nm is incident. The top set of results shows the efficiency for TE polarizations, with different curves in the drawing corresponding to different transmitted and reflected orders and showing how the efficiency varies with grating depth. The bottom set of results similarly shows the efficiency for TM polarizations, with different curves again corresponding to different transmitted and reflected orders. In each of the two drawings, a box is drawn to indicate the depth region over which the efficiency is at least 80%. 
         [0048]    It is preferable to have structural characteristics of the grating that simultaneously maximize the TE and TM polarization efficiencies. This is frequently impossible to achieve, but there are circumstances when both polarization efficiencies can be achieved greater than some acceptable threshold value, such as greater than 70%, greater than 75%, greater than 80%, greater than 85%, greater than 90%, or greater than 95%. In the example, the TE diffraction efficiency is greater than 80% when the grating depth is between about 0.21 and 0.28 μm and the TM diffraction efficiency is greater than 80% when the grating depth is between about 0.17 and 0.22 μm. This provides a window between about 0.21 and 0.22 μm where the diffraction efficiencies for both polarizations may be simultaneously achieved greater than 80%. 
         [0049]    Having fully described several embodiments of the present invention, many other equivalents or alternative embodiments of the present invention will be apparent to those skilled in the art. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.