Abstract:
An optical network is formed of multiple H-tree distribution devices, separated into different waveguide layers. The optical network receives an input optical signal, such as an optical clock signal, and makes duplicate copies of that input signal. The duplicate copies are routed through the connected H-tree distribution devices, which are arranged to produce identical, synchronized copies of the clock signal. The network can take the form of a 1×2 N  device, where 2 N  represents the number of these output signals. The H-tree distribution devices forming the network are of varying size and may be formed in different waveguide layers with different index of refraction differentials between the H-tree devices and surrounding claddings. In some forms, the optical network is integrated with optical-to-electrical converters, i.e., photodetectors, which take the optical output signals and convert them to synchronized electrical signals that may be communicated to digital circuits.

Description:
FIELD OF THE DISCLOSURE  
         [0001]    This patent generally relates to signal propagation and more specifically to H-tree distribution of a signal.  
         BACKGROUND OF THE PRIOR ART  
         [0002]    For digital systems, accurate timing is crucial to data transmission. Clocking signals therefore are crucial to digital systems, because clocking signals set the timing for the components in the systems. A computer motherboard, for example, might have a single master clock signal that is transmitted to and synchronized with integrated circuit boards, chipsets, peripherals, or other components connected to the motherboard. All system components may be synchronized using this master clock signal.  
           [0003]    Various techniques exist for generating and distributing clock signals within a digital system. For example, a primary clock signal might be generated by a ring oscillator or a separate clock chip (e.g., a crystal oscillator). The clock signal may then be routed from the generator to each of the devices connected to the clock. These techniques use electrical clock signals, i.e., clock signals traveling along metallic or semiconductor conduits. Unfortunately, clock signals in the electrical domain present numerous design limitations.  
           [0004]    Ideally, clock signals would have a well defined duration, consistent shape, and zero propagation path dependence. In reality, clock signals have variable rise and fall times, noticeable jitter, and noticeable path-dependent skew, a particular problem that arises from timing differences and waveform variation between clock signals. There is also a sizeable power drain associated with electrical clock signals.  
           [0005]    Typically, clock signals are distributed throughout a system via a distribution network. In theory, the network would make duplicate copies of a clock signal and provide identical paths for each duplicate copy, so that any device connected to the network would receive a synchronized clock signal. In reality, however, skew and jitter problems abound, primarily due to electrical load differences among the various paths and parasitic effects within the network.  
           [0006]    Recently, some have proposed moving away from a purely electrical domain digital clocking system to an optical domain clocking system. Using optical signals, i.e., light pulses, presents some obvious theoretical advantages. Optical signals are not susceptible to load variations or parasitic effects, because they travel through waveguides and not conducting metallic wires. Also, optical signals may propagate at much faster clock rates. Electrical clock signals have a theoretical limit of about 25 GHz for signal transmission of about 5 mm, while optical clock signals may extend into the THz range and travel much longer distances, thus allowing for a digital system with orders of magnitude faster performance capabilities.  
           [0007]    In the optical clock networks proposed for clock signal distribution, a network distributor generates or receives a clock signal, and that signal is then split into multiple signals by a Y-branch splitter, multimode interferometer or similar device. Each copy of the clock signal is then provided to one output waveguide, where all the output waveguides are of equal length to keep the copies of the clock signal in phase.  
           [0008]    While optical networks do not have the impedance load variation and parasitic problems of electrical domain networks, they have their share of shortcomings. One of the main problems affecting optical networks is modal confinement. For an optical signal to propagate and not lose its waveform or intensity, the signal&#39;s mode must be confined to the propagating waveguide. Further still, its mode profile must stay constant over the propagation length of that waveguide. This means that the waveguides must have a higher index of refraction differential with respect to their surrounding cladding layers. This also means that only waveguides of certain bending radii (typically quite large) are used to avoid bending losses. Unfortunately, large bending radii result in large devices and, as such, limit device scalability. The problem is multiplied with network complexity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is an illustration of a device having a clock distribution device and a digital microchip.  
         [0010]    [0010]FIG. 2 is an illustration of an optical distribution device with an H-tree distribution network that may be used as the clock distribution device of FIG. 1.  
         [0011]    [0011]FIG. 3 is an exploded illustration of a portion of the H-tree distribution network of FIG. 2, showing H-tree distribution devices of different size.  
         [0012]    [0012]FIG. 4 is a partial illustration of two waveguide layers each containing at least one H-tree distribution device.  
         [0013]    [0013]FIG. 5 is an illustration of a series of interconnected H-tree distribution devices of differing size.  
         [0014]    [0014]FIG. 6 is a side illustration of the structure of FIG. 5, taken along line AA, along with additional structure, showing evanescent coupling between H-tree distribution devices.  
         [0015]    [0015]FIG. 7 is an illustration of an example coupling between an output waveguide of an H-tree distribution device and a photodetector.  
         [0016]    [0016]FIG. 8 is an illustration of another example coupling between an output waveguide of an H-tree distribution device and a photodetector.  
         [0017]    [0017]FIG. 9 is a side illustration, similar to that of FIG. 6, of another coupling between H-tree distribution devices.  
         [0018]    [0018]FIG. 10 illustrates a system that includes an optical network routing an optical signal to a plurality of subsystems. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Various optical devices are described. Although the descriptions are provided in the context of propagating an optical clock signal, it will be understood by persons of ordinary skill in the art that the examples are not limited to the transmission of optical clock signals. The devices described may be used to distribute any information carrying optical signal. Furthermore, while the techniques described are provided in the context of distributing an input optical signal into a plurality of output signals, the techniques may be used on any number of optical devices to provide increased scalability and performance.  
         [0020]    [0020]FIG. 1 illustrates an example device  100  that includes an integrated chip module  102  mounted on a substrate  104  that may be a printed circuit board, for example. The integrated chip module  102  includes a mounting substrate  106  which may be a DIP package or Ball Grid Array (BGA), for example. The integrated circuit module  102  also includes a clock distribution device  108  and a digital microchip  110  mounted to the clock distribution device  108 . In the illustrated configuration, the clock distribution device  108  is mounted directly to the mounting substrate  106 .  
         [0021]    The digital chip  110  may be any known digital chip including a microprocessor, an application specific integrated chip (ASIC), chipset, or the like. Example digital chips include those from the Intel Corporation family of microprocessors, of which the Intel Pentium® microprocessor is an example. The digital chip  110  may represent a device having multiple subsystems that are each capable of receiving a separate clock signal.  
         [0022]    The clock distribution device  108  is an optical network capable of creating and distributing multiple clock signals in an optical domain. Further, the distribution device  108  is an integrated device with optical-to-electrical (O/E) converters that convert optical domain clock signals into electrical domain clock signals, which may then be provided to the digital chip  110  via a BGA  112 , in the illustrated example. The O/E converter is also termed a photodetector herein, of which a photodiode is an example. Also, the distribution device  108  may include output waveguides without O/E conversion to provide optical clock signals.  
         [0023]    In operation, the distribution device  108  is fed with a master clock signal via an input waveguide  114 . The clock signal may be from an external waveguide such as an optical fiber coupled to a clock generating circuit, not shown. The master clock signal is routed throughout the distribution device  108 , which creates as many copies of the master clock signal as desired. By way of example only, the distribution device  108  may be a 1×2×2 (1 to 4), 1×2×2×2×2 (1 to 16), 1×2×2×2×2×2×2 (1 to 64), or 1×2 N  distribution network, taking in one input clock signal and, in the latter example, generating 2 N  identical versions of that clock signal.  
         [0024]    The distribution device  108 , being an optical distribution network, will propagate clock signals of a much higher frequency than is achievable with traditional electrical distribution devices. Also, the distribution device  108  produces clock signals with reduced skew and reduced jitter, i.e., with less variation between the clock signals as compared to electrical distribution devices due to equal propagation length management by the optical network within the device  108 .  
         [0025]    [0025]FIG. 2 illustrates a top view of an example distribution device  200  that may be used as the distribution device  108 . The distribution device  200  is an optical device with two input optical waveguides  202  and  204 . The input waveguide  202  is connected to an H-tree distribution device  206  formed in a first section  208  and formed of three interconnected Y-branches  210 ,  212 , and  214 . Output branches  216  and  218  of the Y-branch  210  couple as inputs to the Y-branches  212  and  214 , respectively. The Y-branch  212  has output branches  220  and  222 , and the Y-branch  214  has output branches  224  and  226 . The three Y-branches  210 ,  212 , and  214  form the H-tree distribution device  208 , which is a 1×2×2 distribution device.  
         [0026]    With the Y-branches  212 ,  214 , and  216  each being 50/50 splitters and each being substantially the same in shape and dimension, the output branches  220 ,  222 ,  224 , and  226  will transmit substantially identical copies of the input clock signal on the waveguide  202 . Photodetectors  228 ,  230 ,  232  and  234  are used for O/E conversion of the signal on those output branches  220 ,  222 ,  224 , and  226 , respectively. The photodetectors  228 ,  230 ,  232  and  234  may be silicon photodiodes, for example.  
         [0027]    The input waveguide  204  is connected to a first H-tree distribution network  250  in a second section  251  of the device  200 . The network  250  has 32 output waveguides each connected to an O/E converter  252  and each providing an identical copy of an input signal received on the input waveguide  204 . For readability, not all of the illustrated O/E converters  252  in the network  250  are numbered with a reference numeral. Nonetheless, the converters  252  are substantially identical in the illustrated example to ensure that the clock signals produced by the network  250  are in phase when converted to the electrical domain. The H-tree network  250  is formed from a plurality of interconnected individual H-trees distribution devices. As illustrated in FIG. 3, the waveguide  204  is an input waveguide for a first H-tree distribution device  254 , or primary H-tree distribution device. The H-tree distribution device  254  includes four output waveguides  256 ,  258 ,  260 , and  262 , each carrying a substantially identical output signal derived from the input signal coupled from the input waveguide  204 . The output signals from the device  254  are attenuated copies of the input signal but are still in phase with one another. The output waveguide  258  is coupled to a secondary H-tree distribution device  264 , which has two output waveguides  266  and  268  coupled to H-tree distribution devices  270  and  272 , respectively. As with the H-tree distribution device  254 , each H-tree distribution device  264 ,  270 , and  272  receives a signal, an output signal from a next larger H-tree distribution device, and creates four substantially identical output signals. In an example, the input signal may be an optical clock signal, for example, one produced by a mode locked laser source.  
         [0028]    As depicted in the illustrations of FIGS. 2 and 3, the network  250  is formed of individual H-tree distribution devices of differing size. For example, the H-tree distribution device  254  is a level 1 structure that spans a larger area, as shown, than the H-tree distribution device  264 , which is a level 2 structure. Further, the H-tree distribution device  264  spans a larger area, as shown, than the level 3H-tree distribution devices  270  and  272 , which, in the illustrated example, span an identical area. The level indications used herein are for convenience purposes to describe H-tree distribution devices of different size in area and that exist within different waveguide layers, as will be described in more detail below. The area spanned by a particular H-tree distribution device, in plan view, includes at least the three Y-branches forming that device including at least a portion of the input and out waveguides for each of the three Y-branches.  
         [0029]    In the illustrated example, the differences between structures of different levels are not only differences in overall area, but are also differences in Y-branch radius of curvature. The H-tree distribution device  254 , for example, is formed of three Y-branches  274 ,  276 , and  278  (see FIG. 3), each with branches of identical radius of curvature. This radius of curvature is larger than the radius of curvature for three Y-branches  280 ,  282 , and  284  forming the H-tree distribution device  264 . That is, the radius of curvature on the level 1 structure is larger than the radius of curvature on the level 2 structure(s). Similarly, the radius of curvature on the Y-branches  280 ,  282 , and  284  is larger than the radius of curvature of the Y-branches (not labeled) forming the level 3 structures  270  and  272 .  
         [0030]    The network  250  is symmetrical and provides substantially identical input signals at the O/E converters  252 . As a result, although not depicted in FIG. 3, there is a second level 2 (or secondary) H-tree distribution device, substantially identical to assembly  264 , coupled to each of the output waveguides  254 ,  256 , and  262 . Further, each of these level 2 assemblies are connected to four level 3 assemblies that are themselves identical to one another, and for the illustrated example, would be identical to assemblies  270  and  272 . The assembly  264  is shown in FIG. 3 coupled to only two such level 3 structures ( 270  and  272 ) for simplification purposes only. FIG. 2 illustrates that the input waveguide  204  may be coupled to the first H-tree distribution network  250  and a second H-tree distribution network  286  that is identical to the network  250  and, as such, not described further herein.  
         [0031]    In addition to indicating relative size, the level indications on various structures depicted herein may also describe the location of the structure relative to other structures. FIG. 4 is a partial illustration of a device  300  formed of two waveguide layers  302  and  304  disposed adjacent one another. All of the waveguide layers described herein may provide a cladding region and a waveguide core region. The first waveguide layer  302  includes a level 1 structure in the form of an H-tree distribution device  306  (partially shown) that may be formed in the layer  302  through deposition, photolithographic, chemical etch, deposition again, and planarization processing, for example. A first Y-branch  308 , a second Y-branch  310 , and output waveguides  312  and  314  of the H-tree distribution device  306  are shown, and all are formed in the layer  302 . This layer  302  may be deposited, formed, clamped, or bonded on the layer  304 , after a level 2H-tree distribution device  316  (partially shown) has been formed therein. FIG. 4 illustrates a Y-branch  318  of the H-tree distribution device  316  having branching waveguides  320  and  322 . In line with the examples of FIGS. 2 and 3, the H-tree distribution device  306  has a radius of curvature, R 1 . As illustrated, the H-tree distribution device  316  has a radius of curvature, R 2 , where R 1 &gt;R 2 . That is, a tighter radius of curvature is used in the level 2 structure  316 . As the illustration of FIG. 4 shows, the device  300  has the H-tree distribution device  306  formed at a first depth of the device  300  and the H-tree distribution device  316  formed at a second depth of the device  300 , wherein the two depths are different The devices  306  and  316  may have portions that extend into or from different depths. For example, an input waveguide coupled to the first Y-branch  308  may extend from a different depth of the device  300  or from a different waveguide layer, not shown.  
         [0032]    The waveguide layers  302  and  304  may be formed of the same cladding material, for example a SiO 2  material as the cladding layer. Alternatively, each layer  302  and  304  may be formed of a different material. A SiO x N y  material, with x between and including 0 to 2 and y between and including 0 to 1.333, may be used as the waveguide core material. These x and y values are provided by example and apply to all SiO x N y  materials herein. Because the radii of curvature on the devices  306  and  316  are different, different SiO x N y  materials may be used in each of the layers  302  and  304 . For example, the device  316 , having a tighter radius of curvature than device  306 , may have a SiO x N y  material that produces a higher index of refraction differential between the waveguides and surrounding cladding, than would the SiO x N y  material used with the device  306 , which has a larger radius of curvature. As used herein, the term index of refraction differential, also referred to as the index of refraction contrast, refers to the difference between the index of refraction of a waveguide core region and a cladding region surrounding the core region, at a given reference wavelength.  
         [0033]    Coupling of an optical clock signal for the output waveguides  312  and  314  into level 2 structures is achieved by evanescent coupling in the depicted illustration, i.e., by having waveguides adjacent to one another so that energy may flow between the two. Other coupling, such as direct or butt-coupling may also be used. Specifically, the waveguide  312  is disposed above or adjacent the input waveguide  324 , thereby defining an overlapping or coupling length, D. In the illustrated example, the length D is chosen to ensure maximum coupling of energy. The waveguide  314  would similarly share a coupling length with a second level 2 structure (not shown) in the same manner. Furthermore, it will be understood that the level 2 structure  316  may also include output waveguides that are coupled to O/E converters or coupled to other structures like a level 3 structure in a third waveguide layer. In either case, the O/E converters or additional structure may alternatively be formed in the layer  304 . In fact, while FIG. 4 shows two different waveguide layers  302  and  304 , the H-tree distribution devices  306  and  316  may be formed at different depths in a single optical substrate and doped or fabricated to have different indices of refraction if desired.  
         [0034]    [0034]FIG. 5 illustrates a portion of an H-tree distribution network  400  formed of a level 1H-tree distribution device  402 , a level 2H-tree distribution device  404 , and two level 3H-tree distribution devices  406  and  408 . In the illustrated example, each level indication represents a structure of different waveguide dimension, width (FIG. 5) and height (FIG. 6). For example, an output waveguide  410  of the H-tree distribution device  402  may be wider than an input waveguide  412  of the H-tree distribution device  404  coupled thereto. An overlap between the waveguides  410  and  412  is shown for explanation purposes.  
         [0035]    [0035]FIG. 6 is an illustration of a side view of the device  400  taken along line AA of FIG. 5, with additional structure shown. The H-tree distribution device  402  is formed in a first waveguide layer  450 ; the H-tree distribution device  404  is formed in a second, smaller thickness waveguide layer  452 ; and the H-tree distribution device  406  is formed in a third, even smaller thickness waveguide layer  454 . Further, the substrates  450 ,  452 , and  454  are disposed adjacent one another, as illustrated, thereby allowing energy form one layer to couple into an adjacent layer. The waveguide layers  450 ,  452 , and  454  need not be in this configuration. Instead, a cladding or buffer region may extend between layers. In the illustrated example, the H-tree distribution devices  402  and  404  share a coupling region  456 . Similarly, the region  458  couples the H-tree distribution devices  404  and  406 . Evanescent coupling occurs over the regions  456  and  458 , in the illustrated embodiment. Alternatively, coupling between distribution devices may be through directional coupling, such as extending a coupling region between devices that otherwise do not touch one another. FIG. 9, described in more detail below, illustrates an example with a coupler extending between adjacent distribution devices.  
         [0036]    With structures of different size formed in different waveguide layers, a network device can be formed of H-tree distribution devices having different optical properties. In the illustrated example of FIGS. 5 and 6, the H-tree distribution device  402  is a level 1 structure with a level 1 index of refraction that is smaller than a level 2 index of refraction for the H-tree distribution device  404 . Thus, if cladding regions  460  and  462  of the layers  450  and  452 , respectively, have substantially the same index of refraction, the index of refraction differential (or contrast) between waveguide and cladding for the H-tree distribution device  402  (i.e., Δn1) will be smaller than the index of refraction differential (Δn2) for the H-tree distribution device  404 . Similarly, in the illustrated example, Δn3 for the H-tree distribution device  406  in the layer  454  having cladding region  464  is larger than Δn2. The cladding regions  460 ,  462  and  464  in the illustrated example may be made of SiO 2 , for example, and the H-tree distribution devices  402 ,  404 , and  406  may be made of SiO x N y , where x and y are chosen to form the desired Δn ratios. Example values are given above.  
         [0037]    For optical distribution devices of increasingly smaller sizes, the index of refraction differential may be increased to reduce any bending losses on the propagating optical signal. By way of example only, the radii of curvature of the Y-branches described may range from 500 μm to sub  10  μm and the index of refraction differential, An, may range from approximately 0.05 to approximately 1, with a larger differential corresponding to smaller radii of curvature. These ranges are merely examples, however, and the radii of curvature and index of refraction differential may be chosen in any manner to minimize losses and increase scalability for any desired size of the optical distribution structure.  
         [0038]    To provide O/E conversion, a fourth waveguide layer  466  is disposed adjacent the layer  454  and houses a photodetector  468  coupled to the H-tree distribution device  406 . In this three level configuration, the level 3H-tree distribution device  406  provides the output waveguides for the assembly  400 . The photodetector  468  may be a silicon-based photodiode, for example, and is shown evanescently coupled to the H-tree distribution  406 . The photodetector  468  may represent a separate device coupled to the H-tree distribution device  406 . Other photodetection methods may also be used.  
         [0039]    [0039]FIG. 7 shows another example of evanescent coupling in which an output waveguide  500  of an H-tree distribution device is disposed adjacent a photodetector  502  extending into a waveguide layer  504 . The photodetector  502  converts an optical clock signal coupled from the waveguide  500  into an electrical signal, which may be communicated to circuitry through a conducting lead  506 . FIG. 8 shows a similar structure to that of FIG. 7, but with the waveguide  500  direct or butt-coupled to a photodetector  508  formed at least partially extending above a surface of a waveguide layer  510 . An anti-reflection coating may be used between the waveguide  500  and the butt-coupled photodetector  508  to reduce reflection loss. A conducting lead  512  extends from an opposite end of the layer  510  to couple an electrical clock signal to external circuitry. Other techniques, such as angling adjacent faces of the waveguide  500  and the photodetector  508 , may also be used. In both FIGS. 7 and 8, the waveguide  500  may be a stand alone structure or may be formed in a layer (not shown) similar to that shown in FIG. 6.  
         [0040]    [0040]FIGS. 7 and 8 show example techniques for integrating an optical-to-electrical converter into a multilayer distribution network by forming a layer with a photodetector adjacent the output waveguides of an H-tree distribution network. For example, if the smallest structure in a network (e.g., a level 3H-tree distribution device in a three level structure) is formed in a bottom waveguide layer, a photodiode substrate layer may be coupled to this bottom waveguide layer. It will be understood by persons of ordinary skill in the art that other techniques may be used for such coupling and conversion. Furthermore, it will be understood that the photodetectors  468 ,  502 , and  508  may be silicon-based structures or formed of other materials. Germanium-based materials, for example, may be particularly useful for converting optical signals propagating at approximately 1550 nm or 1310 nm, whereas silicon-based photodetectors may be desired for wavelengths such as approximately 850 nm or shorter wavelengths such as 400-650 nm.  
         [0041]    [0041]FIG. 9 shows an optical device  600  with a generic coupling structure, where a first H-tree distribution device  602  is coupled to a coupler  604  that is also coupled to a second H-tree distribution device  606 . The coupler  604  may be a coupling region, like a bulk region through which modal transformation between the structures  602  and  606  occurs. The coupler  604  may be a tapered waveguide or a prism or mirror that reflects energy from the structure  602  into the structure  606 . Other coupling devices, whether based on modal transformation, like an interference-based optical structure, reflection, or the like will be known to persons of ordinary skill in the art. A similar coupler  608  couples the H-tree distribution device  606  with the H-tree distribution device  610 , which is, in turn coupled to a photodetector  612 .  
         [0042]    [0042]FIG. 10 illustrates a system  1000  that provides a clock signal, or other optical signal, to devices within the system  1000  or devices connected thereto. By providing identical copies of the clock signal, these devices may be synchronized together.  
         [0043]    The system  1000  includes a clock signal generator  1002  coupled to an optical network  1040 . The clock signal generator may be a 10 GHz mode locked laser providing an optical clock signal, such lasers are known and may have better than 100 fs jitter. An optical fiber or waveguide may be used in coupling the generator  1002  to the network  1004 . Other optical clock signal generators may also be used. The generator  1002  may alternatively provide an electrical clock signal that may be coupled to either the network  1004  directly or a separate device, where the electrical clock signal is converted to an optical clock signal. For example, an electrical clock signal may be used to modulate a laser to create an optical clock signal. The clock signal generator  1002  and the optical network  1004  may be integrated in a computer motherboard or on a microchip, or they may be separate devices.  
         [0044]    The network  1004  is coupled to a series of subsystems Sub 1   1006 , Sub 2   1008 , Sub 3   1010 ., SubN−2  1012 , SubN−1,  1014 , and SubN  1016 , providing an optical clock signal to each. The subsystems  1006 - 1016  may represent a circuit, microprocessor, chipset, memory, I/O interface, or other device that typically receives a clock signal in a processor system. The generator  1002  provides a clock signal to the network  1004 , which then creates identical copies of the clock signal. The copies are synchronized with the clock signal from the generator  1002  and are substantially identical in intensity and are in-phase. The network  1004  provides at least one copy to each of the subsystems  1006 - 1016 . The network  1004  is also connected to a separate subsystem  1018  that may receive a different clock signal from that sent to the subsystems  1006 - 1016 .  
         [0045]    The network  1004  may be a network of individual H-tree distribution devices like any of those described hereinabove.  
         [0046]    Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalence.