WO2022182777A1

WO2022182777A1 - Integrated optical tap manufactured using femtosecond laser writing

Info

Publication number: WO2022182777A1
Application number: PCT/US2022/017549
Authority: WO
Inventors: Jan Watté; Saurav Kumar; Cristina LERMA ARCE; Vivek PANAPAKKAM VENKATESAN
Original assignee: Commscope Technologies Llc
Priority date: 2021-02-25
Filing date: 2022-02-23
Publication date: 2022-09-01

Abstract

The invention is directed to allowing the fraction of light entering a tap waveguide in an integrated optical device to be adjusted. In some approaches a first substrate has a first waveguide and a second substrate has a second waveguide. The first and second waveguides are placed close to each other so that light can couple from one waveguide to the other. The relative position of the substrates is adjustable, so as to tune the amount of light coupled from one waveguide to the other. In other approaches, that use a splitter to tap a portion of the optical signal, the refractive index of cladding material around one of the branch waveguides is altered, thus altering the fraction of light entering that waveguide.

Description

INTEGRATED OPTICAL TAP MANUFACTURED USING FEMTOSECOND LASER WRITING Cross-Reference to Related Application This application is being filed on February 23, 2022 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Serial No.63/153,696, filed on February 25, 2021, the disclosure of which is incorporated herein by reference it its entirety. Field of the Invention The present invention is generally directed to optical communications, and more specifically to devices for tapping off a portion of an optical signal propagating within an optical fiber network. Background of the Invention Passive optical networks have become prevalent in part because service providers want to deliver high bandwidth communication capabilities to customers. Passive optical networks are a desirable choice for delivering high-speed communication data because they can avoid the use of active electronic devices, such as amplifiers and repeaters, between a central office and a subscriber termination. The absence of active electronic devices can decrease network complexity and/or cost and may increase network reliability. An example of an optical fiber network 100 used in providing optical communications to multiple end users is schematically illustrated in FIG.1. The network 100 includes a central office 102 that may be connected to provide internet service to the end users 104 or to a public switched telephone network (PSTN). The network 100 may connect to the end users 104 via one or more fibers 106 that are located overhead or housed within underground conduits. The central office 102 typically includes an optical transmitter 102a for transmitting signals to the end users 104 and an optical receiver 102b for receiving signals from the end users 104. In this embodiment, the end users 104 are domestic users, and the optical network 100 may be a fiber to the home (FTTH) network. The central office 102 is typically connected to a trunk fiber 106, which can also be referred to as a feeder fiber or feeder cable. A number of break-out locations 108 are located along the length of the feeder cable 106. At a break-out location 108 a portion of the optical signal is tapped from the feeder cable 106 using an optical tap and is then split by a splitter, often a passive splitter network, into a number of user signals that are directed along local fibers 110 to a subset of the end users 104. The fraction of the optical signal that is tapped off the feeder cable 106 may need to be different at different break-out locations. For example, if a first break-out location on the feeder cable that is relatively close to the central office and a second break-out location on the feeder cable that is relatively far from the central office both require to split off the same amount of power form the feeder cable, then the tap at the second break- out location needs to tap off a larger percentage of the optical power flowing along the feeder cable. In another example, a first break-off location may serve a larger number of local users than a second break-off location, in which case the amount of power that needs to be tapped out of the feeder cable at the first break-out location may be significantly higher than that tapped out at the second break-out location. A common current approach to forming an optical tap in a fiber is to use a fused biconical taper (FBT) in which two fibers are twisted and drawn under heating, to form a region where evanescent coupling is possible between the cores of the two fibers. This approach ends up in a package that has a larger footprint than is desired in many situations, because the resulting tap requires a loop of fiber to protect the pulled region, which has been mechanically weakened by the drawing process and needs to be protected. The FBT tap is then spliced into the fiber network, or coupled into the network using fiber connectors. In an alternative approach a tap, which may be viewed as being an asymmetric splitter, may be integrated on an optical chip along with a splitter network. However, this approach requires a dedicated mask for use in the lithographic manufacturing process. While it may be possible to manufacture a number of such devices having different tapping fractions on a single wafer, careful management of diced wafer is imperative. Consequently, there is a need for the operator of the optical fiber network to either maintain a large number of taps that tap off a different amounts of light, i.e. have different tapping fractions, or to have a supply of taps whose tapping fraction is adjustable. With current designs it can be expensive to produce waveguide optical taps that have a precisely engineered, fixed tapping fraction, and requires careful inventory management. Also, current solutions for optical taps having a variable tapping fraction are expansive and unreliable. Additionally, it is preferred in many cases that the optical tap and the splitter be integrated together so as to reduce complexity, coupling losses and footprint. Summary of the Invention One embodiment of the invention is directed to an optical device that has a first substrate comprising a first waveguide. The first waveguide has a first waveguide input and a first waveguide output. The first substrate has a first surface. A second substrate has a first surface facing the first surface of the first substrate. The second substrate has a tap waveguide optically coupled to a waveguide splitter network. The waveguide splitter network comprises a plurality of waveguide splitter outputs. At least a portion of the first waveguide is proximate the first surface of the first substrate and at least a portion of the tap waveguide is proximate the first surface of the second substrate so that, when an optical signal propagates along the first waveguide from the first input to the first output, a portion of the optical signal is coupled to the tap waveguide as a tap signal. The tap signal propagates to the waveguide splitter network to produce split output signals at the waveguide splitter outputs. Another embodiment of the invention is directed to an optical device that has a substrate and an input waveguide on the substrate. There is an optical tap portion at an end of the input waveguide. A main waveguide is coupled at the tap portion to receive light from the input waveguide. The main waveguide proximate the tap portion has an associated first refractive index difference between a refractive index of the main waveguide and a refractive index of a main waveguide cladding portion proximate the tap portion. There is a tap waveguide coupled at the tap portion to receive light from the input waveguide. The tap waveguide proximate the tap portion has an associated second refractive index difference between a refractive index of the tap waveguide and a refractive index of a tap waveguide cladding portion proximate the tap portion. The first refractive index difference is different from the second refractive index difference. Another embodiment of the invention is directed to an optical device that has a substrate. An input waveguide on the substrate has a termination and has a longitudinal axis. A first branch waveguide on the substrate has a first end proximate the termination of the input waveguide and is parallel to the longitudinal axis. The first branch waveguide is disposed on a first side of the longitudinal axis. A second branch waveguide on the substrate has a second end proximate the termination of the input waveguide and is parallel to the longitudinal axis. The second branch waveguide is disposed on a second side of the longitudinal axis. A cladding of the first branch waveguide at the first end of the first branch waveguide has a first refractive index and a cladding of the second branch waveguide at the second end of the second branch waveguide first waveguide has a second refractive index different from the first refractive index. The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. Brief Description of the Drawings The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: FIG.1 schematically illustrates an embodiment of a prior art fiber communication network; FIG.2 schematically illustrates in integrated optical tap/splitter unit according to an embodiment of the present invention; FIGs.3A and 3B schematically illustrate side views of an integrated optical tap/splitter unit having two substrates that are translatable to tune the fraction of light tapped from a first substrate to a second substrate, according to embodiments of the present invention; FIGs.4A and 4B schematically present plan views of the first and second substrates of FIG.3A, respectively, according to an embodiment of the present invention; FIGs.5A and 5B schematically cross-sectional views through the first and second substrates of FIG.3A, illustrating how relative movement between the first and second substrates affects the fraction of light tapped from the first substrate to the second substrate, according to an embodiment of the present invention; FIG.6 schematically illustrates another cross-sectional view through the integrated optical tap/splitter unit of FIG.3A showing the relative positions of the first waveguide in the first substrate and the output waveguides of the second substrate, according to an embodiment of the present invention; FIG.7 schematically illustrates a plan view of the integrated optical tap/splitter unit, according to an embodiment of the present invention; FIG.8 schematically illustrates a side view of an integrated optical tap/splitter unit having two substrates that are translatable to tune the fraction of light tapped from a first substrate to a second substrate, according to another embodiment of the present invention; FIG.9 schematically illustrates a plan view of an integrated optical tap/splitter unit having two substrates that are translatable to tune the fraction of light tapped from a first substrate to a second substrate, according to another embodiment of the present invention; FIG.10 schematically illustrates a plan view of an integrated optical tap/splitter unit having two substrates, showing a mechanism based on compression of a buckling column to translate one substrate relative to the other, according an embodiment of the present invention; FIGs.11A and 11B schematically illustrate end views of the optical tap splitter unit of FIG.10 under two different conditions of buckling column compression, showing movement of the first waveguide relative to the tap waveguide, according to an embodiment of the present invention; FIG.12 schematically illustrates a plan view of an integrated optical tap/splitter unit having two substrates, showing a mechanism based on deflection of two leaf springs to translate one substrate relative to the other, according an embodiment of the present invention; FIGs.13A and 13B schematically illustrate end views of the optical tap/splitter unit of FIG.12 under two different conditions of spring deflection, showing movement of the first waveguide relative to the tap waveguide, according to an embodiment of the present invention; FIG.14 schematically presents an integrated optical tap/splitter unit having an optical tap and a splitter network, illustrating various cladding areas where a refractive index change may be implemented to change the tapping fraction of the optical tap, according to an embodiment of the present invention; FIGs.15A-15C schematically illustrate different process steps that may be used to change the refractive index of cladding proximate a waveguide, based on changing the cladding material, according to an embodiment of the present invention; FIGs.16A-16B schematically illustrate different process steps that may be used to change the refractive index of cladding proximate a waveguide, based on femtosecond laser processing of the cladding material, according to an embodiment of the present invention; FIGs.17A-17C schematically illustrate different process steps that may be used to change the refractive index of cladding proximate a waveguide, based on doping the cladding material, according to an embodiment of the present invention; and FIG.18A schematically illustrates a different embodiment of optical splitter that may be used with the present invention; FIGs.18B and 18C respectively illustrate cross-sectional and plan views of the optical splitter of FIG.18A modified to provide a refractive index change to cladding proximate a waveguide, according to an embodiment of the present invention; FIG.18D schematically illustrates a variation of the optical splitter illustrated in FIG.18C, according to an embodiment of the present invention; FIGs.19A-19F schematically illustrate process steps of a first method for providing a region of modified cladding refractive index proximate a waveguide, according to an embodiment of the present invention; and FIGs.20A-20F schematically illustrate process steps of a second method for providing a region of modified cladding refractive index proximate a waveguide, according to an embodiment of the present invention. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. Detailed Description The present invention is directed to providing optical tap components that provide a desired tap fraction, i.e. that tap off a desired fraction of an optical signal propagating along a main fiber, and that split the tapped signal into a plurality of sub-signals. In particular, the present invention is directed to providing the tapping function and the splitting function in an integrated optical environment, for example in planar lightwave circuits (PLCs) implemented using glass, such as silica or borosilicate glass, or using polymers, or in optical circuits implemented on a semiconductor platform such as silicon or silicon nitride. Such components may be referred to as integrated tap/splitter units. An integrated tap/splitter unit typically includes a tap/splitter integrated optical element (IOE) contained within a housing that is provided with fiber connectors for connecting the integrated tap/splitter unit to the optical fiber network. Within the housing the tap/splitter IOE includes waveguides that are coupled, typically via optical fibers, to the fiber connectors. An exemplary tap/splitter IOE 200 is schematically illustrated in FIG.2. The IOE 200 is formed on a substrate 202 that has a tap portion, within the dashed rectangle 204, and a splitter portion, within the dashed rectangle 206. The substrate 202 may be formed of material suitable for an optical chip, such as silica glass, borosilicate glass, polymer, silicon or silicon nitride. An input waveguide 208 may be coupled to an upstream section of the optical feeder fiber (not shown) and receives the main optical signal onto the substrate 202. Within the tap portion 204, a tap waveguide 210 receives a tap fraction of the main optical signal from the input waveguide 208, while the untapped portion of the main optical signal propagates along the signal waveguide 212 to an output 214 that may be coupled to a fiber connector connected to a downstream portion of the feeder fiber (not shown). The tapped signal received from the input waveguide 208 propagates along the tap waveguide 210 to the splitter portion 206, where the tapped signal is split into split tapped signals by a splitter network 216. In the illustrated embodiment, the splitter network 216 includes two layers of Y-junction splitters 218, the first layer splitting the tapped signal into two portions and the second layer splitting the two portions into four portions. In some embodiments, the split tapped signals may all have the same power. In other embodiments one or more of the split tapped signals may have a larger or smaller power than the other split tapped signals. The splitter network 216 has a number of splitter outputs 220, which may be connected to respective fibers for propagating the split tapped signals to respective destinations. The illustrated embodiment includes a splitter network 216 having four splitter outputs 220, however it will be appreciated that the splitter network 216 may have a different number of splitter outputs, for example 2, 8, 16, or some other number. In some embodiments, it is useful to be able to tune the tapping fraction of the tap unit. An embodiment of an integrated tap/splitter unit that permits the tapping fraction to be tuned is described with reference to FIGs.3-5. FIG.3A schematically illustrates an integrated tap/splitter unit 300 that includes a first substrate 302 and a second substrate 304. The first substrate 302 is positioned with a first surface 306 on, or very close to, a first surface 308 of the second substrate 304. The first substrate 302 has a first waveguide 310 passing therethrough, from an input 312 to an output 314, for carrying the main optical signal. An input fiber 316 may be coupled to the input 312 and an output fiber 318 coupled to the output 314. The input and output fibers 316, 318 may be connected to the first substrate 302 using any suitable technique, for example via an alignment block 320, such as a v-groove alignment block. At least a portion 322 of the first waveguide 310, which may be referred to as the coupling portion, is located close to the first surface 306 of the first surface, at a distance sufficiently close as to permit optical coupling of light from the first waveguide 310 to a tap waveguide 324 in the second substrate 304. Thus, a portion of the main optical signal propagating along the first waveguide 310 may be tapped from the first waveguide 310 to the tap waveguide 324 at the coupling portion 322 of the first waveguide 310. In this embodiment, the coupling portion 322 of the first waveguide 310 is closer to the first surface 306 of the first substrate than are the input 312 and output 314 of the first waveguide 310. In other embodiments, the coupling portion 322 is not closer to the first surface 306 than the input 312 or output 314. The tap waveguide 324 has a coupling portion 322’ where light is coupled between the first waveguide 310 and the tap waveguide 324. In this embodiment, the coupling portion 322’ of the tap waveguide 324 is separated from the first surface 308 of the second substrate by a distance equal to the separation distance between the first surface 308 of the second substrate 304 and a waveguide splitter output 328. A bottom view of the first substrate 302, looking at its first surface 306, is schematically illustrated in FIG.4A, showing the core 326 of the input fiber 316 aligned with the input 312 of the first waveguide 310, and the core 328 of the output fiber 318 aligned with the output 314 of the first waveguide 310. The tap waveguide 324 is positioned close to the first surface 308 of the second substrate 304 so as to allow evanescent coupling from the first waveguide 310 to the tap waveguide 324. FIG.4B schematically illustrates a top view of the second substrate 304, looking at its first surface 308. The tap waveguide 324 feeds into a waveguide splitter network 326, for example a passive splitter network based on cascaded Y-couplers, although other types of splitting elements may be used. The splitter network 326 has a plurality of output waveguides 328 that are coupled to respective split output fibers 330. The split output fibers 330 have cores 332 that are aligned with the respective output waveguides 328. The split output fibers 330 may be aligned to the output waveguide using any suitable method, for example via v-groove alignment block 334, or a ribbon connector. Thus, the tapped signal that couples into the tap waveguide 324 from the first waveguide 310 is split into four separate portions by the splitter network 326, as split tapped signals. In some embodiments, the split tapped signals may all have the same power. In other embodiments one or more of the split tapped signals may have a larger or smaller power than the other split tapped signals. Another embodiment of the tap/splitter unit 300 is schematically illustrated in FIG. 3B. In this embodiment, the first waveguide 310 maintains a constant distance to the first surface 306 of the first substrate 302 along its length, and the tap waveguide 324 has a coupling portion 322’ that lies close to the first surface 308 of the second substrate 304, while the splitter network 326 is at a greater distance from the first surface 308. In this embodiment, the coupling portion 322 of the first waveguide 310, i.e. that portion of the first waveguide 310 that couples light to the tap waveguide 324, is not closer to the first surface 306 than the input 312 or output 314. In the illustrated embodiments, the splitter network 326 is a 1x4 splitter network, in other words the splitter network has one input that feeds four outputs. It will be understood that the splitter network may include a different number of outputs, for example 2, 8, 16 or some other number. The fraction of the optical signal tapped from the first waveguide 310 to the tap waveguide 324 is dependent, at least in part, on how close the first waveguide 310 approaches the tap waveguide 324. Thus, the tapping fraction may be changed by altering the separation distance between the first and tap waveguides 310, 324. A change in separation distance in this embodiment of tap/splitter unit 300 is described with reference to FIGs.5A and 5B. FIG.5A schematically illustrates a cross-sectional view of the first and second substrates 302, 304 at the section AA’, where the first waveguide 310 is positioned directly above the tap waveguide 324. However, lateral translation of one of the first and second substrates 302, 304 relative to the other, in the direction shown by the double-headed arrow, results in an increased distance between the waveguides 310, 324, which can result in a reduction in the tapping fraction. FIG.5B schematically illustrates relative positions of the waveguides 310, 324 after the first substrate 302 has been translated to the right, in a direction that is perpendicular to the axis of the coupling portion 322 of the first waveguide 310. FIG.6 schematically illustrates a cross-sectional view through the tap/splitter unit 300 at the section BB’, showing the relative positioning of the main waveguide 310 and the output waveguides 328. FIG.7 schematically illustrates a top view of the tap/splitter unit 300 from above the first substrate 302, showing the second substrate 304 and its associated optical fibers in dashed lines. In this embodiment, the output fiber 318 of the first substrate 302 is between two of the split output fibers 330 when viewed vertically. As used here, the term “vertically” means viewed in a direction perpendicular to the plane of the substrates. In other embodiments, the distance between the first waveguide 310 and the first surface 306 of the first substrate 302 may be constant across the first substrate 302, while the distance between the first surface 308 of the second substrate 304 and both the tap waveguide 324 and the splitter network 326 is constant, for example as illustrated in FIG. 8. The tap/splitter unit 350 includes a first waveguide 310 that is equidistant from the first surface 306 along its length. In some embodiments, the waveguide output 314 of the first substrate 302 may be located outside the outputs 328 of the second substrate 304 as viewed vertically, for example as is schematically illustrated for the tap/splitter unit 360 in FIG.9. The lower, second substrate 304 and its associated fibers 330 are shown in dashed lines. In an exemplary embodiment of this arrangement, the tap/splitter unit 360 includes a first waveguide 310 that is curved between its input 312 and output 314 such that its associated output fiber 318 is vertically located outside the output fibers 330 of the second substrate 304. In other embodiments, instead of the first waveguide 310 being shaped to permit the output fiber 318 to be vertically separated from the tap output fibers 330, the tap waveguide 324 and/or splitter network 326 may be shaped so that the output fibers 330 of the second substrate 304 lie vertically to the side of the output fiber 318. The tapping fraction may be monitored when the tap/splitter unit is being assembled, for example by passing an optical signal into an input fiber feeding the first waveguide on the first substrate, and monitoring one or more output signals from the output fibers on the second substrate. In some embodiments the tapping fraction is selected as a fixed value. The tapping fraction may be adjusted during assembly by laterally moving one of the substrates relative to the other to reduce or increase the optical coupling between the first waveguide and the tap waveguide. Once a desired tapping fraction has been achieved, the relative positions of the first and second substrates may be fixed, for example by flood curing the device with a UV lamp to cure an optical adhesive that is on the edges of the substrates and, in some cases, between the substrates. Thus the first and second substrates are in a fixed spatial relationship. For a tap/splitter unit having adhesive between the substrates, the refractive index of the adhesive may be advantageously selected to match the refractive index of the substrates, so as to reduce optical losses in the light coupled between the two substrates. In other embodiments the tap/splitter unit has an adjustable tapping fraction, so that technician may be able to “dial in” a desired tapping fraction before installation of the tap/splitter unit in the optical network, or even after it has been installed. In such cases, there may be a layer of index matching liquid between the first and second substrates to reduce coupling losses. An adjustable tap/splitting unit typically includes a mechanism that translates one of the substrates relative to the other so as to tune the tapping fraction. One exemplary embodiment of a tunable tap/splitting unit 1000 is schematically illustrated in FIG.10. In this embodiment, the first substrate 1002 sits on the second substrate 1004. The first substrate 1002 includes a first waveguide 1010 passing between an input 1012 and an output 1014. An input fiber 1016 and an output fiber 1018 are aligned by alignment blocks 1020, such as v-groove blocks, or any other suitable mechanism, to the input 1012 and output 1014 respectively. A tapping fraction of a light signal entering the first substrate 1002 via the input fiber 1016 is coupled from the first substrate 1002 to the second substrate 1004 in a manner like that discussed above, is passed through a splitter network and propagates out of the second substrate 1004 via splitter output fibers 1030. The splitter output fibers 1030 may be aligned to the output waveguides of the splitter network of the second substrate 1004 using any suitable approach, such as a v-groove alignment block 1034 or fiber ribbon connector. The relative positions of the first and second substrates may be adjusted using any suitable method. In the illustrated embodiment a spring mount 1040 is mounted to the first surface 1008 of the second substrate 1004. The spring mount 1040 may be formed of any suitable material, such as glass, semiconductor or the like. The spring mount 1040 may be attached to the first surface 1008 of the second substrate using, for example, a UV curable optical adhesive. One approach for manufacturing the spring mount 1040 integrally with a buckling column 1046 is to contour laser ablate a rectangular block of suitable material, e.g. glass. After the plate has been processed with femtosecond laser writing, the plate can be etched with a suitable etchant, to leave the spring mount integrally formed with a head portion 1042 and the buckling column 1046. The resulting spring mount 1040/buckling column 1046 may then be positioned and glued to the upper surface of the second substrate 1004. In another approach, the spring mount 1040 is formed from a first material, such as glass and the buckling column 1046 formed from a different material, such as polysulfone, polycarbonate or polyvinylidenefluoride. The buckling column 1046 may then be attached at one end 1044 to the spring mount 1040. The second end 1048 of the buckling column 1046 is in contact with a moveable head 1050. The moveable head 1050 is translatable in the directions shown by the double- headed arrow by a translating mechanism 1052, such as a micro-stepper motor or fine micrometer screw. The head portion 1042 and the moveable head 1050 may be provided with recesses to retain the ends 1044, 1048 of the buckling column 1046. As the buckling column 1046 is compressed by the moveable head 1050 translating in the direction towards the first end 1044 of the buckling column 1046, the middle portion 1054 of the buckling column 1046 is deflected outwards, away from the spring mount 1040. The first substrate 1002 is concomitantly translated in a direction away from the spring mount 1040. The first substrate 1002 may be attached to the middle portion 1054 of the buckling column, for example via laser welding. As the buckling column 1046 is released by the movable head translating in a direction away from the first end 1044 of the buckling column 1046, the deflection of the middle portion 1054 of the buckling column 1046 is reduced, and the first substrate 1002 is translated to towards the spring mount 1040. Thus, the first waveguide 1010 of the first substrate 1002 can be laterally translated relative to the tap waveguide 1024 in the second substrate, thus tuning the fraction of light tapped from the first waveguide 1010 into the tap waveguide 1024. In some embodiments, the first substrate 1002 may be held under a spring bias (not shown) that pushes its right side 1056 towards the spring mount 1040 so as to permit the first substrate 1002 to translate both left and right. FIG.11A schematically illustrates an end view of the of the tunable tap/splitter unit 1000, omitting the connecting optical fibers. The first substrate 1002 is in a position such that the first waveguide 1010 is directly above the tap waveguide 1024 in the second substrate 1004. This corresponds to a first position of the movable head 1050. FIG.11B schematically illustrates the same end view of the tunable tap/splitter unit 1000 with the movable head 1050 in a second position that is closer to the head portion 1042 than the first position. Consequently, the buckling column 1046 has been deflected outwards and the first substrate 1002 translated in a direction away from the spring mount 1040. As a result, the distance between the first waveguide 1010 and the tap waveguide 1024 has been increased, as can be seen by comparing the positions of the first waveguide 1010 relative to the vertical dot-dashed lines in FIGs.11A and 11B, which results in a smaller tapping fraction. Thus, controlled movement of the movable head 1050 results in controllable adjustment of the tapping fraction in the tap/splitting unit 1000. The first surface 1008 of the second substrate 1004 may be provided with one or more elements (not shown), such as ribs or the like, to guide movement of the first substrate 1002. Another exemplary embodiment of a tunable tap/splitting unit 1200 is schematically illustrated in FIG.12. In this embodiment, the first substrate 1202 sits on the second substrate 1204. The first substrate 1202 includes a first waveguide 1210 passing between an input 1212 and an output 1214. An input fiber 1216 and an output fiber 1218 are aligned by alignment blocks 1220, such as v-groove blocks, or any other suitable mechanism, to the input 1212 and output 1214 respectively. A tapping fraction of a light signal entering the first substrate 1202 via the input fiber 1216 is coupled from the first substrate 1202 to the second substrate 1204 in a manner like that discussed above, is passed through a splitter network and out of the second substrate 1204 via splitter output fibers 1218. The splitter output fibers 1218 may be aligned to the output waveguides of the splitter network of the second substrate 1204 using any suitable approach, such as a v- groove alignment block 1234. A spring mount 1240 is mounted to the first surface 1208 of the second substrate 1204. The spring mount 1240 may be formed of any suitable material, such as glass, semiconductor or the like. The spring mount 1240 may be attached to the first surface 1208 of the second substrate sing, for example, a UV curable optical adhesive. In other embodiments, the spring mount 1240 may be formed integrally with the second substrate 1204. For example, the upper surface 1208 of the second substrate 1204 may be exposed via an etching process that removes a layer of material from the substrate 1204, while the area of the spring mount 1240 is protected during the etching process. The spring mount 1240 includes sidewall 1242 facing the first substrate 1202. Two leaf springs 1244, 1246 are each attached at one end to the spring mount 1240 and at the other end to the first substrate. Each leaf spring 1244, 1246 is formed of a resiliently deformable material, for example glass, such as silica glass, or a polymer such as polysulfone, polycarbonate or polyvinylidenefluoride. The leaf springs 1244, 1246 are each curved in opposite directions, leaving between them an opening 1248 that overlaps an aperture 1250 through the second substrate 1204. Operation of the tunable tap/splitting unit 1200 is now described with reference to FIGS.13A and 13B. A tapered element 1252, for example in the form of a tapered shaft, is disposed through the opening 1248 between the leaf springs 1244, 1246 and through the aperture 1250 in the second substrate 1204. The vertical position of the tapered element 1252 is adjustable in a via the tapered element drive mechanism 1254, as shown with the vertical double-headed arrow. The tapered element drive mechanism may include a micro-stepper motor, a finer micrometer, or some other mechanism that can advance the tapered element in the desired direction. The tapered element 1252 may be made of any suitable material, such as glass, such as silica glass, a metal, such as aluminum, or a polymer such as polyphenylene sulfide. The dimension of the tapered element 1252 in the direction between the leaf springs 1244, 1246 is greater than the resting separation between the leaf springs 1244, 1246 so that, when the tapered element 1252 is pushed downwards through the second substrate 1204 by the drive mechanism 1254, the tapered element 1252 pushes apart the leaf springs 1244, 1246. As a result, the ends of the leaf springs 1244, 1246 attached to the first substrate 1202 move closer to the spring mount 1240, with a concomitant translation of the first substrate 1202 towards the spring mount 1240. FIG.13A schematically illustrates an end view of the tunable tap/splitting unit 1200 from the input side of the first substrate 1202, without the input fiber present. The tapered element 1252 is in a first position that corresponds to the first waveguide 1210 in the first substrate 1202 being located directly above the tap waveguide 1224 of the second substrate 1204. This first position of the tapered element 1252 corresponds to a first tapping fraction. FIG.13B schematically illustrates the same view, but with the tapered element 1252 in a second position. The width of the tapered element 1252 between the leaf springs 1244, 1246 is now greater than in the first position, which results in a translation of the first substrate 1202 towards the spring mount 1240. Consequently, the first waveguide 1210 has moved to a position to the left of the tap waveguide 1224, increasing the distance between the first waveguide 1210 and the tap waveguide 1224, as shown by comparing the position of the first waveguide 1210 relative to the vertical dot- dashed lines in FIGs.13A and 13B. Thus, controlled movement of the tapered element 1252 results in controllable adjustment of the tapping fraction in the tap/splitting unit 1200. The first surface 1208 of the second substrate 1204 may be provided with one or more elements (not shown), such as ribs or the like, to guide movement of the first substrate 1202. The embodiments for mechanisms that may be used for translating one substrate relative to the other illustrated in the figures above were based on the translation mechanism being anchored to the second substrate which is fixed in place. One result of this approach is that the substrate being moved is attached to only two fibers. It will be appreciated, however, that the translation mechanism may be placed on the first substate, which is fixed in place, in which case the mechanism may be operated to translate the second substrate, or it may be separate from both substrates. Another approach to controlling the tapping fraction of a waveguide tap on an optical chip is to adjust the refractive index of the cladding material around the branches of the tap junction. In general, increasing the refractive index difference between one waveguide branch and its cladding results in a larger fraction of light propagating along that waveguide, while a reduction in the refractive index different results in a reduction in the fraction of light propagating along that branch. FIG.14 schematically illustrates an optical chip 1400 having a tap portion 1402 and a splitter portion 1404. The input waveguide 1406 leads to a tap junction 1408 where a portion of the signal propagating along the input waveguide 1406 is diverted along the tap waveguide 1410. The remainder of the optical signal propagates along the main waveguide 1412. The tap junction 1408 may be any suitable type of waveguide coupler for use in an optical tap, such as a Y- branch coupler, or an adiabatic coupler. That portion of the optical signal propagating along the tap waveguide 1410 passes to a splitter network 1414, where it is split into different components that are presented to the splitter outputs 1416. The portion of the cladding close to, and downstream of, the tap junction 1408, which lies above and/or below the main waveguide 1412, generally shown as area 1418, may be referred to as the main waveguide cladding region. The portion of the cladding close to and downstream of the tap junction 1408, which lies above and/or below the tap waveguide 1410, and generally shown as area 1420 may be referred to as the tap waveguide cladding region. Numerical modelling of a Y-branch coupler has shown that changing the refractive index of one or both of the cladding regions 1418, 1420 can alter the fraction of light that passes into the tap waveguide 1410. A reduction in the refractive index of a cladding region results in a larger refractive index difference, ǻn, between the waveguide core and its cladding. This in turn leads to an increase in the amount of light propagating along that waveguide and less light propagating along the complementary waveguide. For example, if the refractive index of the main waveguide cladding region 1418 is reduced, then a larger fraction of the light passes into the main waveguide 1412, with a concomitant reduction in the amount of light passing into the tap waveguide 1410. In another example, if the refractive index of the tap cladding region 1420 is increased, reducing the ǻn of the tap waveguide 1410, then a smaller fraction of the light passes along the tap waveguide 1410, i.e. the tapping fraction can be reduced. Thus, a change in refractive index in the main waveguide cladding region 1418 results in a change in ǻnm for the main waveguide 1412 within the main waveguide cladding region 1418, where ǻnm is the difference between the refractive index of the material of the main waveguide core (taken as being the average refractive index across the main waveguide if the refractive index is not uniform across the main waveguide) and the cladding around the main waveguide 1412. Likewise, a change in the refractive index in the tap waveguide cladding region 1420 results in a change in ǻnt for the tap waveguide 1412 within the tap waveguide cladding region 1420, where ǻnt is the difference between the refractive index of the material of the tap waveguide core (taken as being the average refractive index across the tap waveguide if the refractive index is not uniform across the tap waveguide) and the cladding around the tap waveguide 1410. In conventional integrated tap units, ǻn_m is the same as ǻn_t. However, by engineering the cladding material to be different for one or both of the tap waveguide 1410 and main waveguide 1412 close to the tap junction 1408, ǻn_m is not the same as ǻn_t. The cladding regions 1418, 1420 are understood to cover those regions of the waveguides 1410, 1412 where the values of ǻnm and ǻnt can affect the fractions of light coupled from the input waveguide 1406 into the tap and main waveguides 1410, 1412. In a tap/splitter chip in which the waveguides 1406, 1410, 1412 are fabricated using conventional lithographic-based diffusion or ion implantation techniques in a silica substrate 1400, one or more of the cladding regions 1418, 1420, 1422, 1424 may be processed by etching a slot, through a mask, proximate the waveguide. This is schematically illustrated in FIG.15A, which shows a cross-sectional view through a substrate 1500 with a waveguide 1502. The waveguide 1502 may be disposed on top of a lower cladding layer 1504. An upper cladding layer 1506 is disposed over the waveguide 1502. The side areas 1508 of the waveguide layer 1510 may also be provided with cladding material to provide lateral confinement of the light propagating along the waveguide 1502. In some embodiments, the waveguide 1502 may be formed in the waveguide layer 1510 by doping selected portions of the cladding layer, e.g. in a low- index difference platform by doping silica with Ge or the like, with cladding layers such as layers 1504, 1506, 1508 formed of undoped material, e.g. undoped silica. In other embodiments, the waveguide 1502 may be formed in a high index difference platform, for example formed of a semiconductor such as silicon or silicon nitride, with cladding, such as layers 1504, 1506, 1508 formed of a lower index material such as silica. A mask is formed on top of the substrate 1500 to define the area where etching will take place. FIG.15B shows a slot 1512 etched in the upper cladding layer 1506. The slot 1512 may be etched using, for example, wet etching or dry etching such as reactive ion etching (RIE). The slot 1512 may then be backfilled to form an altered cladding region 1514 having a selected value of refractive index, as is schematically illustrated in FIG. 15C. The altered cladding region 1514 may be formed using any suitable material that produces the desired value of refractive index, such as a polymer or polymer blend. As an example, Ormocore and Ormoclad are polymers having respective first and second refractive indices, available from micro resist technology GmbH, Berlin, Germany. The two polymers may be used a blend with the ratio of the two polymers selected to produce a refractive index in the range between first and second refractive indices. The polymer or polymer blend may be applied via spin-coating and photo-cured to achieve the required value of refractive index at the coupler branch. Photocuring may be performed while passing an optical signal through the chip in order to monitor the tapping fraction in real time. After photocuring, the polymer or polymer blend may be planarized using a chemical polish so that the altered cladding region is coplanar with the upper cladding layer 1506. It may also be possible to alter the region of the lower cladding layer 1504 that lies below the waveguide 1502, for example using a method discussed below. Femtosecond laser processing of silica glass is known to be an effective method of increasing the refractive index of the glass. In cases where it is desired to increase the refractive index of a cladding material, for example to achieve a cladding refractive index that is closer to the core index, the cladding region 1418, 1420, 1422, 1424 may be processed after formation of the waveguide. FIG.16A schematically illustrates a substrate 1600 having a waveguide 1602 which is below an upper cladding layer 1606, and may also be above a lower cladding layer 1604. The waveguide 1602 may also be disposed between regions of cladding material 1608 in the waveguide layer 1610. A femtosecond laser system 1620 generates femtosecond light pulses which are focused by a focusing element 1622 into the region of the structure on the substrate 1600 that is to be processed. Moving the focus 1624 around the region in three dimensions results in femtosecond laser processing the region. In the embodiment illustrated in FIG.16B, the region 1626 of the upper cladding layer 1606 that lies above the waveguide 1602 has been processed, as is shown by hatching. The region 1628 of the lower cladding layer 1604 that lies below the waveguide 1602 may also be processed. Femtosecond laser processing may be used to increase the refractive index of silica by up to around 0.4-0.5%. A typical refractive index difference between the core and cladding of a waveguide in a low index difference platform such as silica may be in the range 0.8%-1.2%. Thus, femtosecond laser processing can make a significant change in the refractive index difference between the core and cladding. If the refractive index difference between the core and cladding refractive indices, ǻn, is 1%, i.e. the cladding refractive index has a value that is 99% of the waveguide core refractive index, then processing the cladding1604, 1606 may result in an increase in refractive index of the cladding of up to 0.5%, so that the resulting value of ǻn may be reduced to about 0.5%. Such a reduction in ǻn will tend to reduce the amount of light coupled into the waveguide 1602 if the waveguide 1602 forms one of the two output branches of a coupler. In another embodiment, the waveguide 1602 may be subject to femtosecond laser processing so as to increase the value of ǻn. For example, if the value of ǻn is 1% before femtosecond laser processing, and the femtosecond laser processing adds 0.05%, then ǻn may be as large as 1.5% after laser processing. Such an increase in ǻn will tend to increase the amount of light coupled into the waveguide 1602 if the waveguide 1602 forms one of the two output branches of a coupler. Another approach to tailoring the cladding refractive index is via doping, which uses lithographic techniques. FIG.17A schematically illustrates a substrate 1700 having a waveguide 1702 in a waveguide layer 1704. The waveguide 1702 may be formed using lithographic techniques along with ion implantation or diffusion to increase the refractive index of the waveguide 1702. FIG.17B shows the substrate 1700 and its accompanying layers after an upper cladding layer 1706 has been disposed over the waveguide 1702. A mask may then be placed over the substrate 1700 to define an area where the cladding refractive index is to be adjusted. The region of cladding exposed through the mask may then have its refractive index changed by, e.g., ion implantation or diffusion to form a modified cladding region 1708, as is schematically illustrated in FIG.17C. In a tap/splitter chip in which the waveguides 1406, 1410, 1412 are fabricated using femtosecond 3D laser writing, one or more of the cladding regions 1418, 1420, 1422, 1424 may be processed by etching a slot, through a mask, proximate the waveguide. This process is similar to that illustrated in FIGs.15A-15C, in which a mask covers the cladding region beside the waveguide that is to be modified. A slot is etched into the cladding, for example using RIE. The slot may then be backfilled using a sol-gel having selected value of refractive index. The sol-gel may then be photo-cured to achieve the required value of refractive index at the coupler branch. Photocuring may be performed while passing an optical signal through the chip in order to monitor the tapping fraction in real time. In another approach, the cladding may be modified using femtosecond laser writing, where the cladding modification results in a change in refractive index that is less than the change in refractive index adopted when writing the waveguide. Such an approach may be similar to that illustrated in FIGs.16A and 16, in which a cladding region beside the laser-written waveguide is processed using a femtosecond laser to increase its refractive index. Another type of Y-branch coupler is schematically illustrated in FIG.18A. A chip 1800 is formed with an input waveguide 1804 on a substrate 1802. The input waveguide 1804 may terminate at an optional tapered section 1806. First and second branch waveguides 1808, 1810 extend from the taper laterally displaced from the axis 1812 of the input waveguide 1804. In embodiments where the first and second branch waveguides 1808, 1810 are each separated from the axis 1812 by the same amount, d, and the widths of the first and second branch waveguides 1808, 1810 are the same, the Y-branch coupler is a 50:50 coupler. If one of the branch waveguides 1808, 1810 is separated from the axis 1812 by an amount that is greater than the other branch waveguide 1808, 1810, then the branch waveguide lying closer to the axis 1810 receives a greater fraction of the optical power propagating along the input waveguide 1804 than the branch waveguide lying further away from the axis 1810. Also, if one of the branch waveguides 1806, 1808 is wider than the other, then wider waveguide receives a larger fraction of the optical power from the input waveguide 1804. The fractions of light passing into the branch waveguides 1808, 1810 may be adjusted after the chip 1800 has been fabricated, or while the chip 1800 is being fabricated. A cross-sectional view through the chip 1800 is provided in FIG.18B. In one illustrative embodiment, the input waveguide 1804 is provided in a first waveguide layer 1814, which is above a first cladding layer 1816 and below a second cladding layer 1818. The branch waveguides 1808, 1810 are provided in a second waveguide layer 1820 that lies above the second cladding layer 1818 and below a third cladding layer 1822. The first, second and third cladding layers 1816, 1818, 1822 have refractive indices that are lower than the refractive indices of the waveguides they surround. The first, second and third cladding layers 1816, 1818, 1822 may be formed from different cladding materials or may be formed the same cladding materials. In some embodiments, the cladding material used for the cladding layers 1816, 1818, 1822 may be a polymer, or mixture of polymers, or a thermal oxide such as silica. Those parts 1824 of the first and second waveguide layers 1814, 1820 that lie laterally outside the waveguides 1804, 1808, 1810 may also contain cladding material, i.e. material whose refractive index is lower than that of the waveguides 1804, 1806, 1810 In the illustrated embodiment, the first, second and third cladding layers 1816, 1818, 1822 are each made using a first cladding material. FIG.18C schematically illustrates that a modified region of cladding 1826 in the third cladding layer 1822, comprised of a second cladding material and located above the second branch waveguide 1810, has replaced part of the first cladding material. In some embodiments, there is a modified region of cladding 1828 in the second cladding layer 1818, located below the second branch waveguide 1810. The modified region of the cladding 1828 in the second cladding layer 1818 may be formed of the second cladding material or a third cladding material. A plan view of the chip 1800 showing the modified region of cladding 1826 above the second branch waveguide is schematically illustrated in FIG.18C. The cross- sectional view in FIG.18B is taken at the cross-sectional line AA’. It should be appreciated that the modified cladding regions 1826, 1828 do not necessarily extend along the entire length of the second branch waveguide 1810, but may extend along only a part of the length. It will be appreciated that the first branch waveguide 1808 may also be provided with modified cladding regions. As a result of this structure, the refractive index difference between the waveguide and the cladding experienced by a light signal propagating along the waveguide with a modified cladding region is not constant, but changes with propagation distance along the waveguide. The branch waveguides 1808, 1810 need not have constant width, but may include respective tapered sections 1808’, 1810’, as is schematically illustrated in FIG.18D. Additionally, the input waveguide 1804 may terminate a point where there is lateral overlap between the input waveguide 1804 and the first and second branch waveguides 1808, 1810, as is schematically illustrated in FIG.18D. Thus, in a direction along the longitudinal axis 1812, the first and second branch waveguides 1808, 1810 have ends that overlap the termination of the input waveguide. In some embodiments, the substrate 1802 is a semiconductor substrate, such as a silicon substrate. The first, second and third cladding layers 1816, 1818, 1822 may be a thermal oxide such as silica, or a doped thermal oxide, or may be formed from a polymer or polymer blend, such as a uv-curable polymer, or from any other suitable material. The cladding material used for the modified cladding regions 1826, 1828, may be a thermal oxide such as silica, or a doped thermal oxide, or may be formed from a polymer or polymer blend. Polymers and polymer blends may be preferred as they provide a range of well controlled refractive index for the modified cladding region. The waveguides 1804, 1808, 1810 may be formed from a semiconductor material, for example silicon or silicon nitride, or may also be implemented in a silica-based device. A first process to provide modified cladding regions around a waveguide is now described with reference to FIGs.19A-19F. The process is described in the context of fabricating a structure like that shown in FIGs.18A-18C, although it will be appreciated that the process may be used with other waveguide architectures. A first waveguide 1804 is formed in a first waveguide layer 1814 over a first cladding layer 1816, with a second cladding layer 1818 over the first cladding layer 1816, as is schematically illustrated in FIG.19A. The second cladding layer 1818 is etched to form a well 1818a where a modified cladding region is desired, as is schematically illustrated in FIG.19B. The well 1818a may be formed using standard photolithographic methods including, for example, the formation of a photoresist mask, followed by an etching step which removes the unwanted material from the second cladding layer that is not protected by the mask, followed by rinsing and removal of the photoresist layer. A layer 1928 of the material of which the modified cladding region 1828 is to be made is next disposed over the second cladding layer 1818, filling the well 1818a in the process, as is schematically illustrate in FIG.19C. The layer 1928 of material may be deposited on the second cladding layer 1818 using any suitable method. Where the layer 1928 of material is a polymer or polymer blend, the layer 1928 may be deposited using spin-coating. Following a curing step, for example uv-curing of the polymer, the layer 1928 is removed, leaving the modified cladding region 1828 in the second cladding layer 1818.. One approach to removing the layer 1928 is chemical mechanical polishing. Next, a second waveguide layer 1820 is deposited over the second cladding layer 1818, and the first and second branch waveguides 1808, 1810 formed therein. A third cladding layer 1822 may then be deposited over the second waveguide layer 1820, as is schematically illustrated in FIG.19E. The third cladding layer 1822 may then be processed to include a modified cladding region 1826 in a manner similar to that described above for the modified cladding region 1828 in the second cladding layer 1818. The modified cladding regions 1826, 1828 may be formed using the same cladding material, or may be formed using respectively different cladding materials. A second process to provide modified cladding regions around a waveguide is now described with reference to FIGs.20A-20F. The process is described in the context of fabricating a structure like that shown in FIGs.18A-18C, although it will be appreciated that the process may be used with other waveguide architectures. A first waveguide 1804 is formed in a first waveguide layer 1814 over a first cladding layer 1816, with a second cladding layer 1818 over the first cladding layer 1816, and a second waveguide layer 1820 disposed over the second cladding layer 1818, as is schematically illustrated in FIG.20A. No waveguide has been formed in the second waveguide layer 1820 at this point. The second cladding layer 1818 is etched below the second waveguide layer 1820 at the point where it is desired to locate a modified cladding region in the second cladding layer 1818, to form a hollow cavity 1828a, as is schematically illustrated in the FIG.20B. The hollow cavity may be formed in buffered hydrofluoric acid following a photolithographic step. A layer 2028 of the material of which the modified cladding region 1828 is to be made (shown as hatched) is next disposed over the second cladding layer 1818, which also fills the cavity 1828a to form the modified cladding layer 1828, as is schematically illustrate in FIG.20C. The layer 2028 of material may be deposited on the second waveguide layer 1820 using any suitable method. Where the layer 2028 of material is a polymer or polymer blend, the layer 2028 may be deposited using spin-coating. Following a curing step, for example uv-curing of the polymer, the layer 2028 is removed from the second waveguide layer 1820, leaving the modified cladding region 1828 in the second cladding layer 1818, as is schematically illustrated in FIG.20D. One approach to removing the layer 2028 is chemical mechanical polishing. The first and second branch waveguides 1808, 1810 are then formed in the second waveguide layer 1820. A third cladding layer 1822 may be deposited over the second waveguide layer 1820 as is schematically illustrated in FIG.20E. The third cladding layer 1822 may then be processed to include a modified cladding region 1826 in a manner similar to that described above, as is schematically illustrated in FIG.20E. The modified cladding regions 1826, 1828 may be formed using the same cladding material, or may be formed using respectively different cladding materials. Any of the waveguides in the tap unit, including the input waveguide, the tap waveguide and the main waveguide, may be provided with one modified cladding region, either above or below, or two modified cladding regions, both above and below in order to tune the fraction of light that passes from the input waveguide to the main waveguide and to the tap waveguide. The waveguides in the tap/splitter units described herein may be formed using any suitable method. For example, in a glass substrate the waveguides may be formed using lithographic techniques, including diffusion and implantation through lithographically masked regions of the substrate surface, or by 3D femtosecond laser processing. 3D femtosecond laser processing is particularly suited to the embodiment illustrated in FIG.3 where the depth of the main waveguide 310 relative to the first surface 306 varies across the substrate 302. In semiconductor substrates, such as silicon substrates, waveguides may be made using standard semiconductor lithographic techniques to form waveguides, including buried waveguides, surface waveguides and ridge waveguides. A waveguide may be described as being on a substrate if it is formed within the substrate, for example via doping a part of the substrate, or if it is disposed within a layer on top of the substrate. The layer containing the waveguide need not be directly on the substrate surface, but there may be one or more intervening layers between the waveguide and the substrate. The intervening layers may also be described as being on the substrate. In the present description of the invention it has generally been assumed that light signals propagate from left to right across a figure, in which case ports on the left side of a device have been referred to as inputs, and those on the right as outputs. It will be appreciated that in many optical devices optical signals may propagate through the device in opposing directions. For example, in one direction a tap may operate to split out a portion of the main optical signal propagating along the feeder fiber from the central office for directing to one or more users. In the reverse direction, however, the tap operates as a combiner, combining signals from the one or more users to the main optical signal propagating along the feeder fiber to the central office. Accordingly, although various waveguides, fibers, and ports may be labeled as “input” and “output” in this description, it should be understood that these labels are used only for ease of description. It should be understood that the waveguides, optical fibers and ports described herein may carry operate as inputs for optical signals propagating in one direction but as outputs for signals operating in the opposite direction. It should be noted that the illustrations are not drawn to scale, but have been drawn to allow easy understanding of the locations of various elements. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices. As noted above, the present invention is applicable to fiber optical communication and data transmission systems. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. List of Elements in Specification 100 Network 102 Central office 102a Optical transmitter 102b Optical receiver 104 End users 106 Feeder cable 108 Break-out location 110 Local fibers 200 Tap/splitter IOE 202 Substrate 204 Tap portion 206 Splitter portion 208 Input waveguide 210 Tap waveguide 212 Signal waveguide 214 Output 216 Splitter network 218 Y-junction splitters 220 Splitter outputs 300 Tap/splitter unit 302 First substrate 304 Second substrate 306 First surface 308 First surface 310 First waveguide 312 Input 314 Output 316 Input fiber 317 Core 318 Output fiber 319 Core 320 Alignment block 322 Coupling portion 324 Tap waveguide 326 Splitter network 328 Output waveguide 330 Output fibers 332 Cores 334 V-groove alignment block 350 Tap/splitter unit 360 Tap/splitter unit 1000 Tunable tap/splitter unit 1002 First substrate 1004 Second substrate 1008 First surface 1010 First waveguide 1012 Input 1014 Output 1016 Input fiber 1018 Output fiber 1020 Alignment blocks 1024 Tap waveguide 1030 Splitter output fibers 1034 V-groove alignment block 1040 Spring mount 1042 Head portion 1044 First end 1046 Buckling column 1048 Second end 1050 Moveable head 1052 Translating mechanism 1054 Middle portion 1200 Tunable tap/splitter unit 1202 First substrate 1204 Second substrate 1208 First surface 1210 First waveguide 1212 Input 1214 Output 1216 Input fiber 1218 Output fiber 1220 Alignment blocks 1224 Tap waveguide 1230 Splitter output fibers 1234 V-groove alignment block 1240 Spring mount 1242 Sidewall 1244 Leaf spring 1246 Leaf spring 1248 Opening 1250 Aperture 1252 Tapered element 1254 Drive mechanism 1400 Optical chip 1402 Tap portion 1404 Splitter portion 1406 Input waveguide 1408 Tap junction 1410 Tap waveguide 1412 Main waveguide 1414 Splitter network 1416 Splitter outputs 1418 Cladding regions/area 1420 Cladding regions/area 1422 Claddings regions/area 1424 Cladding regions/area 1500 Substrate 1502 Waveguide 1504 Lower cladding layer 1506 Upper cladding layer 1508 Side area 1510 Waveguide layer 1512 Slot 1514 Altered cladding region 1600 Substrate 1602 Waveguide 1604 Lower cladding layer 1606 Upper cladding layer 1608 Cladding material 1610 Waveguide layer 1620 Femtosecond laser system 1622 Focusing element 1624 Focus 1626 Processed region 1628 Processed region 1700 Substrate 1702 Mask 1704 Waveguide 1706 Upper cladding layer 1708 Cladding region 1800 Chip 1802 Substrate 1804 Waveguide 1806 Waveguide 1808 Branch waveguide 1810 Branch waveguide 1812 Axis 1814 Waveguide layer 1816 First cladding layer 1818 Second cladding layer 1818a Well 1820 Second waveguide layer 1822 Third cladding layer 1824 Parts of waveguide layers 1826 Modified region of cladding 1828 Modified region of cladding 1828a Hollow cavity 1928 Layer of material 2028 Layer

Claims

Claims What we claim as the invention is: 1. An optical device, comprising: a first substrate comprising a first waveguide, the first waveguide having a first waveguide input and a first waveguide output, the first substrate having a first surface; a second substrate having a first surface facing the first surface of the first substrate, the second substrate comprising a tap waveguide optically coupled to a waveguide splitter network, the waveguide splitter network comprising a plurality of waveguide splitter outputs; wherein at least a portion of the first waveguide is proximate the first surface of the first substrate and at least a portion of the tap waveguide is proximate the first surface of the second substrate so that, when an optical signal propagates along the first waveguide from the first input to the first output, a portion of the optical signal is coupled to the tap waveguide as a tap signal, the tap signal propagating to the waveguide splitter network to produce split output signals at the waveguide splitter outputs.

2. The optical device as recited in claim 1, wherein the portion of the first waveguide proximate the first surface of the first substrate is separated from the first surface by a first separation distance which is less than a separation distance between at least one of i) the first waveguide input and ii) the first waveguide output, and the first surface of the first substrate.

3. The optical device as recited in claim 1, wherein the portion of the tap waveguide proximate the first surface of the second substrate is separated from the first surface of the second substrate by a distance less than a distance of between a waveguide splitter output and the first surface of the second substrate.

4. The optical device as recited in claim 1, wherein the first substrate and the second substrate are in a fixed spatial relationship.

5. The optical device as recited in claim 1, wherein one of the first and second substrates is moveable relative to the other of the first substrate and the second substrate so as to change a separation distance between the first waveguide and the tap waveguide.

6. The optical device as recited in claim 5, further comprising a translation mechanism fixed on the other of the first substrate and the second substrate, the translation mechanism being coupled to the one of the first substrate and second substrate, the translation mechanism being operable to translate the one of the first and second substrate relative to the other of the first substrate and the second substrate.

7. The optical device as recited in claim 6, wherein the translation mechanism comprises a compressible leaf spring, the one of the first and second substrate being coupled to the leaf spring so that when the leaf spring is compressed, the one of the first substrate and the second substrate is translated relative to the other of the first substrate and the second substrate.

8. The optical device as recited in claim 6, wherein the translation mechanism comprises a pair of leaf springs forming an opening therebetween and a tapered element, the one of the first and second substrate being coupled to the pair of leaf springs leaf spring so that when the tapered element in the opening displaces the leaf springs, the one of the first substrate and the second substrate is translated relative to the other of the first substrate and the second substrate.

9. The optical device as recited in claim 8, wherein other of the first and second substrates is provided with an aperture aligned with the opening between the pair of leaf springs to receive the tapered element.

10. The optical device as recited in claim 1, wherein the first waveguide has a coupling portion where light is couplable from the first waveguide to the tap waveguide, and the translation mechanism is operable to translate the one of the first substrate and the second substrate in a direction substantially perpendicular to an axis of the coupling portion of the first waveguide.

11. The optical device as recited in claim 1, further comprising an input fiber having a core coupled to the input of the first waveguide, and output fiber having a core coupled to the output of the first waveguide and a plurality of splitter output fibers having cores coupled to respective splitter waveguide outputs.

12. The optical device as recited in claim 11, wherein the input fiber and the output fibers are aligned with the respective input and output of the first waveguide by alignment blocks attached at respective ends of the first substrate.

13. An optical device, comprising a substrate; an input waveguide on the substrate, an optical tap portion at an end of the input waveguide; a main waveguide coupled at the tap portion to receive light from the input waveguide, the main waveguide proximate the tap portion having an associated first refractive index difference between a refractive index of the main waveguide and a refractive index of a main waveguide cladding portion proximate the tap portion; and a tap waveguide coupled at the tap portion to receive light from the input waveguide, the tap waveguide proximate the tap portion having an associated second refractive index difference between a refractive index of the tap waveguide and a refractive index of a tap waveguide cladding portion proximate the tap portion; wherein the first refractive index difference is different from the second refractive index difference.

14. The optical device as recited in claim 13, wherein the tap portion comprises a Y-coupler.

15. The optical device as recited in claim 13, wherein the tap portion comprises an adiabatic coupler.

16. The optical device as recited in claim 13, wherein the substrate is formed of a first material, and wherein one of the main waveguide cladding region, and the tap waveguide cladding region comprises an etched region that has been backfilled with a second material having a refractive index different from the refractive index of the first material.

17. The optical device as recited in claim 13, wherein the at least one of the main waveguide cladding region and the tap waveguide cladding region comprises femtosecond laser processed cladding material.

18. The optical device as recited in claim 13, wherein at least one of the main waveguide cladding region and the tap waveguide cladding region comprises doped cladding material.

19. The optical device as recited in claim 13, further comprising a splitter network on the substrate, a second end of the tap waveguide being coupled to the splitter network.

20. An optical device, comprising a substrate; an input waveguide on the substrate having a termination, the input waveguide having a longitudinal axis; a first branch waveguide on the substrate, the first branch waveguide having a first end proximate the termination of the input waveguide and parallel to the longitudinal axis, the first branch waveguide disposed on a first side of the longitudinal axis; a second branch waveguide on the substrate, the second branch waveguide having a second end proximate the termination of the input waveguide and parallel to the longitudinal axis, the second branch waveguide disposed on a second side of the longitudinal axis; wherein a cladding of the first branch waveguide at the first end of the first branch waveguide has a first refractive index and a cladding of the second branch waveguide at the second end of the second branch waveguide first waveguide has a second refractive index different from the first refractive index.

21. The optical device recited in claim 20, wherein the cladding of the first branch waveguide at the first end of the first branch waveguide lies in a cladding layer and the cladding of the second branch waveguide at the second end of the second branch waveguide first waveguide lies in the cladding layer.

22. The optical device as recited in claim 21, wherein the first and second branch waveguides are disposed between the cladding layer and the substrate.

23. The optical device as recited in claim 21, wherein the cladding layer is disposed between i) the first and second branch waveguides and ii) the substrate.

24. The optical device as recited in claim 20, further comprising an upper cladding layer above the first branch and second waveguides and a lower cladding layer below the first and second branch waveguides, wherein the cladding of the first branch waveguide comprises cladding material in both the upper and lower cladding layers and the cladding of the second branch waveguide comprises cladding material in both the upper and lower cladding layers.

25. The optical device as recited in claim 20, wherein the input waveguide is disposed in a first waveguide layer on the substrate and the first and second branch waveguides are disposed in a second waveguide layer on the substrate.

26. The optical device as recited in claim 25, wherein the first waveguide layer is disposed between the second waveguide layer and the substrate.

27. The optical device as recited in claim 20, wherein sides of the input waveguide proximate the termination are parallel.

28. The optical device as recited in claim 20, wherein sides of the input waveguide proximate the termination form a taper.

29. The optical device as recited in claim 20, wherein sides of the first branch waveguide proximate the first end and sides of the second branch waveguide proximate the second end are parallel.

30. The optical device as recited in claim 20, wherein sides of the first branch waveguide proximate the first end form a taper and sides of the second branch waveguide proximate the second end form a taper.

31. The optical device as recited in claim 20, wherein, in a direction along the longitudinal axis, the first and second ends of the first and second branch waveguides overlap the termination of the input waveguide.