Waveguides for high-density optical fanout, and corresponding systems, assemblies, and methods

Abstract

Systems and methods of forming a waveguide are presented herein. The system comprises a waveguide assembly comprising a substrate defining a substrate refractive index. The system further comprises a waveguide formed within the substrate defining a waveguide refractive index, which is different than the substrate refractive index. The waveguide comprises a first end defining a first width, a second end defining a second width, and a curved section extending between the first end and the second end. The curved section defines a third width extending between an inside edge and an outside edge. The third width is different than at least one of the first width and the second width.

Description

WAVEGUIDES FOR HIGH-DENSITY OPTICAL FANOUT, AND CORRESPONDING SYSTEMS, ASSEMBLIES, AND METHODS RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 63 / 668,564 filed on July 8, 2024, the content of which is relied upon and incorporated herein by reference in its entirety. FIELD

[0002] Embodiments herein relate generally to optical-electric printed circuit boards, and more particularly, to waveguide assemblies for high-density optical fan-out between optical elements. BACKGROUND

[0003] Benefits of optical fiber include extremely wide bandwidth and low noise operation. Because of these advantages, optical fiber is increasingly being used for a variety of applications, including, but not limited to, broadband voice, video, and data transmission. Connectors are often used in data center and telecommunication systems to provide service connections to rack- mounted equipment and to provide inter-rack connections. Accordingly, optical connectors are employed in both optical cable assemblies and electronic devices to provide an optical-to-optical connection wherein optical signals are passed between an optical cable assembly and an electronic device. The respective optical connectors of the optical cable assembly and the electronic device define an optical connector assembly.

[0004] However, as optical components and photonic integrated circuits decrease in size, the optical components may comprise varying number of optical ports and may define different pitches. To account for the variability between components, waveguides may be positioned between components to provide a connection therebetween.

[0005] Waveguides for connecting optical components, may require optical fanouts to provide the requisite number of connections on either side. A photonic integrated circuit (PIC) may have more ports than a fiber connector, and further the PIC may have a smaller pitch in comparison to the fiber connector. Thus, to transition between the pitch of the PIC and the pitch of the fiber connector, the waveguide may have a fanout which includes bends. However, curved (or bent)waveguides accumulate losses that are much larger than a straight waveguide, or may require a larger footprint (e.g., area) to reduce the losses while maintaining a curve. Thus, there exists a need to create a waveguide that provides an optical fanout while producing losses that are comparable to or less than a straight waveguide. BRIEF SUMMARY

[0006] Embodiments of the present disclosure are directed towards waveguide configurations which incur similar losses to or less losses than a straight waveguide, while taking up a smaller or similar footprint. The waveguide may be fabricated through an ion-exchange process and may comprise one or more loss mitigating designs.

[0007] In some embodiments, the waveguide may comprise a gradual change in the refractive index profile along the path of the waveguide. In this regard, the waveguide may define multiple widths along the path, with the widest part being along a curved section or the “bend” of the waveguide.

[0008] In some embodiments, the waveguide may utilize non-circular bends along the curved section. The curvature of the bend may gradually vary along the waveguide path lengths, and may include Euler curves, Bezier curves, or combinations thereof. The inside edge and the outside edge of the waveguide, specifically of the curved section may define different parametric profiles.

[0009] In some embodiments, the waveguide may utilize lateral offset between the first end, second end, and the curved section. The lateral offset improves mode matching between straight waveguides (e.g., the profile at the first end and the second end) and the curved section. The lateral offset may be used in similar embodiments between two intersections of two inversed covered portions, for example in an “S” curve.

[0010] In some embodiments, the waveguide may utilize a tapered transition between the straight section and the curved section. The taper may ease the refractive index transition, thereby preventing mismatch losses within the waveguide.

[0011] In some embodiments, the waveguide may comprise a compensation structure. The compensation structure may be formed on the substrate before, or in connection with the waveguide. The compensation structure may be formed on the inside edge of the waveguidethereby changing the effective refractive index profile and pulling the signal toward the inside edge of the waveguide.

[0012] In an example embodiment a waveguide assembly is provided. The waveguide assembly comprises a substrate defining a substrate refractive index. The assembly further comprises a waveguide formed within the substrate. The waveguide comprises a first end defining a first width, and a second end defining a second width. The waveguide further comprises a curved section extending between the first end and the second end. The curved section further defines a third width which is different than at least one of the first width and the second width. The waveguide defines a waveguide refractive index which is different than the substrate refractive index.

[0013] In some embodiments, the first end and the second end may each define single mode ports. In some embodiments, the first width and the second width may be the same.

[0014] In some embodiments, the first end may define a first axial centerline relative to the first width and the second end may define a second axial centerline relative to the second width. The curved line made define a curved centerline relative to the third width, and the curved centerline of the curved section may be offset from at least one of the first axial centerline and the second axial centerline.

[0015] In some embodiments, the difference between the substrate refractive index and the waveguide refractive index may be about 0.01. In some embodiments, the waveguide assembly may further comprise a compensation structure. The compensation structure may extend from the inside edge of the curved section to a compensation edge. The compensation structure may produce a refractive index gradient. In some embodiments, the waveguide may define a first interface between the first end and the curved section and a second interface between the curved section and the second end. The first width may change to the third width at the first interface, and the third width may change to the second width at the second interface. In some embodiments, the first end may be tapered so as to increase from the first width to the third width at a first interface, and the second end may be tapered from the second width to the third width.

[0016] In some embodiments, the third width may be uniform along the curved section. In some embodiments, the curved section may define a curved centerline with a radius of curvature in the range of 3 mm to 15 mm. In some embodiments, the inside edge may define a first radius ofcurvature, and the outside edge may define a second radius of curvature, and the first and second radii of curvature may be different.

[0017] In some embodiments, the inside edge may be defined by a first function and the outside edge may be defined by a second function, and the first and second functions may be different. In some embodiments, one of the first function and / or the second function may be one of a Euler curve or a Bezier curve. In some embodiments, the third width may vary along a path of the waveguide extending between the first end and the second end. In some embodiments, the first width and the second width may be minimum widths, and the third width may reach a local maximum where the outside edge may define a minimum radius of curvature.

[0018] In another example embodiment a system is provided. The system comprises a first element and a second element. The system further comprises a waveguide disposed between the first element and the second element. The waveguide assembly provides optical communication between the first element and the second element. The waveguide assembly comprises a substrate defining a substrate refractive index. The assembly further comprises a waveguide formed within the substrate. The waveguide comprises a first end defining a first width, and a second end defining a second width. The waveguide further comprises a curved section extending between the first end and the second end. The curved section further defines a third width which is different than at least one of the first width and the second width. The waveguide defines a waveguide refractive index which is different than the substrate refractive index.

[0019] In some embodiments, the first end and the second end may each define single mode ports. In some embodiments, the first width and the second width may be the same.

[0020] In some embodiments, the first end may define a first axial centerline relative to the first width and the second end may define a second axial centerline relative to the second width. The curved line made define a curved centerline relative to the third width, and the curved centerline of the curved section may be offset from at least one of the first axial centerline and the second axial centerline.

[0021] . In some embodiments, the waveguide assembly may further comprise a compensation structure. The compensation structure may extend from the inside edge of the curved section to a compensation edge. The compensation structure may produce a refractive index gradient. In some embodiments, the waveguide may define a first interface between the first end and the curved section and a second interface between the curved section and the second end. The firstwidth may change to the third width at the first interface, and the third width may change to the second width at the second interface. In some embodiments, the first end may be tapered so as to increase from the first width to the third width at a first interface, and the second end may be tapered from the second width to the third width.

[0022] In some embodiments, the third width may be uniform along the curved section. In some embodiments, the inside edge may be defined by a first function and the outside edge may be defined by a second function, and the first and second functions may be different.

[0023] In yet another embodiment a method of forming a waveguide is provided. The method comprises providing a substrate defining a substrate refractive index, and applying a diffusion mask onto a surface of the substrate. The method further comprises engaging in ion exchange diffusion between a salt bath and the surface through the mask opening. The ion exchange forms a waveguide below the surface of the substrate, defining a waveguide refractive index. The waveguide comprises a first end defining a first width, and a second end defining a second width. The waveguide further comprises a curved section extending between the first end and the second end. The curved section further defines a third width which is different than at least one of the first width and the second width. Notably there is a difference between the waveguide refractive index and the substrate refractive index. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0024] Reference will now be made to the accompanying drawings, which are not drawn to scale, and wherein:

[0025] FIG.1 illustrates an example waveguide fanout between a photonic integrated circuit (PIC) and optical connectors, as discussed herein;

[0026] FIG.2 illustrates a graph depicting a relationship between a mask opening size during fabrication and the refractive index, in accordance with some embodiments discussed herein;

[0027] FIG.3A illustrates a graph depicting the relationship between waveguide width and the refractive index, in accordance with some embodiments discussed herein;

[0028] FIGs.3B-C illustrate graphs depicting the relationship between waveguide width and losses, in accordance with some embodiments discussed herein;

[0029] FIG.4A illustrates an example waveguide, in accordance with some embodiments discussed herein;

[0030] FIG.4B illustrates a heat map showing the refractive index distribution across the waveguide of FIG.4A, in accordance with some embodiments discussed herein;

[0031] FIG.4C illustrates a graph depicting the relationship between the radius of curvature of the waveguide and the losses therefrom, in accordance with some embodiments discussed herein;

[0032] FIG.5 illustrates an example waveguide, in accordance with some embodiments discussed herein;

[0033] FIG.6A illustrates an example waveguide, in accordance with some embodiments discussed herein;

[0034] FIG.6B illustrates a graph depicting the relationship between the waveguide length and the losses, in accordance with some embodiments discussed herein; and

[0035] FIG.7 is a flowchart of example methods of forming a waveguide, in accordance with some embodiments discussed herein. DETAILED DESCRIPTION

[0036] Some example embodiments will not be described more fully herein with reference to the accompanying drawings, in which some, but not all example embodiments are shown. Indeed, the examples described and pictured herein should not be construed as being limiting to the scope, applicability or configuration of the present disclosure. Rather, these example embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout.

[0037] Directional terms as used herein- for example, up, down, left, right, front, back, top, bottom, vertical, and horizontal – are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0038] Optical systems provide high speed communication systems for transmitting, receiving, and processing optical signals with high efficiency and reliability. Integrating optical components into other systems may increase the bandwidth, and processing ability, while reducing the footprint of the system. As systems require greater processing power, maintaining efficiency and minimizing losses is a priority. However, as the size of the optical systems decreases, new connections may be required to provide similar results.

[0039] Optical systems provide data communication between different optical components. The optical system may be used to convert between optical signals and electrical signals. In thisregard, the optical system may receive one or more optical signals from an optical fiber and transmit the optical signal to a photonic integrated circuit (PIC) or similar processing device, where the optical signal is converted to an optical or electrical signal, transmitted to another optical device or otherwise processed. The components of the optical system may define different pitches, and a differing number of ports. In this regard, the optical system may be designed to guide the optical signals between the optical components.

[0040] FIG.1 illustrates an optical connection system utilizing a waveguide formed in a glass substrate to connect a photonic integrated circuit (PIC) and three optical connectors. The optical connection system is a fanout system 100 which provides data communication between a PIC 105 to one or more connectors 110a, 110b, 110c for example a multifiber push on (MPO) ferrule. The system 100 may be used in high performance-systems, for example data centers, or computer clusters which are used in connection with artificial intelligence.

[0041] The PIC 105 may comprise optical ports with a pitch of about 50 microns or less, taking up a first distance D1. In some embodiments, the first distance D1 may be about 2.4 mm.

[0042] The one or more connectors may comprise 16 connection (e.g., optical fibers) at a pitch of about 250 microns. Each of the connectors may define a width of about 7mm. Since the number of connection ports, and the pitch of the connections are different between the PIC 105 and the connectors 110a, 110b, 110c, there is not a one-to-one connection, and the components cannot be directly connected.

[0043] In order to connect the PIC 105 to the one or more connectors 110a, 110b, 110c a waveguide fan out 125 may be formed on a substrate 120. The waveguide 125 may be used to transmit the optical signals between one or more connectors 110a, 110b, 110c and the PIC 105 accounting for the connection difference and the pitch differential therebetween. Typically waveguides guide the electromagnetic waves from the PIC 105 to the respective connector 110a, 110b, 110c, which in turn guides the electromagnetic wave into an optical fiber cable 111a, 111b, 111c. In addition to guiding the signal between the two components, the waveguides 125 may route the location of the signal to account for the difference in pitch and size between the two components.

[0044] In order to fan out the signals between the PIC 105 and the connectors 110a, 110b, 110c, the waveguides 125 may each define a radius of curvature. The radius of curvature is the radius that fits the circle which most closely relates to the curve at that instance. For a constant curve,the radius of curvature is constant, while a changing curve defines multiple radii of curvature along the curve. The radius of curvature of the waveguide defines the length D2of the substrate 120 required for the connection between the optical components. In the illustrated embodiment, when the radius of curvature is 10mm, the required lengthof the substrate 120 is 23mm, and when the radius of curvature is 15mm, the required length D2 is 30 mm. In this regard, a larger radius of curvature requires a greater length D2.

[0045] Although waveguides may be utilized to connect optical elements, a curved waveguide may incur greater losses than, and require a larger footprint as compared to a similar straight waveguide. In the illustrated configuration, the waveguide 125 incurred losses between 0.3-1.0 dB / cm at a wavelength of 1310nm, while requiring up to 30 mm of space. As discussed, the losses would increase when including coupling losses (e.g., between the PIC 105 and the waveguide 125, and between the waveguide 125 and the connector 110a, 110b, 110c). In contrast, a straight waveguide may exhibit losses of less than 0.1 dB / cm.

[0046] Thus, there exists a need for a waveguide fanout which reduces losses while reducing or maintaining a smaller footprint across the waveguide, as electronics, and other implementations of the waveguide become smaller.

[0047] In order to reduce the footprint length of the waveguide, a novel waveguide may be utilized as discussed herein. A waveguide may be produced using the ion-exchange process. First, a wafer, for example a glass substrate, may be cleaned and polished to remove any surface contaminates. Then a diffusion mask, for example, a photoresist material may be deposited on to the surface of the glass substrate in a pattern which defines the waveguide structure. The substrate is then subjected to a molten salt bath, wherein the ions of the salt exchange with the ions in the glass substrate. The substrate is quenched and cooled to terminate the diffusion process, wherein the waveguide structure remains below the substrate surface. This process may be repeated one or more times to form the waveguide structure or form multiple waveguide patterns.

[0048] The ion-exchange process provides a customizable waveguide technology to make low loss integrated waveguides across the surface of a glass substrate with potential to reduce the size of optical systems.

[0049] Using the ion-exchange process, the refractive index of the waveguide (e.g., the portion which underwent ion-exchange) may be changed. In this regard, there is a difference in therefractive index between the substrate and the waveguide. In some embodiments, the ion exchange process may cause the waveguide to have a gradient refractive index along the path. In this regard, waveguide refractive index may be influenced by the width of the waveguide. In this regard, a wider waveguide width may create a higher refractive index than in a narrower waveguide. In an example embodiment, the waveguide may define a refractive index of at least 0.005, at least 0.01, or even at least 0.015. In other embodiments, the waveguide refractive index, may be about 0.005, about 0.01, or even about 0.015. In this regard, the greater the refractive index of the waveguide, the lower the bend loss, or a smaller bend radius may be achievable. Parameters within the ion-exchange process influence the amount of ions being exchanged throughout the process, thereby ions of different sizes and charges may be exchanged between the solution and the waveguide. The difference of methods of ion exchange may include the processing temperature, the duration of the exchange, the concentration of the ion source, the diffusion mask, mechanical agitation, and / or post treatment steps. Each of these parameters may influence the refractive index of the waveguide, and thus, the index difference between the waveguide and the substrate. Multiple ion-exchange processes with and without mask may be combined to integrate waveguides and waveguide compensation structures.

[0050] FIG.2 illustrates a graph 200 illustrating the relationship between the mask opening size and the difference in the refractive index n of the substrate and the waveguide, when formed under a first set of operating conditions. When the operating conditions are different the refractive index n may be different at different mask openings. In this regard, under a first set of operating conditions, a low contrast waveguide, with an index difference of 0.005 may be achieved with a diffusion mask opening of 3 microns. In some embodiments, a low contrast waveguide may define a refractive index n difference between 0.0042 – 0.0074, and a high contrast waveguide may define a refractive index n difference of greater than 0.0074.

[0051] While under a different set of operating conditions, a high contrast index difference of 0.01 may be achieved with a mask opening of 3 microns. In comparison to achieve an index difference of 0.01 under the first operating conditions of ion-exchange, illustrated in graph 200, the mask opening would have to be greater than 12 microns.

[0052] In some embodiments, a greater refractive index n may result in a higher acceptance angle of the total internal reflection of the waveguide. The higher acceptance angle provides a broader range of incident light rays to be effectively captured and guided through the system.Additionally, the greater refractive index n may result in mode confinement within the bends of the waveguide resulting in lower loss for tighter bends. Notably, when the light rays are maintained within the system the radiation losses and bend losses incurred through the waveguide are decreased.

[0053] Due to the birefringence of the waveguides in the order of 10-5or lower, the waveguide designs discussed herein may be applicable to both transverse electric (TE) and transverse magnetic (TM) polarization.

[0054] In TE polarization the electric field vector of the electromagnetic wave is perpendicular or transverse to the direction of propagation. Thus, in the optical waveguide, the light waves where the electric field oscillates perpendicular to the plane of incidence. The TE mode in the waveguide have the electric field predominantly confined within the waveguide, with minimal electric field in the substrate.

[0055] In TM polarization the magnetic field vector of the electromagnetic wave is perpendicular to the direction of propagation, rather than the electric field. The TM mode within the wave guide causes the magnetic field to be confined within the waveguide, while the electric field is distributed through both the waveguide and the substrate.

[0056] FIGs.3A-C illustrate the effect of the waveguide width on different properties and losses of the waveguide. The example effects were simulated with a guided mode at 1310 nm and with a glass waveguide with a low index contrast, i.e., a refractive index n difference between 0.0042 – 0.0074.

[0057] FIG.3A illustrates a graph 300 demonstrating the relationship between waveguide diffusion mask width and the effective refractive index at a first polarization 115 and a second polarization 117. In some embodiments, the first polarization 115 is TE polarization and the second polarization 117 is TM polarization. The effective refractive index is the average refractive index over the path of the waveguide. Here for both the first polarization 115 and the second polarization 117 the effective refractive index increases as the waveguide width increases.

[0058] FIG.3B illustrates a graph 400 depicting the relationship between the waveguide diffusion mask width and radiation losses for a straight waveguide for both the first polarization 115 and the second polarization 117. Radiation losses occur due to imperfect total internal reflection, bend losses, absorption, scattering, and mode mismatch. The radiation losses for boththe first polarization 115 and the second polarization 117 decrease at similar rates as the diffusion mask width of the waveguide increases. However, the loss from the first polarization 115 is slightly lower than the second polarization.

[0059] FIG.3C illustrates a graph 500 depicting the relationship between the waveguide diffusion mask width during ion-exchange and the total losses for a waveguide which includes a 90 degree bend with a radius of curvature of 30mm. Thus, in addition to the propagation loss of 0.47dB, the radiation loss decreases significantly with an initial widening of the waveguide diffusion mask width. The total losses of the waveguide decrease significantly when the waveguide diffusion mask width is widened from 2 microns to 4 microns. However, when the waveguide is widened to a diffusion mask width larger than about 4 microns, the losses stay constant as defined by the constant propagation loss and other sources of loss except bend radiation loss.

[0060] A waveguide which combines the loss reduction of a wider waveguide, and of higher index contrast may produce losses similar to or less than a straight waveguide, and beneficially may define a similar sized or smaller footprint (e.g., D2FIG. 1) than a straight waveguide while providing the requisite number of connections to each optical component. In some embodiments, the waveguide 130 may be used to connect different a first optical element and a second optical element. In some embodiments, the first optical element may be a PIC, a connector or similar. Similarly, the second optical element may be a PIC, a connector or similar. In this regard the waveguide 130 may be used to create connect a PIC to a connector, connect tow PIC’s or even connect two connectors.

[0061] FIG.4A illustrates a top-down view of an example waveguide 130 formed within a glass substrate. The waveguide 130 defines a curved section 135 extending between a first end 131 and a second end 133. The curved section 135 interfaces with the first end 131 at a first interface 127, and interfaces with the second end 133 at a second interface 128.

[0062] The curved section 135 of the waveguide 130 is bound by an inside edge 135a and an outside edge 135a. The curved section 135 further defines a curved centerline 136, which extends between the inside edge 135a and the outside edge 135b.

[0063] In some embodiments, the curved section 135 may define a circular 90-degree bend between the first interface 127 and the second interface 128. In some embodiments, the curved centerline 136 may define a radius of curvature, wherein the radius of curvature may be between3 – 20 mm, between 5 mm – 15 mm preferably between 5 mm – 10 mm, and more preferably between 3 mm – 7 mm. In other embodiments, the curved section 135 may define a non-circular profile where the curvature gradually changes along the path length. In still further embodiments, the curved section 135 may comprise multiple curved portions, for example, an s-bend. In this regard, each of the multiple curved portions may define the same curvature, or may define variable curvature.

[0064] The first end 131 may define a first width W1. The first width W1may allow for single mode operation. In an example embodiment, the first width W1may be 3 microns. In some embodiments, the first width W1may be sized to minimize coupling losses, for example, between a PIC and the waveguide 131.

[0065] The second end 133 may define a second width W2, which allows for single mode operation. In some embodiments, the second width W2 may be the same as the first width W1, while in other embodiments the second width W2 may be different than the first width W1.

[0066] In some embodiments, the curved section 135 may define a third width W3. The third width W3may be the distance between the inside edge 135a and the outside edge 135b. In some embodiments, the third width W3may be constant along the entire curved section 135, while in other embodiments, the third width W3may be variable along the curved section 135.

[0067] In some embodiments, at the first interface 127 the waveguide 131 may transition from the first width W1 to the third width W3, and at the second interface, the waveguide 130 may transition from the third width W3 to the second width W2.

[0068] In the illustrated embodiment the waveguide 130 may be a high contrast waveguide. In this regard the difference between the refractive index of the substrate and the waveguide may be greater than 0.074. The first end 131 and the second end 133 may be straight, and thus, may not incur bend losses, thus, the first width W1and the second width W2may be smaller than the third width W3. The curved section 135 may include radiation losses due to the curvature thereof, thus, to reduce the losses, the third width W3may be widened to reduce the losses incurred therein.

[0069] To further reduce losses the curved section 135 may be offset from each of the first end 131 and the second end 133. To explain, the first end 131 may define a first axial centerline 137 centered within the first width W1 and the second end 133 may define a second axial centerline 138 which is centered within the second width W2. In the illustrated embodiments, the curved centerline 136 of the curved section 135 is offset from at least one of the first axial centerline 137and / or the second axial centerline 138 of the first end 131 and the second end 133. In this regard, at least one of the first axial centerline 137 and the second axial centerline 138 is positioned closer to the outside edge 135b than the inside edge 135a. The offset provides maximal modal overlap, while minimizing mismatch losses between the different refractive index profiles. In embodiments comprising multiple curved sections (e.g., a “S” curve) the offset may also be used to reduce the mismatch losses.

[0070] To further reduce losses, a compensation structure 141 may be formed within the waveguide 120. The compensation structure 141 may be an additional diffusion mask which affects the background refractive index gradient, as illustrated in FIG.4B. The waveguide 130 has a first refractive index 130a, the substrates have a second refractive index 145 and the compensation structure has a third refractive index 142. The compensation structure 141 may define a separation distance S1 which extends between a compensation edge 141a and the curved centerline 136 of the curved section 135. The compensation structure 141 may induce bend compensation within the curved section 135 by causing pulling the signal towards the compensation structure 141. In some embodiments, the separation distance S1may be between 20-60 microns, between 30-50 microns or even between 35-45 microns. In some embodiments, the separation distance S1may be 45 microns.

[0071] Without the compensation structure 141, in the illustrated image when light propagates in the first end 131 the curved section 135 pushes the light towards the outside edge 135b. The compensation structure 141 may help drive the light towards the curved centerline 136 to prevent losses at the edges of the curved section 135.

[0072] The waveguide 130 may produce losses which are comparable to a straight waveguide as illustrated in FIG. 4C. In this regard, FIG.4C illustrates a graph 600 which demonstrates the relationship between the radius of curvature of the curved section to the losses incurred. The graph 600 shows a total loss 180 of the waveguide and breaks down the total loss 180 into a propagation loss 182, a mismatch loss 184, and a negligible radiation loss due to the bend 186.

[0073] The graph 600 illustrates that the total losses 180 increase as the radius of curvature increases. The main source of the loss which corresponds to the curvature is the propagation loss 182. Propagation losses occur when the waves travel through a medium. Additionally, the mismatch loss 184 due to the sudden change in width (e.g., at the first interface 127 and the second interface 128) is constant across various radius of curvatures, however, the mismatch loss184 accounts for a large portion of the total losses 180. The mismatch loss 184 may be mitigated, or otherwise reduced by introducing a taper in the first end 131 and / or the second end 133 as illustrated in FIG.5.

[0074] FIG.5 illustrates a waveguide 130’ which rather than including a straight first end 131 and a straight second end 133 as depicted in FIG.4A, utilizes a tapered first end 132 and a tapered second end 134. In this regard, the first width W1 gradually transitions to the third width W3. The gradual change in width also causes the refractive index profile to gradually change along the first end 131, which reduces the mismatch losses due to the small or negligible difference in the refractive index profile at the first interface 127 between the first end 132 and the curved section 135. Similarly, the losses may be reduced at the second interface 128 by introducing a taper into the second end 134, such that the third width W3, and refractive index profile gradually transitions to the second width W2. Thus, the losses due to the mismatch in of the refractive indices is minimized.

[0075] To explain, as discussed with reference to FIG.2 the mask width corresponds to the refractive index difference between the cladding and the core of the waveguide. Thus, as the width of the waveguide changes, the refractive index of the waveguide changes. Thus, when the waveguide 130 defines a rapid change between two widths there will be greater mismatch losses, than when compared to when the waveguide gradually changes widths.

[0076] In some embodiments, the slope of the taper of the first tapered end 132 and the second tapered end 134 may be the same, while in other embodiments the tapers may be different.

[0077] In some embodiments, the taper of the inside edge may define a first slope, and the taper of the outside edge may define a second slope. In some embodiments, the first slope may be the inverse of the second slope, in this regard the absolute values of the first slope and the second slope may be the same, while in other embodiments, the first slope and the second slope may define different absolute values.

[0078] To further mitigate and reduce losses within the waveguide a fully adiabatic waveguide may be formed, as illustrated in FIG. 6A-B. A waveguide 230 may be formed with a first end 231, a second end 233 with a curved section 235 therebetween. The first end 231 may define a fourth width W4 and the second end 233 may define a fifth width W5. The curved section 235 may be bound by an inside edge 235a and an opposite outside edge 235b. The curved section may define a curved centerline 236 which is centered relative to the inside edge 235a and theoutside edge 235b along the path of the waveguide. The curved section 235 may define a sixth width W6which may change. In some embodiments, one or both of the outside edge 235b, or the inside edge 235a may define a non-circular bend profile. In this regard, the curve may be a Euler bend, a Bezier curve, a parametric function, an exponential curve, or similar.

[0079] In the illustrated embodiment, the outside edge 235b may be bound by a Euler bend, which follows Fresnel integrals. In this regard, even though the inside edge 235a defines a constant curve, the gradual change of the outside edge 235b causes the sixth width W6to vary along the waveguide path.

[0080] In some embodiments, the sixth width W6may be defined by a function of the path length t, or in other terms as W(t). The function may be quadratic, linear, exponential, etc. In the illustrated embodiment shown in FIG.6A, the function is a pseudo-logarithmic function such that sixth width W6 increases from the fourth width W4, a local minimum, to a maximum width while the radius of curvature of the outside edge (e.g., the Euler bend) decreases from a maximum bend radius at the first end 231 to a minimum bend radius. In some embodiments, the maximum width of the sixth width W6may occur where the radius of curvature of the outside edge is at a minimum bend radius.

[0081] In some embodiments, to further decrease the losses a compensation structure 241 may be applied onto the substrate. The compensation structure 241 may define a second separation distance S2 extending from the inside edge 241a of the compensation structure to the centerline 236 of the curved section 235. The second separation distance S2 may vary along the path of the waveguide as a function S(t). In the illustrated embodiment the second separation distance S2 may be at a minimum near the minimum bend radius. The second separation distance S2 further introduces another degree of adiabaticity to the waveguide.

[0082] FIG.6B illustrates a graph 700 illustrating the accumulation of losses of the waveguide illustrated in FIG. 6A along the path length. In the illustrated embodiment the losses are within a fully adiabatic bend with an effective radius of 9.35mm, where the fourth width W4and the fifth width W5are 3 microns, and the sixth width W6at the maximum is 9 microns, and where the bend radius minimum is 5 mm, in a high contrast profile operating at 1310nm. As shown in the graph 700 the total loss of 0.15 dB closely resembles the propagation loss shown in FIG.4C, due to radiation-less nature.

[0083] The adiabatic propagation of the waveguide 230 provides for perfect mode matching, thereby reducing propagation losses, and provides for reduced radiation losses due to the higher index contrast within the tapering and the radiation-less bend, with the total loss dominated by only the background propagation loss. Example Flowchart(s)

[0084] FIG.7 is a flowchart illustrating an example method 800 for forming a waveguide within a substrate. At operation 810, a substrate is provided. The substrate may be a glass substrate. The substrate defines a surface. Optionally the substrate may be cleaned and polished. At operation 820, a diffusion mask is applied to the surface of the substrate. The diffusion mask may be a photoresist material. At operation 830, the substrate and the diffusion mask may engage in ion exchange diffusion. In some embodiments, ion exchange diffusion may consist of subjecting the substrate and diffusion mask to a molten salt bath, wherein the ions of the salt exchange with the ions in the glass substrate. Optionally, at operation 840 the substrate may be quenched and cooled to terminate the diffusion process, wherein the waveguide structure remains on the substrate surface. This process may be repeated one or more times to form multiple waveguide patterns.

[0085] Notably, the ion exchange process may be repeated multiple times with different ions. For example, a first ion exchange may be conducted utilizing K+, and a second ion exchange may be conducted using Ag-. In this regard, a first diffusion mask may be applied to the substrate, and the substrate may undergo ion exchange with the first ion. The diffusion mask may be removed from the substrate, and optionally the substrate may be quenched and cleaned prior to applying a second diffusion mask and undergoing a second ion exchange process with the second ion. After the second ion exchange the second diffusion mask may be removed from the substrate to reveal the waveguide. As mentioned, the ion exchange process may be repeated one or more times, with one or more different ions to create the desired waveguide, and compensation structure.

[0086] Notably, the above operations for FIG.7, while described in a certain order, may be performed in a different order and / or some of the operations may be performed simultaneously.Conclusion

[0087] It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements.

Claims

Claims:

1. A waveguide assembly comprising: a substrate defining a substrate refractive index; a waveguide formed within the substrate, wherein the waveguide comprises: a first end defining a first width; a second end defining a second width; and a curved section extending between the first end and the second end, wherein the curved section defines a third width extending between an inside edge and an outside edge, wherein the third width is different than at least one of the first width and the second width, wherein the waveguide defines a waveguide refractive index, wherein there is a difference between the waveguide refractive index and the substrate refractive index.

2. The waveguide assembly of claim 1, wherein the first end and the second end define single mode ports.

3. The waveguide assembly of any of claims 1-2, wherein the first width and the second width are the same.

4. The waveguide assembly of any of claims 1-3, wherein the first end defines a first axial centerline relative to the first width, wherein the second end define a second axial centerline relative to the second width, and wherein the curved section defines a curved centerline relative to the third width, and wherein the curved centerline of the curved section is offset from at least one of the first axial centerline and the second axial centerline.

5. The waveguide assembly of any of claims 1-4, wherein the difference between the substrate refractive index and the waveguide refractive index is about 0.

01.

6. The waveguide assembly of any of claims 1-5, further comprising a compensation structure, wherein the compensation structure extends from the inside edge of the curved section to a compensation edge, wherein the compensation structure produces a refractive index gradient.

7. The waveguide assembly of any of claims 1-6, wherein the waveguide defines a first interface between the first end and the curved section, and a second interface between the curved section and the second end, and wherein the first width changes to the third width at the first interface, and the third width changes to the second width at the second interface.

8. The waveguide assembly of claim 7, wherein the first end is tapered so as to increase from the first width to the third width at a first interface, and the second end is tapered from the second width to the third width.

9. The waveguide assembly of any of claims 1-8, wherein the third width is uniform along the curved section.

10. The waveguide assembly of any of claims 1-9, wherein the curved section defines a curved centerline with a radius of curvature within the range of 3 mm to 15 mm.

11. The waveguide assembly of any of claims 1-10, wherein the inside edge defines a first radius of curvature and the outside edge defines a second radius of curvature, wherein the first radius of curvature and the second radius of curvature are different.

12. The waveguide assembly of any of claims 1-7 wherein the inside edge is defined by a first function and the outside edge is defined by a second function, wherein the first function and the second function are different.

13. The waveguide assembly of claim 12, wherein the first function or the second function is one of a Euler curve or a Bezier curve.

14. The waveguide assembly of any of claims 12-13, wherein the third width varies along a path of the waveguide extending between the first end and the second end.

15. The waveguide assembly of any of claims 12-14, wherein the first width and the second width are minimum widths, and wherein the third width reaches a local maximum where the outside edge defines a minimum radius of curvature.

16. A system comprising: a first element; a second element; and a waveguide assembly disposed between the first element and the second element, wherein the waveguide assembly provides optical communication between the first element and the second element, wherein the waveguide assembly comprises: a substrate defining a substrate refractive index; a waveguide formed within the substrate, wherein the waveguide comprises: a first end defining a first width; a second end defining a second width; and a curved section extending between the first end and the second end, wherein the curved section defines a third width extending between an inside edge and an outside edge, wherein the third width is different than at least one of the first width and the second width, wherein the waveguide defines a waveguide refractive index, wherein there is a difference between the waveguide refractive index and the substrate refractive index.

17. The system of claim 16, wherein the first end and the second end define single mode ports.

18. The system of any of claims 16-17, wherein the first width and the second width are the same.

19. The system of any of claims 16-18, wherein the first end defines a first axial centerline relative to the first width and the second end defines a second a second axial centerline relative to the second width, and , and wherein the curved section defines a curved centerline relative to the third width, and wherein the curved centerline of the curved section is offset from at least one of the first axial centerline and the second axial centerline.

20. The system of any of 16-19, further comprising a compensation structure, wherein the compensation structure extends from the inside edge of the curved section to a compensation edge, wherein the compensation structure produces a refractive index gradient.

21. The system of any of claims 16-20, wherein the waveguide defines a first interface between the first end and the curved section, and a second interface between the curved section and the second end, and wherein the first width changes to the third width at the first interface, and the third width changes to the second width and the second interface.

22. The system of claim 21, wherein the first end is tapered so as to increase from the first width to the third width at a first interface, and the second end is tapered from the second width to the third width.

23. The system of any of claims 16-22, wherein the third width is uniform along the curved section.

24. The system of any of claims 16-23, wherein the inside edge is defined by a first function and the outside edge is defined by a second function, wherein the first function and the second function are different.

25. A method of forming a waveguide, the method comprising: providing a substrate defining a substrate refractive index; applying a diffusion mask on to a surface of the substrate; engaging in ion exchange diffusion between a salt bath and the surface through the mask opening, wherein the ion exchange forms a waveguide below the surface of the substrate, wherein the waveguide defines a waveguide refractive index, wherein the waveguide comprises: a first end defining a first width; a second end defining a second width; and a curved section extending between the first end and the second end, wherein the curved section defines a third width extending between an inside edge and an outside edge, wherein the third width is different than at least one of the first width and the second width, wherein there is a difference between the waveguide refractive index and the substrate refractive index.

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