Self-Supporting Flexible PWG Containing films: Waveguide containing films can have a broad range of waveguide density, waveguide dimensions, clad film thicknesses, fully connectorizable, and have been imaged with 12 by 18 inch photomasks. They are micromachinable for connectorization, and can be applied to diverse substrates or left stand alone. They are thermal mechanically stable and space qualified RAD hard and have a large range of properties summarized elsewhere. They have undergone repeatable bending 4000 times with 180 degree hairpin bends with 3mm radius of curvature by Rogers Corporation with no adverse performance impact. The flexible film in the photo has waveguides every 125 microns 50 microns high and alternating between 50 microns wide and 6 microns wide across the entire strip. One is illuminated See 10 for details on performance characteristics.
Extremely Sharp Waveguide Junctions: The monomer diffusion light induced self-development process to create the waveguides use an exposure process with direct contact to photomasks. Waveguide intersecting junctions for splitters and combiners do not require polymer removal to form the junction and thus are able to provide low loss splitters with very sharp junctions etc. An example of a sharp junction with a 1x16 splitter is in the photo.
Small millimeter range waveguide radius of curvatures (ROC) are produced with OILs technology: Having the smallest ROC is important for compact PWG routing and devices for both in-plane and out-of-plane bending. The curved waveguides in the photo were part of the HDP board level PWG study that included ROC threshold evaluation. OIL’s monomer diffusion based PWG formation produces very smooth side walls having no contact with mechanical tools or wet chemical or reactive ion contact to form the waveguides defining sidewalls. This results in OIL having very small ~5 millimeter or less in-plane ROCs before onset of bending loss as the bending radius decreases. OIL’s extremely smooth sidewall surfaces are determined by the photomask quality and minimally to the guide formation process which is very different to the other waveguide formation processes. For out-of-plane ROCs, OILs film coating process for the waveguide forming film layer creates even smoother top and bottom waveguide surfaces and thus even shorter/smaller out-of-plane ROC’s before bending loss occurs. The waveguide width in the plane of the bend is proportional to the ROC loss threshold value so that thinner guide widths produce the smallest low loss ROC. Details are reviewed in ref 10.
Closely Spaced Waveguide Splitter Arrays: Splitter arrays as shown in the photo can create star couplers to mix signals and distribute them. They can be configured in an N by M format and thus enable novel splitters where N and or M are continuous integers. This allows for example 1 to 5 splitting not achievable with standard tree branch splitting that requires even number outputs. If mixing region dimensions incur undesired balance outputs then internally imaged mode scramblers can be used to ensure full mode excitation in the mixing region and thus balanced outputs.
Mode Scrambling Low Refractive Index Deflectors Inside Waveguides: Multimode waveguide splitting requires a reasonable mode fill or encircled flux conditions. Empirically this is of the order 60 to 70% of higher angle mode excitation. Adequate mode fill is needed for stable coupling between different waveguides or fiber and waveguides to avoid thermal/mechanical throughput fluctuations or modal noise due to slight axially misaligned guides or guide interface variations in size etc. Even more importantly proper mode fill is needed for balanced stable waveguide splitting that does not fluctuate with thermal variations or mechanical /input conditions. The photo shows an enlarge region of a 50 micron wide waveguide with one set of three defectors. They are approximately 4 microns wide and about 40 microns deep through the entire waveguide. A cross section through the scrambler structure shows the low index deflectors through the waveguide. More detail is shown in Ref 10.
Low Refractive Index Deflecting Structures Inside waveguides for WDM’s and Crossovers: In addition to creating low index internal deflectors for mode scrambling and sensors described elsewhere, OIL technology has the ability to create internal guiding walls to continuously extend waveguide side walls into or within adjoining or crossing waveguides. This enables OIL to create lower loss crossing waveguides particularly for small angles less than 90 degrees in the 25 to 60 degree range. Additionally, inserting a dielectric filter at the narrow waveguide crossing region junction permits separation of different wavelengths for reflection into a returning waveguide and transmission of selected wavelengths through the dielectric filter into the waveguide on the other side.
Intersecting/crossing waveguides: Crossing waveguides can pass light signals through each other with no interference unlike electrically conducting wires. In other words light beams can pass through each other in space or when in waveguides. Multimode waveguides require mode filled operation for device / coupling interface stability and low modal noise. With most allowed higher angles (higher order modes) propagating light in the multimode waveguide the likelihood of leakage or even crosstalk at crossing junctions is high. Thus having internal low refractive index barriers as the waveguides pass through each other, that allows for light guiding through what is usually an unguided region in the crossing region, reduces the leakage or waveguide to waveguide crosstalk interference. OIL has found that shallow 25 to 60 or so crossing angles with the internal guiding barriers are more effective than even the 90 degree crossing angle for improving performance.
OIL’s graded refractive index profiles for GI optical fiber coupling: Exposure parameter modifications enable the OIL process to produce graded refractive index profiles more compatible with coupling with low loss to graded index optical fibers. Currently the best graded profile is achieved in the plane of the waveguide forming film. This will be shown in greater detail in ref 10. Refractive index profiles can be defined using a shearing interference microscope as in the photo with details elsewhere. The interference microscope provides interference fringe patterns that are indicate lower refractive index at the guide edges. For step index profiles the fringes are flat across the wave guide width. These two results are shown in the photo and elsewhere. Some initial results have been demonstrated with refractive index grading at the top and bottom of waveguides in addition to side to side in the waveguide forming layer but the process is not yet optimized.
OIL’s graded index relative to step index profiles can be visualized in the PWG near field: Graded refractive index waveguides have a higher index near the center of the waveguide. In optical fibers the index profile is close to being parabolic with high power propagated in the fiber center relative to the fiber core edges. The same is shown for OIL’s PWG with the power concentrated near the center for graded index profiles and relatively uniform near fields for step index profiles We can control the diffusion profile within the layer by parameter modifications during exposure. Details are reviewed elsewhere ref `10.
OIL Graded Waveguide Refractive Index. OIL is able to modify the multiple monomer ratios to control guiding properties: OIL can design and control the maximum waveguide refractive index or numerical angle (NA) propagation properties for matching optical fibers or other needs. Formulation modifications of the ratios of several monomers in our formulation does not alter the unique self-development diffusion process for creation of waveguides but does change the maximum refractive index. NA of 0.23 to closely match OM3 and OM4 max NA for optical fibers; and similarly NA near 0.28 to 0.3 to match the max NA of OM1 fibers is achieved. For special applications we have demonstrated NA’s near 0.4 or more in studies underway to more closely match silicon chip waveguides to assist coupling of integrated photonic chip generated signals to off chip connectivity. The photo demonstrates the PWG refractive index difference for the two formulations. The shearing interference microscope photos show fringe deflection of about 1.2 on the left to nearly 2 for a similar thickness waveguide. This fringe shift difference corresponds to an NA shift from approximately 0.22 to 0.3 both with graded index profiles. Guide dimension and shearing microscope parameters make the photos look somewhat different dimensionally. The key point is that OIL can modify guiding properties with very slight changes in the ratios of several monomers and not have to change the basic formulation.
OIL Wavegujide Eye Diagram. Capability for propagating high data rates from 10GB/s to 30GB/s and beyond is essential for data center and high speed computing: As signals are wrapped electronically they produce “eye diagrams” as in the photo. When there is little modal noise or signal distortion eye diagrams provide clean patterns with a wide opening in the center indicative of excellent performance at the data rate evaluated. The results shown here produced by Cisco are for 10 GB/s operation for OIL’s PWG starting at the top with high index or NA ~ 0.3, in the middle with lower index or NA ~ 0.23 and in the bottom eye diagram for a graded index OM4 optical fiber. They compare favorably. Tests are planned for 20 and 30 GB/s.
OIL’s technology can produce single mode waveguides and devices: initial development of OIL’s technology emphasized single mode waveguides and splitters that were connectorized using some unique techniques. This work was reported in presentations and publications. The most complete review is a chapter in a reference text edited by Hornak. (ETC ) OIL’s dedicated single mode formulations emphasized 1300nm and 1550 nm spectral regions with 6 micron square waveguides and a relative index over the surround of 0.0055 to 0.006 to produce coupling overlap (diagonal of 8.5 microns for 9 micron OF) and an NA to practically match standard optical fiber. For the 850 nm region the guide index needed to be lowered to 0.0033 or so for ideal single mode coupling with 850 nm optimized optical fiber. As shown in the photo the PWG near fields at the top and optical fiber near fields are at the bottom. On the left for 850 nm operation both PWG and fiber are slightly double moded as they are designed for the longer wavelengths. The 1300nm is in the middle and 1550 nm range on the right. For these longer wavelengths the PWG match with single mode optical fiber near and far fields(not shown) is excellent. Very novel coupling techniques are all described in the reference text. Since the work was done 20 to 25 years ago other connectorization approaches would likely be considered at this time. The important thing to remember is that single mode guides have an evanescent field that extends out some 20 microns so that clad layers and guide spacing needs to take this into account. On the other hand multimode waveguides that are reasonably mode filled (encircled flux) can be a few microns apart with no cross talk as discussed in ref 10. The point being that applications for either single and/or multimode must be considered with this in mind.