Multimode Polymer Waveguides Research Articles

Epoxy resin multimode optical waveguide fabricated by non-contact photolithography

Multimode polymer waveguides were created using epoxy resin materials. The fabrication process used non-contact photolithography, which is a simple and cost-effective method. During the experiments, a reliable and efficient developer was obtained. The samples were analyzed using scanning electron microscopy and the results showed that the ridge waveguides had a smooth surface and vertical sidewalls. The cross-section size was approximately 50 × 55 μm2. The waveguides had a transmission loss was about 0.08 dB/cm at a wavelength of 850 nm, which was measured using the cut-back method. Additionally, the waveguides were found to be stable in thermal and damp heat conditions.

Optical printed circuit boards with multimode polymer waveguides and pluggable connectors for high-speed optical interconnects.

We demonstrate the development of optical printed circuit boards (OPCBs) containing multimode polymer waveguides and pluggable optical connectors. The basic optical characteristics of the PCB-embedded waveguides, waveguide connectors, and high-speed performance were comprehensively evaluated. The fabricated OPCB comprises eight electrical layers and one optical layer. Waveguides are terminated at both ends with MT/MPO connectors. The optical channels comprising 10 cm-long waveguides embedded in OPCBs with two connectors show an average insertion loss of 6.42 dB. The resulting coupling loss is 0.77 dB per interface, which is very low and to our knowledge is among the lowest reported to date for waveguides embedded in rigid PCBs. 30 Gbps per channel NRZ data transmission was demonstrated with a measured waveguide bandwidth of 23 GHz × m, which gives a possible data traffic of 720 Gb/s for such 24-channel parallel optical link. Our efforts lay the foundation for the further development of OPCBs with higher performance.

Investigation on mode dispersion and lamination stability of multimode polymer waveguides for an optical backplane.

In this paper, two noteworthy issues of mode dispersion and lamination stability of multimode polymer waveguides for optical backplane are investigated. In the case of center launching by 50-µm graded-index (GI) multimode fiber (MMF), mode dispersion of polymer waveguides with different widths is analyzed theoretically and measured in the view of bit error rate (BER) curves. Compared with the waveguide with the width of 40 µm, 1-dB power penalty is observed by the 70-µm-width waveguide due to its larger mode dispersion. On the other hand, waveguide stability after laminating process with high temperature and pressure is measured experimentally. No significant changes in core shape and size are observed. The average insertion loss of 80 channels before and after lamination are 0.137 dB/cm and 0.192 dB/cm, respectively. Error-free transmission at 25 Gb/s is obtained by laminated waveguides. The results imply the feasibility and potential of multimode waveguides for optical backplane.

Direct bandwidth measurement of multimode waveguides based on an optical sampling technique.

We demonstrate direct bandwidth measurement of 11-cm-long multimode polymer waveguides based on an optical sampling technique for the first time, to our knowledge. The pulse shape can be recovered after transmission due to the advantages such as high bandwidth and high refresh rate of optical sampling technology. A reduction in averaged bandwidth (bandwidth-length product) from 241 GHz (27 GHz·m) to 180 GHz (20 GHz·m) of straight waveguides is observed when using mode scramblers to fully stimulate the higher-order modes. The effects such as bending and crossing structure of the waveguides on the bandwidth are also investigated. The proposed method is effective for measuring the bandwidth and dispersion of meter- and centimeter-long waveguides, fibers, and optical devices.

Bend- and Twist-Insensitive Flexible Multimode Polymer Optical Interconnects

Polymer multimode optical waveguides can enable high-speed short-reach optical interconnection at low cost within high performance electronic systems. The formation of such waveguides on flexible substrates can offer important additional advantages such as light weight, ability to be tightly bent, and reconfigurability which are particularly important in environments where space, weight, and shape conformity are critical, for instance in vehicles and aircraft. The ability of such flexible optical interconnects to be tightly bent and twisted with low excess loss is crucial in enabling their use in systems with limited space and with movable parts. As a result, in this work, we present a new design of such flexible polymer multimode waveguides that achieves improved bending loss performance over the conventional waveguide design. It is experimentally shown that the proposed design achieves a very low excess loss of 0.5 dB for a 3 mm radius bend under a 50 μm MMF launch. In comparison, flexible waveguides with the conventional design exhibit a 2 dB excess loss under the same launch and bend conditions. Additionally, useful rules that associate the twisting loss performance of flexible polymer waveguide samples with their geometric characteristics are derived. It is shown that negligible twisting losses (<0.1 dB for a 50 μm MMF input) can be achieved when the dimensions of the waveguide samples are appropriately selected. The results demonstrate the strong potential of such bend- and twist-insensitive flexible polymer waveguides for use in next-generation vehicles and aircraft.

SPR Biosensor Based on Polymer Multi-Mode Optical Waveguide and Nanoparticle Signal Enhancement.

We present a surface plasmon resonance (SPR) biosensor that is based on a planar-optical multi-mode (MM) polymer waveguide structure applied for the detection of biomolecules in the lower nano-molar (nM) range. The basic sensor shows a sensitivity of 608.6 nm/RIU when exposed to refractive index changes with a measurement resolution of 4.3 × 10−3 RIU. By combining the SPR sensor with an aptamer-functionalized, gold-nanoparticle (AuNP)-enhanced sandwich assay, the detection of C-reactive protein (CRP) in a buffer solution was achieved with a response of 0.118 nm/nM. Due to the multi-mode polymer waveguide structure and the simple concept, the reported biosensor is well suited for low-cost disposable lab-on-a-chip applications and can be used with rather simple and economic devices. In particular, the sensor offers the potential for fast and multiplexed detection of several biomarkers on a single integrated platform.

Edge-Coupling of O-Band InP Etched-Facet Lasers to Polymer Waveguides on SOI by Micro-Transfer-Printing

O-band InP etched facets lasers were heterogeneously integrated by micro-transfer-printing into a 1.54 μm deep recess created in the 3 μm thick oxide layer of a 220 nm SOI wafer. A 7 × 1.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cross-section, 2 mm long multimode polymer waveguide was aligned to the ridge post-integration by e-beam lithography with <; 0.7 μm lateral misalignment and incorporated a tapered silicon waveguide. A 170 nm thick metal layer positioned at the bottom of the recess adjusts the vertical alignment of the laser and serves as a thermal via to sink the heat to the Si substrate. This strategy shows a roadmap for active polymer waveguide-based photonic integrated circuits.

Ball Lens Embedded Through-Package Via To Enable Backside Coupling Between Silicon Photonics Interposer and Board-Level Interconnects

Development of an efficient and densely integrated optical coupling interface for silicon photonics based board-level optical interconnects is one of the key challenges in the domain of 2.5D/3D electro-optic integration. Enabling high-speed on-chip electro-optic conversion and efficient optical transmission across package/board-level short-reach interconnections can help overcome the limitations of a conventional electrical I/O in terms of bandwidth density and power consumption in a high-performance computing environment. In this context, we have demonstrated a novel optical coupling interface to integrate silicon photonics with board-level optical interconnects. We show that by integrating a ball lens in a via drilled in an organic package substrate, the optical beam diffracted from a downward directionality grating on a photonics chip can be coupled to a board-level polymer multimode waveguide with a good alignment tolerance. A key result from the experiment was a 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m chip-to-package 1-dB lateral alignment tolerance for coupling into a polymer waveguide with a cross-section of 20 × 25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^2$</tex-math></inline-formula> . An in-depth analysis of loss distribution across several interfaces was done and a −3.4 dB coupling efficiency was measured between the optical interface comprising of output grating, ball lens and polymer waveguide. Furthermore, it is shown that an efficiency better than −2 dB can be achieved by tweaking few parameters in the coupling interface. The fabrication of the optical interfaces and related measurements are reported and verified with simulation results.

Directly inscribed multimode polymer waveguide and 3D device for high-speed and high-density optical interconnects.

A 75-cm-long circular-core polymer waveguide compatible with standard 50-μm-core multimode fibers (MMFs) is designed and fabricated by using a direct inscribing method for high-speed and high-density optical interconnects. The fabricated waveguide has low loss (<0.044 dB/cm at 850 nm) and low crosstalk (<-34 dB with a core pitch of 62.5 μm) with a negligible coupling loss with the MMFs. It also exhibits a low bending loss (<0.08 dB/mm with a bending radius of 4 mm), which agrees well with calculated results. Error-free NRZ data transmission over the 75-cm-long waveguide at 25 Gb/s is demonstrated, and 4 × 25 Gb/s short wavelength division multiplexing (SWDM) is realized on a straight waveguide. Moreover, a two-layer waveguide and a 3-dimensional (3D) Y-splitter/combiner are also fabricated for 3D integration.

Polymer multimode waveguide bend based on a multilayered Eaton lens.

Reducing the bending radius of low-index-contrast waveguides is essential in reducing the size of the integrated optical components. A polymeric multimode waveguide bend is presented based on the Eaton lens. Ray-tracing calculations are utilized to truncate the Eaton lens in order to improve the performance of the bend. The truncation of the lens decreases the footprint of the bend as well. A designed waveguide bend with a radius of 18.4μm is implemented by a concentric cylindrical multilayer structure. Average bend losses of 0.69 and 0.87dB are achieved for the TM0 and TM1 modes, respectively, in the C-band of optical communication. The bend loss is lower than 1dB in a bandwidth of 1520-1675nm for both modes.

Ultra-Low Cost High-Density Two-Dimensional Visible-Light Optical Interconnects

Visible light communications have attracted considerable interest in recent years owing to the ability of low-cost light emitting diodes (LEDs) to act both as illumination sources and data transmitters with moderate data transmission rates. In this paper, we propose the formation of ultra-low cost visible-light integrated optical links by interfacing dense micro-pixelated LED arrays with matching multi-layered multimode polymer waveguide arrays. The combination of these two optical technologies can offer relatively high aggregate data densities ≥ 0.5 Tb/s/mm2 using very low cost components that can be directly interfaced with CMOS electronics and integrated onto standard printed circuit boards. Here, we present the basic system design and report the first proof-of-principle demonstration of such a visible light system employing 4×4 μLED arrays on a pitch matching four-layered waveguide array samples. Different interconnection topologies and light coupling schemes are investigated and their performance in terms of loss and crosstalk is compared. Data transmission of 2.5 Gb/s with a bit error rate within the forward-error correction threshold of 3.8×10−3 is achieved over a single μLED-waveguide channel using PAM-4 modulation and equalization. The results presented here demonstrate the potential of such ultra-low cost visible-light optical interconnects.

Practical Evaluation of Polymer Waveguides for High-Speed and Meter-Scale On-Board Optical Interconnects

We comprehensively evaluated optical characteristics, environmental stabilities, fiber optics compatibilities, and high-speed performances of multimode polymer waveguides fabricated in small batch with a length of 946 mm. The waveguides exhibit a low transmission loss (0.046 dB/cm at a wavelength of 850 nm), low interchannel crosstalk (≤ −58 dB), and a large misalignment tolerance (±20 μm) between fiber and waveguide. The waveguides show good environmental stability and the increase of propagation loss is as small as 0.005 dB/cm and 0.004 dB/cm after a 1000-h ageing test and five solder reflow cycles, respectively. The waveguides have good compatibilities with fibers having different core diameters and index profiles that may be encountered in the real application environment. High-speed transmission performances were evaluated using both nonreturn to zero (NRZ) transmission at data rates of 25 Gb/s and 30 Gb/s, and PAM4 transmission at data rates of 20 Gb/s and 56 Gb/s, respectively. There is no obvious degradation on the eye diagram due to the insertion of the waveguide. Error free transmission were successfully obtained for NRZ transmission at data rates of both 25 Gb/s and 30 Gb/s. The BER for PAM4 transmission reached a level of 10−4 at data rates of both 10 Gbaud (20 Gb/s) and 28 Gbaud (56 Gb/s), which is well below the forward error correction limitation. Moreover, it is the bandwidth of the test equipment rather than the polymer waveguide itself that limits the high-speed transmission performances in our experiments. The results demonstrate that the polymer waveguides are good candidates for high-speed and meter-scale on-board optical interconnect applications.

High-Speed Data Transmission Over Flexible Multimode Polymer Waveguides Under Flexure

Polymer multimode waveguides on flexible substrates enable the formation of bendable low-cost optical interconnects that can be deployed in a wide range of applications. However, the highly multimoded nature of such guides in combination with the stress and mode mixing induced due to sample bending raise important concerns about the effect that sample flexure has on their bandwidth performance and potential to support high-speed data transmission. In this letter, we present data transmission studies on a 1-m long flexible spiral waveguide when flexure is applied. The flexible polymer sample is bent 180° around a cylindrical mandrel, and the loss and frequency response of the waveguide are obtained for radii of curvature down to 4 mm and are compared with the performance obtained when no flexure is applied. The bit-error-rate (BER) performance of the respective optical link is also recorded at data rates up to 40 Gb/s. A flat frequency response up to at least 30 GHz is demonstrated for all bending radii applied, and error-free (BER<10−12) data transmission is achieved for all data rates studied up to 40 Gb/s. The results clearly demonstrate that sample flexing does not result in any significant transmission impairments in such links and highlight the strong potential of this technology for use in high-speed board-level interconnections.

The properties of free-standing epoxy polymer multi-mode optical waveguides

The paper reports on the fabrication and characterisation of free-standing multimode optical epoxy polymer waveguides consisting of a core made of EpoCore and EpoClad polymer cladding and cover protection layers. The 50 × 50 μm2 rectangular waveguides are intended for short-reach optical interconnection and optimised for an operating wavelength of 850 nm. The waveguides of the proposed shapes were fabricated by a standard photolithography process on a silicon substrate provided with a Poly(vinyl alcohol) thin layer. The free-standing structure was then achieved by peeling the deposited EpoClad/EpoCore/EpoClad structures of that substrate. The optical scattering losses of the created planar waveguides, measured by the fibre probe technique at 632.8 and 964 nm, were 0.30 dB cm−1 at 632.8 nm and 0.17 dB cm−1 at 964 nm. Propagation optical loss measurements for rectangular waveguides were performed by the cut-back method and the best samples had optical losses below 0.55 dB cm−1 at 850 and 1310 nm.

Flexible Multimode Polymer Waveguide Arrays for Versatile High-Speed Short-Reach Communication Links

Optical interconnects play an important role in enabling high-speed short-reach communication links within next-generation high-performance electronic systems. Multimode polymer waveguides in particular allow the formation of high-capacity optical backplanes and cost-effective board-level optical interconnects. Their formation on flexible substrates offers significant practical advantages and enables their deployment in environments where shape and weight conformity become particularly important, such as in cars and aircraft. However, bending and twisting of flexible multimode waveguides can have a significant effect on their optical transmission performance due to mode loss and mode coupling. Moreover, the magnitude of the induced effects strongly depends on the launch conditions employed. In this paper, therefore, we present detailed loss, crosstalk, and bandwidth studies on flexible multimode waveguide arrays for different launch conditions regarding their use in real-world systems. The minimum radius to achieve a 1 dB excess bending loss is obtained for each launch condition as well as the crosstalk degradation due to bending. It is shown that for a 50 μm MMF input, a 6 mm bending radius is required for excess losses below 1 dB, while crosstalk below −35 dB is obtained in adjacent waveguides even under strong bending. Moreover, it is found that twisting has a small effect on the loss and crosstalk performance of the samples. Excess twisting loss less than 0.1 dB and crosstalk of −40 dB are obtained for three full 360° turns under a 50 μm MMF launch. Finally, the bandwidth studies carried out on the samples indicate than no significant bandwidth degradation is induced due to sample bending, with bandwidth-length product values larger than 150 GHz × m obtained for restricted launches. The set of results presented herein specify the deployment of this flexible polymer multimode waveguide technology in real-world systems and demonstrate its strong potential to implement low-loss high-bandwidth optical interconnects.

Experiment and Prediction of Ablation Depth in Excimer Laser Micromachining of Optical Polymer Waveguides

Extending the data transfer rates through dense interconnections at inter‐ and intraboard levels is a well‐established technique especially in consumer electronics at the expense of more cross talk, electromagnetic interference (EMI), and power dissipation. Optical transmission using optical fibre is practically immune to the aforementioned factors. Among the manufacturing methods, UV laser ablation using an excimer laser has been repeatedly demonstrated as a suitable technique to fabricate multimode polymer waveguides. However, the main challenge is to precisely control and predict the topology of the waveguides without the need for extensive characterisation which is both time consuming and costly. In this paper, the authors present experimental results of investigation to relate the fluence, scanning speed, number of shots, and passes at varying pulse repetition rate with the depth of ablation of an acrylate‐based photopolymer. The depth of ablation essentially affects total internal reflection and insertion loss, and these must be kept at minimum for a successful optical interconnection on printed circuit boards. The results are then used to predict depth of ablation for this material by means of adaptive neurofuzzy inference system (ANFIS) modelling. The predicted results, with a correlation of 0.9993, show good agreement with the experimental values. This finding will be useful in better predictions along with resource optimisation and ultimately helps in reducing cost of polymer waveguide fabrication.

Spectral response of Bragg gratings in multimode polymer waveguides.

A means to calculate the multimodal spectral response of Bragg gratings in general non-circular multimode waveguides is proposed. To illustrate the power of the technique, the spectra of two Bragg temperature sensors are numerically calculated in which coupling between 100 modes is considered. It is shown how the Bragg wavelength in multimode Bragg grating waveguides is affected by the number of modes and energy distribution among them. Good matching of the simulated spectrum of a multimode Bragg grating on a planar inverted rib waveguide to the measured spectrum is seen.

Silicon-free, low-loss and high contrast polymer multimode waveguides

The fabrication and characterization of SU-8 multimode optical waveguides on fused quartz and silicon oxide substrates were successfully realized and analyzed. Optical losses for the transverse electric (TE) mode polarization of 0.58 dB cm−1 and 1.44 dB cm−1 and transverse magnetic (TM) mode polarization of 0.73 dB cm−1 and 1.16 dB cm−1 were measured for SU-8 waveguides on fused quartz and silicon oxide substrates, respectively. The fabrication process for SU-8 waveguides on quartz developed herein could be applied for SU-8 optical integrated devices on other substrate materials.

Wavelength division multiplexing module with large core optical polymer planar splitter and multilayered dielectric filters

The paper reports on the design, fabrication and characterization of low cost and simple fabrication method of the planar wavelength division multiplexing modules with large core input/outputs multimode optical polymer waveguides. The modules are consisting on optical 1x2Y splitter assembled with multilayered dielectric filters and large core plastic fibers. The splitters were designed by beam propagation method using BeamPROP software for input/outputs polymer fibers with 1 mm diameter. Acrylic-based polymers were used as core optical waveguides and poly(methyl methacrylate) were used as substrate and protection cover. Multilayered dielectric filters for wavelengths 532 and 650 nm were used for wavelength division multiplexing. Measurement of the optical insertion losses proved that the insertion optical loss could be lower than 8.1 dB at 650 nm and 8.7 dB at 532 nm. The best module had insertion losses 6.8 dB at 650 nm and 6.9 dB at 532 nm. The wavelength division multiplexing modules can be applied for new application in low cost short distances optical networks.

High-Bandwidth and Large Coupling Tolerance Graded-Index Multimode Polymer Waveguides for On-Board High-Speed Optical Interconnects

Optical interconnects have attracted significant research interest for use in short-reach board-level optical communication links in supercomputers and data centres. Multimode polymer waveguides in particular constitute an attractive technology for on-board optical interconnects as they provide high bandwidth, offer relaxed alignment tolerances, and can be cost-effectively integrated onto standard printed circuit boards (PCBs). However, the continuing improvements in bandwidth performance of optical sources make it important to investigate approaches to develop high bandwidth polymer waveguides. In this paper, we present dispersion studies on a graded-index (GI) waveguide in siloxane materials designed to deliver high bandwidth over a range of launch conditions. Bandwidth-length products of >70 GHzxm and ~65 GHzxm are observed using a 50/125 um multimode fibre (MMF) launch for input offsets of +/- 10 um without and with the use of a mode mixer respectively; and enhanced values of >100 GHzxm are found under a 10x microscope objective launch for input offsets of ~18 x 20 um^2. The large range of offsets is within the -1 dB alignment tolerances. A theoretical model is developed using the measured refractive index profile of the waveguide, and general agreement is found with experimental bandwidth measurements. The reported results clearly demonstrate the potential of this technology for use in high-speed board-level optical links, and indicate that data transmission of 100 Gb/s over a multimode polymer waveguide is feasible with appropriate refractive index engineering.