Single-mode Polymer Waveguides Research Articles

Polymer/silica hybrid 3D waveguide thermo-optic mode switch based on cascaded asymmetric directional couplers.

A polymer/silica hybrid 3D waveguide thermo-optic (TO) mode switch based on cascaded asymmetric directional couplers (ADCs) is theoretically designed and simulated, where the spatial modes of a few-mode silica waveguide can be switched to various single-mode polymer waveguides placed above the few-mode silica waveguide. A beam propagation method is employed to optimize the dimensional parameters of the mode switch to convert the LP11a and LP11b modes of the few-mode silica waveguide to the LP01 mode of two single-mode polymer waveguides using the cascaded ADC 1 and ADC 2. The coupling ratios are higher than 96.4% (93.4%) and 95.1% (92.8%) for the ADC 1 and ADC 2, respectively, under the TE (TM) polarization within the wavelength range from 1530 to 1570 nm, which shows good wavelength independence. Furthermore, the monolayer graphene is introduced as the heating electrode and buried on the surface of the polymer core to increase the heating efficiency and reduce the power consumption. The power consumption for ADC 1 and ADC 2 is 16.69 mW and 17.35 mW, respectively. Compared to the traditional TO switch with an aluminum (Al) heating electrode, the heating efficiency of the presented device can be improved by ∼30%. Moreover, the response speed of the TO mode switch with a 3D waveguide structure was also significantly improved. Compared to the device with Al electrodes, the introduced graphene electrodes can improve the switching speed of the device by ∼60%. The presented TO mode switch with its small size and easy integration should find applications in reconfigurable mode division multiplexing systems.

Investigation on roughness-induced scattering loss of small-core polymer waveguides for single-mode optical interconnect applications.

We investigated the roughness-induced scattering loss (LossR) of small-core polymer waveguides fabricated using the photolithography method, both theoretically and experimentally. The dependence of LossR on the roughness parameter, waveguide dimension, operation wavelength, refractive index difference and distribution, polarization sensitivity, sidewall angle, and bending radius were studied. The surface roughness of both the sidewall and the top/bottom of the fabricated waveguides were measured using laser confocal microscope, and the results showed that the averaged sidewall roughness was approximately 60 nm, which is 3 times that of the top/bottom surface. As a result, the sidewall roughness-induced LossR is 9 times that induced by the top/bottom roughness. The calculated value of LossR agrees well with the measured value. LossR increases rapidly with the decrease in the waveguide width, especially when the waveguide width is reduced below 10 µm, at which the LossR is approximately 0.3 dB/cm. On the other hand, the dependence of LossR on the waveguide height is negligible. Our results provide guidance for developing single-mode polymer waveguides with low loss for optical interconnect applications.

Cointegration of Single-Mode Waveguides and Embedded Electrical Interconnects for High-Bandwidth Communications

This article presents the modeling, development, and demonstration of glass interposer technology with single-mode waveguides (SMWGs) for high-bandwidth communications and embedded trenches for ultrafine copper traces for high-speed electronics. Glass as the substrate for the integration of photonics and electronics exhibits a unique combination of superior properties over silicon, laminates, and polymers. In conjunction with the advantages of glass, shape control of photoimageable dielectric lines during development and cure was developed and optimized. Single-mode polymer waveguides on glass were fabricated and characterized. Sample preparation methods and their impact on the sample edge quality and coupling loss will also be discussed in this article. Overall, this article presents the successful process development and demonstration of 3- $\mu \text{m}$ embedded trenches in the dielectric film for ultrafine copper traces and 2- $\mu \text{m}$ microvias for interlayer interconnects.

Circular-core single-mode polymer waveguide for high-density and high-speed optical interconnects application at 1550 nm.

We demonstrate circular-core 1550 nm single-mode polymer waveguides with a graded-index profile fabricated by commercially available UV-curable epoxies using so-called mosquito method. The relative index difference ∆n of the waveguides was designed to be 0.46% in order to guarantee both single-mode operation and good compatibility with standard single-mode fiber. Accurate refractive index tuning of monomer for core construction was realized by mixing the core and cladding epoxies properly. The core pitch of the waveguides was chosen to be 50 μm to satisfy the requirements for high-density on-board optical interconnects. Both the optical characteristics and high-speed performances of the waveguides were comprehensively studied at 1550 nm. The measured transmission and coupling loss are 0.79 dB/cm and 0.78 dB, respectively. The waveguides exhibit an inter-channel crosstalk as low as -45 dB, and a 3 dB misalignment tolerance larger than ± 4 μm on the input and output facet in both horizontal and vertical directions. NRZ signal at a data rate of 25 Gb/s was transmitted over a 10 cm-long waveguide. There is no obvious degradation on the eye diagram due to the insertion of the waveguide and error free transmission was successfully obtained. Our results imply that the fabricated single-mode polymer waveguides have good potential in high-density and high-speed optical interconnects application.

Quasi-vertical tapers for polymer-waveguide-based interboard optical interconnects

A mode transformer based on the quasi-vertical taper is designed to enable high coupling efficiency for interboard-level optical interconnects involving single-mode polymer waveguides and standard single-mode fibers. A triangular region fabricated above the waveguide is adopted to adiabatically transform the mode from the fiber into the polymer waveguide. The effects of the geometrical parameters of the taper, including width, height, tip width, etc., on the coupling efficiency are numerically investigated. Based on this, a quasi-vertical taper for the polymer rib waveguide system is designed, fabricated, and characterized. Coupling losses of 1.79±0.30 and 2.23±0.31 dB per coupler for the quasi-TM and quasi-TE mode, respectively, are measured across the optical communication C and L bands (1535 to 1610 nm). Low-cost packaging, leading to widespread utilization of polymeric photonic devices, is envisioned for optical interconnect applications.

Laser direct writing of complex radially varying single-mode polymer waveguide structures

Increasing board-to-board and chip-to-chip computational data rates beyond 12.5 Gbs will require the use of single-mode polymer waveguides (WGs) that have high bandwidths and are able to be wavelength division multiplexed. Laser direct writing (LDW) of polymer WGs provides a scalable and reconfigurable maskless procedure compared to common photolithography fabrication. LDW of straights and radial curves are readily achieved using predefined drive commands of the two-axis direct drive linear stage system. Using the laser direct write process for advanced WG structures requires stage-drive programming techniques that account for specified polymer material exposure durations. Creating advanced structures such as WG S-bends into single-mode polymer WG builds provides designers with the ability to affect pitch control, optical coupling, and reduce footprint requirements. Fabrication of single-mode polymer WG segmented radial arcs is achieved through a smooth radial arc user-programmed defined mathematical algorithm. Cosine and raised-sine S-bends are realized through a segmentation method where the optimal incremental step length and bend dimensions are controlled to achieve minimal structure loss. Laser direct written S-bends are compared with previously published photolithographic S-bend results using theoretical bend loss models. Fabrication results show that LDW is a viable method in the fabrication of advanced polymer WG structures.

Multi-level single mode 2D polymer waveguide optical interconnects using nano-imprint lithography

Single and multi-layer passive optical interconnects using single mode polymer waveguides are demonstrated using UV nano-imprint lithography. The fabrication tolerances associated with imprint lithography are investigated and we show a way to experimentally quantify a small variation in index contrast between core and cladding of fabricated devices. 1x2 splitting devices based on directional couplers and multimode interference interferometers are demonstrated to have less than 0.45 dB insertion loss with 0.02 ± 0.01 dB power imbalance between the outputs. We demonstrate an 'optical via' with an insertion loss less than 0.45 dB to transfer light from one optical signal plane to another. A 1x4 two-dimensional optical port is experimentally demonstrated to spatially split the input power with an insertion loss of 1.2 dB.

Polymer waveguides for electro-optical integration in data centers and high-performance computers.

To satisfy the intra- and inter-system bandwidth requirements of future data centers and high-performance computers, low-cost low-power high-throughput optical interconnects will become a key enabling technology. To tightly integrate optics with the computing hardware, particularly in the context of CMOS-compatible silicon photonics, optical printed circuit boards using polymer waveguides are considered as a formidable platform. IBM Research has already demonstrated the essential silicon photonics and interconnection building blocks. A remaining challenge is electro-optical packaging, i.e., the connection of the silicon photonics chips with the system. In this paper, we present a new single-mode polymer waveguide technology and a scalable method for building the optical interface between silicon photonics chips and single-mode polymer waveguides.

Bragg Grating Sensors in Laser-written Single Mode Polymer Waveguides

We present a technology for integrating Bragg gratings with single mode polymer waveguides fabricated in the EpoCore/EpoClad material system. The gratings were inscribed in a photosensitive polymethyl methacrylate (PMMA) coatingusing a phase mask and then transferred in the lower cladding layer using reactive ion etching maintaining compatibility withstandard waveguide fabrication technologies. Subsequently, the waveguide core was patterned on top using laser direct-write lithography of a spin-coated polymer layer. When exciting the waveguides with a broadband spectrum around 1550nm, 2 reflection peaks around 1580nm were found corresponding to the fundamental TE and TM mode in the polymer waveguide. Eventually, this technology will be used for structural health monitoring in concrete constructions or composite materials.

High-Precision, Self-Aligned, Optical Fiber Connectivity Solution for Single-Mode Waveguides Embedded in Optical PCBs

A fully passive, optical fiber connectivity solution for polymer waveguides embedded in electro-optical printed circuit boards (EOCB) is described and its preliminary results for single-mode applications demonstrated. The connectivity solution is based on a pluggable glass-fiber connector interface and a self-alignment packaging technology using high-precision silicon V-grooves. The V-grooves provide precise positioning of the glass-fiber relative to the polymer waveguide through alignment structures on the EOCB, patterned in the same laser direct writing fabrication step as the waveguide core. We realized an EOCB module with an LC-connector pluggable adaptor interface assembled on one side of the EOCB. With this module, we were able to prove for the first time the usability of our connectivity solution for single-mode applications. Coupling losses as low as 1.2 dB between a standard LC-connector and the single-mode polymer waveguide embedded into the EOCB have been reached. This passive packaging solution provides a cost-effective optical connectivity to wave-guides in EOCBs, which will be required in future generations of optical interconnects.

Fan-out routing and optical splitting techniques for compact optical interconnects using single-mode polymer waveguides

Polymer waveguide (WG) S-bends are necessary for fan-out routing techniques and optical splitting in high-density optical interconnects. Designing and manufacturing of optimal S-bends are critical for minimizing optical link loss while maintaining overall size and layout constraints. Complete structural loss analysis is demonstrated theoretically and shown experimentally utilizing both radial and transitional loss in single-mode (SM) polymer WG radial arc, cosine, and raised-sine S-bend profiles. SM polymer WG straights were first fabricated to measure standard propagation loss. SM WG S-bends were fabricated incorporating straight lead-in and lead-out sections to incorporate transitional loss present in workable designs. S-bend designs were measured at different dimensions and matched to theoretical losses. Compact cosine and radial arc S-bends exhibited the lowest structure loss for low and high NA WGs, respectively. High-speed performance of SM WG straights and S-bends was measured at 10 Gbit/s demonstrating low error rate. Optical splitters designed with S-bends and tapers were also evaluated and fabricated. Trade-off between optimal loss and minimal device size is discussed.

Development of high-density single-mode polymer waveguides with low crosstalk for chip-to-chip optical interconnection

High-density single-mode polymer waveguides were fabricated for chip-to-chip optical interconnection. The waveguides were designed as minimized mode field diameters for the lowest inter-channel crosstalk caused by mode coupling. The optimum relative index difference chosen was 1.2% to ensure compatibility with low crosstalk and wide fabrication tolerances. The 60-mm-length linear waveguides demonstrated a low propagation loss of 0.6 dB/cm and -45 dB crosstalk at 1310 nm. Also, a new crosstalk mechanism for a curved waveguide was revealed.

Flip-chip optical couplers with scalable I/O count for silicon photonics

A scalable and tolerant optical interfacing method based on flip-chip bonding is developed for silicon photonics packaging. Bidirectional optical couplers between multiple silicon-on-insulator waveguides and single-mode polymer waveguides are designed and fabricated. Successful operation is verified experimentally in the 1530-1570 nm spectral window. At the wavelength of 1570 nm, the coupling loss is as low as 0.8 dB for both polarization states and the planar misalignment loss is less than 0.6 dB for TE and 0.3 dB for TM polarization in a lateral silicon-polymer waveguide offset range of ± 2 µm. The coupling loss does not exhibit any temperature dependence up to the highest measurement point of 70°C.

Polymer waveguides featuring isotropic and anisotropic sections: Application to the fabrication of polarization splitters

We present the fabrication of single-mode polymer waveguides featuring both isotropic and anisotropic sections on the same substrate. The core of the waveguide is made of a polymerizable liquid crystal, which is thermally patterned into isotropic and anisotropic regions before etching. Simulation is performed in order to design polarization-separating couplers. The technological fabrication is then presented as well as a first realization.

Simple way of fabrication of Epoxy Novolak Resin optical waveguides on silicon substrate

AbstractIn this paper we report about design, fabrication and properties of the polymer optical waveguides deposited on silica‐on‐silicon substrate. The design of the waveguides is based on a concept that geometric dimensions of the single mode polymer waveguide are determined by geometrical parameters of the silica layer. The design of the waveguides was schemed for 650 nm, 850 nm, 1310 nm and 1550 nm wavelength. The design of the presented planar waveguides was realized on the bases of modified dispersion equation while the ridge waveguides design was proposed following the Fischbeck concept. Both designs were refined applying RSoft software using beam propagation method. Proposed shapes of the waveguides were etched by standard photolithography process into the silica layers and polymer waveguide layers were subsequently deposited into the treated substrate by spin coating. Epoxy Novolak Resin was used as the waveguide core material and polymethylmethacrylate was used as a cover protection layer. Propagation optical loss measurements were done by using the cut‐back method. The best samples had optical losses lower than 1.5 dB/cm at 650 nm and 0.5 dB/cm at 1310 nm (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Narrowband 1.5-μm Bragg filter based on a polymer waveguide with a laser-written refractive-index grating

We report the fabrication of narrowband frequency-selective filters for the 1.5-μm telecom window, which include a single-mode polymer waveguide with a submicron Bragg grating inscribed by a helium-cadmium laser. The filters have a reflectance R > 98 % and a nearly rectangular reflection band with a bandwidth Δλ≈0.4nm. They can be used as components of optical multiplexers/demultiplexers for combining and separating signals in high-speed dense wavelength-division multiplexed optical fibre communication systems.

Nanoporosity effect in optical loss of single-mode polymer waveguides

We fabricate single-mode polymer waveguide structures exhibiting polarization-independent ultra-low loss of 0.04 and 0.05 dB/cm at the 1310 and 1550 nm bands, respectively, with a Deltan of 1.6%. A porous structure that arises during the fabrication process is studied by considering its implications in the propagation loss based on the Rayleigh-Mie scattering loss mechanism. We demonstrate that the porous structure is to be reduced to the nanoscale (i.e., <10 nm) to realize waveguide structures with ultra-low propagation losses based on fabrications and measurements of morphologies with various degrees of porosity. Further, the bending loss and its respective polarization-dependent propagation loss behavior are analyzed to realize compact devices with ultra-low-loss and polarization-independent features, where a 4 mm bending radius is found to be adequate for such a performance over the 1550 nm band.

Electrooptic Polymer Modulator With Single-Mode to Multimode Waveguide Transitions

Grayscale lithography has been used to create refractive index (RI) tapers in an electrooptic (EO) polymer modulator. These RI tapers are used to convert a passive single-mode photobleached polymer waveguide to the fundamental mode of an unbleached, active multimode polymer waveguide. The benefits of this modulator design include a coupling loss reduction of 2.6 dB per interface and an increased confinement factor of 0.83. Using this design, an EO polymer modulator was fabricated with a half-wave voltage of 1.9 V, modulation extinction ratio of 15 dB, and a fiber to lens insertion loss of 11.6 dB.

Fabrication and characterization of low-loss polymeric waveguides and micro-resonators

We realize low loss, single-mode polymer waveguides and microring resonators in SU-8 (MichroChem) using direct E-beam lithography. We also present and demonstrate a novel and simple approach for accurately extracting the propagation loss in the waveguides as well as the coupling between the waveguide and the microring. The demonstrated approach is insensitive to the I/O coupling efficiency to the optical chip and does not require any pre-calibration of the experimental setup.

Variable optical attenuator based on large-core single-mode polymer waveguide

To improve the reproducibility of passive alignment, large-core single-mode waveguides are demonstrated, which can be connected to thermally expanded core fibers with increased alignment tolerance. It is shown that polymer waveguides with a core dimension of 25 /spl times/25 /spl mu/m/sup 2/ and an index contrast less than 0.001 can satisfy single-mode condition. As a novel functional device incorporating the large-core waveguide, variable optical attenuators (VOAs) are designed and fabricated. For the fabrication of the thick core structure, a soft molding process is developed. Due to the small index contrast of the waveguide, efficient attenuation is expected for small electrical power consumption, which is confirmed by three-dimensional beam propagation method. From the fabricated VOA, more than 20 dB of attenuation is obtained by applying 20 mW.