Optical wireless communication (OWC) promises high-speed data transmission of multiple Gb/s per user and aggregate capacities beyond Tb/s. To achieve such data rates in an eye-safe environment, we envisage an array of arrays of optical emitters, with each emitter addressing a small atto-cell. To realise this vision, each array emitter must provide uniform illumination of the desired atto-cell while minimising interference to adjacent cells. Vertical Cavity Surface Emitting Laser (VCSEL) is an attractive optical emitter for such a design due to their high modulation bandwidth (BW), circular beam waist, low cost and commercial availability of low cost arrays in the near-infrared spectrum. However, available arrays are typically multi-mode devices developed for data communications, often exhibiting a doughnut-shaped beam profile. If used with a simple lens arrangement, the resulting illumination shows non-uniform SNR over the intended atto-cell area and interference into adjacent cells. In this work, a 5×5 VCSEL-array-based OWC multi-beam transmitter using microlens arrays is designed to homogenise each VCSEL output beam intensity at the receiver plane. The performance of the proposed transmitter is verified in simulation and experiments, demonstrating a beam intensity uniformity of up to 90% over a 1 m2 square area and 25.5 mW/m2 uniform irradience distribution for each atto-cell area. We demonstrate the data transmission capability of a single array element achieving an 8 Gb/s data rate for a single channel using OOK modulation and Decision Feedback Equalization (DFE) with a Silicon Photomultiplier (SiPM) over a 3-metre free space link.
KEYWORDS: Data transmission, Telecommunications, Phase shift keying, Optical components, Optical communications, Modulation, Data storage, Data communications, Data centers, Amplitude modulation
In this paper, we review recent work on the development of a novel low-complexity equalizer that can enable single-lane <100 Gb/s short-reach optical links based on carrierless amplitude and phase modulation. This equalizer, named the CAP equalizer, can mitigate the transmission impairments in the link due to a non-ideal channel frequency response, providing significant performance advantage over conventional FFE and DFE equalizers and enabling higher data rates and longer reach. Its use is demonstrated in a VCSEL-based MMF link achieving data transmission of 112 and 124 Gb/s over 100 m OM4 MMF.
Multimode polymer waveguides have attracted considerable interest for use in high-speed on-board communication links as they provide low loss (<0.04 dB/cm at 850 nm) and high bandwidth (>40 GHz×m) and can be cost-effectively integrated onto standard PCBs. The fabrication of such waveguides on flexible substrates can provide additional advantages: shape flexibility, lightweight and reduced thickness which are particularly important in the aviation and automotive industries. Such flexible and lightweight optical connections will play an important role in next-generation airplanes and driverless cars connecting the multitude of peripheral sensors with the central processing unit at high speed and low latency. However, in such applications, flexible polymer waveguides are required to be bent to meet their stringent space requirements and twisted or stretched when connecting movable parts. Under sharp flexure, the bending or twisting loss dominates the waveguide loss limiting their practical use. In this work therefore, we present a new waveguide design for flexible polymer waveguides with improved bending performance and derive useful layout rules for minimizing twisting losses in such samples. The proposed waveguide structure only requires one additional fabrication step and achieves bending losses below 0.5 dB for a 3 mm bend. In comparison, the conventional waveguide design yields a 2 dB loss under the same bending radius and launch condition. Additionally, useful equations relating the maximum allowed number of twisting turns for low excess loss with sample thickness and width are proposed. Bending and twisting measurements on flexible waveguide samples are presented validating these methods and demonstrating the potential of this technology.
Optical technologies are increasingly considered for use in high-performance electronic systems to overcome the performance bottleneck of electrical interconnects when operating at high frequencies and provide high-speed communication between electronic chips and modules. Polymer waveguides are leading candidates for implementing board-level optical interconnections as they exhibit favourable mechanical, thermal and optical properties for direct integration onto conventional printed circuit boards (PCBs). Numerous system demonstrators have been reported in recent years featuring different types of polymer materials and opto-electronic (OE) PCB designs. However, all demonstrated polymer-based interconnection technologies are currently passive, which limits the length of the on-board links and the number of components that can be connected in optical bus architectures. In this paper therefore, we present work towards the formation of low-cost optical waveguide amplifiers that can be readily integrated onto standard PCBs by combining two promising optical technologies: siloxane-based polymers and ultra-fast laser plasma implantation (ULPI). Siloxane-based waveguides exhibit high-temperature resistance in excess of 300°C and low loss at different wavelength ranges, while ULPI has been demonstrated to produce very high dopant concentrations in glass thin films with values of 1.63×1021 cm−3 recently reported in Er-doped silica layers. Here we present detailed simulation studies that demonstrate the potential to achieve a internal gain of up to 8 dB/cm from such structures and report on initial experimental work on Er-doped films and waveguides demonstrating photoluminescence and good lifetimes.
Multimode polymer waveguides have attracted strong interest for use in high-speed board-level interconnections as they can be cost-effectively integrated onto standard printed circuit boards (PCBs) and flexible substrates using conventional methods of the electronics industry and provide low-loss (<0.04 dB/cm at 850 nm) and high-bandwidth (>30 GHz×m) interconnection. Various high-capacity passive optical backplanes have been demonstrated using this technology while data transmission up to 40 Gb/s using NRZ modulation has been reported. Despite however the intensive research in this technology, very few studies have been reported on mode mixing effects in such multimode waveguides. Mode mixing is a very important phenomenon in highly-multimoded systems as it greatly affects the mode power distribution and therefore, light propagation inside these waveguides. Important transmission characteristics such as their loss and bandwidth performance are affected as well as the behaviour of passive waveguide components such as bends, crossings and couplers due to the different mode distribution at their input. In this work therefore, we present theoretical and experimental studies on mode mixing in polymer multimode waveguides used in board-level optical interconnects. Measurements are carried out on 24-cm long flexible waveguide samples to assess the strength of mode mixing using two common methods used in multimode fibre: mandrel wrapping and micro-bending, while a simple ray tracing model is developed to correlate mode mixing strength with waveguide sidewall roughness. The combination of experimental and theoretical studies can indicate the strength of the effect over the practical range of lengths (~1 m) which are relevant to the application.
In recent years, light emitting diodes (LEDs) have gained renewed interest for use in visible light communication links (VLC) owing to their potential use as both high-quality power-efficient illumination sources as well as low-cost optical transmitters in free-space and guided-wave links. Applications that can benefit from their use include optical wireless systems (LiFi and Internet of Things), in-home and automotive networks, optical USBs and short-reach low-cost optical interconnects. However, VLC links suffer from the limited LED bandwidth (typically ~100 MHz). As a result, a combination of novel LED devices, advanced modulation formats and multiplexing methods are employed to overcome this limitation and achieve high-speed (>1 Gb/s) data transmission over such links. In this work, we present recent advances in the formation of high-aggregate-capacity low cost guided wave VLC links using stacked polymer multimode waveguides and matching micro-pixelated LED (μLED) arrays. μLEDs have been shown to exhibit larger bandwidths (>200 MHz) than conventional broad-area LEDs and can be formed in large array configurations, while multimode polymer waveguides enable the formation of low-cost optical links onto standard PCBs. Here, three- and four-layered stacks of multimode waveguides, as well as matching GaN μLED arrays, are fabricated in order to generate high-density yet low-cost optical interconnects. Different waveguide topologies are implemented and are investigated in terms of loss and crosstalk performance. The initial results presented herein demonstrate good intrinsic crosstalk performance and indicate the potential to achieve ≥ 0.5 Tb/s/mm2 aggregate interconnection capacity using this low-cost technology.
Multimode polymer waveguides have attracted great interest for use in high-speed short-reach communication links as they can be cost-effectively integrated onto standard PCBs using conventional methods of the electronics industry and provide low loss (<0.04 dB/cm at 850 nm) and high bandwidth (>30 GHz×m) interconnection. The formation of such waveguides on flexible substrates can further provide flexible low-weight low-thickness interconnects and offer additional freedom in the implementation of high-speed short-reach optical links. These attributes make these flexible waveguides particularly attractive for use in low-cost detachable chip-to-chip links and in environments where weight and shape conformity become important, such as in cars and aircraft. However, the highly-multimoded nature of these waveguides raises important questions about their performance under severe flex due to mode loss and mode coupling. In this work therefore, we investigate the loss, crosstalk and bandwidth performance of such waveguides under out-of plane bending and in-plane twisting under different launch conditions and carry out data transmission tests at 40 Gb/s on a 1 m long spiral flexible waveguide under flexure. Excellent optical transmission characteristics are obtained while robust loss, crosstalk and bandwidth performance are demonstrated under flexure. Error-free (BER<10-12) 40 Gb/s data transmission is achieved over the 1 m long spiral waveguide for a 180° bend with a 4 mm radius. The obtained results demonstrate the excellent optical and mechanical properties of this technology and highlight its potential for use in real-world systems.
Polymeric multimode waveguides are of particular interest for optical interconnections in short-reach data links. In some applications, for example in space-borne systems, the use of advanced materials with outstanding performance in extreme environments is required (temperature and radiation). In this paper therefore, we present novel siloxane polymers suitable for these applications. The materials are used to form straight, 90° bent and spiral polymer waveguides by low-cost conventional photolithographic techniques on FR4 substrates. The samples have been tested to investigate their propagation characteristics and demonstrate their potential for high-speed data links. Overall, there is strong evidence that these multimode waveguides can be successfully employed as high-speed short-reach data links. Their excellent thermal properties, their low cost and the simple fabrication process indicate their suitability for a wide range of space applications.
Multimode polymer waveguides are being increasingly considered for use in short-reach board-level optical interconnects as they exhibit favourable optical properties and allow direct integration onto standard PCBs with conventional methods of the electronics industry. Siloxane-based multimode waveguides have been demonstrated with excellent optical transmission performance, while a wide range of passive waveguide components that offer routing flexibility and enable the implementation of complex on-board interconnection architectures has been reported. In recent work, we have demonstrated that these polymer waveguides can exhibit very high bandwidth-length products in excess of 30 GHz×m despite their highly-multimoded nature, while it has been shown that even larger values of > 60 GHz×m can be achieved by adjusting their refractive index profile. Furthermore, the combination of refractive index engineering and launch conditioning schemes can ensure high bandwidth (> 100 GHz×m) and high coupling efficiency (<1 dB) with standard multimode fibre inputs with relatively large alignment tolerances (~17×15 μm2). In the work presented here, we investigate the effects of refractive index engineering on the performance of passive waveguide components (crossings, bends) and provide suitable design rules for their on-board use. It is shown that, depending on the interconnection layout and link requirements, appropriate choice of refractive index profile can provide enhanced component performance, ensuring low loss interconnection and adequate link bandwidth. The results highlight the strong potential of this versatile optical technology for the formation of high-performance board-level optical interconnects with high routing flexibility.
Glass and polymer interstacked superlattice like nanolayers were fabricated by nanosecond-pulsed laser deposition with a 193-nm-ultraviolet laser. The individual layer thickness of this highly transparent thin film could be scaled down to 2 nm, proving a near atomic scale deposition of complex multilayered optical and electronic materials. The layers were selectively doped with Er 3+ and Eu 3+ ions, making it optically active and targeted for integrated sensor application.
An optical transceiver formed onto a conventional low-cost printed circuit board with integrated optical waveguides is
presented. The transceiver incorporates an optical multimode polymer Y-splitter formed directly on a low-cost singlelayered
FR4 substrate enabling duplex transmission along a single optical fibre. The transmitter and receiver assemblies
are mounted onto the board using methods common to conventional PCB manufacturing. Simple through-board
connectors, compatible with pick-and-place assembly technology, are used to interface the electrical and optical layers of
the board. This approach allows end-fired optical coupling between the active devices and optical waveguides on the
board. The demonstrated transceiver, intended as a board-level optical network unit, achieves error-free data
transmission for both Tx and Rx modules at 10 Gb/s.
This paper presents an overview of multimode waveguides and waveguide components formed from siloxane polymer
materials which are suitable for use in optical interconnection applications. The components can be cost-effectively
integrated onto conventional PCBs and offer increased functionality in optical transmission. The multimode waveguides
exhibit low loss (0.04 dB/cm at 850 nm) and low crosstalk (< -30 dB) performance, large alignment tolerances and
negligible mode mixing for short waveguide lengths. Error-free data transmission at 10 Gb/s over 1.4 m long waveguides
has been successfully demonstrated. Waveguide crossings exhibit very low excess losses, below 0.01 dB/crossing, and
excellent crosstalk performance. Low loss is obtained for waveguide bends with radii of curvature larger than 8 mm and
6 mm for 90° and S-shaped bends respectively. High-uniformity splitting is achieved with multimode Y-splitters even in
the presence of input misalignments. Y-combiners are shown to benefit from the multimode nature of the waveguides
allowing low loss combining (4 dB for an 8×1 device). A large range of power splitting ratios between 30% and 75% is
achieved with multimode coupler devices. Examples of system applications benefiting from the use of these components
are briefly presented including a terabit capacity optical backplane, a radio-over-fibre multicasting system and a SCM
passive optical network.
This paper presents a design for rectangular ring resonators based on low-index polymers operating at an 850 nm
wavelength. Multimode interference couplers are used to enable signal splitting and combining and air trench assisted
90° mirrors are used as beam turning elements. 3D full-vector, 2D FDTD and parameter models are combined to
simulate the ring resonator design. This design, suitable for spectral shaping, achieves a FSR of 0.29 nm, a FWHM of
0.066 nm and an on-off ratio of approximately 20 dB.
In this work the recent interest in waveguides for use in short optical links has motivated a study of the modal noise
dependence on launch conditions in short-reach step-index multimode polymer waveguides. Short optical links,
especially those with several connection interfaces and utilising a restricted launch are likely to be subject to a modal
noise power penalty. We therefore experimentally study the modal noise impact of restricted launches for a short-reach
optical link employing a 50 x 50 μm polymer multimode waveguide. Lens launches resulting in small diameter input
spots are investigated as are restricted launches from an 8 μm core optical fibre. For a launch spot of 10 μm diameter no
impairment is observed for up to 9 dBo of mode selective loss, and for a fibre launch with a dynamic input movement of
6 μm no impairment is seen for up to 8 dBo of mode selective loss.
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