Towards coherent O-band data center interconnects

Upcoming generations of coherent intra/inter data center interconnects currently lack a clear path toward a reduction of cost and power consumption, which are the driving factors for these data links. In this work, the tradeoffs associated with a transition from coherent C-band to O-band silicon photonics are addressed and evaluated. The discussion includes the fundamental components of coherent data links, namely the optical components, fiber link and transceivers. As a major component of these links, a monolithic silicon photonic BiCMOS O-band coherent receiver is evaluated for its potential performance and compared to an analogous C-band device.TU Berlin, Open-Access-Mittel - 2021BMBF, 13N14932, Verbundprojekt: Photonic Embedding of Active Region LASER Chips on Silicon (PEARLS) - Teilvorhaben: Entwurf und Charakterisierung von eingebetteten, horizontal-gekoppelten Laser-Strukturen auf SiliziumEC/H2020/822002/EU/Lasercom-on-chip for next generation, high-speed satellite constelation interconnectivity/ORIONA

Characterization And Optimization Of Avalanche Photodiodes Fabricated By Standard Cmos Process For High-Speed High-Speed Photoreceivers

A dissertation presented on the characterization and optimization of avalanche photodiodes fabricated by standard CMOS process (CMOS-APD) for high-speed photoreceivers, beginning with the theory and principle related to photodetector and avalanche photodiodes, followed by characterization,optimization, and wavelength dependence of CMOS-APD, and finally link up with the transimpedance amplifier. nMOS-type and pMOS-type silicon avalanche photodiodes were fabricated by standard 0.18 μm CMOS process, and the currentvoltage characteristic and the frequency response of the CMOS-APDs with and without the guard ring structure were measured. CMOS-APDs have features of high avalanche gain below 10 V, wide bandwidth over 5 GHz, and easy integration with electronic circuits. In CMOS-APDs, guard ring structure is introduced for high-speed operation with the role of elimination the slow photo generated carriers in a deep layer and a substrate. The bandwidth of the CMOS-APD is enhanced with the guard ring structure at a sacrifice of the responsivity. Based on comparison of nMOS-type and pMOS-type APDs, the nMOS-type APD is more suitable for high-speed operation. The bandwidth is enhanced with decreasing the spacing of interdigital electrodes due to decreased carrier transit time and with decreasing the detection area and the PAD size for RF probing due to decreased device capacitance. Thus, an nMOS-type APD with the electrode spacing of 0.84 μm, the detection area of 10 x 10 μm², the PAD size for RF probing of 30 x 30 μm² along with the guard ring structure was fabricated. As a results, the maximum bandwidth of 8.4 GHz at the avalanche gain of about 10 and the gain-bandwidth product of 280 GHz were achieved. Furthermore, the wavelength dependence of the responsivity and the bandwidth of the CMOS-APDs with and without the guard ring structure also revealed. At a wavelength of 520 nm or less, there is no difference in the responsivity and the frequency response because all the illuminated light is absorbed in the p+-layer and the Nwell due to strong light absorption of Si. On the other hand, a part of the incident light is absorbed in the Psubstrate and the photo-generated carriers in the P-substrate are eliminated by the guard ring structure for the wavelength longer than 520 nm, and then bandwidth was remarkably enhanced at the sacrifice of the responsivity. In addition, to achieve high-speed photoreceivers, two types of TIA which are common-source and regulated-cascode TIAs were simulated by utilizing the output of the CMOSAPDs.The figure of merits of gain-bandwidth product was used to find the ideal results of the transimpedance gain and bandwidth performance due to trade-offs between both of them. The common-source TIA produced the transimpedance gain of 22.17 dBΩ, the bandwidth of 21.21 GHz and the gain-bandwidth product of 470.23 THz × dBΩ. Besides that, the simulated results of the regulated-cascade TIA configuration demonstrate 79.45 dBΩ transimpedance gain, 10.64 GHz bandwidth, and 845.35 THz × dBΩ gain-bandwidth product. Both of these TIA results meet the target of this research and further encouraging this successful CMOS-APDs to realize high-speed photoreceivers

Distributed Circuit Analysis and Design for Ultra-wideband Communication and sub-mm Wave Applications

This thesis explores research into new distributed circuit design techniques and topologies, developed to extend the bandwidth of amplifiers operating in the mm and sub-mm wave regimes, and in optical and visible light communication systems. Theoretical, mathematical modelling and simulation-based studies are presented, with detailed designs of new circuits based on distributed amplifier (DA) principles, and constructed using a double heterojunction bipolar transistor (DHBT) indium phosphide (InP) process with fT =fmax of 350/600 GHz. A single stage DA (SSDA) with bandwidth of 345 GHz and 8 dB gain, based on novel techniques developed in this work, shows 140% bandwidth improvement over the conventional DA design. Furthermore, the matrix-single stage DA (M-SSDA) is proposed for higher gain than both the conventional DA and matrix amplifier. A two-tier M-SSDA with 14 dB gain at 300 GHz bandwidth, and a three-tier M-SSDA with a gain of 20 dB at 324 GHz bandwidth, based on a cascode gain cell and optimized for bandwidth and gain flatness, are presented based on full foundry simulation tests. Analytical and simulation-based studies of the noise performance peculiarities of the SSDA and its multiplicative derivatives are also presented. The newly proposed circuits are fabricated as monolithic microwave integrated circuits (MMICs), with measurements showing 7.1 dB gain and 200 GHz bandwidth for the SSDA and 12 dB gain at 170 GHz bandwidth for the three-tier M-SSDA. Details of layout, fabrication and testing; and discussion of performance limiting factors and layout optimization considerations are presented. Drawing on the concept of artificial transmission line synthesis in distributed amplification, a new technique to achieve up to three-fold improvement in the modulation bandwidth of light emitting diodes (LEDs) for visible light communication (VLC) is introduced. The thesis also describes the design and application of analogue pre-emphasis to improve signal-to-noise ratio in bandwidth limited optical transceivers

Monolithic electronic-photonic integration in state-of-the-art CMOS processes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 388-407).As silicon CMOS transistors have scaled, increasing the density and energy efficiency of computation on a single chip, the off-chip communication link to memory has emerged as the major bottleneck within modern processors. Photonic devices promise to break this bottleneck with superior bandwidth-density and energy-efficiency. Initial work by many research groups to adapt photonic device designs to a silicon-based material platform demonstrated suitable independent performance for such links. However, electronic-photonic integration attempts to date have been limited by the high cost and complexity associated with modifying CMOS platforms suitable for modern high-performance computing applications. In this work, we instead utilize existing state-of-the-art electronic CMOS processes to fabricate integrated photonics by: modifying designs to match the existing process; preparing a design-rule compliant layout within industry-standard CAD tools; and locally-removing the handle silicon substrate in the photonic region through post-processing. This effort has resulted in the fabrication of seven test chips from two major foundries in 28, 45, 65 and 90 nm CMOS processes. Of these efforts, a single die fabricated through a widely available 45nm SOI-CMOS mask-share foundry with integrated waveguides with 3.7 dB/cm propagation loss alongside unmodified electronics with less than 5 ps inverter stage delay serves as a proof-of-concept for this approach. Demonstrated photonic devices include high-extinction carrier-injection modulators, 8-channel wavelength division multiplexing filter banks and low-efficiency silicon germanium photodetectors. Simultaneous electronic-photonic functionality is verified by recording a 600 Mb/s eye diagram from a resonant modulator driven by integrated digital circuits. Initial work towards photonic device integration within the peripheral CMOS flow of a memory process that has resulted in polysilicon waveguide propagation losses of 6.4 dB/cm will also be presented.by Jason S. Orcutt.Ph.D

Modelling, Analysis and Design of Optimised Electronic Circuits for Visible Light Communication Systems

This thesis explores new circuit design techniques and topologies to extend the bandwidth of visible light communication (VLC) transmitters and receivers, by ameliorating the bandwidth-limiting effects of commonly used optoelectronic devices. The thesis contains detailed literature review of transmitter and receiver designs, which inspired two directions of work. The first proposes new designs of optically lossless light emitting diode (LED) bandwidth extension technique that utilises a negative capacitance circuit to offset the diode’s bandwidth-limiting capacitance. The negative capacitance circuit was studied and verified through newly developed mathematical analysis, modelling and experimental demonstration. The bandwidth advantage of the proposed technique was demonstrated through measurements in conjunction with several colour LEDs, demonstrating up to 500% bandwidth extension with no loss of optical power. The second direction of work enhances the bandwidth of VLC receivers through new designs of ultra-low input impedance transimpedance amplifiers (TIAs), designed to be insensitive to the high photodiode capacitances (Cpd) of large area detectors. Moreover, the thesis proposes a new circuit, which modifies the traditional regulated cascode (RGC) circuit to enhance its bandwidth and gain. The modified RGC amplifier efficiently treats significant RGC inherent bandwidth limitations and is shown, through mathematical analysis, modelling and experimental measurements to extend the bandwidth further by up to 200%. The bandwidth advantage of such receivers was demonstrated in measurements, using several large area photodiodes of area up to 600 mm^2, resulting in a substantial bandwidth improvement of up to 1000%, relative to a standard 50 Ω termination. An inherent limitation of large area photodiodes, associated with internal resistive elements, was identified and ameliorated, through the design of negative resistance circuits. Altogether, this research resulted in a set of design methods and practical circuits, which will hopefully contribute to wider adoption of VLC systems and may be applied in areas beyond VLC

Parallel reconfigurable single photon avalanche diode array for optical communications

There is a pressing need to develop alternative communications links due to a number of physical phenomena, limiting the bandwidth and energy efficiency of wire-based systems or economic factors such as cost, material-supply reliability and environmental costs. Networks have moved to optical connections to reduce costs, energy use and to supply high data rates. A primary concern is that current optical-detection devices require high optical power to achieve fast data rates with high signal quality. The energy required therefore, quickly becomes a problem. In this thesis, advances in single-photon avalanche diodes (SPADs) are utilised to reduce the amount of light needed and to reduce the overall energy budget. Current high performance receivers often use exotic materials, many of which have severe environmental impact and have cost, supply and political restrictions. These present a problem when it comes to integration; hence silicon technology is used, allowing small, mass-producible, low power receivers. A reconfigurable SPAD-based integrating receiver in standard 130nm imaging CMOS is presented for links with a readout bandwidth of 100MHz. A maximum count rate of 58G photon/s is observed, with a dynamic range of ≈ 79dB, a sensitivity of ≈ −31.7dBm at 100MHz and a BER of ≈ 1x10−9. We investigate the properties of the receiver for optical communications in the visible spectrum, using its added functionality and reconfigurability to experimentally explore non-ideal influences. The all-digital 32x32 SPAD array, achieves a minimum dead time of 5.9ns, and a median dark count rate (DCR) of 2.5kHz per SPAD. High noise devices can be weighted or removed to optimise the SNR. The power requirements, transient response and received data are explored and limiting factors similar to those of photodiode receivers are observed. The thesis concludes that data can be captured well with such a device but more electrical energy is needed at the receiver due to its fundamental operation. Overall, optical power can be reduced, allowing significant savings in either transmitter power or the transmission length, along with the advantages of an integrated digital chip

THE USE OF TUNED FRONT END OPTICAL RECEIVER AND PULSE POSITION MODULATION

The aim of this work is to investigate the use of tuned front-ends with OOK and PPM schemes, in addition to establish a theory for baseband tuned front end receivers. In this thesis, a background of baseband receivers, tuned receivers, and modulation schemes used in baseband optical communication is presented. Also, the noise theory of baseband receivers is reviewed which establishes a grounding for developing the theory relating to optical baseband tuned receivers. This work presents novel analytical expressions for tuned transimpedance, tuned components, noise integrals and equivalent input and output noise densities of two tuned front-end receivers employing bi-polar junction transistors and field effect transistors as the input. It also presents novel expressions for optimising the collector current for tuned receivers. The noise modelling developed in this work overcomes some limitations of the conventional noise modelling and allows tuned receivers to be optimised and analysed. This work also provides an in-depth investigation of optical baseband tuned receivers for on-off keying (OOK), Pulse position modulation (PPM), and Di-code pulse position modulation (Di-code PPM). This investigation aims to give quantitative predictions of the receiver performance for various types of receivers with different photodetectors (PIN photodetector and avalanche photodetector), different input transistors (bi-polar junction transistor BJT and field effect transistor FET), different pre-detection filters (1st order low pass filter and 3rd order Butterworth filter), different detection methods, and different tuned configurations (inductive shunt feedback front end tuned A and serial tuned front end tuned B). This investigation considers various optical links such as line of sight (LOS) optical link, non-line of sight (NLOS) link and optical fibre link. All simulations, modelling, and calculations (including: channel modelling, receiver modelling, noise modelling, pulse shape and inter-symbol interference simulations, and error probability and receiver calculations) are performed by using a computer program (PTC Mathcad prime 4, version: M010/2017) which is used to evaluate and analyse the performance of these optical links. As an outcome of this investigation, noise power in tuned receivers is significantly reduced for all examined configurations and under different conditions compared to non-tuned receivers. The overall receiver performance is improved by over 3dB in some cases. This investigation provides an overview and demonstration of cases where tuned receiver can be optimised for baseband transmission, offering a much better performance compared to non-tuned receivers. The performance improvement that tuned receivers offers can benefit a wide range of optical applications. This investigation also addresses some recommendations and suggestions for further work in some emerging applications such as underwater optical wireless communication (UOWC), visible light communication (VLC), and implantable medical devices (IMD). Keyword: Optical communications, Baseband receivers, Noise modelling, tuned front end, pulse position modulation (PPM)