Free-space Optical Interconnects Research Articles

Heterogeneous 3-D circuits: Integrating free-space optics with CMOS

A three-dimensional (3-D) integrated circuit combining free-space optical interconnects (FSOI) with CMOS devices has been developed. An overview of the combined optical and CMOS system is described. Experimental data and simulated results are provided. A 3-D integrated test circuit merging the free-space optical network with gallium arsenide based vertical cavity surface emitting lasers (VCSELs) and germanium based photodetectors has been experimentally tested. The prototype 3-D IC test circuit exhibits a 3.3GHz bandwidth and a 5.1mW total power consumption per link. The bandwidth is limited due to the use of 5GHz bandwidth commercial VCSELs and the large inductive impedance of the wirebonds attaching the VCSELs and photodetectors to the I/O pads. The design of transmitter circuits for the 3-D integrated free-space optical interconnect system is discussed, and simulated extrapolated results on operating frequency and bandwidth are provided. The microprocessor operates at 333MHz and includes a bus width of 64bits, requiring a FSOI bandwidth of 10.65Gbps after serialization.

Improved OFDM Allocation Scheme for Crosstalk Mitigation in Multiple Free-Space Optical Interconnection Links

This study considers the use of the dc-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) resource allocation technique to mitigate the crosstalk effect in a free-space optical interconnect (FSOI) system. The optimization of DCO-OFDM parameters is a promising approach for reducing the crosstalk effect of dense FSOI. However, determining the optimal DCO-OFDM subcarrier allocation and the corresponding bit-and-power loading requires to solve an intractable discrete optimization problem, which is nonconvex and difficult to solve explicitly. In this study, we use cross-entropy optimization (CEO) to address this problem. Simulation results reveal that the proposed CEO-based allocation scheme obtains better bit rates that are less susceptible to the crosstalk effect than the traditional greedy-based algorithm and other state-of-the-art stochastic optimization techniques.

Hermite-Gaussian Mode Division Multiplexing for Free-Space Optical Interconnects

Hermite-Gaussian Mode Division Multiplexing for Free-Space Optical Interconnects

OFDM Allocation Optimization for Crosstalk Mitigation in Multiple Free-Space Optical Interconnection Links

The growing demand for high interconnection speed in next-generation computers is driving the technology shift for communication from the electronic to the optic domain. One of the favored interconnection technologies for this task is the freespace optical interconnect (FSOI). FSOI technology uses laser links between computer components and provides a lower bound on propagation delay due to the low index of refraction of air, when compared with the indexes common in waveguide technologies. FSOIs based on dc-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) may provide excellent data throughput in intensity modulation/direct detection systems. However, the main drawback limiting the implementation of FSOIs is the inevitable tradeoff between interconnection density and the crosstalk level, resulting from the diffraction effect and from optical misalignment. The purpose of this paper is to promote improved interconnection density of such FSOIs by use of inherent DCO-OFDM resource allocation capabilities. The crosstalk-resulted interference was formulated as joint multilink bit-and-power allocation optimization. The theoretical analysis reveals general guidelines for dense FSOI. Further, a reduced-complexity numerical suboptimal algorithm for joint multilink bit-and-power allocation was proposed. The simulation results show that the proposed suboptimal algorithm outcome is close to the theoretical optimal performance.

Experimental Evaluation of Free Space Links in the Presence of Turbulence for Server Backplane

Free-space optical interconnection (FSOI) technology enables high interconnection speed in next-generation servers. FSOIs use laser links between server components and provide a lower bound on propagation delay due to the shortest line-of-sight path and the low index of refraction of air, when compared with the indices common in waveguide technologies. One of the main problems of FSOI systems is the inevitable turbulence effect that results from the air cooling of server components. Cooling fans produce a flow of hot air, due to excessive thermal emission from electronic components inside the chassis; this produces turbulence-induced fading and may influence the communication performance. The objective of this paper is to experimentally examine the influence of the turbulence effect on FSOI channels inside a server chassis. This includes studying scintillation statistics and characterizing temporal and spatial in-chassis turbulence. The results of the analysis reveal that the in-chassis optical channel preserves the well-known log-normal scintillation distribution. However, some temporal and spatial characteristics do not fit the commonly accepted turbulence theory predictions.

Spectral switching-based fan-out architecture and information processing in free-space

In this paper, we elaborate different type of spectral switching techniques and spectral switching-based information processing (SSBIP) schemes. On the basis of theoretical, experimental and empirical studies carried out so far on spectral switching, we demonstrate novel type of interference-induced 1×N (1 input N outputs) and 1×N×M (1 input N×M outputs) spectral switching-based fan-out architectures for information transmission in free-space. In addition, a comprehensive analysis is presented to explore the feasibility of SSBIP scheme in contrast to the recent technological advancements. The spectral switching-based techniques are contrived ideas but might find potential applications in optical computing, state-of-the-art technique like SIMO (single-input multiple-output), free-space optical (FSO) interconnects and polychromatic light-based FSO communications.

Approximate analytical method to evaluate diffraction crosstalk in free-space optical interconnects systems that use circular microlenses with finite uniform apertures

We propose an accurate analytical model to calculate the optical crosstalk of a first-order free space optical interconnects system that uses microlenses with circular apertures. The proposed model is derived by evaluating the resulted finite integral in terms of an infinite series of Bessel functions. Compared to the model that uses complex Gaussian functions to expand the aperture function, it is shown that the proposed model is superior in estimating the crosstalk and provides more accurate results. Moreover, it is shown that the proposed model gives results close to that of the numerical model with superior computational efficiency.

Real-time validation of receiver state information in optical space-time block code systems.

Free space optical interconnect (FSOI) systems are a promising solution to interconnect bottlenecks in high-speed systems. To overcome some sources of diminished FSOI performance caused by close proximity of multiple optical channels, multiple-input multiple-output (MIMO) systems implementing encoding schemes such as space-time block coding (STBC) have been developed. These schemes utilize information pertaining to the optical channel to reconstruct transmitted data. The STBC system is dependent on accurate channel state information (CSI) for optimal system performance. As a result of dynamic changes in optical channels, a system in operation will need to have updated CSI. Therefore, validation of the CSI during operation is a necessary tool to ensure FSOI systems operate efficiently. In this Letter, we demonstrate a method of validating CSI, in real time, through the use of moving averages of the maximum likelihood decoder data, and its capacity to predict the bit error rate (BER) of the system.

Crosstalk reduction in free space optical interconnects systems using microlenses with Gaussian transmittance

A novel method to reduce the diffraction crosstalk for micro-lens based free space optical interconnects is presented. Instead of using microlenses with uniform transmittance apertures, the use of microlenses with non-uniform transmittance apertures is proposed. It is shown that the diffraction crosstalk which exists in the free space interconnects systems that use microlenses with uniform transmittance apertures can be substantially reduced by using microlenses with Gaussian transmittance. The optical field at the detectors array using both the uniform and Gaussian apertures have been derived and used to calculate the crosstalk. Numerical results have been introduced to show the improvement of the signal-to-crosstalk ratio when using the Gaussian transmittance for the microlens.

Performance Analysis of Paraxial Free Space Optical Interconnects System that Uses Bessel Beams and Micro-Lenses with Circular Apertures

Performance Analysis of Paraxial Free Space Optical Interconnects System that Uses Bessel Beams and Micro-Lenses with Circular Apertures

Surface-Normal Ge/SiGe Asymmetric Fabry–Perot Optical Modulators Fabricated on Silicon Substrates

We demonstrate the first vertical-incidence Ge/SiGe quantum well reflection modulators fabricated entirely on standard silicon substrates. These modulators could help enable massively parallel, free-space optical interconnects to silicon chips. An asymmetric Fabry-Perot resonant cavity is formed around the quantum well region by alkaline etching the backside of the Si substrate to leave suspended SiGe membranes, upon which high-index-contrast Bragg mirrors are deposited. Electroabsorption and electrorefraction both contribute to the reflectance modulation. The devices exhibit greater than 10 dB extinction ratio with low insertion loss of 1.3 dB. High-speed modulation with a 3 dB bandwidth of 4 GHz is demonstrated. The moderate-Q cavity (Q ~ 600) yields an operating bandwidth of more than 1 nm and permits operation without active thermal stabilization.

Design and fabrication of microlens arrays as beam relay for free-space optical interconnection

In this paper we report the design and fabrication of a beam relay for free space optical interconnection using microlens arrays. Multiple microlens arrays with same focal lengths were designed and fabricated in an out-of-plane layout. This design can be easily integrated with silicon-based optical interconnection devices. The beam relay was fabricated using direct lithography of SU-8 photoresist, and then replicated using UV curable polymer molded with a PDMS intermediate mold. The optical performance was tested and the experimental results show that the optical performances are mainly limited by the aberration of microlenses. Further study needs to be conducted to improve the surface quality of the lenses to reduce the aberrations.

Optimizing bandwidth density in free space optical interconnects under the use of error correcting codes

The bandwidth density of error corrected free space optical interconnects is considered in this article. Analyses for the bandwidth density with and without error correcting codes are presented and compared. It is shown that the bandwidth density can be improved using error correcting codes with code rate as a design parameter. Moreover, it is shown that the fill factor of the detector array can be used as a parameter to further improve the bandwidth density for the coded system. Numerical results on the effect of using the code rate and fill factor to optimize the bandwidth density are presented and discussed.

Ge/SiGe asymmetric Fabry-Perot quantum well electroabsorption modulators

We demonstrate vertical-incidence electroabsorption modulators for free-space optical interconnects. The devices operate via the quantum-confined Stark effect in Ge/SiGe quantum wells grown on silicon substrates by reduced pressure chemical vapor deposition. The strong electroabsorption contrast enables use of a moderate-Q asymmetric Fabry-Perot resonant cavity, formed using a film transfer process, which allows for operation over a wide optical bandwidth without thermal tuning. Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 V drive swings, respectively, with insertion loss less than 4.5 dB. For 60 ?m diameter devices, large signal modulation is demonstrated at 2 Gbps, and a 3 dB modulation bandwidth of 3.5 GHz is observed. These devices show promise for high-speed, low-energy operation given further miniaturization.

Analytical models for channel crosstalk in short-range free space optical interconnects

Analytical models for optical channel crosstalk in short-range free space optical interconnect systems are proposed. The diffraction crosstalk noise is modeled as a function of many parameters of the interconnect system, including the interconnect distance, the interconnect spacing, and the photo-detector size. The dependence of crosstalk noise on different systems' parameters is discussed and evaluated using the proposed models. It is shown that optimum parameters for the optical system and the detectors array can be easily determined to optimize and improve the interconnect performance. The effect of high modes of vertical cavity surface emitting lasers (VCSELs), VCSEL aperture, and the incident angle of the VCSEL beam on crosstalk are also discussed.

Dual-prism coupler for board-level free-space optical interconnects: design and simulations

An optical design for coupling light from off-chip lasers to on-chip surface-normal modulators is proposed in order to implement chip-to-chip free-space optical interconnects. The method uses a dual-prism module constructed from prisms made of two different glasses. The various alignment tolerances of the proposed system were investigated. For the off-chip lasers, vertical cavity surface emitting lasers (VCSEL) are proposed. The rationale behind using on-chip modulators rather than VCSELs is to avoid VCSEL thermal loads on the chip, and because of higher reliability of modulators than VCSELs. Particularly above 10 Gbit/sec (Gbit/sec⁡), an empirical model shows rapid decrease of VCSEL median time to failure vs. data rate. Thus the proposed interconnect scheme which utilizes continuous wave VCSELs that are externally modulated by on-chip multiple quantum well modulators is applicable for chip-to-chip optical interconnects at 20 Gbit/sec⁡ and higher line data rates.

3-D integrated heterogeneous intra-chip free-space optical interconnect

This paper presents the first chip-scale demonstration of an intra-chip free-space optical interconnect (FSOI) we recently proposed. This interconnect system provides point-to-point free-space optical links between any two communication nodes, and hence constructs an all-to-all intra-chip communication fabric, which can be extended for inter-chip communications as well. Unlike electrical and other waveguide-based optical interconnects, FSOI exhibits low latency, high energy efficiency, and large bandwidth density, and hence can significantly improve the performance of future many-core chips. In this paper, we evaluate the performance of the proposed FSOI interconnect, and compare it to a waveguide-based optical interconnect with wavelength division multiplexing (WDM). It shows that the FSOI system can achieve significantly lower loss and higher energy efficiency than the WDM system, even with optimistic assumptions for the latter. A 1×1-cm2 chip prototype is fabricated on a germanium substrate with integrated photodetectors. Commercial 850-nm GaAs vertical-cavity-surface-emitting-lasers (VCSELs) and fabricated fused silica microlenses are 3-D integrated on top of the substrate. At 1.4-cm distance, the measured optical transmission loss is 5 dB, the crosstalk is less than -20 dB, and the electrical-to-electrical bandwidth is 3.3 GHz. The latter is mainly limited by the 5-GHz VCSEL.

Performance optimization of a free space optical interconnect system with a metal-semiconductor-metal detector

In this paper we study the possibility and the potentiality of using metal semiconductor-metal photodetector (MSM-PD) in three-dimensional parallel free space optical interconnect (FSOI) systems. The signal-to-noise ratio (SNR) and time response are used as performance measures to optimize the geometry of MSM-PD used in FSOI systems. Both SNR and time response are evaluated, analyzed, and their dependence on feature parameters of the MSM-PD, including finger size, spacing, and number of fingers, are considered. Based on the results obtained, we show that the use of MSM-PD in FSOI improves the interconnect speed at a given acceptable SNR.

Size optimization of a quad-cell alignment detector for short range free space optical interconnects

In this paper we consider geometry optimization of a quad-cell alignment detector for short range free space optical interconnects. The alignment technique uses alignment beams and quad detectors to detect different misalignment errors. We show that the size of the quad detector cells can be used as a design parameter to optimize the sensitivity of the alignment system. The sensitivity analysis and error signal generation are presented assuming standard Gaussian beam models. Results on optimum sensitivity for the case of transverse and angular misalignments are presented and discussed.

Two-Axis MEMS Lens Alignment System for Free-Space Optical Interconnect

We present a two-axis microelectromechanical systems (MEMS) lens aligner with a 260 μm × 220 μm translation range that positions a 6.35 mm lens with focal length f = 12.1 mm for alignment compensation of free-space optical interconnects (FSOIs) between computer servers separated by 50 mm spacing. Efficient ultrasonic linear piezoelectric motors (PMs) provide actuation with zero power required to hold the lens alignment. A four-channel FSOI is demonstrated with 1 × 4 vertical cavity surface-emitting laser (VCSEL) and photodiode (PD) arrays capable of 10 Gb/s transmission bandwidth. Demonstrated minimum step size of 1.68 μm is sufficient for aligning the 20 μm VCSEL spots onto the 40 μ m PD receivers. Force transmission between PMs and a silicon MEMS flexure stage is accomplished using a 1-mm steel ball bearing in a magnetic groove, providing compliance in the nondriven axis. The ball-coupling design has 15 μm backlash and induces a maximum of 8 μm of cross-axis motion.