Free-space Optical Interconnect System Research Articles

Bandwidth Density Analysis of Coded Free-Space Optical Interconnects

The performance of free-space optical interconnects (FSOIs) system is significantly influenced by noise, similar to any wireless communication system. This noise has a notable impact on both the bandwidth density and data rate of FSOIs system. To address these challenges, this study proposes the utilization of vertical-cavity-surface-emitting laser (VCSEL) arrays on the transmitter side and photodetector arrays on the receiver side for FSOIs. The study investigates the bandwidth density of the system with and without coding while maintaining a specific bit error rate. An analysis is conducted in the presence of higher-order modes in the laser beams of the FSOIs system and a fundamental Gaussian operating mode. The presence of the higher-order modes leads to degradation in the performance of the FSOIs system in terms of bandwidth density. In addition, we examine the impact of the signal-to-noise ratio (SNR) on the system’s bandwidth density for each considered operating mode. The provided simulation results clearly demonstrate that coding significantly enhances the bandwidth density of the systems, with the extent of improvement being closely tied to the employed code rate and codeword length.

Bandwidth density optimization of misaligned optical interconnects

In this paper, the bandwidth density of misaligned free space optical interconnects (FSOIs) system with and without coding under a fixed bit error rate is considered. In particular, we study the effect of using error correction codes of various codeword lengths on the bandwidth density and misalignment tolerance of the FSOIs system in the presence of higher order modes. Moreover, the paper demonstrates the use of the fill factor of the detector array as a design parameter to optimize the bandwidth density of the communication. The numerical results demonstrate that the bandwidth density improves significantly with coding and the improvement is highly dependent on the used codeword length and code rate. In addition, the results clearly show the optimum fill factor values that achieve the maximum bandwidth density and misalignment tolerance of the system.

Bessel Beams and Gaussian Beams as information carriers in free space optical interconnects systems: A comparison study

<p><span style="font-family: Times New Roman;">We introduce a comparison study for the performance of a lens-based free space optical interconnects system assuming Bessel Beams and Gaussian Beams as information carriers. The optical field at the detector plane was derived for the two beam profiles. In both cases the expressions for the output optical filed are expressed in terms of complex Gaussian functions. The performance of the system for the two beams is evaluated and compared. Using simulation results we show that the use of Bessel beam gives superior results to that of using Gaussian beam for large interconnects distance.</span></p>

Models for optical crosstalk in lens-based free-space optical interconnects system that use Gaussian beams as information carriers

We introduce three crosstalk models in lens-based free-space optical interconnects systems. In these models, we use the Gaussian beams as the information carriers. In the first and the second models, the resulted finite integral of the optical field at the detectors plane was solved by expanding the lens’ finite aperture in terms of complex Gaussian functions and in terms of an infinite series of Bessel functions, respectively. In the third proposed model, the resulted finite integral was solved by approximating the Gaussian function in the integrand by zeroth-order and first-order Bessel functions without the need to expand the aperture function. We show that the third proposed model compares favorably with the other models with superior computational efficiency.

Crosstalk Modeling in Lens Based Free Space Optical Interconnects Systems with Circular Symmetry

We introduce convenient model and an optimization scheme to optimize the signal-to-crosstalk ratio (SCR) in a free space optical interconnects (FSOIs) system that uses microlenses with finite circular apertures. In this model, we consider both the stray light crosstalk and the crosstalk due to the diffraction at the microlens apertures to evaluate the SCR. Using cylindrical form of Collins diffraction integral and the Laguerre–Gaussian (LG) beam model, we derive an approximate closed form formula for the optical field of a multimode LG beam propagating through circular apertured FSOIs by expanding the hard edge circular aperture function of the microlens in terms of complex Gaussian functions. The analyses indicate that the size of the detector is an important factor to optimize the SCR for both the apertured and the unapertured misaligned FSOIs system. The effect of higher order mode of the laser source on the SCR is also considered.

Aberrated Microlenses to Reduce Crosstalk in Free Space Optical Interconnects Systems

A free space optical interconnects system that uses Gaussian beams and focusing microlenses with spherical aberration has been considered. Numerical results show that the use of microlenses with negative spherical improves the signal-to-crosstalk ratio of the interconnects system at some optimum positions of the detector array. An expression for output field at the detectors plane is derived and used to evaluate the signal-to-crosstalk ratio.

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.

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.

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

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.

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.

Noise analysis of a transimpedance amplifier for short-distance free-space optical interconnects

We consider the design of a metal-oxide semiconductor field effect transistor-based transimpedance amplifier that represents the front end of an optical receiver for optimum noise performance in a lensless, free-space optical interconnects system. The optical diffraction noise and the electrical front-end circuitry noise are considered as the two main noise sources for the design optimization. We show that the capacitance of the photodetector diode as well as the input capacitance of the transimpedance amplifier can be used as design parameters to minimize the total noise.

Crosstalk analysis of aligned and misaligned free-space optical interconnect systems

We present an analysis of crosstalk in aligned and misaligned free-space optical interconnect (FSOI) systems. On the basis of a generalized diffraction integral formula, an analytical expression of irradiation distribution for FSOI systems and a convenient approach to calculate crosstalk noise signal ratios (CNSR) are proposed. Simulations are performed to analyze the factors affecting the CNSR. The analyses indicate that small beam quality factor and wide channel pitch will significantly improve performance of the FSOIs. Furthermore, the displacement of the transmitter microlens array will affect the interconnection distance much more significantly than that of the receiver microlens array.

Robust free space board-to-board optical interconnect with closed loop MEMS tracking

We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on silicon-on-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise.

Analytic Solution of Free Space Optical Beam Steering Using Risley Prisms

This paper provides some mathematical formulae for Risley beam steer control system that is used for free space optical interconnection system. Given the prisms' orientations, the analytic formulae can be used to calculate the pointing position. For the inverse problem, i.e., given the desired pointing position, the required prisms' orientations can be obtained efficiently by applying any numerical method for systems of nonlinear equations related to the formulae. Unlike some existing methods, these formulae are not first-order approximation but an accurate closed analytical form. It does not need a good initial guess on the motor positions required by some other methods. It also does not need a feedback loop to get the correct pointing position but it can be used in feedback implementation. This feature is especially imporutant for systems that need to slew the beam to the desired position quickly and accurately.