Free-space Interconnects Research Articles

Surface-Normal Free-Space Beam Projection via Slow-Light Standing-Wave Resonance Photonic Gratings.

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles, and nonvertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light-matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing, and complex free-space interconnect. The large coupler size challenges general inverse design techniques, and solutions obtained by them are often difficult to physically understand and generalize. Here, by posing the problem to a carefully constrained computational inverse-design algorithm capable of large area structures, we discover a qualitatively new class of grating couplers. The numerically found solutions can be understood as coupling an incident photonic slab mode to a spatially extended slow-light (near-zero refractive index) region, backed by a reflector. The structure forms a spectrally broad standing wave resonance at the target wavelength, radiating vertically into free space. A reflectionless adiabatic transition critically couples the incident photonic mode to the resonance, and the numerically optimized lower cladding provides 70% overall theoretical conversion efficiency. We have experimentally validated an efficient surface normal collimated emission of ≈90 μm full width at half-maximum Gaussian at the thermally tunable operating wavelength of ≈780 nm. The variable-mesh-deformation inverse design approach scales to extra large photonic devices, while directly implementing the fabrication constraints. The deliberate choice of smooth parametrization resulted in a novel type of solution, which is both efficient and physically comprehensible.

Metasurface-Integrated Photonic Platform for Versatile Free-Space Beam Projection with Polarization Control

Densely integrated photonic circuits enable scalable, complex processing of optical signals, including modulation, multiplexing, wavelength conversion, and detection. Directly interfacing such integrated circuits to free-space optical modes will enable novel optical functions, such as chip-scale sensing, interchip free-space interconnect and cooling, trapping, and interrogation of atoms. However, doing this within the limits of planar batch fabrication requires new approaches for bridging the large mode scale mismatch. Here, by integrating a dielectric metasurface with an extreme photonic mode converter, we create a versatile nanophotonic platform for efficient coupling to arbitrary-defined free-space radiation of 780 nm wavelength with well-controlled spatially-dependent polarization, phase, and intensity. Without leaving the chip, the high index photonic mode is converted first to a ≈ 200 μm wide, precisely collimated, linearly-polarized Gaussian beam, which is then modified by a planar, integrated, low-loss metasurface. We demonstrate high numerical aperture, diffraction limited focusing to an ≈ 473 nm spot at an ≈ 75 μm working distance, and combine it with simultaneous conversion from linear to elliptical polarization. All device components are lithographically defined and can be batch fabricated, facilitating future chip-scale low-cost hybrid photonic systems for bio-sensing, nonlinear signal processing and atomic quantum sensing, frequency references and memory.

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.

Free-space interconnect for optical switch and analogue neural network

In this paper, the architecture of a vector-matrix multiplier (MVM) is simulated. The optical design can be made compact by the use of GRIN lenses for the optical fan-in. The intended application area was in storage area networks (SANs) but the concept can be applied to a neural network.

Performance Comparison and Overview of Different Approaches for VLSI Optoelectronic Interconnects

Although a great deal of research has been carried out over the last decade in which many different approaches for devising optoelectronic interconnects have been proposed, more detailed analysis of the performance parameters such as power, latency, and area of different optical technologies is yet to be done. Optoelectronics offer potential for new approaches to the production of VLSI interconnects, but previous works have made overly simplified assumptions for optoelectronic interconnections. In this paper, we present a preliminary analysis of the system parameters of different optoelectronic interconnect technologies. Based on our analysis, we conclude that the three-dimensional free-space optoelectronic interconnect network has the best speed area product performance compared with fiber-optical interconnects and optical microelectromechanical systems (MEMS) interconnects. Second, although the optoelectronic interconnect offers higher data rates and less power per bit compared with electronic interconnects, the bipolar encoding scheme on the source plane and the detector plane means that a larger area and volume will be needed.

Early experience with in situ chip-to-chip alignment characterization of Proximity Communication flip-chip package

Proximity Communication (PxC) facilitates the integration of VLSI chips in a package using near-field capacitive coupling between chips, eliminating the need for solder or wires for I/O at the chip-to-chip interface. PxC provides chip-to-chip interconnect with bandwidth density and energy per bit similar to on-chip I/O, enabling system on a chip performance within a package. We have built early packages to explore assembly concepts and developed test methods for verification of the PxC design space. This package started with an adhesively-bonded three-chip subassembly of two Island chips and one Bridge chip. The two outer Island chips were reflowed to an alumina ceramic substrate. In the resulting package, communication between chips was achieved using PxC from Island to Bridge and then from the Bridge to the other Island. We demonstrated the ability to detect the X, Y, and Z chip-to-chip relative location and the ability to steer PxC data to optimize signal integrity within the package. This paper describes the first demonstration of active monitoring of chip-to-chip alignment during thermal cycling, in a PxC-enabled package. Leveraging this work, future packages will better exploit PxC benefits such as free-space electrical interconnect and re-workability of multi-chip modules.

Large Scale, All-Fiber Optical Cross-Connect Switches for Automated Patch-Panels

Transparent, all-optical cross-connect switches provide non-blocking, one-to-one reconfigurable interconnections between any of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> input fibers and any of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> output fibers. Cross-connects based on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">all-fiber</i> rather than free-space interconnections have the potential for low opto-mechanical complexity, miniaturization, high optical power handling and ideal optical performance. In this paper, we propose and analyze novel architectures for large-scale, all-fiber cross-connects based on the application of theory of Knots and Braids to fiber switching and compare them on the basis of scalability, modularity and reconfiguration time. A prototype 140 input by 140 output, automated all-fiber cross-connect is developed to demonstrate these concepts.

Assessment of integration of off-axis Fresnel lenses into a free-space interconnect

The integration of multiple optical functions into a single optical element is often seen as advantageous to improve practicality and/or performance. In free-space devices, a common way to achieve this is to use off-axis Fresnel lenses to simultaneously collimate and redirect light. We study the impact of this type of integration on assembly tolerances and performance of a free-space interconnect designed to format a one-dimensional set of inputs into a two-dimensional array. Two systems are built and compared: one where light collimation and redirection are performed by different elements and one where these functions were combined into off-axis Fresnel lenses. The integrated system has lower losses but exhibits higher polarization sensitivity.

Trade-offs between lens complexity and real estate utilization in a free-space multichip global interconnection module

The FAST-Net (Free-space Accelerator for Switching Terabit Networks) concept uses an array of wide-field-of-view imaging lenses to realize a high-density shuffle interconnect pattern across an array of smart-pixel integrated circuits. To simplify the optics we evaluated the efficiency gained in replacing spherical surfaces with aspherical surfaces by exploiting the large disparity between narrow vertical cavity surface emitting laser (VCSEL) beams and the wide field of view of the imaging optics. We then analyzed trade-offs between lens complexity and chip real estate utilization and determined that there exists an optimal numerical aperture for VCSELs that maximizes their area density. The results provide a general framework for the design of wide-field-of-view free-space interconnection systems that incorporate high-density VCSEL arrays.

Optical Look-ahead Carry Full-adder Using Dual-rail Coding

In this paper, a new optical parallel binary arithmetic processor (OPBAP) capable of computing arbitrary n-bit look-ahead carry full-addition is proposed and implemented. The conventional Boolean algebra is considered to implement OPBAP by using two schemes of optical logic processor. One is space-variant optical logic gate processor (SVOLGP), the other is shadow-casting optical logic array processor (SCOLAP). SVOLGP can process logical AND and OR operations different in space simultaneously by using free-space interconnection logic filters, while SCOLAP can perform any possible 16 Boolean logic function by using spatial instruction-control filter. A dual-rail encoding method is adopted because the complement of an input is needed in arithmetic process. Experiment on OPBAP for an 8-bit look-ahead carry full addition is performed. The experimental results have shown that the proposed OPBAP has a capability of optical look-ahead carry full-addition with high computing speed regardless of the data length.

System-level modeling and simulation of the 10G optoelectronic interconnect

Mixed-signal multidomain systems present a challenge for computer-aided design tools. Optical and electronic simulation tools are available as separate entities. However, to date, successful system-level cosimulation has not been implemented, leading to expensive refabrication. We present a unique system-level simulation tool for mixed electrooptical systems. We apply our tool Chatoyant to the simulation of an optical high-speed free-space interconnect system designed for 10-GHz speeds. The 10G free-space optical interconnect module has optical, optoelectronic, and microwave components and thus is an ideal vehicle to use as a test system. We demonstrate how Chatoyant, a mixed-signal multidomain simulator, has been used to evaluate end-to-end performance of this complex system, including the exploration of design tradeoffs and mechanical tolerancing.

Refractive microlens on fiber using UV-curable fluorinated acrylate polymer by surface-tension

A new novel fabrication method for lensed fiber has been proposed utilizing a viscosity-optimized UV-curable fluorinated acrylate polymer. The structure is composed of three segments; single-mode fiber, coreless silica fiber and UV-curable polymer lens-tip. Flexible control of the curvature of lens-tip was realized by control of deposited volume and surface tension of the liquid polymer. Free-space interconnection performances such as working distance, spot size at beam waist, and misalignment tolerances are characterized.

Design and implementation of a modulator-based free-space optical backplane for multiprocessor applications.

Design and implementation of a free-space optical backplane for multiprocessor applications is presented. The system is designed to interconnect four multiprocessor nodes that communicate by using multiplexed 32-bit packets. Each multiprocessor node is electrically connected to an optoelectronic VLSI chip which implements the hyperplane interconnection architecture. The chips each contain 256 optical transmitters (implemented as dual-rail multiple quantum-well modulators) and 256 optical receivers. A rigid free-space microoptical interconnection system that interconnects the transceiver chips in a 512-channel unidirectional ring is implemented. Full design, implementation, and operational details are provided.

Scalability analysis of diffractive optical element-based free-space photonic circuits for interoptoelectronic chip interconnections

An interchip free-space optical interconnection module is investigated to solve the pin-input-output bottleneck at the interface of silicon integrated circuits. The scalability of the photonic circuit is theoretically analyzed by use of the minimum feature size requirement of each diffractive element used. The study showed that interconnection densities of 1000-2000 channels/cm is possible for a 40-mm interconnection length with a 3-mm-thick optical substrate. Diffraction-limited imaging capability has been demonstrated using a fabricated prototype, confirming its applicability for interchip free-space interconnections. Photonic circuit insertion losses of -23.4 dB for TE polarization and -25.9 dB for TM polarization as well as a polarization-dependent loss of 2.5 dB are found to be caused primarily by a pair of binary linear gratings used for beam deflections. Design modifications aiming at insertion loss reduction and further improvement of tolerance capabilities are also discussed.

Free-space parallel multichip interconnection system

A parallel data-communication scheme is described for interchip communication with free-space optics. We present a proof-of-concept and feasibility demonstration of a practical modular packaging approach in which free-space optical interconnect modules can be simply integrated on top of an electronic multichip module (MCM). Our packaging architecture is based on a modified folded 4-f imaging system that is implemented with off-the-shelf optics, conventional electronic packaging techniques, and passive assembly techniques to yield a potentially low-cost packaging solution. The prototype system, as built, supports 48 independent free-space channels with eight separate laser and detector chips, in which each chip consists of a one-dimensional array of 12 devices. All chips are assembled on a single ceramic carrier together with three silicon complementary metal-oxide semiconductor chips. Parallel optoelectronic (OE) free-space interconnections are demonstrated at a speed of 200 MHz. The system is compact at only 10 in.(3) (~164 cm(3)) and is scalable because it can easily accommodate additional chips as well as two-dimensional OE device arrays for increased interconnection density.

Plastic microoptical interconnection modules for parallel free-space interand intra-MCM data communication

We design and fabricate a prototype scalable multichannel free-space interconnection module with the potential for Tb/s/spl middot/cm/sup 2/ aggregate bit-rate capacity over inter- and intra-MCM interconnection distances. The component is fabricated in a high quality optical plastic, PMMA, using deep proton lithography, an ion-based rapid prototyping technology. As a feasibility demonstration, data communication is achieved at 622 Mb/s per channel with a bit error rate smaller than 10/sup -13/ for 16 channels with an interchannel crosstalk lower than -22 dB. We perform a sensitivity analysis for misalignments and fabrication errors and study the fabrication issues of these components with injection molding techniques. Finally, we provide evidence that these modules can be mass fabricated with the required precision.

Interchip free-space interconnection implemented with planar optical elements

Interchip free-space interconnection implemented with planar optical elements

Angular-precision enhancement in free-space micromachined optical switches

Free-space micromachined optical-switching technology has emerged as a promising means of building the large-scale optical crossconnects that will be needed in future optical-transport networks. Although this technology has rapidly demonstrated promising optical performance, its ability to expand to the required port-count has yet to be demonstrated, In particular, the tight angular tolerances imposed by free-space interconnection present a significant challenge, In this letter, we demonstrate integrated mechanical structures that are capable of achieving better than 0.1/spl deg/ micromirror angular repeatability, thus opening the path to high-port-count optical crossconnects that live within cross-office optical-loss budgets.

Single-pass thin-film electro-optic modulator based on an organic molecular salt

Single crystal films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with excellent optical quality have been prepared by modified shear method. Electro-optic modulation for light propagating perpendicular to the film surface was measured at a wavelength (720 nm) where absorption is small. Very large electro-optic modulation (20%) was demonstrated in a 3 μm thick film at a low ac field (1 V/μm, at 4 kHz). Electro-optic modulation in such thin films is suitable for a wide range of applications in free-space interconnects and various modulator structures with light propagating perpendicular to the film. The magnitude of the electro-optic coefficient at 720 nm, r11∼530 pm/V, is significantly larger than that of bulk crystals.

FAST-Net optical interconnection prototype demonstration

The first prototype of the free-space accelerator for switching terabit networks (FAST-Net) program is described. The optical interconnection module consists of an array of matched high-resolution, wide-angle (f/1.1) imaging lenses that effect a global point-to-point shuffle link pattern across a multichip smart pixel array. A mirror is used to fold the shuffled links back onto the array to facilitate single-plane multichip packaging and simplified opto-mechanical alignment. The test system incorporates several monolithically integrated two-dimensional arrays of vertical-cavity surface-emitting lasers (VCSELs) and photodetectors (PDs) in the multichip plane. In initial experiments the prototype showed good overall optical efficiency and sufficient channel separation for VCSEL/PD arrays with pitches as small as 100 m. These results suggest that high bisection bandwidth optical free-space interconnection fabrics are feasible for high-throughput switching and multiprocessor applications.