Maggie M. Li

Design limitations of highly parallel free-space optical interconnects based on arrays of vertical cavity surface-emitting laser diodes, microlenses, and photodetectors

We utilize a novel diffraction formalism to study the crosstalk effect in a highly parallel free-space optical interconnect based on two-dimensional arrays of surface-emitting laser diodes, microlenses, and photodetectors. The diffraction induced crosstalk between adjacent laser diodes in each detector to the system limitations is investigated. Optimum design rules and formulas are given for the first time, to include the relation of channel packaging density and interconnect length to the design parameters of the optical interconnect components. The design formulas developed here yield an optimum detector size and indicate a tradeoff between channel packaging density and interconnect length. The feasibility of such a free-space interconnect with a channel packaging density of 3460 channels/cm/sup 2/ and 2.0 cm interconnection length is determined using typical parameters of detector radius from /spl sim/5 to /spl sim/45 /spl mu/m, lens radius of 85 /spl mu/m, and laser diode radius of /spl sim/5 pm operating at wavelength 0.67 pm for signal-to-noise ratio above 17 dB. Some experiments were conducted to measure the diffraction induced crosstalk and optical link efficiency. >

1‐to‐12 surface normal three‐dimensional optical interconnects

We present a surface normal optical interconnect with a 1‐to‐12 collinear fan‐out. Two types of polymer‐based holograms were fabricated to provide a collinear 1‐to‐12 fan‐out from guided mode to substrate modes and twelve 1‐to‐1 surface normal interconnects. Fluctuation of up to 7.2 dB for the 1‐to‐12 fan‐out hologram was observed due to the oscillating and the film saturation effects of the transmission hologram. Diffraction efficiency better than 70% was observed for all the total internal reflection holograms. The result reported herein greatly enhanced optical signal processing capability of guided wave optical interconnects. The advantages of free space optical interconnect, such as global interconnect, three dimension, massive fan‐out/fan‐in capabilities, and surface normal optical interconnect, can be realized using the demonstrated architecture. The coupling from waveguide to fiber can be realized from the surface rather than the edge of a photonic integrated circuit.

A novel wavelength-division-demultiplexer with optical in-plane to surface-normal conversion

We present a novel surface normal optical wavelength-division-demultiplexer (WDDM), working at 750, 770, 790, 810, 830, and 850 nm wavelengths. The device is based on the integration of a polymer-based planar waveguide, a substrate waveguiding plate, and waveguide holograms. The unique optical in-plane to surface-normal conversion converts the difficult three spatial and three angular edge coupling problem into a planar surface one, resulting in a reliable miniaturized optoelectronic packaging containing a photodetector array and the WDDM. A six-channel wavelength-division-demultiplexer with equally spaced collinear surface-normal outputs are designed and demonstrated in a polymer-based planar waveguide in conjunction with holograms on a glass substrate.<<ETX>>

FIVE-CHANNEL SURFACE-NORMAL WAVELENGTH-DIVISION DEMULTIPLEXER USING SUBSTRATE-GUIDED WAVES IN CONJUNCTION WITH A POLYMER-BASED LITTROW HOLOGRAM

We report on a five-channel wavelength-division demultiplexer using substrate-guided waves in conjunction with a polymer-based Littrow hologram operating at 700, 710, 720, 730, and 740 nm. An average cross talk of −40 dB between adjacent channels is measured. Diffraction efficiencies of 69%, 78%, 83%, 77%, and 69% are both experimentally and theoretically confirmed for the five-channel device. We also present further study aimed at reducing the wavelength channel separation to 1 nm and find that achieving such a goal requires a device length of 6.4 cm corresponding to a propagation distance of 9.05 cm.

Two‐channel surface‐normal wavelength division demultiplexer using substrate guided waves in conjunction with multiplexed waveguide holograms

We report a wavelength division demultiplexer (WDDM) using collinear surface‐normal input and output coupling. The reported device employs polymer‐based multiplexed waveguide holograms in conjunction with substrate guided waves. A two‐channel WDDM device operating at 700 and 738 nm with diffraction angles of 45° and 50° are demonstrated. The peak diffraction efficiencies of 80% and 77% are measured for these two channels. A crosstalk of −31 dB between the two channels is measured. Variations of the diffraction efficiencies and of the bandwidths as a function of film thickness and index modulation are further considered. A good agreement between theoretical analysis and experimental results is obtained.

Angular limitations of polymer‐based waveguide holograms for 1‐to‐many V‐shaped surface‐normal optical interconnects

Experimental results of 1‐to‐2 intraplane and of 1‐to‐32 interplane V‐shaped fanouts are delineated. Coupling efficiencies of 48% for surface‐normal and of 45% for near‐surface‐normal interplane fanout beams are theoretically and experimentally confirmed. The influence of the angular fluctuation of a device having two multiplexed waveguide holograms with film thickness of 15 μm and index modulation of 0.04 is studied. The angle between the two grating vectors is determined to be less than 26° to keep the near‐surface‐normal fanout beams.

Design limitations of highly parallel free-space optical interconnects based on arrays of vertical-cavity surface-emitting laser diodes, microlenses, and photodetectors

The packing density of a highly parallel free-space multi-stage optical interconnect network involving arrays of vertical cavity surface-emitting lasers (VCSELs), microlenses, and photodetectors is analyzed for the first time, based on a crosstalk model involving the diffraction-induced crosstalk among pixels. Variations of channel packing density, interconnection distance, and optical transmission efficiency are evaluated to provide optimum design parameters for the arrays of VCSELs, microlenses, and photodetectors. It is shown that it is pivotal to optimize the photodetector diameter in such an optical interconnect network. Photodetector array misalignment effects on system performance are further investigated, providing both transverse and longitudinal alignment tolerance. Several experiments are also conducted to verify the theory developed herein.

A three-dimensional (3-D) substrate-guided-wave to free-space multistage optoelectronic interconnection using wavelength division multiplexing and space division demultiplexing

An integrated 3-D guided-free-space four-stage optoelectronic fan-out (6/spl times/6, 2/spl times/6, 6/spl times/6: and 2/spl times/6) interconnect using wavelength division multiplexing (WDM) is proposed and then demonstrated together with 40 (2/spl times/4/spl times/5) 3-D optoelectronic fan-outs using space division demultiplexing (SDDM). This channel separation is one order of magnitude smaller than that using wavelength-selective detecting techniques in WDM. A signal to noise ratio of 57 dB is experimentally determined, with two channel 40 (2/spl times/4/spl times/5) fan-outs having a channel separation of 600 /spl mu/m in SDDM. The interconnection scheme presented herein allows each pixel in a transmitting plane to communicate simultaneously and reconfigurably with many pixels in the subsequent planes in a truly 3-D feature. This system can utilize vertical cavity surface emitting laser diodes, photo detecting planes, and planar compact guided-free-space fan-out interconnects, allowing compact multistage integration. By using 2-D spatially separated or multiplexed hologram arrays on a thin light guiding plate, the interconnection capability is greatly enhanced as compared to other techniques. This novel optoelectronic interconnect technology may have widespread applications in microelectronic systems and fiber-optic communication networks.

Nd3+-doped graded index single-mode polymer waveguide amplifier working at 1.06 and 1.32 μm

In this paper, we report a graded index (GRIN) polymer waveguide amplifier working at 1.06 micrometers wavelength using Nd-doped photolime gel as the active medium. Fluorescent spectrum at 1.32 micrometers region was also observed for the first time. The polymer introduced is soluble in water. As a result, the chemical compounds containing rare earth ions (REIs) can be mixed with the host polymer as long as they are hydrophilic. But the doping concentrations should be below the level of microscopic clustering which quenches the active ions. There are other quenchers which shorten the lifetime of metastable states of active ions.

Design considerations for high packaging density optical bus array

In this paper, a crosstalk model is developed to study the packing density and interconnect distance limitations of an optical interconnect system employing polymer-based single-mode bus arrays. The upper limit of channel packing density (1250 channels/cm at interconnect distance of 5 cm) is determined for the first time using the crosstalk model, in which channel cross-coupling among an infinite number of waveguides is considered. Computer simulations are provided together with the proven experimental results. It is shown that there is a threshold of channel separation due to channel cross-coupling, which results in a tradeoff between channel packing density and interconnect distance. Waveguide dimension closer to the cutoff boundary of second mode (E12X) is preferred for an optimum design.