Roger King

Design and analysis of single-mode oxidized VCSELs for high-speed optical interconnects

Oxide-confined vertical-cavity surface-emitting laser diodes (VCSELs) are fabricated for applications in high-performance optical interconnects. Both 980-nm as well as 850-nm wavelength devices in one- and two-dimensional arrays are investigated. Noise properties of single- and multimode devices under different operation conditions are relative intensity noise of single-mode devices can be as low as -150 dB/Hz at output powers of about 1 mW and feedback levels up to -30 dB. Data rates up to 12.5 Gb/s with bit error rates below 10/sup -11/ are achieved with VCSELs showing stable single-mode emission at large-signal modulation, combined with modulation bandwidths exceeding 10 GHz. Arrays with 4/spl times/8 elements flip-chip mounted on Si CMOS driver chips ready for use in parallel data transmission systems are presented.

Improved output performance of high-power VCSELs

The intention of this paper is to report on state-of-the-art high-power vertical-cavity surface-emitting laser diodes (VCSELs), single devices as well as two-dimensional (2-D) arrays. Both approaches are studied in terms of electrooptical characteristics, beam performance, and scaling behavior. The maximum continuous wave (CW) output power at room temperature of large-area bottom-emitting devices with active diameters up to 320 /spl mu/m is as high as 0.89 W, which is to our knowledge the highest value reported for a single device. Measurements under pulsed conditions show more than 10-W optical peak output power. Also, the CW performance of 2-D arrays has been increased from 0.56 W for 23 elements to 1.55 W for 19 elements due to significantly improved heat sinking. The extracted power densities spatially averaged over the area close to the honeycomb-like array arrangement raised from 0.33 kW/cm/sup 2/ to 1.25 kW/cm/sup 2/. Lifetime measurements have proven acceptable reliability for over 10000 h at a degradation rate of less than 1% per 1000 h. The emission wavelength of bottom-emitting devices is restricted to about 900 nm or higher due to fundamental absorption in the GaAs substrate. Windowing of the substrate has been studied to allow for shorter wavelength emission.

RCE photodetectors based on VCSEL structures

We have fabricated top illuminated resonant cavity enhanced (RCE) detectors by partially removing top-mirror pairs of a vertical-cavity surface-emitting laser (VCSEL) structure by etching. The observed maximum quantum efficiency is 73% with a spectral full-width at half-maximum (FWHM) of about 1.7 nm. This particular RCE detector was fabricated by etching away 8 top-mirror pairs of a VCSEL structure, with 15.5 top-mirror pairs remaining. Spectral FWHMs as wide as 6.5 nm, with a peak efficiency of 36%, have been measured in the case of 4.5 top mirrors remaining. An increase in the 3-dB detector bandwidth up to 2.8 GHz is observed by reducing the active device diameter from 90 to 20 /spl mu/m.

POF-based interconnects for intracomputer applications

The technology and the optical characteristics of /spl times/8 optical interconnects based on plastic optical fibers (POFs) are presented. The MT-compatible interconnects have been realized by using ribbonized 120-1125-/spl mu/m multimode step index POF. Two-dimensional POF arrays with a pitch of 250 /spl mu/m have been fabricated either by using precision drilled PMMA-hole-plates or by embedding the POFs in grooved copper plates and piling them up subsequently. The transmission loss of a 20 cm-long 8/spl times/8 interconnect including two 900 bends was 1.5 dB at 650 nm, 3.5 dB at 870 nm, and 4.5 dB at 980 nm.

High volume production of single-mode VCSELs

Up to now applications for singlemode VCSELs were in low volume and high prized applications like tunable diode laser absorption spectroscopy (TDLAS, [1,2]) or optical interferometers. Typical volumes for these applications are in the range of thousands of pcs per year, with pricing levels of several 100 USD/pcs. New applications for singlemode VCSELs in consumer markets require manufacturing in very high volumes and at very low cost. Examples are laser-based optical mouse sensors, optical encoders, and rubidium atomic clocks for GPS systems [3,4]. U-L-M photonics presents manufacturing aspects, device performance and reliability data for these devices. The first part of the paper is dealing with high volume manufacturing of 850 nm singlemode VCSEL chips with very high efficiency and low operation current. Special processing technologies have been developed to achieve yields on 3 inch wafers of more than 90%. Wafer qualification procedures are discussed as well. The second part of the paper covers high volume packaging in TO and SMT type packages where very high packaging yields must be achieved. In the last part of the paper reliability issues are discussed, focused on the very high susceptibility of these devices to electrostatic discharge.

Oxide-confined 2D VCSEL arrays for high-density inter/intra-chip interconnects

We have designed and fabricated 4 X 8 vertical-cavity surface-emitting laser (VCSEL) arrays intended to be used as transmitters in short-distance parallel optical interconnects. In order to meet the requirements of 2D, high-speed optical links, each of the 32 laser diodes is supplied with two individual top contacts. The metallization scheme allows flip-chip mounting of the array modules junction-side down on silicon complementary metal oxide semiconductor (CMOS) chips. The optical and electrical characteristics across the arrays with device pitch of 250 micrometers are quite homogeneous. Arrays with 3 micrometers , 6 micrometers and 10 micrometers active diameter lasers have been investigated. The small devices show threshold currents of 600 (mu) A, single-mode output powers as high as 3 mW and maximum wavelength deviations of only 3 nm. The driving characteristics of all arrays are fully compatible to advanced 3.3 V CMOS technology. Using these arrays, we have measured small-signal modulation bandwidths exceeding 10 GHz and transmitted pseudo random data at 8 Gbit/s channel over 500 m graded index multimode fiber. This corresponds to a data transmission rate of 256 Gbit/s per array of 1 X 2 mm2 footprint area.

Volume production of polarization controlled single-mode VCSELs

Over the past 3 years laser based tracking systems for optical PC mice have outnumbered the traditional VCSEL market datacom by far. Whereas VCSEL for datacom in the 850 nm regime emit in multipe transverse modes, all laser based tracking systems demand for single-mode operation which require advanced manufacturing technology. Next generation tracking systems even require single-polarization characteristics in order to avoid unwanted movement of the pointer due to polarization flips. High volume manufacturing and optimized production methods are crucial for achieving the addressed technical and commercial targets of this consumer market. The resulting ideal laser source which emits single-mode and single-polarization at low cost is also a promising platform for further applications like tuneable diode laser absorption spectroscopy (TDLAS) or miniature atomic clocks when adapted to the according wavelengths.

Demonstrating optoelectronic interconnect in a FPGA-based prototype system using flip-chip mounted 2D arrays of optical components and 2D POF-ribbon arrays as optical pathways

Architectural studies have identified field-programmable gate arrays (FPGA) as a class of general-purpose very large scale integration components that could benefit from the introduction at the logic level of state-of-the-art massively parallel optical inter-chip interconnections. In this paper, we present a small-scale optoelectronic multi-FPGA demonstrator in which three optoelectronic enhanced FPGAs are interconnected by 2D Plastic Optical Fiber (POF) ribbon arrays. The full-custom FPGA chips consisting of an 8 X 8 array of very simple programmable logic cells are equipped with two optical sources and two receivers per FPGA cell yielding a maximum of 256 optical links per chip. The optical links are designed for signaling rates of 80 to 100 Mbit/s (160 to 200 Mbaud using Manchester coded data) compatible with the maximum clock frequency of the, in 0.6 micrometers CMOS implemented, FPGA chips. The results of parallel link experiments between such modules with both VCSELs and LEDs as sources will be shown. A large scale parallel bit error rate experiment at 90 Mbit/s/channel between two half-populated VCSEL-based FPGA modules with 112 of their 128 channels operational at bit error rates below 10-13 on all active channels (approximately equals 10 Gbit/s/chip) proves the feasibility of this approach. We first briefly discuss the general architecture and the realization of the optoelectronic FPGA demonstrator system. We then present measurement results on the available modules, followed by some conclusions on this work.

Commercial VCSELs reach 0.1-W cw output power

Following the success in fiber based DataCom, VCSELs start to conquer additional market shares in a variety of other applications like free space optics (FSO), lighting, printing, and sensing. U-L-M photonics presents a new family of commercial high power VCSELs emitting powers of up to 50 mW cw at RT based on top-emitting technology. The devices are available at 850 nm emission wavelength. All devices can be operated passively cooled and provide modulation bandwidths of up to 1 GHz. Wallplug efficiencies are in excess of 25 %. Even higher output power of 250 mW cw from a 80 μm active diameter bottom-emitting VCSEL operating at 980 nm has already been obtained although just beeing passively cooled. Further power up scaling is achieved by arrangement of multiple VCSELs in 2D arrays. For the first time we demonstrate cw output power of 10 Watt cw at RT from compact monolithic VCSEL module of 14 mm2 chip area. Transfer of the technology to other wavelengths, e.g. 808 nm and 945 nm, is presented, too, and shows perspectives towards homogeneous optical pumping of solid state lasers. Almost identical device performance levels can be presented for the entire wavelength span. All discussed results are based on highest quality epitaxy optimized for maximum intrinsic efficiency and differential slope efficiency. Oxide confinement is used for current constriction that provides most efficient electrical pumping of the active area. In combination with advanced mounting techniques all mentioned aspects sum up to allow for cost effective VCSEL products in the medium and high power laser regime. The circular output beam in addition to simple heat sinking offers attractive solutions for advanced system integration.

45% quantum-efficiency light-emitting diodes with radial outcoupling taper

10We have investigated efficient light outcoupling from light- emitting diodes (LEDs) by introducing lateral tapers. The concept is based on light generation in the very central area of a circularly symmetric structure. After propagating between two highly reflecting mirrors light is outcoupled in tapered mesa region. By proper processing we achieve quantum and wallplug efficiencies of almost 30% for outcoupling via a planar surface or, respectively, 45% and 44% for encapsulated devices.