Hendrik Roscher

Small-Pitch Flip-Chip-Bonded VCSEL Arrays Enabling Transmitter Redundancy and Monitoring in 2-D 10-Gbit/s Space-Parallel Fiber Transmission

We demonstrate novel pixel architectures in 2-D vertical-cavity surface-emitting laser (VCSEL) arrays offering additional functionality without sacrificing efficient fabrication, compactness, and low module cost. Very high flip chip VCSEL packing densities enable both a built-in 3-per-channel VCSEL redundancy as well as simple intracell VCSEL monitoring. Each pixel has three individually addressable oxide-confined and substrate- removed 850-nm-wavelength VCSELs directly flip bonded to the mesas that are extremely close-spaced to permit equal butt coupling to 50-mum-core-diameter multimode fibers (MMFs) without the need for external coupling optics. Built-in radial VCSEL-to- fiber launch offsets as small as 7.8 mum were reached, leading to offset coupling penalties of only 0.4 dB. Quasi-error-free transmission of 10 Gbit/s signals is achieved over 500-m-long MMFs even under offset launch conditions. New opportunities for a range of further applications offered by high-density VCSEL arrays are discussed.

Comparison of approaches to 850-nm 2D VCSEL arrays

There is a wide variety of reasons why future high-performance datacom links are believed to rely on two-dimensional VCSEL arrays suitable for direct flip-chip hybridization. Some typical are as follows: highest interconnect density, high-frequency operation, self alignment for precise mounting, productivity at high number of channels per chip. In this paper the latest approaches to flip-chip VCSELs are presented. In particular we will asses the properties of transparent substrate VCSEL arrays which are soldered light-emitting side up as well as VCSEL arrays which are soldered light-emitting side down, e.g., onto a CMOS driver chip. The VCSEL arrays are designed for bottom- or top-emission at 850 nm emission wavelength and modulation speeds up to 10 Gbps per channel.

Novel concepts of vertical-cavity laser-based optical traps for biomedical applications

Using vertical-cavity surface-emitting lasers (VCSELs) as light sources in optical traps offers various advantages compared to the common approaches. In particular, these are small dimensions, a circularly symmetric output beam, and the simple fabrication of two-dimensional laser arrays. We investigate the application of VCSELs in a standard tweezers setup, where trapping forces of up to 4.4 pN are achieved with 15 μm polystyrene particles and a transverse multi-mode VCSEL. The latter has improved trapping characteristics compared to a single-mode device. By introducing a small-spaced array of three VCSELs in the setup, non-mechanical movement with average velocities of up to 3 μm/s is demonstrated with 10 μm particles. Furthermore, the novel concept of the integrated optical trap is presented. By integrating a microlens directly on the VCSEL output facet, two-dimensional optical trapping is achieved in a small-sized system without any external optics. Elevation and trapping of 10 μm polystyrene particles is demonstrated at optical output powers of about 5 mW. In order to improve the beam quality of the lasers, the inverted surface relief technique is applied, which eliminates a previously observed offset between laser center and trapped particle.

Low thermal resistance flip-chip bonding of 850 nm 2-D VCSEL arrays capable of 10 Gbit/s/ch operation

In this work, 8/spl times/8 and 4/spl times/8 arrays of 850 nm oxide-confined VCSELs with 250 /spl mu/m device pitch are directly hybridized onto silicon fanout chips by means of an indium solder based flip-chip technology. Using this structure, digital data transmission experiments are conducted, which demonstrates that quasi error-free (bit error rate <10/sup -12/) 10 Gbit/s transmission can be achieved.

Monolithically integrated transceiver chips for bidirectional optical interconnection

We report on the design, fabrication and test results of monolithically integrated transceiver chips consisting of GaAs metal-semiconductor-metal photodiodes and 850nm wavelength vertical-cavity surface-emitting lasers. These chips are well suited for low-cost and compact bidirectional optical interconnection at Gbit/s data rates in mobile systems and industrial or home networks employing large core size multimode fibers.

Toward more efficient fabrication of high-density 2-D VCSEL arrays for spatial redundancy and/or multi-level signal communication

We present flip-chip attached high-speed VCSELs in 2-D arrays with record-high intra-cell packing densities. The advances of VCSEL array technology toward improved thermal performance and more efficient fabrication are reviewed, and the introduction of self-aligned features to these devices is pointed out. The structure of close-spaced wedge-shaped VCSELs is discussed and their static and dynamic characteristics are presented including an examination of the modal structure by near-field measurements. The lasers flip-chip bonded to a silicon-based test platform exhibit 3-dB and 10-dB bandwidths of 7.7 GHz and 9.8 GHz, respectively. Open 12.5 Gbit/s two-level eye patterns are demonstrated. We discuss the uses of high packing densities for the increase of the total amount of data throughput an array can deliver in the course of its life. One such approach is to provide up to two backup VCSELs per fiber channel that can extend the lifetimes of parallel transmitters through redundancy of light sources. Another is to increase the information density by using multiple VCSELs per 50 μm core diameter multimode fiber to generate more complex signals. A novel scheme using three butt-coupled VCSELs per fiber for the generation of four-level signals in the optical domain is proposed. First experiments are demonstrated using two VCSELs butt-coupled to the same standard glass fiber, each modulated with two-level signals to produce four-level signals at the photoreceiver. A four-level direct modulation of one VCSEL within a triple of devices produced first 20.6 Gbit/s (10.3 Gsymbols/s) four-level eyes, leaving two VCSELs as backup sources.

Design and communication applications of short-wavelength VCSELs

We report on recent progress in the design of short-wavelength vertical-cavity surface-emitting lasers (VCSELs) for 10 Gbit/s datacom applications. Topics of interest include differential mode delay characterizations of high-performance multimode fibers and their interplay with transverse single- and multimode VCSELs, flip-chip integrated two-dimensional arrays at 850 nm wavelength, as well as experiments toward the realization of optical backplanes. In the latter case, reliable 10 Gbit/s data transmission has been achieved over low-loss integrated polymer waveguides with up to 1 meter length. Moreover we present VCSELs with output powers in the 10 mW range that are employed in multi-beam transmitters for free-space optical data transmission with Gbit/s speed over distances of up to about 2 km.

Toward redundant 2D VCSEL arrays for optical datacom

We have developed technologies that bring functional redundancy to flip-chip mounted 850nm backside-emitting high-speed VCSEL arrays with 250μm channel pitch. A self-aligned dry-etch process results in vertical mesa sidewalls facilitating extremely narrow gaps as small as 2μm between adjacent mesas, and even smaller gaps seem possible with this technology. Very dense intra-cell arrangements of oxide-confined circular and pie-shaped VCSELs were fabricated with minimal current aperture center-to-center distances of 20μm and 17μm, respectively. The paper also gives a first theoretical approximation of the additional coupling loss introduced by the inevitable radial VCSEL--fiber displacement for a 10μm VCSEL coupling into a standard 50μm fiber channel under varying offset launch conditions. We also present 4x8 and 8x8 regular arrays directly hybridized onto silicon carrier chips by means of an indium solder based flip-chip technology. The elimination of thermal bottlenecks by direct mesa bonding cuts the thermal resistance by half to about 1.3K/mW for 10μm devices as compared to offset-bonded devices.

Fabrication-Efficient Flip-Chip-Bondable 850-nm VCSELs With an X-Shaped Vertical Cavity

We present a novel approach to flip-chip-bondable vertical-cavity surface-emitting lasers and 2-D arrays emitting at 850 nm, the standard for multimode fiber optical interconnects. A unique sequence of wet-etching steps renders laser fabrication particularly efficient and allows the incorporation of near-cavity heat spreaders. Record-low thermal resistances of substrate-removed devices are achieved without compromising the dynamic properties.

Recent progress in short-wavelength VCSEL-based optical interconnections

We report on recent progress in the design and application of vertical-cavity surface-emitting lasers (VCSELs) for optical interconnect applications in the 850 nm emission wavelength regime. Ongoing work toward parallel optical interconnect modules with channel data rates of 10 Gbit/s is reviewed and performance results of flip-chip integrated two-dimensional VCSEL arrays are presented. 10 Gbit/s speed as well as low thermal resistance of the lasers has been achieved. As a possible alternative to graded-index multimode fibers, we show 10 Gbit/s data transmission over 100 m length of a novel, entirely undoped multimode photonic crystal fiber. The use of VCSELs with output powers in the 10 mW range is demonstrated in a 16-channel free-space optical (FSO) module and VCSELs with even higher output power are shown to provide possible FSO connectivity up to data rates of 2.5 Gbit/s.