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.
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.
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.
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.
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.
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.