High Speed VCSELs for Optical Interconnects

Abstract

The forecast for the serial transmission speeds used for data communication s ystems is a continued exponential increase with time, directly in concert with silicon integrated circuit scaling and in response to human society's perpetual hunger for massive increases in bandwidth. Electrical interfaces for single channel bit rates beyond 10 Gbit/s are being standardized for a variety of applications, including for example (with an expected data -rate): Fibre Channel FC32G (34 Gbit/s), InfiniBand (20 Gbit/s), common electrical interface CEI (25-28 Gbit/s), and universal serial bus protocols USB 3.0 (up to 25 Gbit/s). As a result, the fundamental electro-magnetic limitations of copper wire-based links at bit rates >10 Gbit/s make fiber based optics for data communication indispensable at distances >1 m. At shorter distances, problems associated with electrical transmission lines at such high frequencies, e. g. high power consumption, strong signal attenuation, signal distortion and electromagnetic interference, lead to unstoppable and progressive penetration of optical communication links into traditional copper interconnect markets. These trends greatly expand the applications of vertical cavity surface emitting lasers (VCSELs) and VCSEL arrays as very inexpensive, efficient, reliable, readily manufacturable and compact laser light sources for next-generations of fiber-optic, free-space, board-to-board, module-to-module, chip-to-chip and on-chip interconnects and related information systems and networks.Already today oxide-confined GaAs-based VCSELs emitting at 850 nm are the key components for low cost high speed local and storage area network (LAN/SAN) data communication systems. Furthermore, active optical cable links for short-reach computer and consumer applications, for example USB, DisplayPort, and HDMI standards, are increasingly based on VCSELs operating in the near-infrared spectral range. Immense research and development effort all around the world resulted in the great progress in the area of highspeed GaAs-based VCSELs in the last few years. At the standard wavelength of 850 nm room temperature error free data transmission at the bit rate of 32 Gbit/s has been demonstrated in 2009. Also at longer wavelengths bit rates of 35 Gbit/s (980 nm) and 40 Gbit/s (1100 nm) have been achieved at room temperature using GaAs-based VCSELs, while the latter device was based on the buried tunnel junction, requiring additional epitaxial growth step and making the laser fabrication more complicated. While the wavelength of 850 nm is the current standard for LAN/SAN applications, potential competitive standards at 980 and 1100 nm have many critical advantages for very and ultra short reach systems. This includes smaller operational voltages due to the lower photon energy, which are decisive for complementary metal oxide semiconductor (CMOS) drivers, transparency of the GaAs substrate, which is important for bottom-emitting lasers, and deeper potential wells, suppressing the escape of injected non-equilibrium carr...

Single-chip photonic transceiver based on bulk-silicon, as a chip-level photonic I/O platform for optical interconnects

“…The direct-modulation VCSELs-on-Si is employed as a monolithic-integrated photonic transmitter (Tx) on the same bulk-silicon wafer with the Ge V-PDs, and can solve the compact chip-level light source problem, which is one of the main obstacles to implementing silicon-based optical interconnects at present. VCSELs, emitting at various wavelengths, are well-established, compact, reliable, low-power laser light sources and widely used in all levels of optical interconnects 47 48 49 50 51 52 53 54 . In this scheme, high performance VCSEL devices based on recent progress can be transplanted and utilized on a bulk silicon substrate, as an integrated compact reliable light source, without much degradation of device performance.…”

“…This single-chip photonic TRx scheme can simultaneously increase the level of photonic integration and reliability, as well as performance levels in the applied systems, and has many advantages for realistic, implementable, cost-effective applications. With properly designed VCSEL epi-structures and optimizations of VCSEL-on-Si device fabrication processes, the single-chip transceivers are capable of high performance up to ~50 Gb/s 47 48 49 50 51 52 53 . Moreover, since the effective absorption wavelength range of germanium is wide, from λ ~650 nm to λ ~ 1600 nm, any type of vertical emitting lasers within this wavelength range, for example, 980 nm, 1060 nm, 1310 nm, or 1550 nm and so on, can be used as the counterpart chip-level light source in the proposed single-chip TRx scheme for various types of applications.…”

Design principles and realization of electro-optical circuit boards

“…VCSELs and photodiodes emit, respectively absorb, light via the optical aperture on the chip surface [11]. This requires appropriate placement in the micrometer range to obtain acceptable and reproducible coupling losses.…”

Progress on High-Speed 980 nm VCSELs for Short-Reach Optical Interconnects

“…In 2008 we continued the development of high-speed high-temperature-stable 980 nm VCSELs based on the active region grown in the SML growth mode, since this type of active region has demonstrated its high-temperature stability in the first generation of our 980 nm VCSELs [50,51] described above. Similarly to the first generation, the second generation of the 980 nm VCSELs developed in our group [52][53][54] has been grown by molecular beam epitaxy (MBE) on n+ GaAs (100) substrates and utilized a triple stack of highly strained SML grown InGaAs layers. The stack was placed into the middle of the low-index Al 0.80 Ga 0.20 As cavity, since the Al content of 80% provides a better thermal conductivity and reduces the mesa capacitance due to the smaller dielectric constant as compared to for example, ∼30% Al used conventionally.…”

“…Based on the previous results described above we have decided to develop the next generation of high-speed hightemperature-stable VCSELs emitting at 980 nm [54,60,61]. To increase the relevance of our research for future commercial applications, we have decided to change the growth platform from MBE to metalorganic chemical vapor deposition (MOCVD), which is the standard and wellestablished growth technique for large-scale mass production of VCSELs.…”