30 Gb/s Directly Modulated 850 nm Datacom VCSELs Ralph H. Johnson Finisar, Advanced Optical Components Division, 600 Millennium Drive, Allen TX 75013
[email protected]
Daniel M. Kuchta IBM T. J. Watson Research Center , 1101 Kitchawan Rd. M.S. 38-123, Yorktown Heights NY 10598
[email protected] Abstract: Error free data transmission at 30 Gb/s has been demonstrated using a 6 micron oxide aperture AlGaAs based VCSEL with GaAs quantum wells and conductive AlGaAs mirrors operating at 850nm. This is comparable to the best results for VCSELs at any wavelength to date. The devices were grown using MBE and processed using the standard Advance Optical Components process. The design methodology used a combination of 1D and 2D optical, and electrical simulations to minimize absorption and resistance simultaneously as well as to optimize speed. ©2008 Optical Society of America OCIS codes: 140.7260
1. Introduction High volume data centers have reached the point where there is a need for 100Gb/s aggregate bandwidth from individual links. The options currently under consideration by various standards bodies include 4x25Gb/s, 10x10Gb/s and 10x12Gb/s. The 850nm laser and detector technology for the short wavelength 10Gb/s solutions exists commercially, however, the 4x25Gb/s technology does not. In addition, the next generation of Fibre Channel serial links will operate at 17Gb/s. For short haul datacom, 850 nm VCSELs have become the ubiquitous optical solution and the standard for lower speed short haul links. To be backwards compatible these higher serial bitrate solutions, 17Gb/s and 25Gb/s, need to also operate at 850 nm. However, the >10Gb/s high speed VCSEL results presented to date have primarily been at longer wavelengths, 980-1100nm which take advantage of the reduced density of states in InGaAs, and the improved electrical and thermal characteristics of GaAs in the DBR [1-3]. These recently demonstrated results at longer wavelengths and the absence of similar results at 850 nm may suggest that it might be necessary to adopt a new ‘standard’ wavelength to achieve the desired higher datarates. This work addresses this issue with comparable results for 850 nm devices which can be standards compatible. The implied requirements for an 850 nm VCSEL are a low series resistance for driver headroom, high reliability, impedance matching, and minimization of parasitic RC time constants. At the same time the aperture diameter needs to drop to achieve maximum bandwidth for an applied current. This creates severe design constraints because of the simultaneous small size and small resistance requirements. It is not simply a matter of increasing the doping because free carrier absorption, especially in the p-mirror causes an increase in threshold, and decreases the slope efficiency. Thus, there are several aspects of the design which address the interaction of absorption and resistance. In addition to resistance, capacitance must also be minimized as the RC time constants also limit high bitrate performance. 2. Design and fabrication The layout uses the standard Advanced Optical Components wagon wheel layout [4]. As in [4], the capacitance across the native oxide is minimized in two ways; the isolation implant reduces the radius affecting capacitance and the taper on the oxide increases the thickness of the capacitor at high radii where the area is large. The parasitic time constants are further reduced from the results in [4] using lower mirror resistance and thicker silicon dioxide dielectric under the bond pad. The lower mirror consists of 37 AlGaAs n-DBR pairs. The upper p-DBR consists of 24 AlGaAs pairs. The current confinement is a nominally 6 micron oxide aperture near the second null above the active region. The epitaxial growth method was MBE using a Gen 3 MBE system. The mirror design uses conductive mirrors with digital alloys to simulate ramps with careful design to minimize both absorption and resistance using proprietary methods. The process, except for minor modifications to reduce capacitance and accommodate the modified epi, was identical to the standard commercial process. The design methodology used a combination of a 1D optical stack simulator combined with a 1D and 2D VCSEL simulator. DC and transient simulations were used to optimize the structure for the relevant DC and AC characteristics. The simulated relaxation oscillation frequency (ROF) from a transient simulation for a 5 mA bias was 12 GHz. For indium containing quantum wells at 850 nm the simulated ROF is 17 GHz. The disadvantages of 850 nm versus longer wavelength solutions are (1) the higher density of states resulting in lower gain curves, (2) the use of AlGaAs instead of GaAs for the higher index layer and (3) the difficulty in making a suitable non absorbing tunnel junction at 850 nm. The use of AlGaAs instead of GaAs or AlAs leads to a higher thermal impedance. The lower electron and hole mobilities can to some extent be compensated by using highly sophisticated schemes to heavily dope at the optical nulls, also negating the need for a tunnel junction. The density of states issue can potentially be addressed by design of indium containing quantum wells optimized for 850 nm. One advantage that 850 nm does have is the reduced band offsets which makes vertical conduction easier. Another advantage is lower free carrier absorption for two reasons: the structures are thinner, and the absorption coefficients are lower. Lower absorption is very important since the primary task is to make a small aperture with low resistance and low absorption. Other advantages of the 850nm wavelength band are a well established record of high reliability, low cost, and availability of standard multimode fibers optimized for 850nm (OM3 and its higher performance successors).
3. Measurement and Results The devices have a series resistance of 125 Ohms measured between 3 and 6mA. The room temperature threshold current is 0.75mA. For high speed characterization the VCSELs are mounted on a brass block with a short wirebond to a K sparkplug connector. Optical eyes were characterized up to 30Gb/s where the measurement was limited by the photodiode. Figure 1 shows a 25Gb/s unfiltered eye diagram taken at room temperature with a 6mA bias and 500mVpp swing. The data pattern is PRBS7. The extinction ratio is 2.8 (4.5dB). Figure 2 shows a 30Gb/s, 8 mA, 700mVpp eye diagram of the same 6 micron device. The extinction ratio is 3.0 (5dB) under these conditions. The measured 20-80% edge rates of 18ps are limited by the detector. From small signal measurements, the 3dB bandwidth reaches a maximum of 19GHz at 8mA. Figure 3 shows BER curves vs. OMA for 26Gb/s and 30Gb/s through 2m and 106m of OM3 fiber. The 106m link consisted of a single 100m span with 6 patch cords and 6 connectors. The link penalty was 0.8dB and 1.3dB for 26Gb/s and 30Gb/s respectively. Part of the low penalty observed can be attributed to the RMS spectral width of
