Modulation properties of high-speed vertical cavity surface emitting lasers

Abstract

Vertical cavity surface emitting lasers (VCSEL) are expected to achieve large modulation bandwidths at low drive currents, resulting from their small active volume. However, the high speed performance is currently limited by the parasitic capacitance and resistance of these devices. The small signal modulation and large signal dynamic characteristics are reported for an oxide-confined VCSEL [ 11 in which the structure has been optimized to reduce these parasitics.The MOVPE grown structure is shown in Fig. 1. The equivalent small signal circuit is also indicated. Coplanar waveguides were evaporated on 5 pm thick' polyimide, resulting in a 50fF capacitance between the bonding pad and conducting substrate. The mesa area surrounding the active region was proton implanted to reduce the capacitance contribution from the thin oxide layer. Finally, a -3OWsquare lateral sheet resistance was obtained with an upper n-doped mirror, necessitating growth on a p type substrate.The 850nm lasers exhibited sub-milliamp threshold, and rollover at several times above threshold, properties desirable for high speed operation. The small signal modulation is shown for several currents in Fig. 2 for a 4x4pm2 device, with a threshold current of 0.5mA. A maximum 3db bandwidth frequency of 21 GHz was obtained for a drive current of 4.8mA. The resonance frequency, 3db bandwidth and damping frequency are shown in Fig. 3. The slope of the 3db bandwidth gives a modulation current efficiency factor (MCEF) of 14.2 GHu'dmA. The contributions of the individual transverse modes to the modulation response in larger multimode devices will also be addressed.The measured turn-on delay for a device with an oxide aperture area of Xx8pm' is shown in Fig. 4. The VCSEL was operated under conditions of no bias and large pulse currents, from which the carrier lifetime can be extracted. A value of 1.6 11s was determined from the fit of the data, resulting in a threshold carrier density of qi*5x10'scmg. qi is the current injection efficiency, which has been estimated as .6 for proton implanted VCSEL's.[2] The obtained carrier density is comparable to values observed in GaAs quantum well edge emitting lasers. The role of the oxide aperture on recombination mechanisms will also be dicussed.

Power complexity of multiplexer-based optoelectronic crossbar switches

“…The memory changes state relatively infrequently. Each I/O port processes data at the rate of 40 Gb/s, so that the power used by all memory in one port can be modeled as equivalent to charging/discharging one basic inverter at the aggregate data rate, given by memory (28) Equations (27) and (28) will overestimate the area and power of memory, since static and dynamic memory cells are highly optimized for low area and low power dissipation. They do yield a useful upper bound.…”