Development of an electroabsorption loss-modulated VCSEL for high-speed data transfer

The possibilities to increase modulation bandwidth for high-speed data transfer using vertical cavity surface emitting lasers (VCSEL) are explored using intracavity loss-modulation instead of the traditional current modulation technique. Theory shows that a larger bandwidth is possible, but at the cost of having a large (+20 dB) resonance peak. The high-frequency roll-off slope of these devices is shown to have an intrinsic slope of -20 dB/decade, and an additional -20 dB/decade is observed due to parasitics. Sources of electrical parasitics were suspected to be one or more of the following: (1) carrier time-of-flight (TOF), (2) carrier accumulation at heterointerfaces, (3) modulator p-i-n junction capacitance and (4) contact metallization capacitance and resistance. An analysis was done to find the sources of the parasitics, and through methods such as (1) variation of modulator i-region thickness, (2) low temperature growth of the i-region, (3) reduction of metallization capacitance and blue shifting of the modulator absorption wavelength. The effects of these measures were estimated by fitting the high frequency roll-off slope of the optical response. Reduction of metallization and junction capacitance have shown to be the most beneficial measures, partly determined by fitting and measurements of frequency dependent S-parameters.;A novel type of VCSEL has been proposed and demonstrated based on research done on an intracavity loss-modulated VCSEL. This device uses a duo-cavity architecture to optically decouple the AC modulation component from the VCSEL section. Devices were fabricated and showed a nearly resonance-free high-frequency response up to or beyond 20 GHz. To improve bandwidth and device parasitics, intracavity loss-modulated devices were used as a test-bed for the modulator section. To increase the modulation depth of these devices from the initial 20% shown by the proof-of-concept design, additional multiple quantum well (MQW) sets were added to a modulator i-region of increased thickness. The final duo-cavity device presented in this work shows a 40% modulation depth and a 17 GHz bandwidth at a drive current of 7 mA. In this device, the bandwidth is believed to be limited by a large RC value due to a high modulator junction series resistance.