UC Santa Barbara

Solid-state lighting based on III-nitride by light emitting diodes (LEDs) has been developed as a high efficient light source and has been successfully commercialized in the market. In addition to the function of lighting, visible light communication (VLC) has gained momentum to solve the data traffic of the radio frequency (RF) wireless communication. Many progresses in LED based VLC have been reported previously. However, since LEDs are limited by the carrier lifetime of its spontaneous emission and RC parasitics, the modulation bandwidth is typically on the order of MHz.

For white lighting communication, the bandwidth and data rate are significantly affected by a slow phosphor response due to its relaxation decay time on the order of 100 ns ~ 1 μs.

As an alternative to LED technology, the use of laser diodes (LDs) as a transmitter presents a viable approach to overcome the limitations of modulation bandwidth associated with transmitters and phosphors. Despite of these merits, only a few studies have been reported for LD based VLC without a significant improvement in modulation bandwidth and data rate. In this dissertation, I discuss the materials, fabrication, and device characterization of III-nitride visible laser diodes on semipolar (202 ̅1 ̅) orientation for high-speed performance. Furthermore, I demonstrated laser based white lighting communication system containing different phosphors in supporting the merits in high-speed data transmission resuling in world-record of high speed performance.

In the first part of this thesis, the initial VLC system by using commercial high-power laser diodes is discussed. It is important to study the capability of data transmission by utilizing the high-power designed laser diode because not only it suggests the guideline beyond the LED based VLC, but also high-power operation is necessary for LiFi application. Moreover, the bandwidth limiting effect by the phosphor commonly observed in LED based system is investigated for laser based system and an understanding of these effects is used to achieve high data rate of white lighting communication without filtering phosphor converted photons. In the second part of this thesis, the optimization of epitaxial structure, optical mode, and fabrication process of semipolar laser diodes are discussed. These include the relatively conventional structure with well-optimized topside ohmic n-type contact for high-speed microwave probing rather than backside n-type contact. Dynamic characteristics and a high-speed performance of LDs emitting at 410 nm are investigated resulting in a 3-dB bandwidth of 5 GHz and 5 Gbit/s direct modulation with on-off keying (OOK), which were limited by the bandwidth of the photodetector used for the measurements. The differential gain of the LDs was determined to be 2.5 ± 0.5 × 10-16 cm2 by comparing the slope efficiency for different cavity lengths. Analysis of the frequency response showed that the K-factor, the gain compression factor, and the intrinsic maximum bandwidth were 0.33 ns, 7.4 × 10-17 cm3, and 27 GHz, respectively.

Finally, I will conclude with a demonstration of white lighting gigabit data transmission by utilizing red-, green-, and blue-emitting phosphors based on processed semipolar 410 nm LD. The advantage of near UV or violet LD in white lighting data communication involves lowest ambient light that can be significant noise level in other spectrum range.