Zugl.: Darmstadt, Techn. Univ., Diss., 2006

Ulm, Univ., Diplomarbeit, 2007.

Zugl.: Zürich, Techn. Hochsch., Diss.

Stuttgart, Univ., Diss., 2004.

One of the key advances in photonic technology in recent decades was the development of a new type of diode lasers emitting in the visible and infrared region. These vertical cavity surface-emitting lasers (VCSELs) emerged from a laboratory curiosity in 1977 [13] to an object of industrial mass production [14] and are currently used in many applications. The applications include communication, printing, and absorption spectroscopy [15]. Their rise in credibility has largely been motivated by the rapid evolution of their performance, the more sweeping recognition of their compatibility with low-cost wafer-scale fabrication, and their possible formation into specific arrays with no change in the fabrication procedure. Various applications such as advanced chemical sensors and high-density optical storage require coherent and small-size ultraviolet-emitting devices (below 400nm). Therefore, to extend the VCSEL emission to the ultraviolet (UV) region, intensive efforts have been made in the VCSEL technology. However, the achievement of such UV VCSEL is very challenging because of the various limitations and issues. The issues noticeably include the carrier injection, optical confinement, and highly reflective distributed Bragg reflectors (DBR) structures with a broad bandwidth operating in the UV region [16]. In this context, motivated by the reported large refractive index induced by boron incorporation [7], we propose to introduce the boron-based material systems (BAlGaN) as an innovative solution to address some of the encountered difficulties. The objective of the proposed research is to investigate and optimize new wide-bandgap BAlGaN material systems and illustrate their incorporation into the building blocks of vertical cavity surface-emitting laser structures for operation in the UV spectral range (<400nm). Toward this goal, we have focused our research activities in three main directions. The first direction is devoted to the simulation of DBRs reflectivity by taking into consideration the experimental refractive indexes. Once the materials needed in the different components of the VCSEL are well defined, the second direction lies in the achievement of growth conditions optimization and characterization of the new wide-bandgap BAlGaN material systems. The study has led to the structural and morphological quality improvement of (B,Al,Ga)N materials. Unique optical properties of the BGaN and BAlN materials were also demonstrated. Upon demonstrating the materials' promising optical characteristics, the final direction consists of the epitaxial growth and characterization of the highly reflective DBRs and active region of the UV VCSEL structure.

In this dissertation, the combined optical link, where the baseband digital signal and wireless signal are transmitted simultaneously over a multi-mode fiber (MMF) using an VCSEL and received through a photoreceiver and a feed forward equalizer (FFE). For this hybrid optical link, a new type of combiner is developed using the multi-layer organic (MLO) process. For the overall link simulation, a rate-equation-based VCSEL model is developed with circuit components. This model describes the high-speed modulation characteristic as well as the thermal effect on the L-I (light vs. bias current) characteristic. Additionally, The FFE is developed to further extend a MMF distance by compensating differential modal delay (DMD) in MMF. Two different implementation approaches are taken for the FFE by passive LC ladder based delay line and active inductance peaking delay line structure. To overcome the voltage headroom limitation of the conventional Gilbert cell architecture, modified Gilbert cell is presented and implemented as a multiplier cell for both FFEs. The FFEs are fully integrated on a single chip and fabricated by a standard 0.18 ?m CMOS process. The developed FFE successfully rebuild the distorted signal form the MMF at 10 Gbps data rate.

This work investigated the characteristics of transverse modes and polarization of a VCSEL and attempted to control its transverse mode with optical feedback. The fundamental studies indicated that this solitary VCSEL's output consisted of two mutually orthogonal linear polarizations with different threshold currents and different transverse mode distributions. Furthermore, the temperature dependence of the transverse modes of the VCSEL revealed a better stability than that of edge-emitted lasers. In the mode controlling experiments, an iris was employed to select a specific transverse mode for feedback, resulting in an enhancement in the power of the feedback mode. A significant improvement was accomplished in the feedback of the fundamental mode of the major polarization component: the laser oscillated in a single mode with a side mode suppression ratio greater than 19.8 dB and with a depression of the relative intensity noise more than 20 dB/Hz. Nevertheless, a week feedback in the minor polarization component would stimulate more competition among transverse modes and degraded the performance of the VCSEL. Finally, no effect was observed in the orthogonal-polarization optical feedback experiment due to the excessively small of feedback ratio.

Zugl.: Zürich, Techn. Hochsch., Diss., 2009

Techn. Universiẗat, Diss., 2002--Berlin.

Techn. Universiẗat, Diss., 2004--Berlin.