| dc.description.abstract | In today′s optical communications (OI) market, vertical-cavity surface-emitting lasers (VCSELs) with central wavelengths at 850 nm have mainly been used as the primary signal emitters for short distances (<300 m). This is because VCSELs have a low-cost of fabrication, small device area, low power consumption, and most importantly have the capability of high-speed direct modulation. Data centers require an enormous number of links, which makes cost and energy efficiency critical issues. Therefore, many research teams are trying to solve this problem, some by using shortwave wavelength division multiplexing (SWDM) technology which utilizes multiple VCSELs at different wavelengths coupled into one multimode fiber (MMF). Consequently, the number of MMFs can be reduced. In addition to the cost of fiber, the data rate of each channel is also a very important issue. The targeted the on-off keying (OOK) modulation speed of a VCSEL is 56 Gbit/s to meet the requirements of the next generation of OI channels.
Up till now, almost all high-speed VCSELs utilize the wet oxidation process for current confinement, but this thin oxide layer causes significant parasitic capacitance, which is one of the factors limiting bandwidth. In order to overcome this problem, we demonstrate an oxide-relief technique which uses selective wet chemical etching to remove AlOx and replace it with air, whose dielectric constant is lower than that of AlOx. As the parasitic capacitance is reduced, the speed will increase. In addition, the lattice constant of AlOx does not match that of the surrounding AlGaAs layers, which may cause some defects during high-current operation. The oxide-relief technique can solve this problem and increase the reliability of the devices. Therefore, the oxide-relief technique not only can enhance the bandwidth but also can increase the reliability. In addition to the problem of speed, transmission distance is also very important. With the rapid development of the Internet of Things and the Cloud, the amount of data that needs to be handled has increased enormously and data centers are becoming bigger and bigger, making long-distance transmission an issue. Under long-distance transmission, problems of mode and chromatic dispersion arise, which distorts the data to be transmitted. Therefore, we utilize the Zn-diffusion technique, which disorders the top Distributed-Bragg-Reflector (DBR) mirrors and reduces the reflectivity in the diffused area, to suppress the higher-order modes. Since the number of modes becomes less, the influence of modal dispersion will thus decrease, which will help to achieve long-distance transmission without distortion.
In this dissertation, we incorporate the Zn-diffusion and oxide-relief technique to fabricate a VCSEL. First, we demonstrated single-mode 850 nm VCSELs with a side-mode suppression ratio (SMSR) of more than 30 dB, and obtained a maximum data rate up to 26 Gbit/s. Under OOK modulation formats we successfully demonstrated a high bit rate-distance product (14 Gbit/s × 2.0 km) for OM4 MMF transmission. In addition, we have demonstrated a single-mode 850 nm VCSEL array structure with excellent lasing performance. A stable (invariable) near circular far-field pattern with a narrow full-width half maximum (FWHM) divergence angle (~4°) under the full range of bias current and a high maximum single-lobe output power (187.4 mW) under continuous wave (CW) operation can be achieved. This has enabled the development of high-speed VCSELs, one of which is a 850 nm VCSEL whose electrical-to-optical (E-O) bandwidth achieves 29 GHz. In addition by using forward error correction (FEC) and decision feedback equalization (DFE) processing, at room-temperature (RT) we were able to obtain error free data transmission for 54 Gbit/s back-to-back (BTB) through a 1 km OM4 fiber. With another 940 nm VCSEL, an E-O bandwidth of 31 GHz was achieved with and 50 Gbit/s BTB data transmission under RT. Error-free transmission over a 50 meter OM5 fiber can be successfully achieved without using pre-emphasis or equalization techniques. These high-speed VCSELs have been applied to SWDM systems. A Zn-diffusion/oxide-relief VCSEL structure with a novel active layer design, which can achieve invariant high-speed performance from RT to high temperature (85 °C), has been studied. When the ambient temperature increases to 150 °C, it can achieve 25 Gbit/s error-free data transmission. | en_US |