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Title: Design, fabrication, and investigation of 2 μm GaSb-based diode lasers
Authors: Li, Xiang
Keywords: Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
Engineering::Electrical and electronic engineering::Semiconductors
Issue Date: 2019
Source: Li, X. (2019). Design, fabrication, and investigation of 2 μm GaSb-based diode lasers. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: High-performance light sources operating in the 2 µm range are key components for a number of applications such as medical diagnostics, eye-safe light detection and ranging (LIDAR), wind velocity measurement, free space and advanced optical communications, plastic material processing, optical pumping for solid-state lasers and optical parametric oscillators (OPO), etc. In this wavelength range, laser emission was achieved in many different host crystals and fibers with Tm3+ and Ho3+ ions as well as InP- and GaSb-based semiconductor materials. Among these lasers, semiconductor diode lasers have their specific advantages: flexible wavelength tuning by engineering the bandgap of the active region, compactness, high efficiency, easy electrical pump, integration opportunities with silicon photonic circuits, which make them very desirable. For the two semiconductor material systems, GaSb-based materials are preferred since relatively high strain is inevitable for InP-based materials to emit light in the 2 µm range, which brings difficulties in material growth. Two different GaSb-based laser structures were used to fabricate all the devices in this work. The main difference between these two structures is one uses AlGaAsSb barrier with higher aluminum composition to improve the carrier confinement. According to their lasing wavelength, they are referred to as 1960 nm laser structure and 2020 nm laser structure, respectively in this thesis. First, Fabry-Pérot (FP) ridge waveguide lasers were fabricated with a developed standard fabrication process based on our process capability. The threshold current densities of the fabricated lasers are as low as ~96 A/cm2, which is among the lowest values for a 2 μm GaSb-based diode laser. The characteristic temperatures T0 and T1 are 103 and 606 K, respectively near room temperature, which are also very impressive. In addition to these performance parameters, the carrier recombination behaviors within the fabricated lasers were systematically investigated both electrically and optically. Ideality factor (electrical, via voltage current characteristics) and Z parameter (optical, via spontaneous emission) were used for the investigations as they are good reflections of the three main recombination processes (Shockley-Read-Hall (SRH), radiative, Auger). Besides Z parameter, through the analysis of the activation energy derived from the spontaneous emission, it is found that severe thermal loss of holes at large injection currents cannot be ignored. The findings in the ideality factor and spontaneous emission studies are in very good agreement with each other. On the other hand, high-frequency optical pulse trains are desirable for a number of applications in chemistry, telecommunications, medicine, and military. For example, real-time monitoring of chemical reactions, ultra high bit rate optical communications, ultrafast electro-optical sampling and optical coherence tomography (OCT). There are mainly three techniques to obtain ultrafast pulses: gain switching, Q-switching and mode locking. Compared to the other two techniques, mode locking technique allows the generation of pulses with shorter pulse duration and higher repetition frequencies, which makes it more desirable for many applications. First, shorter pulse duration provides higher resolution. It can slow down some fast-moving objects such as molecules or electrons and therefore measure some ultrafast processes, e.g., the relaxation processes of carriers in semiconductors, chemical reaction dynamics, and electro-optical sampling of high-speed electronics. Second, higher repetition frequency means higher transmission capacities in communications. One example is ultrashort pulse train emitted from a mode-locked laser can be used to realize optical time division multiplexing (OTDM). This is one form of time division multiplexing (TDM), which is widely used in optical fiber communications to increase the overall data transmission capacities. In addition, many longitudinal modes (different frequencies) are included in the light emission from a mode-locked laser, which enables the lasers to work as multi-wavelength light sources by combining appropriate optical filters. Since these modes (frequencies) are precisely locked to longitudinal mode spacing, they are very promising for another capacity improvement technique: wavelength division multiplexing (WDM). Due to the above attractions, monolithic two-section mode-locked lasers (MLLs) were fabricated and fully characterized. Stable mode locking was achieved in these lasers under a variety of bias conditions up to 80 ℃. Hysteresis was observed in a fabricated laser at temperatures higher than 60 ℃ with small negative absorber bias voltages (Va). Cavity length and gain/absorber length ratio were analyzed. After this, mode gain, different working regimes (instabilities), phase noise/timing jitter, frequency tuning of these MLLs were systematically investigated. All these characterization works were the first time done on a 2 µm mode-locked diode laser. Furthermore, tunable single frequency operations are favorable in sensing and optical communication applications, e.g., tunable diode laser absorption spectroscopy (TDLAS), wavelength division multiplexing (WDM) technique. Another very trending filed is III-V/silicon integration. To make the reason short, it combines the light emitting capability of III-V with the high design freedom and complementary metal-oxide-semiconductor (CMOS) compatibility of silicon (Si) photonics, which is very promising for large bandwidth intra- and inter-chip connections, compact and low-cost sensing and communication devices, etc. Two III-V/Si integration methods: wafer bonding and edge coupling have been investigated. For the wafer bonding method, a tapered waveguide GaSb-based laser, bonded onto silicon on insulator (SOI) circuits with an Al2O3 bonding layer was designed and simulated via the beam propagation method (BPM) simulations. The thermal property of the GaSb-on-Si laser has been calculated using a constant heat spreading model, and the results show that the devices using Al2O3 bonding layer have a much lower (~70%) thermal resistance as compared to the one using SiO2 bonding layer. For the edge coupling method, a 2 µm external cavity tunable laser with Si photonic chip as the wavelength selective component is designed, fabricated, and characterized. The semiconductor optical amplifiers (SOAs) were fabricated from the 1960 nm laser wafer with 4.5° to 6.5° tilted waveguides. Si vernier wavelength filter was simulated and optimized using Lumerical softwares. At the juncture of writing this thesis, a tuning range of ~60 nm with side mode suppression ratio (SMSR) of higher than 10 dB was achieved in one fabricated laser. 60 nm is among the largest tuning ranges for a tunable diode laser in the 2 μm wavelength range.
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