Abstract
To development short-reach applications in optical communication system, reliable, high-speed and low cost optical sources are very impotant issue. Traditionally, for high-speed operation in long-distance transmission, DFB lasers are usually adopted for their superior output performance. However, the demand of second epitaxial regrowth will induce the complicated process and lower yield. The DFB laser also usually needs an additional optical isolator to avoid reflective light. These reasons lead to the cost of the module for the DFB laser is essentially high. Directly modulated semiconductor Fabry-Perot lasers offer several advantages over DFB lasers, including higher yield, low cost, and no need for optical isolators and therefore are the better choice. To design high-speed, uncooled LDs, three categories that limit laser bandwidth should be considered: 1. Active region design: optical gain characteristics and transport effects 2. Device structure: optical waveguide design, cavity length, mirror reflectivity, power dissipation and thermal resistivity 3. Electrical contact design: parasitic RC roll off In this thesis, we will discuss the design consideration of FP LDs. Although the basic output characteristics of LDs is determined by the device structure. However, for the operation speed of LDs, the parasitic resistance-capacitance (RC) roll-off is also a key limiting factor for the 3-dB modulation bandwidth. We will propose a new planarization technique called self-terminated oxide polish (STOP) technology to reduce parasitic RC. The STOP technique is by depositing a thick SiO2 passivation film instead of the polyimide layer on the ridge-structure wafer surface and planarizes the resulting corrugated oxide surface. The flat oxide enables ridge tops to be exposed uniformly and thus to effectively lower the parasitic RC value. Three device structures were used to fabricate LDs by STOP technique. The lasing wavelength of the first device structure is 1.55 μm. The LDs with 4-μm width and a 300-μm length and a 90%- and 30%-reflectivity facet coating exhibit a threshold current of 22 mA, and a light output power of 16 mW at 100 mA and 20℃. The characteristic temperature T0 is 80.6 K from -10 to 80℃. The 3-dB modulation bandwidth of the LDs is 11 and 14.5 GHz at 50 and 100 mA, respectively. The lasing wavelength of the second device structure is 1.3 μm. The LDs with 4-μm width and a 400-μm length a 90%- and 30%-reflectivity facet coating exhibit a threshold current of 8.5 mA, and a light output power 25.9 at 100 mA and 20℃. The characteristic temperature T0 is 82.6 K from -30 to 80 ºC and 55.9 K from 80 to 110 ºC. The 3-dB modulation bandwidth of the LDs at 50 mA is 12.1 and 9.44 GHz at 20 and 90 ºC, respectively. The lasing wavelength of the third device structure is 1.3 μm. The LDs with 2-μm width and a 400-μm length and 30% facet coating for two mirrors exhibit a threshold current of 8.5 and 57.5 mA at 0 and 130 ºC. The characteristic temperature T0 is 85.5 K from 0 to 80 ºC and 54 K from 80 to 130 ºC. The 3-dB modulation bandwidth of the LDs is 12.7 and 16.9 GHz at 50 and 100 mA, respectively. Although the DC output characteristics of these three LDs are different for their different device structures, the frequent response of all these three LDs can obtain more 10 GHz at 50 mA bias current. Therefore STOP technique can be successfully proven to effective reduce the parasitic resistance-capacitance of LDs.