Characteristics and Applications of Semiconductor Lasers and Semiconductor Optical Amplifiers with Nonidentical Multiple Quantum Wells

With the increase of blooming information and data transmission of internet, the perspective of fiber-optic communication becomes more and more promising in the future. In this dissertation, we focus on the use and design of non-identical multiple quantum wells (MQW) to achieve broadband emission characteristics, less temperature dependence, and higher modulation bandwidth of semiconductor laser diodes and semiconductor optical amplifiers (SOA).
Nonidentical MQW is designed and made by InGaAsP/InP suitable for optical communication wavelength. By proper adjusting the width and sequence of quantum wells and the width of separate-confinement-heterostructure (SCH) layer, we can improve the uniformity of 2D carrier distribution inside quantum wells and broaden the emission spectrum of SOA. Furthermore, we propose a new simple method to measure the broadband gain spectrum with a two-section device. Without any other external tunable lasers and complicated setup, broadband gain spectrum is obtained immediately for a spectral range of 290nm with the gain of above 30 cm-1.
Using nonidentical MQWs structure, temperature sensitivity of long-wavelength semiconductor lasers can be efficiently reduced. Carrier redistribute when temperature increases due to temperature-dependent Fermi-Dirac distribution. In nonidentical MQWs, carriers favor short wavelength QWs as ambient temperature increases. The temperature-induced carrier redistribution among nonidentical MQWs has been observed to contribute larger characteristic temperature and less temperature dependence compared to conventional InGaAsP/InP semiconductor lasers. For temperature varies from 33 K to 260 K, its corresponding energy changes less than 5 meV, while the bandgap energy changes more than 50 meV. And we first observed the “minus characteristics temperature” of semiconductor lasers due to carrier redistribution among nonidentical MQWs.
In addition, we propose a new mechanism for direct modulation of laser diode by using carrier redistribution inside nonidentical MQWs. With proper design of nonidentical MQWs structure, a device with two-section waveguide Fabry-Perot laser diodes can be switched between two widely separated lasing wavelengths at high frequency. The switched intensity can have extinction ratio of 20dB within 5mA of current variation. Carriers redistribute inside nonidentical MQWs and contribute to different lasing wavelengths. Because the transport time between quantum wells is much smaller than the diffusion and drift time in SCH layer of carriers, the modulation bandwidth of two-section laser is expected to surpass the relaxation frequency of conventional laser diode. This new mechanism will greatly improve the transmitter speed and lower the cost in optical communication system.