Theoretical investigation on intrinsic linewidth of quantum cascade lasers

by Tao Liu

Institution: Nanyang Technological University
Year: 2014
Keywords: DRNTU::Science::Physics::Optics and light
Record ID: 1382377
Full text PDF: http://hdl.handle.net/10356/59550


Quantum cascade lasers (QCLs) are unipolar laser sources relying on intersubband transitions in coupled multiple quantum well systems. The light emission can be tuned across the mid-infrared (mid-IR, from 3 to 20 micrometer) and Terahertz (THz, from 1.2 to 5 THz, or 60 to 250 micrometer) ranges of the electromagnetic spectrum. Since their invention in 1994 for mid-IR QCLs and in 2002 for THz QCLs, respectively, these lasers have undergone tremendous improvement, and have become probably the most prominent coherent light sources in the mid-IR and THz spectral ranges. However, many important applications of mid-IR and THz coherent light sources, e.g. spectroscopy and high-speed free-space data communication, are greatly influenced by intrinsic laser fluctuations and noises. Although intrinsic linewidth and noise in semiconductor diode lasers have been widely investigated, theoretical and experimental studies in noise dynamics and linewidth of QCLs have only attracted interests recently. This field is still in its infancy. Since the operational principle of QCLs is totally different from that of semiconductor diode lasers (resonant tunneling effect and coherent interactions in QCLs are unique owing to the intersubband transitions), models on noise and linewidth investigations of diode lasers cannot be directly applied to QCLs. New physical models must be developed on both noise dynamics and linewidth for QCLs. This thesis discusses many aspects of intrinsic linewidth and noises of QCLs, and it is divided into three main parts. Since optical gain spectrum can greatly influence the intrinsic linewidth of lasers, in the first part, we report the study on optical gain of QCLs. The optical gain of QCLs is investigated based on a developed microscopic density-matrix (DM) model. Intersubband semiconductor-Bloch equations are established by incorporating many-body Coulomb interaction, non-parabolicity and coherence of resonant-tunneling transport effects in a quantitative way. The calculations demonstrate the importance of many-body interaction, non-parabolicity and resonant-tunneling transport on optical gain spectrum of mid-IR and THz QCLs. The results show that the gain peak in frequency calculated by the developed microscopic DM model is closer to the experimentally measured lasing frequency compared with the existing macroscopic DM model. In addition, the dependence of optical gain of THz and mid-IR QCLs on device parameters such as the injection and extraction coupling strengths, energy detuning and doping density are also systematically studied in details. This model provides a comprehensive picture of optical properties of THz and mid-IR QCLs and potentially enables a more accurate and faster prediction of the device performance e.g. the laser linewidth enhancement factor and current characteristics. In the second part of the thesis, we report the intrinsic linewidth of QCLs caused by the spontaneous emission, thermal photon and fundamental thermodynamic fluctuation. The linewidth induced by spontaneous…