Theory of the Intrinsic Linewidth of Quantum-Cascade Lasers: Hidden Reason for the Narrow Linewidth and Line-Broadening by Thermal Photons

• Published in: IEEE journal of quantum electronics Vol. 44; no. 1; pp. 12 - 29
• Main Authors: Yamanishi, M., Edamura, T., Fujita, K., Akikusa, N., Kan, H.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.01.2008; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Black body radiation; Computational efficiency; Exact sciences and technology; Frequency; Fundamental areas of phenomenology (including applications); General laser theory; Joining; Laser modes; Laser stability; Laser theory; Lasers; Lasing; Linewidth; nonradiative relaxation; Optical coupling; Optics; Photons; Physics; Quality control; quantum cascade (QC) laser; Quantum cascade lasers; Quantum mechanics; Semiconductor lasers; Semiconductor lasers; laser diodes; Spontaneous emission; Theory; thermal photon; Linewidth; Line widths; thermal photon; nonradiative relaxation; Nonradiative transitions; Quantum cascade laser; Injection current; Laser theory; Line broadening; THz range; Spontaneous emission; Semiconductor lasers; Feedback; quantum cascade (QC) laser
• ISSN: 0018-9197; 1558-1713
• DOI: 10.1109/JQE.2007.907563

We have developed a theory of the intrinsic linewidths of laser output of single-mode quantum-cascade (QC) lasers in mid-infrared and terahertz (THz) ranges. In the theoretical treatment, the concept of an effective coupling efficiency of spontaneous emission, given by a fractional rate of spontaneous emission coupled into a lasing mode to total nonlasing relaxation, is introduced to clarify a hidden reason for the narrowness of the linewidths. A narrow linewidth (12-kHz) reported with a frequency-stabilized 8.5- distributed-feedback QC laser is successfully interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. The present theory predicts the presence of a minimum ldquolinewidth floorrdquo in a high-injection-current region and the independence of linewidth on detuning between gain-peak and emission wavelengths. The theoretical treatment is expanded to derive the further modified Schawlow-Townes formula including the line-broadening by black body radiation in a THz QC laser. The linewidth of a THz QC laser is predicted to be considerably broadened by black body radiation.