Quantum cascade lasers: ultrahigh-speed operation, optical wireless communication, narrow linewidth, and far-infrared emission

TLDR

Quantum cascade lasers, whose band‑structure engineering has enabled operation across most of the mid‑IR and part of the far‑IR, are reviewed for their recent advances that promise significant progress in optical communications, ultrahigh‑resolution spectroscopy, and highly sensitive gas‑sensing. The authors describe how QC lasers exhibit a unique, relaxation‑oscillation‑free frequency response that allows high‑speed digital modulation and mid‑IR optical wireless links, demonstrate ultrashort pulse generation via gain switching and mode‑locking, and report sub‑Hz relative frequency stability when two lasers are locked to optical cavities. Recent measurements show free‑running QC lasers have ~150 kHz linewidths that can be narrowed to 10 kHz, and that metallic waveguides supporting surface‑plasmon modes extend their operating wavelength into the far‑IR.

Abstract

Following an introduction to the history of the invention of the quantum cascade (QC) laser and of the band-structure engineering advances that have led to laser action over most of the mid-infrared (IR) and part of the far-IR spectrum, the paper provides a comprehensive review of recent developments that will likely enable important advances in areas such as optical communications, ultrahigh resolution spectroscopy and applications to ultrahigh sensitivity gas-sensing systems. We discuss the experimental observation of the remarkably different frequency response of QC lasers compared to diode lasers, i.e., the absence of relaxation oscillations, their high-speed digital modulation, and results on mid-IR optical wireless communication links, which demonstrate the possibility of reliably transmitting complex multimedia data streams. Ultrashort pulse generation by gain switching and active and passive modelocking is subsequently discussed. Recent data on the linewidth of free-running QC lasers (/spl sim/150 kHz) and their frequency stabilization down to 10 kHz are presented. Experiments on the relative frequency stability (/spl sim/5 Hz) of two QC lasers locked to optical cavities are discussed. Finally, developments in metallic waveguides with surface plasmon modes, which have enabled extension of the operating wavelength to the far IR are reported.