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.

Mid-infrared (MIR) free space optical communication has seen renewed interest in recent years due to advances in quantum cascade lasers. We present data from a multi-wavelength test-bed operated in the New York metropolitan area under realistic weather conditions. We show that a mid-infrared source (8.1 μm) provides enhanced link stability with 2x to 3x greater transmission over near infrared wavelengths (1.3 μm & 1.5 μm) during fog formation and up to 10x after a short scavenging rain event where fog developed and visibility reduced to ~ 1 km. We attribute the improvement to less Mie scattering at longer wavelengths. We confirm that this result is generally consistent with the empirical benchmark Kruse model at visibilities above 2.5 km, but towards the 1 km eye-seeing limit we measured the equivalent MIR visibility to be > 10 km.

Quantum cascade (QC) lasers, based on intersubband transitions in semiconductor quantum wells, are characterized by ultrafast (picosecond) carrier lifetimes. An important consequence of this unique property is the expected absence of relaxation oscillations in the transient response of these devices. Here, we discuss and experimentally verify this prediction by measuring the modulation response of several 8-μm-QC lasers, properly processed and packaged for high-speed operation, up to 10 GHz.

Experimental results for an optical free-space high-speed link using direct modulated mid-infrared (λ=8.1 μm) quantum cascade lasers are presented. A total of 800 digitally encoded multimedia channels were transmitted. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (λ=0.85 μm) link.

The high-speed direct modulation response of mid-infrared quantum cascade (QC) lasers is investigated up to a frequency of 2 GHz, showing high-frequency data transmission capabilities. The application of QC lasers to optical wireless communications is discussed and demonstrated in a free-space television link over a distance of 70 m.

High-speed mid-infrared quantum cascade lasers with direct modulation bandwidth of approximately 7 GHz have been developed. Error-free digital data transmission at 2.5 Gbit/s is demonstrated with devices emitting at 8 µm and operating at temperatures up to 85 K.