High power edge-cum-surface emitting terahertz laser arrays phased locked by vacuum guided plasmon waves

Quantum cascade detectors (QCD) are photovoltaic mid-infrared detectors based on intersubband transitions. Owing to the sub-picosecond carrier transport between subbands and the absence of a bias voltage, QCDs are ideally suited for high-speed and room temperature operation. Here, we demonstrate the design, fabrication, and characterization of 4.3 µm wavelength QCDs optimized for large electrical bandwidth. The detector signal is extracted via a tapered coplanar waveguide (CPW), which was impedance-matched to 50 Ω. Using femtosecond pulses generated by a mid-infrared optical parametric oscillator (OPO), we show that the impulse response of the fully packaged QCDs has a full-width at half-maximum of only 13.4 ps corresponding to a 3-dB bandwidth of more than 20 GHz. Considerable detection capability beyond the 3-dB bandwidth is reported up to at least 50 GHz, which allows us to measure more than 600 harmonics of the OPO repetition frequency reaching 38 dB signal-to-noise ratio without the need of electronic amplification.

Space-to-ground high-speed transmission is of utmost importance for the development of a worldwide broadband network. Mid-infrared wavelengths offer numerous advantages for building such a system, spanning from low atmospheric attenuation to eye-safe operation and resistance to inclement weather conditions. We demonstrate a full interband cascade system for high-speed transmission around a wavelength of 4.18 µm. The low-power consumption of both the laser and the detector in combination with a large modulation bandwidth and sufficient output power makes this technology ideal for a free-space optical communication application. Our proof-of-concept experiment employs a radio-frequency optimized Fabry–Perot interband cascade laser and an interband cascade infrared photodetector based on a type-II InAs/GaSb superlattice. The bandwidth of the system is evaluated to be around 1.5 GHz. It allows us to achieve data rates of 12 Gbit/s with an on–off keying scheme and 14 Gbit/s with a 4-level pulse amplitude modulation scheme. The quality of the transmission is enhanced by conventional pre- and post-processing in order to be compatible with standard error-code correction.

Interband cascade lasers (ICLs), especially valued for their low power consumption, are particularly appealing for portable, compact, and battery-driven trace gas sensors. However, their performance notably degrades outside the 3-4 µm region. Here, a solution to overcome current performance limitations is presented. Simulation results based on the eight-band k⋅p method employing a generalized momentum matrix element model identify resonant intersubband absorption in the valence band as the causative underlying mechanism. Experimentally, a direct dependence of this resonant absorption on the thickness of the Ga 1−x In x Sb hole-quantum well (h-QW) is confirmed. This is reflected in the improvement of the laser's characteristic temperature T 0 , threshold current density J th , slope efficiency 𝜼, and output power. Extracted waveguide losses from length-dependent measurements substantiate the key role of the valence intersubband absorption. While the performance improvement is experimentally verified at 4.35 µm, the simulation results additionally show how to mitigate undesired absorption at longer wavelengths, paving the way towards high-performance continuous-wave (cw) operation above 6 µm.

GaSb‐based interband cascade lasers (ICLs) emitting at a center wavelength of 6.12 µm at 20 °C in continuous‐wave operation up to a maximum operating temperature of 40 °C are presented. Pulsed measurements based on broad area devices show improved performance by applying the recently published approach of adjusting the Ga1−x$_{1-x}$InxSb layer thickness in the active region to reduce the valence intersubband absorption. The W‐quantum well design adjustment and the optimization of the electron injector, to rebalance the electron and hole concentrations in the active quantum wells, improved the device performance, yielding room temperature current densities as low as 0.5 kA cm−2 for broad area devices under pulsed operation. As a direct result of this improvement together with optimizations of the waveguide design, the long wavelength limit for GaSb‐based ICLs in continuous‐wave operation could be extended. For an epi‐side down mounted 23 µm wide and 2 mm long device with nine active stages and high‐reflectivity back facet, the threshold power is below 1 W and the optical output power is over 25 mW at 20 °C in continuous‐wave mode. Such low‐threshold and low‐power consumption ICLs are especially attractive for mobile and compact sensing systems.