Air Force

Broad area quantum cascade laser with fundamental transverse mode operation

Devices that emit in the mid-infrared wavelength range (about 2 μm-20 μm), as well as long-infrared wavelength ranges, (terahertz spectral range) to attain higher brightness in a single beam

Photonics

Aircraft countermeasures are one application of quantum cascade lasers. The laser can blind heat-seeking missiles. (Troy Carter/TechLink)

Quantum cascade lasers (QCLs) use optical transitions between electronic sub-bands to produce light. QCLs can be designed to emit in the mid-infrared and long-infrared wavelength ranges of the electromagnetic spectrum. Unlike typical inter-band semiconductor lasers that emit electromagnetic radiation through the recombination of electron-hole pairs across a material band gap, QCLs are unipolar, and laser emission is achieved through the use of inter-sub-band transitions in a repeated stack of semiconductor multiple quantum well heterostructures. By way of example, QCLs may be used in the areas of remote sensing, long-wave imaging, communications, aircraft countermeasures, and the like.

Thus, QCLs may be the preferred choice for mid-infrared and long-infrared emission from a compact source, due to their relatively high efficiency at room temperature. Power scaling in QCLs is possible by fabricating BA devices. However, Broad Area QCLs (BA-QCL) with cavity widths that exceed approximately ten μm typically exhibit modal instability, non-linear interactions, beam steering, and loss of brightness. When the cavity width is very large, e.g., greater than 30 μm, high order transverse modes generally result in a far field profile that is double-lobed, with each lobe propagating at large angles from the optical axis. Multi-lobed emission is a large obstacle to producing practical BA-QCL devices with high brightness. As such, although fabricating BA devices may be the simplest method to scale the power of QCLs, high-order transverse modes become prevalent and may inhibit single beam emission and reduce brightness.

Air Force researchers have now identified methods to suppress high-order transverse modes by forming excavations into the laser cavity that change the lateral index profile in a BA-QCL. In other words, in certain implementations, a BA-QCL may be modified by placing a local perturbation in the lateral refractive index profile in a manner that selectively favors lasing in the fundamental transverse mode. These perturbations take the form of excavations formed in the top surface of the optical cavity and are configured to select a fundamental transverse mode of light in the cavity by constricting a lateral refractive index profile. These techniques may be translatable to all commonly practiced semiconductor fabrication methods and may result in the scaling of brightness in BA-QCLs.

Experiments with prototype devices have revealed the optimal depth and configuration of the excavations.

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