technology lasers
QCLs take the leap from toys to tools
A revolutionary active region is the driver behind the record single-facet output powers emanating from Pranalytica’s quantum cascade lasers (QCLs). This advance will spur the launch of compact, lightweight, multi-watt, mid-wave infrared lasers, say the company’s
Richard Maulini, Arkadiy Lyakh, Alexei Tsekoun and Kumar Patel.
Q
CLs are a novel class of laser that can plug critical gaps in the mid-wave and long- wave infrared spectral regions that are currently served by very few continuous wave (CW), room temperature solid- state sources. QCLs can operate in this spectral range because their emission is not based on conduction band to valence bands transitions that govern the emission of conventional laser diodes. Instead, they generate laser emission from transitions between confined intersubband states formed within a superlattice of alternating layers of materials with lower and higher bandgaps, known as quantum wells and barriers. The emission wavelength is then dictated by properties of the superlattice, such as the thickness of the wells and barriers, and this opens up a range of wavelengths that can be reached through bandgap engineering.
The fundamental idea behind the QCLs is not new and dates back to the early 1970s. However, practical realization of this device took nearly 25 years, due to the extreme demand that the laser structure puts on epitaxial quality. Even after the first working QCL was produced, this class of laser remained little more than a laboratory curiosity for a decade. Initial performance was poor, and the first generation of QCLs were available only in the form of individual chips, or chips on carrier assemblies. Consequently, integrating this class of laser into a system required expertise in QCL handling, powering and packaging. In addition, early designs had to be cooled to cryogenic temperatures - CW, room-temperature performance was only realized in 2002.
At Pranalytica, our mission has been to improve the performance of QCLs and their packaging, so that they can make the transition from laboratory devices to commercial lasers that can serve a host of applications. Thanks in part to funding from the US Defense Advanced Research Projects Agency, we have made significant strides in this direction, including a recent room- temperature demonstration of 3W, CW output from one single facet of a 4.6 µm laser. This record-breaking laser,
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www.compoundsemiconductor.net April/May 2010
which has a wall-plug efficiency of 13%, was the result of multiple advances that span the entire QCL production chain, from fundamental design of the active region through to thermal management of the chip.
Beckoning applications
Thanks to these improvements, our QCLs are now attractive candidates for real world applications. In the defense market space, they are being explored for protection of military and civilian aircraft, and high-power handheld devices are being tested as target illuminators. In addition, several non-defense QCL applications are imminent, including free-space optical communications, ultra-sensitive trace-gas sensing based on photo-acoustic spectroscopy and other detection techniques and remote sensing.
There is no denying that it has taken the QCL community a long time to get to the stage where its lasers are commercially viable. That’s partly because this class of laser has a relatively complex design, consisting of hundreds of superlattice layers, each with a thickness of just a few nanometers. Imperfections in the hetero- interfaces can cause undesirable carrier scattering, and in
Fig.1. A typical QCL design employs a two- phonon active region. Longitudinal optical phonons are needed for transitions between levels 3 and 2, and 2 and 1 (left). Pranalytica uses an alternative
approach with a non-resonant extraction active region that vastly increases the freedom of QCL design (right)
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