The mid-IR (MIR) spectral range between 2 and 12μm is well-suited for security and defense applications. This region contains emission bands of man-made substances, absorption fingerprints of security- and defense-related chemicals, and windows of vulnerability of optronic sensors. MIR laser-based devices are scarcely used, however, although they exhibit several advantages compared to their shorter-wavelength counterparts. These long wavelengths are eye-safe, exhibit a variety of light/matter-interaction characteristics, and can penetrate natural and man-made obscurants. In addition, unwanted background emission from both the sun and the Earth's atmosphere is minimal.
We are developing field-deployable laser modules, combining optically pumped semiconductor disk lasers (OPSDLs) operating at 2–2.5μm and quantum-cascade (QC) lasers suitable for operation in the 3.8–5 and 8–12μm spectral regimes.1 At room temperature, these modules convert electrical to IR-optical power with reasonable efficiency (≥10%), moderate average power consumption (≥1W), and selectable modulation formats. They exhibit adequate beam quality (beam-propagation factor M2<2.5) for remote sensing and power projection. In addition, the modules are compact, robust, and reliable. These laser sources open up new perspectives for development of MIR laser-based devices for modulated power projection, active sensing, free-space communication, laser-illuminated imaging, telemetry, and testing. Figure 1 shows the basic layout of our gallium antimonide-based OPSDL.2,3 Continuous-wave (cw) output power of 1.3W at a heat-sink temperature of +20°C (3.4W at −10°C) is delivered to a high-quality circular output beam (M2≤2.6) at a wavelength of 2.25μm.
Figure 1.Schematic layout of an optically pumped semiconductor disk laser (OPSDL). Its chip acts simultaneously as the back mirror of the laser cavity and as gain medium.
For the wavelength ranges 3.8–5 and 8–12μm, QC lasers currently represent the best-suited semiconductor-laser type.4 A QC laser comprises several active regions that are connected in a cascading scheme by so-called injector regions (see Figure 2). QC lasers deliver cw and pulsed power of a few Watts at room temperature with a high-quality (M2<2), strongly divergent beam.5 The inherent drawback of all single-element QC lasers is the limited power level when good beam quality for remote power delivery is required. Spectral-beam combining (SBC) in an external cavity is used for applications aiming at high brightness and wavelength diversity in compact and robust modules (see Figure 3).6 The average output power we obtained from a compact module comprising a six-emitter chip was more than three times larger than that of a single Fabry-Perot QC laser from the same wafer. In addition to scaling up the output power, SBC preserves the good beam quality of single emitters. For both fast and slow axes, M2<2 has been achieved.7
Figure 2.Typical conduction-band profile and squared moduli of the wavefunctions involved in the lasing transition (wavy arrows) of a quantum-cascade (QC) laser. The two active regions are connected by an injector region. The shaded area indicates the injector miniband. F: Fluence.
Figure 3.(left) External-cavity setup for spectral-beam combing and (right) wavelength distribution of eight spectrally combined QC emitters. QCL: QC laser. λ: Wavelength. f: Focal length. norm.: Normalized.
A prominent application of OPSDL/QC-laser modules is directed IR countermeasures (DIRCM). Such systems protect aircraft against IR-guided missiles launched, for example, by man-portable air-defense systems (known as MANPADS).8 To defeat an incoming missile, a laser beam is directed toward its seeker equipment and injects false information into its tracking sensor, thus driving it away from the aircraft. We constructed two portable MIR laser modules based on OPSDL and QC/SBC-laser technologies.9 Power levels, beam-propagation parameters, and modulation capabilities of the modules are appropriate for DIRCM. (The technology-readiness level of the laser modules is in the range of 4–5, i.e., ‘ready to prove feasibility’ to ‘technology development.’) We integrated the OPSDL/QCL modules into a pointing-and-tracking system and tested them in three field trials over a period of several weeks. These trials proved the operability and reliability of the laser modules and clearly showed that MIR semiconductor-laser sources will be the first choice for advanced DIRCM systems.
An application particularly suited for QC lasers is standoff detection of hazardous substances such as explosives dispersed on surfaces. The detection principle is based on multiwavelength IR backscattering spectroscopy. Our active sensor system combines a widely tunable external-cavity QC laser with a (cooled or uncooled) IR camera, the associated electronics, and image-processing software. The laser is spectrally tuned through characteristic absorption bands of the anticipated substance (see Figure 4) and a multispectral datacube is constructed. Data-analysis and image-processing methods are employed toextract the spatially resolved spectroscopic signature of the species contaminating the surface. We recently demonstrated detection of surface contamination of 40μg/cm2trinitrotoluene (better known as TNT) on standard car paint measured at a distance of 3m.10
Figure 4.(left) Sketch of the external-cavity setup in Littrow configuration. (right) Tuning range of 200cm- 1 of a 15×3500μm2QCL chip in such a setup operating in pulsed mode at room temperature. AR: Antireflection. arb.: Arbitrary.
In summary, to open up technical and market opportunities for active MIR systems, current developments focus on three key technologies, including high-power semiconductor lasers, fiber-optic coupling, and large-format multifunctional cameras. Our ongoing work concentrates on two MIR semiconductor-laser technologies, in particular OPSDL and SBC/external-cavity QC lasers. They enable realization of compact, efficient, and easy-to-integrate laser modules with sufficient power and beam quality for standoff detection and power projection.
The authors and their colleagues at Diehl BGT Defence GmbH and the Fraunhofer Institute of Applied Solid State Physics acknowledge support from the German Ministries of Defense and Education and Research (BMBF).
Hans D. Tholl
Diehl BGT Defence GmbH & Co. KG
Hans Tholl is head of the optronics and laser-techniques section. He also teaches laser theory, laser techniques, and wave optics at the University of Applied Sciences in Ravensburg-Weingarten.
Fraunhofer Institute of Applied Solid State Physics
Joachim Wagner is deputy director and heads the business unit Semiconductor Lasers and LEDs. He also lectures at the Physics Department of Freiburg University. His research interests include IR semiconductor-laser modules and systems.
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