Advancements in Laser Research Lead to Better Stand-off Detection for the Warfighter and Nation

Courtesy Photo | Two-dimensional far-field intensity distribution (normalized) of the tapered QCL with plasmonic collimator, measured using a mid-infrared microbolometer array (160 x 120 pixels) with no optics placed directly in front of the laser facet. In (a) and (b), the pump current is, respectively, 1.5 Ith and 2.5 Ith, where Ith is the threshold current of the device. The angles are computed from the known pixel pitch (52 μm) and the distance between the facet and the sensor (14 mm). Images courtesy of Professor Federico Capasso and Dr. Patrick Rauter from Harvard University, under DTRA-funded research.  

FORT BELVOIR, VA, UNITED STATES

05.24.2016

Courtesy Story

Defense Threat Reduction Agency's Chemical and Biological Technologies Department

Fort Belvoir, Va. Harvard University principal investigator Professor Federico Capasso, one of the inventors of the quantum cascade lasers (QCLs), and Harvard researcher, Dr. Patrick Rauter, report recent progress in the development of multi-wavelength QCL arrays citing significant extensions in lengths. This recent Defense Threat Reduction Agency-funded research program highlighted impressive scientific accomplishments enabling high selectivity spectroscopy, which can be used for more accurate stand-off detection systems in defense of the nation.

Two-dimensional far-field intensity distribution (normalized) of the tapered QCL with plasmonic collimator, measured using a mid-infrared microbolometer array (160 x 120 pixels) with no optics placed directly in front of the laser facet. In (a) and (b), the pump current is, respectively, 1.5 Ith and 2.5 Ith, where Ith is the threshold current of the device. The angles are computed from the known pixel pitch (52 μm) and the distance between the facet and the sensor (14 mm). Images courtesy of Professor Federico Capasso and Dr. Patrick Rauter from Harvard University.

Conventional semiconductor lasers rely on recombination of electrons and holes across the fundamental band gap of the light emitting material to generate the wavelength of interest. The QCLs are based on only one type of carrier and inter-sub-band transitions across engineered energy levels using multiple quantum well heterostructures.

Simple quantum mechanics principles combined with material properties can be readily used to design laser emission at desirable wavelengths using engineered sub-bands. This frees the designer from the need to find a naturally occurring material with the required fundamental energy band gap.

The QCLs wavelengths now are extending below 3 micrometers over the long-wavelength mid-infrared to the far-infrared, also known as terahertz regime. In spectroscopy, lasers are used as optical sources for exciting molecular states for detection and identification.

The QCL is a compact solid state coherent source. The laser power generation however is not as high as desired for spectroscopy and it is accomplished at single wavelengths. Multiple wavelengths are needed since many molecules can show response for similar identification of molecules in mixtures and realistic environments.

Professor Capasso and Dr. Rauter report master-oscillator power-amplifier (MOPA) QCL arrays as a more powerful alternative to distributed-feedback (DFB) devices. They achieve multi-watt power levels for tunable single-mode emission while preserving the spectral purity and high beam quality of narrow DFB lasers. A wide tuning range can be achieved by pulsing in sequence tens of QCL lasers on the same chip, each designed to emit a different wavelength. Professor Capasso and his research team improved the QCL power by 100 fold.

They designed a MOPA to boost the QCL-generated laser beam to powers as high as 10 W during peak power. The MOPA QCL arrays were integrated with quarter-wave-shifted DFB gratings selecting a different frequency for each element of the QCL arrays on a single chip. The developed process allows for large scale integration on single semiconductor substrates.

In addition, the Harvard group used a novel flat plasmonic lens of their design directly fabricated on the laser facet to collimate the laser output into a diffraction-limited angle of only a few degrees, more than ten times smaller than the divergence of a typical semiconductor laser.

Professor Capasso’s team also demonstrated single-mode tapered QCL devices and arrays, hyperspectral imaging with arrays of single mode tapered QCL, widely tunable mid-infrared QCLs using sampled grating mirrors and mode switching in a multi-wavelength-DFB quantum cascade laser using an external micro-cavity.

The demonstrated multi-wavelength QCLs have high peak powers of up to 10 W, superior beam quality, electrical means of wavelength tuning independently of mechanical components and compact dimensions. Consequently the MOPA QCL arrays are highly suited as a robust, powerful and tunable mid-infrared source for spectroscopy and stand-off detection systems.

Spectroscopy and detection systems based on multi-wavelength QCL arrays as tunable mid-infrared sources have tremendous potential to outperform conventional systems in a variety of fields and to open up new applications calling for fast, compact and mechanically robust solutions for civilian and defense use.

To learn more about this important research, refer to “Multi-wavelength Quantum Cascade Laser Arrays,” in Laser & Photonics

POC: Dr. Kiki Ikossi; [e-mail]

• Date Taken: 05.24.2016
• Date Posted: 05.24.2016 10:06
• Story ID: 198895
• Location: FORT BELVOIR, VA, US