The quantum-cascade laser (QCL) is a testament to human ingenuity, in which a seemingly unchangeable property of materials (laser wavelength being a function of bulk material composition) is overcome by structuring matter in an innovative way (a superlattice consisting of hundreds of layers of materials in a repeating order, forming a series of multiple-quantum-well heterostructures).
Unlike the single photon produced by an excited electron combining with a hole in an ordinary semiconductor laser at a p-n junction, an electron in a QCL makes numerous intersubband transitions between quantized subbands in one band, producing a "cascade" of photons as it does so. Tailoring the superlattice properties in a QCL tailors the emission wavelength. Note: a somewhat similar type of laser called the interband-cascade laser (ICL), which produces photons from interband rather than intersubband transitions, is quite interesting in its own right, but will not be discussed in this article.
The QCL is one of the most versatile mid-infrared (mid-IR) lasers today (and is a capable emitter in the terahertz radiation region as well), as it is far more compact and rugged than mid-IR sources such as optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs). As a result, the QCL is a perfect fit as a light source for spectroscopic instruments meant for use in the IR molecular "fingerprint" spectral region, as well as many other uses (see table). If need be, such instruments can be small, battery-operated, and durable for use in the field.
"Mid-IR applications span multiple markets including scientific research, life sciences, industrial process control, environmental monitoring, and defense and security," says Leigh Bromley, director of scientific products at Daylight Solutions (San Diego, CA), which makes QCLs and QCL-based instruments. "The diverse nature of mid-IR applications, which range from sensitive molecular detection and imaging to protection of aircraft from missile attack, is driving various mid-IR laser system requirements."
Range and tunability
The flexibility of QCLs—with center wavelengths that cover less than 4 to greater than 12 μm, their ready adaptability to pulsed or continuous-wave (CW) operation, the availability of broad-gain-bandwidth chips tuning over a range of more than 500 cm-1, and their ability to power-scale to multiple watts—makes them ideally suited to many uses, notes Bromley.
Thanks to constantly evolving QCL chip designs, tuning range per QCL has increased rapidly over the years. "Today, some QCLs can routinely provide tuning ranges exceeding 500 cm-1, or greater than 30% of the center wavelength," explains Bromley. "The ongoing thirst for wider tuning range in many applications has, however, driven the development of multi-QCL laser architectures."
He notes that one of these, the Daylight Solutions MIRcat, incorporates up to four tunable QCL modules in one sealed laser head. Good optomechanical design and coaxial boresighting of all QCL beams ensures that the output is a single highly collimated output beam (see Fig. 1). MIRcat can provide pulsed and/or CW operation; peak or average output powers up to about 1 W; relative intensity noise (RIN) as low as -145 dBc/Hz; and mid-IR tuning ranges to >900 cm-1 (or >6000 nm).
The MIRcat QCL is at the heart of the company's IR microscope platform, which combines a multi-QCL laser source and an IR camera to enable video-rate discrete frequency and hyperspectral mid-IR imaging of biological tissue, polymers, and semiconductor materials, as well as a molecular sensor that incorporates a rapid-scan laser engine for high-refresh-rate, multispecies molecular detection, says Bromley.Block Engineering (Marlborough, MA) produces pulsed external-cavity QCLs that are fabricated as compact, sealed packages (see Fig. 2) that are then included as the "engines" inside the company's line of instruments, which include the LaserWarn Detector, an open-path gas-sensing system that can be used for protection of critical facilities against chemical attacks or toxic leaks, according to Petro Kotidis, Block Engineering's CEO.
The LaserTune QCL can operate either in a sweeping mode or at selected wavelengths anywhere within the 5.4–12.8 μm range, says Kotidis. Tuning across this range is achieved in less than 40 ms with a spectral resolution of approximately 1–2 cm-1. The system has a beam-pointing stability of better than 1 mrad and pulse rate up to 3 MHz with 3–300 ns pulses.
The QCL can be coupled with an external detector (also produced by Block) to create a laboratory tool for IR analysis and spectroscopic evaluations, or can be integrated with sampling and a gas or liquid flow cell for process, industrial, and environmental applications. Key applications that Kotidis lists for LaserTune include: IR microscopy, in which analysis of miniaturized samples is achieved by imaging the targets at diffraction-limited dimensions; fiber coupling; and atomic-force microscopy, which takes advantage of the emitter's wide spectral coverage and rapid spectral scanning.
Individual QCL lasers
While QCLs are available as above in modules for long life and hard use in the field, they can also be had for experimental use in other forms. In addition to sealed and collimated QCL modules, Alpes Lasers (St-Blaise, Switzerland) provides QCLs as bare chips (unsoldered and unconnected), chip-on-carriers (soldered on a submount), and a laboratory housing with a replaceable laser.
The last version contains a Peltier cooler that can reach a temperature of -30°C, an antireflection-coated zinc selenide window with high transmission between 3.5 and 12 μm for use with various pulsed or CW QCL chip-on-carriers, and a temperature sensor (the package must be externally air- or water-cooled to remove heat from the Peltier cooler). In addition, Alpes also produces inexpensive, pulsed-only QCLs that are available in small TO-3 cans only 38.8 × 25.4 × 20.5 mm in size.
Stratium (Cardiff, Wales) makes the Bruar, a 50 mW peak pulsed QCL chip on a submount. The emitter is available at wavelengths of 2.8, 3.3, and 10.0 μm and is intended for gas sensing, precision metrology, and spectroscopy applications, enabling accurate detection of methane (CH4), hydrogen chloride (HCL), formaldehyde (CH2O), carbon monoxide (CO), carbon dioxide (CO2), nitric oxide (NO), and other trace gases.
As an example of the many wavelengths that stock nontunable QCLs can provide, AdTech Optics (City of Industry, CA) produces distributed-feedback (DFB) QCLs as chips-on-submount with stock wavelengths of 4.34, 4.54, 4.58, 4.72, 5.26, 6.23, 7.83, 9.47, or 10.26 μm, with the ability to custom-design and fabricate QCLs with other wavelengths on request. AdTech's QCLs are available in similar wavelength ranges as hermetically sealed packages or TO-3 cans, as well as VHL packages (small housings with a window on top that transmits uncollimated light from the QCL).
Thorlabs (Newton, NJ) produces both Fabry-Perot cavity QCLs, which have a broadband output suitable for medical imaging, illumination, and microscopy, and DFB QCLs, which have a much narrower-band output, and are also tunable over a narrow frequency range. The metal C-mount packages are only 6.4 × 4.3 × 7.9 mm in size, with the lasers electrically isolated from their C-mounts. These QCLs are quite flexible in their uses, with mounts, drivers, and temperature control up to the user, but available from Thorlabs if desired. Because the QCLs have no built-in monitor photodiode, they must be operated in constant-current mode.
While a QCL built into a module for field use is generally resistant to a certain amount of abuse, the laboratory versions of QCLs must be handled properly to avoid damage. Thorlabs recommends avoiding electrostatic shock and flux fumes from soldering, as well as dust and other particulates—these lab QCLs should be paired with the proper current source and require temperature regulation hardware. And don't drop the laser package.Many QCL manufacturers provide their own laser drivers and temperature controllers for QCLs. However, Wavelength Electronics (Bozeman, MT) is a specialist in this area, providing drivers, temperature-control equipment, and other accessories for a range of lasers, including QCLs. In one example, the company produces its QCL2000 2 A QCL driver in different forms that include a laboratory instrument version with a display and knobs you can twiddle, and a much smaller sealed box with heat fins (see Fig. 3). Both versions can be networked with computers for automatic control and monitoring.
Application-ready QCL-based systems
As mentioned earlier, some QCL companies integrate their own QCLs into spectroscopic and other instrumentation ready for use in science, industry, and defense. For example, the earlier-described Daylight Solutions MIRcat QCL, in a pulsed version, has been incorporated by the company into its Spero imaging microscope. Spero is designed to measure high-definition IR imaging and spectral data for the identification and quantitation of molecular and chemical components of complex, heterogeneous samples, according to Bromley.
"In contrast with FTIR [Fourier-transform infrared]-based microscope systems, which employ low-brightness incoherent thermal sources that require lengthy scans to build spectra and images, the high output power and brightness of the MIRcat laser enables the use of large-format, room-temperature microbolometer cameras," says Bromley. "In turn, the use of these cameras enables video-rate discrete-frequency imaging. Similarly, hyperspectral data cubes can be built quickly by tuning the MIRcat in discrete wavelength steps and recording a mid-IR image at each wavelength. The ability to image in real time and parse image content according to mid-IR spectral response (chemical components) is bringing new levels of image analysis throughput and chemical specificity to IR microscopy, and promises new, game-changing capabilities for fields such as histopathology and cancer diagnosis."
Block Engineering's LaserWarn open-path gas-sensing system uses eye-safe, invisible QCLs to create a "trip wire" against threatening gaseous chemicals, says Kotidis. Covering distances as long as 2–3 km, the system alarms in less than a second when a dangerous chemical crosses the "trip wire" beams anywhere in the protected area.
"The system provides the most-sensitive standoff chemical detection available today and can operate indoors or outdoors on a continuous 24/7 mode with no consumables," explains Kotidis. "Examples of commercial applications of this product include monitoring of plant emissions for better process control, reduction of environmental pollution, and compliance with environmental release requirements. Examples of government and security applications include protection from chemical attacks of transportation terminals, embassies, government facilities, and forward-operating military bases."
Kotidis adds that the properties of the QCL that make this application possible include ultrawide tunability, which ensures the ability to detect large numbers of chemicals with very few lasers; the subsecond scanning of the QCLs across the mid-IR, which provides real-time detection; and the ability to operate with no consumables, which is essential for continuous operations in remote locations.
Quantum-cascade lasers have a bright future ahead of them-one version of this future is being developed at Pendar Technologies (Cambridge, MA), which has created a single chip that contains many QCLs of different wavelengths.
"QCL arrays continue to develop as a potentially game-changing solution for inclusion into the most modern and most compact sensors," says Mark Witinski, vice president of the chemical analysis and security unit at Pendar. "Current frontiers include broadening the wavelength coverage to accommodate a wider portion of the IR, monolithic and miniaturized beam combining techniques for robust integration, and customized electronics that are compact enough for new instrumentation, but powerful enough to drive rather low-wall-plug-efficiency QCLs appropriately. Pendar Technologies is currently developing a monolithic, electronically tuned QCL array for covering the full 'fingerprint' longwave IR region from 6.7 to 11 μm. Once complete, it will underpin both the gas- and condensed-phase portable sensing platforms that the company will launch in 2017."
1. C. A. Kendziora et al., "Broadband infrared imaging spectroscopy for standoff detection of trace explosives," Proc. SPIE, 9836, 98362G (May 25, 2016); doi:10.1117/12.2224049.
2. R. Furstenberg et al., Appl. Phys. Lett., 93, 224103 (2008).
For More Information
Companies mentioned in this article include:
City of Industry, CA
San Diego, CA
For a complete listing of companies making quantum-cascade lasers, visit the Laser Focus World Buyers Guide (http://buyersguide.laserfocusworld.com/index.html).