Deeper Dive into Quantum Cascade Lasers
Groundbreaking new QCL Wavelength Range from 10mµ to 17µm
Mid-IR molecular spectroscopy is a rapidly developing and promising technique, enabling high-performance chemical detection and analysis for industrial or environmental purposes, with new wavelength ranges becoming commercially available. The essential component for such applications is the laser source, adapted to the specific spectral lines (the fingerprint) of the target molecule. Quantum Cascade Lasers (QCLs) are a perfectly suited solution to build such analysis systems. Until now, QCLs only covered the wavelength range from 4 to 10µm, where many chemical species are detected (NO, N2O, NH3, CH4, COH2, CO, CO2, SO3, etc.). However, the fingerprint region extends much farther in the infrared, and current laser technologies don’t allow for the analysis of many molecules of interest.
Among these molecules of interest are an important class, aromatics (benzene, toluene, ethylbenzene, and xylenes, known as BTEX), which have their main fingerprint absorption lines at wavelengths ranging from 12µm to 15µm. BTEX are toxic, volatile, organic compounds that are principally emitted by the petrochemical industry. Benzene is naturally present in crude oil. It is used primarily as a precursor to manufacture chemicals with a more complex structure, widely used in the chemical industry. Benzene and toluene are used as additives in gasoline and are often detected at filling stations, a significant source of exposure for humans. BTEX molecules are also frequently produced and released in the atmosphere during large fires or industrial accidents. The detection of BTEX is then a major issue for environmental monitoring and safety.
A new class of Distributed Feedback (DFB) QCLs has been developed at the University of Montpellier, France, in a joint laboratory with CNRS. The unique aspect is the use of different III-V semiconductor materials to fabricate the active region of the lasers. These materials are part of the antimonide family, a specialty of the University of Montpellier for decades. The foundation of QCLs consists of artificial heterostructure alternating nanometer-scale quantum wells and barrier layers. While the standard QCL uses InGaAs and AlInAs on an InP substrate – the basis of the optoelectronics telecom industry – this novel QCL technology is based on InAs and AlSb. These materials’ specific properties enabled the expansion of QCL wavelength range from 2.6µm to 25µm. High-performance laser sources have been demonstrated recently in the range 10 to 20µm, with powers in the milliwatts range emitted at room temperature in a continuous wave. An efficient DFB process was also demonstrated for the fabrication of single-frequency tunable lasers suitable for spectroscopy applications.
Tunable diode laser absorption spectroscopy (TDLAS) with temperature tuning (λ ≈ 14.9µ)
mirSense is a well-known and established entity, with 20 years of expertise in the QCL industry for defense and spectroscopy and now offers this new InAs-based QCL technology. Their groundbreaking UniMir line of long-wavelength, single-frequency DFB QCLs are now commercially available in a sealed High Heat Load (HHL) package, with integrated collimating lens, thermistor, and thermoelectric cooler (TEC). The UniMir is well suited for integration into systems, or as a stand-alone turnkey system for R&D and detection applications. These lasers, operated in CW or in pulsed mode, emit a maximum power of 5 to 10mW (<15µm) at room temperature. By controlling the chip’s operating temperature through the Peltier element inside the laser’s package, customers tune the emission wavelength without mode hopping, while maintaining single longitudinal mode operation. The technology’s versatility allows them to address any wavelength between 10 and 18µm in CW and up to 21µm in pulsed mode, opening the way for high-resolution spectroscopy applications in this spectral range, notably for the detection of BTEX, but also CH3I, HCN and many other molecules. Compared to wavelengths produced by previous QCL technology, these new QCLs will enable much stronger absorption rates by these molecules, with less cross interference from other molecules in the 10m-17µm range.
Showing the difference in energy gap among the various QCL substrate materials
Below we’ve listed a couple of example configurations and their target application:
Benzene:
Perfect for benzene environmental monitoring, this ~14.9µm (~674 cm-1) UniMir model is a single-mode DFB QCL, operating in CW mode with 5mW output power (with the base plate of the HHL-package at +20oC). The full tunable range is >2cm-1, while the continuous tuning range, free from ‘mode hopping’ is >0.5cm-1.
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The curves on the left indicate the voltage of the laser as a function of the applied DC current and laser chip temperature. The curves on the right indicate the output power as a function of the applied DC current and laser chip temperature |
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The curves indicate the typical laser single-mode emission wavelength as a function of the applied DC current and laser chip temperature. Lasers with slightly shifted wavelength are also available. |
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Xylene and Propane:
Perfect for xylene and propane monitoring in the chemical industry, this ~13.4µm (~746 cm-1) UniMir model is a single-mode DFB QCL, operating in CW mode with 5mW output power (with the base plate of the HHL-package at +20oC). The full tunable range is ~3cm-1, while the continuous tuning range, free from ‘mode hopping’ is >1cm-1.
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The curves indicate the voltage (left) and output power (right) of the laser as a function of the applied DC current and laser chip temperature. |
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Emission spectra as a function of the temp. of the laser chip. |
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Unprecedented QCL Wavelengths for Enhanced Molecular Spectroscopy
Steady growth in the nuclear industry has led to an increase in demand for more accurate, efficient, and reliable detection and monitoring of critical compounds, like Uranium hexafluoride (UF6) assay or Methyl Iodide (CH3i). This has led to the development of new technologies, enhancing the capabilities of molecular spectroscopy. Entities worldwide are developing advanced spectroscopy-based technologies and methods, aiming to decrease accidents with better safeguards, enable the rapid and precise assessment of nuclear plant incidents, and assist organizations like the International Atomic Energy Agency (IAEA) in developing nuclear safeguards. Measuring gaseous radioactive CH3i is very important to quickly assess nuclear plant incidents and avoid shutting down a nuclear power plant, which may cost millions of USD per day. In case of a severe nuclear accident like Three Mile Island or Fukushima, the ability to rapidly and accurately measure CH3i is critical to assess the nature and severity of the incident and gain valuable information about how to respond. Having accurate information when you need it can mitigate costly, disruptive shutdowns and empower facility personnel and inspectors to make quicker and better-informed decisions.
To assist the nuclear industry in the detection and monitoring of these critical compounds, a new class of Distributed Feedback (DFB) Quantum Cascade Laser (QCL) has been developed. While the standard QCL uses InGaAs and AlInAs on an InP substrate – the basis of the optoelectronics telecom industry – this novel QCL technology is based on InAs and AlSb. These materials’ specific properties enabled the expansion of the QCL wavelength range from 4µm to 25µm, which better encompasses the easily discernible absorption peaks of the compounds of interest.
Read the full article here.
QCLs & New Low-Cost IR Sensors Open Door for Many OEM Opportunities
Simpler production processes mainly drive the reasons for this massive cost reduction. Producing HgTe detectors does not require the same very costly Molecular Beam Epitaxy (MBE) equipment or the ultra-high vacuum conditions required to manufacture traditional semiconductor thin films. The production time to manufacture HgTe is also much faster, typically only one day. HgTe involves the production of a solution of nanoparticles in traditional chemistry glassware, like the one held by Emmanuel in the picture in his left hand. He is holding a HgTe detector in his right hand that can be used for infrared sensing.
Emmanuel and his team utilize a 4.4 µm (4400 nm) quantum cascade laser (QCL) system from mirSense that can generate pulse width down to 30 nanoseconds to characterize the response time of their detectors. Their detectors are most responsive in the short-wave infrared (SWIR), but have some responsiveness in the mid-infrared (MWIR), which is where the 4.4 µm laser comes in. Additionally, the QCL system provides the ability to excite some intraband transitions of the nanocrystals (see more on this in the list of publications below).
Read the full article here.
Quantum Cascade Lasers (QCLs) for Infrared Countermeasures
Heat-seeking missiles, also known as air-to-air guided missiles, have been a part of the United States Air Force’s (USAF) arsenal for nearly 70 years. The USAF first introduced heat-seeking technology in the 1950s, with the acquisition of the Falcon (AIM-4) and Sidewinder (AIM-9) missiles, and by the outbreak of the Vietnam War, heat-seeking missiles had become a staple of aerial combat [1]. Since nearly the invention of heat-seeking missile technology, engineers have been simultaneously developing countermeasures to trick the sensor into missing its target. At first, these countermeasures were relatively crude in design, consisting of flares that the pilot could deploy when under attack. As the technology evolved, flares were starting to be phased out, and “heat-seeking missile countermeasures based on hot sources” began to be phased in [2]. These early devices were essentially larger versions of the Globar used in modern infrared (IR) spectrometers. These hot sources consisted of a mass of silicon carbide, which, when current passes through, heats up to the point where it emits large quantities of IR radiation from roughly 4 microns to 15 microns in wavelength. Coupled with a rotating, mechanical shutter that would modulate the thermal output, these sources make it impossible for the heat-seeking missile to lock on to the target [2].
Unlike flares, which can only be deployed once and required the pilot to deploy them once targeted actively, these so-called “hot sources” could remain active during the entire mission, significantly improving the effectiveness of the countermeasure. As laser technology improved, it eventually became viable to replace the Globar with an IR laser source, reducing the weight and complexity of the system. In 1997, D. B. Meeker, working for the United States Navy, patented one of the earliest known aerial laser IR countermeasures, designed for use onboard an aircraft [3].
It is by no coincidence that this patent application coincides with the invention of the Quantum Cascade Laser (QCL) at Bell Labs in the mid-1990s, which is now capable of producing light anywhere between 4 microns and 12 microns. Now that it has been close to a quarter-century since the invention of the QCL, they are the source of choice for modern IR countermeasure systems. This article will go on to explain why the mid-IR spectral region is so vital for heat-seeking missiles, and it will further elaborate on the importance of QCLs with examples of modern commercially available devices.
When choosing the right QCL for IR countermeasures, there are a few major points to consider. Most commercially available QCLs are designed for spectroscopic sensing applications, which require single longitudinal mode (SLM) operation. While these lasers have excellent spectral characteristics, their output powers are often limited to a few milliwatts. Therefore, these lasers are far from ideal for tricking heatseeking missiles, where the laser needs to be powerful enough to overcome the thermal emissions from the jet engine. This brings us to our next consideration, Quasi Continuous-Wave (QCW) operation of QCL’s. The QCW output of theses lasers generate less heat within the system and allows for a much better wall plug efficiency than that of continuous wave (CW) systems. This is because in QCW operation, the laser is only turned on for some percentage of the time, depending on the percentage of duty cycle set, greatly reducing overall power consumption. Additionally, QCW repetition rates are very high (in the hundreds of kHz range), which cannot be detected by the missile’s sensors. The final major consideration to mention is beam quality. Lasers for countermeasure applications need to be single spatial mode to allow for optimal collimation and beam steering.
Read the full article here.
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