High Sensitive Gas Sensing with Low Power Consumption Quantum Cascade Lasers

Recently, gas sensing is performed in various fields such as environmental protection, industrial process, agricultural analysis, medical care, and security and safety. In these fields, portable gas sensing apparatuses that enable high-sensitive, real-time, and/or on-site measurements are required. Figure 1 shows the correlation between the detection sensitivity and measurement time of existing gas sensing apparatuses. High-sensitive methods, such as Fourier transform infrared spectroscopy (FT-IR) and mass spectrometry, take time for measurement. Meanwhile, the sensitivity of non-dispersive infrared absorption (NDIR) and semiconductor sensors (considered to be real-time solutions) is only below about 0.1%. In addition, in the case of FT-IR and mass spectrometry, gas samples must be brought back to a laboratory for measurement; conducting on-site measurement is difficult. To overcome these problems, gas sensing using a quantum cascade laser (QCL), which is a kind of semiconductor laser, as a light source, has attracted much attention. This is because gas sensing using a QCL is expected to enable high-speed, high-sensitivity, and high-portability sensing, which has been difficult with conventional gas sensing methods. High-sensitive measurement is achieved primarily because QCLs can lase in the mid-infrared region (wavelength: 3–20 μm) as discussed below. Gaseous molecules absorb mid-infrared light and are subject to vibrational excitation. The wavelength at which absorption occurs depends on the molecules, and the absorption intensity depends on the gas concentration. Therefore, gas sensing can be achieved by measuring the absorption spectrum of the target gas. Many molecules have absorption lines in the mid-infrared region (i.e. fingerprint region of molecules), and absorption of mid-infrared light is much larger than the other wavelength regions due to absorptions by the fundamental vibrations of molecules. Besides, as named “atmospheric window,” absorption by water that is present in large amounts in the atmosphere is also low in the mid-infrared region. These characteristics help achieve high-sensitive sensing. Furthermore, gases, ranging from those with low molecular weight and prevalent in the atmosphere to those consisting of complicated organic molecules, are subject to absorption at around 7–12 μm in the mid-infrared region. Thus, gas sensing in the mid-infrared region is highly useful, and its applications in various fields have been actively examined. It should be noted, however, that until QCLs were developed, there had been few lasers that lased even at midinfrared wavelengths. The mainstream solution was to use an FT-IR, which utilizes an interferometer and white light derived from ceramics, tungsten, or dispersive infrared spectroscopy in which a diffraction grating is used for spectrometry. QCLs can operate in a single mode by introducing a DFB structure with a diffraction grating into a chip.(1)-(4) This enables the measurement exclusively for the High Sensitive Gas Sensing with Low Power Consumption Quantum Cascade Lasers