A variety of applications of external cavity quantum cascade lasers
the combination of quantum cascade media and external cavity laser structure realizes the near-infrared quantum cascade laser output. It has the characteristics of narrow-band, ultra wideband single-mode tuning, high power and excellent beam quality, and is very suitable for field applications
eric Takeuchi, Kyle Thomas, Timothy day
the remarkable progress made in the performance and manufacturing level of quantum cascade (QC) materials has prompted people to develop the wide application of this technology in the mid infrared field. At the same time, a large number of innovative behaviors have emerged in the design and manufacture of micro external cavity lasers (ECL). By integrating QC gain medium into ECL structure to form external cavity quantum cascade laser (ecqcl), the industry is realizing the great value of these technologies. At present, external cavity quantum cascade lasers are expected to have higher power, higher efficiency and wider tuning range
literally, combining QC and ECL into ecqcl seems to be just a simple combination of the two, but the fact is far from the case. It should be emphasized that in the external cavity structure, the QC material is only used as the gain medium, and the external cavity can be made around it. This change in method has achieved all the advantages that QC technology can provide
the arrival of QC technology
before that, researchers initially used QC technology to detect and display mid infrared wavelengths of light in the laboratory. However, in the past few years, the development of epitaxial growth technology of QC gain media has made this technology commercialized from research institutes and university laboratories. In many cases, wafer manufacturers use their production experience, processes and capabilities in III-V semiconductor materials (such as gallium arsenide and indium phosphide) to obtain high-quality QC chips with excellent performance. At present, quantum cascade wafers can be grown by metal chemical vapor deposition (MOCVD), which is the first choice in the industry. Compared with molecular beam epitaxy (MBE), MOCVD has many advantages.  For example, faster growth can rapidly develop the mass production process, significantly increase production and reduce costs
similarly, the performance of QC gain media has also made great progress. The reported ultra wideband gain range in a single QC laser is 5 ~ 8 m (750 wavenumbers).  Through the complete waveguide and thermal theoretical model, the QC structure optimized for continuous optical operation can output high power; The output power of high efficiency FP cavity laser (simple cut surface) on one side has exceeded 1.3W at room temperature.  , 
due to the recent mature development of QC chip manufacturing technology, it is now possible to manufacture various types of lasers (see Figure 1)
Figure 1: the basic structure of QC laser includes fp-qcl (above), dfb-qcl (middle) and ecqcl (below). The gain medium is gray, the wavelength selection mechanism is blue, the coating surface is orange, and the output beam is red
the simplest structure is F-P cavity laser (fp-qcl). In the F-P structure, the cutting surface provides feedback for the laser, and sometimes a dielectric film is used to optimize the output
the second structure is to directly etch the distributed feedback grating on the QC chip. This structure (dfb-qcl) can output a narrow spectrum, but the output power is much lower than that of fp-qcl structure. Through the maximum range of temperature tuning, dfb-qcl can also provide limited wavelength tuning (10~20cm-1 tuning range can be obtained through slow temperature tuning, or 2~3cm-1 tuning range can be obtained through rapid injection current heating tuning)
the third structure combines the QC chip with the external cavity to form ecqcl.  ,  this structure can not only provide narrow spectral output, but also provide fast tuning (faster than 10ms) over the whole gain bandwidth (hundreds of cm-1) of QC chip. Because the ecqcl structure uses low loss components, it can operate efficiently under the condition of portable battery power supply
ecqcl's main component
qc semiconductor chip can only be used as gain medium, and the laser cavity must be designed with it as the center. Many factors, such as die bonding, thermal management, dielectric film, micro optical components, wavelength control, electronic drive and packaging, play a key role in ecqcl to reliably provide the above advantages
proper selection of welding and heat sink materials is very important for QC bare sheet bonding. In addition, long-term reliability largely depends on the correct implementation of QC design processes, including implantation of heterojunction and ridge waveguides, as well as table and epitaxial mounting configurations
in addition, a high-performance dielectric film must be used to optimize the output. At present, dielectric films with reflectivity less than 0.001 have been used in ecqcl devices. These films provide a wide bandwidth (hundreds of wavenumbers) while maintaining high performance at high temperatures and pressure loads
combined with high-efficiency electronic devices, ecqcl can now be driven by ordinary battery sources. Using a simple AA battery (one charge) can make the compact "thermal laser pen" work for several hours at a power of more than 50MW, and no cooling function is required. Even if integrated into the sensor system, a single battery charge can make it work continuously for more than 4 hours to provide real-time data
since the output light has a large divergence angle, which is the characteristic of most semiconductor lasers, it is generally necessary to use optical elements with large numerical aperture (NA) to provide output characteristics near the diffraction limit. In a typical example, using large Na (greater than 0.8) micro optical elements to collimate ecqcl, the far-field intensity distribution shows that its M2 is better than 1.2The most attractive thing about QC technology is that it can obtain a wide wavelength tuning range from a single device while maintaining excellent spectral purity.  At present, the single-mode output tuning range of commercial ecqcl products can reach 300 wavenumbers (see Figure 2).  At the same time, the tuning accuracy of each wavelength is better than 1/104. The whole gain band can be tuned within 10ms and the narrow-band output characteristics can be maintained
Figure 2: ultra wideband single-mode tuning can be achieved using ecqcl structure (right figure) (left figure)
ecqcl is integrated into a sealed butterfly package with thermoelectric temperature control. Inside the package, the QC gain medium is made into a component. This structure can concentrate all optical power into a single high-power near diffraction limit output beam (see Figure 3). The total volume of the package is approximately 0.8in 3. The weight is less than 2 ounces
Figure 3: ecqcl (right) in closed package can output higher power. The data show that ecqcl can output 4~5 m laser at room temperature
the application of external cavity QCL technology is expanding to many market areas. In the field of medical diagnosis, QCL technology is being used to develop portable and wearable sensors for noninvasive glucose detection; Ecqcl based sensors are being used in standard breath detectors and non-invasive medical diagnosis.  In the field of personal safety, QCL technology is being used to ensure that workers in industrial environment can continuously monitor the level of toxic gases in their environment. In addition, QCL technology can also be used to measure nitrogen oxides, sulfur oxides, carbon dioxide, carbon monoxide, ammonia and other combustion gases in real time, so as to minimize fuel consumption and emissions in marine and industrial environments
all these sensors can be integrated with wireless functions to realize real-time data monitoring and storage in a central server. In addition, an environmental monitoring network can be established to provide wide area or local coverage, and monitor air quality in time and space (see Figure 4). 
Figure 4: the environmental monitoring device was used to detect the air quality during the 2008 Beijing Olympic Games
ecqcl technology is also being used in the field of lighting. Researchers in the field of national defense are testing "thermal laser pens". When they are applied to new low-cost thermal imaging cameras, these devices are ideal "flash lamps", which can improve the sensitivity of volatile gas emission detection, and can also be used as a long-range application light source for explosive detection and identification
at present, the power of high-power ecqcl has been increased to several watts. The advantages of ecqcl in size, weight and power consumption make it an ideal substitute for some applications, including infrared countermeasures for military and commercial aircraft protection, as well as active infrared imaging and long-distance remote detection applications
1 Z. Liu et al., IEEE Phot. Tech. Lett. 18(12) (2006).
2. C. Gmachl et al., Nature 415, p. 883 (2002).
3. S. Howard et al., IEEE Sel. Top. Quant. Elect. 13(5) p. 1054 (2007).
4. Y. Bai et al., Appl. Phys. Lett. 92, p. (2008).
5. G. The larger the size of Wysocki et al., appl Phys. B 92, p.305 (2008).
6. T. Day et al., "miniaturized external cavity quantum cascade lasers for broad tuna officially announced today that it plans to invest an additional $550million with its suppliers to support the new product probability in the mid infrared," Cleo and 2006 QELS (May 2006)
7. R. Maulini et al., Appl. Phys. Lett. 88, p. 201113, 2006.
9. T. Risby and S. Solga, Appl. Phys. B 85, p. 421 (2006).
10. MIRTHE (mid-InfraRed Technologies for the Health and Environment), an NSF Engineering Research Center headquartered at Princeton University (end)消化不良怎么治疗
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