Quantum cascade laser (QCL) is a semiconductor laser based on sub-band electronic transition, which results in a broad emitting wavelength covering from 3 to 300 μm. QCL is an ideal light source in the fields of gas sensing, environmental monitoring, medical diagnosis, and photoelectric countermeasures. However, the relatively low output power (1-3 W) of the single transverse mode QCL is a major limitation for its applications. Laser beam combining technology is an effective way to enhance the output power. At present, the power beam combining of mid-infrared and long-wave infrared QCLs is heavily limited by the low-loss optical materials and component preparations. Beam combining with high efficiency and low loss is challenging, and few research results have been reported. Therefore, the fiber combining of long-wave infrared QCLs in the 7.6-7.8 μm wavelength band was studied in this paper. The laser power was combined with a 4-in-1 single-mode hollow-core fiber bundle.

In order to realize the high-efficiency single-mode fiber coupling of QCLs, the optical fiber coupling system was designed. The optical fiber system was composed of a QCL collimator and a fiber coupler. Due to the large QCL emitting area and large divergence angle, an aspheric collimator with a large numerical aperture was designed and fabricated. During the optical design and optimization, the QCL was assumed to be an extended light source. Using the optimized collimator, a fiber coupling efficiency of 88.9% was obtained. To combine the laser beams from individual QCL, a 4-in-1 fiber combiner was fabricated using AgI/Ag single-mode hollow-core fiber, which had a high damage threshold and low transmission loss. During the preparation, the outer protective layer of the fiber was stripped away, and the four fibers were tightly arranged in the sleeve and fixed. Finally, the fiber was protected by metal armor. The input terminals of the fiber combiner were four independent SMA905 fiber connectors, and a unified SMA905 connector was made at the output end.

The optical fiber coupling experiments are conducted using the designed optical fiber coupling system and the prepared long-wave single-mode hollow-core fiber combiner. When the QCL output power is 642 mW, the laser power throughout the single-mode fiber is 438 mW. The corresponding fiber coupling efficiency is 68%. In addition, we experimentally compare the coupling efficiency using a point-source collimator and an extended-source collimator. Using the extended-source collimator with a large numerical aperture, the fiber coupling efficiency is increased from 59% to 68%, as shown in Fig. 10. An infrared camera is used to observe the collimated QCL spot and the beam spot out of the single-mode fiber. In addition, the beam propagation quality factor M² after the fiber coupling is calculated. After the fiber coupling, a symmetric Gaussian distribution is observed, and the beam quality is improved to 1.2, compared to the M² in Table 7. On the basis of the single-channel optical fiber coupling experiment, the optical fiber combining experimental setup of a four-channel long-wave infrared QCL is built. When the total input power from four QCLs is 2.27 W, the fiber combining power is 1.5 W. The corresponding combining efficiency is 66%. In order to evaluate the beam quality of the combined beam, the lens is used to focus the output light, and the intensity distribution of the output spot of the beam combiner is measured within two times Rayleigh distance. The results are shown in Figs. 14 and 15. The transmission quality factors of the combined beam are calculated as MX2=2.67 and MY2=2.56, which meant a good beam quality.

In this paper, the long-wave infrared QCL beam combining technology based on single-mode hollow-core fiber is studied. Considering the large emitting area and big divergent angle of the fundamental transverse mode long-wave infrared QCL, a QCL collimator with a large numerical aperture is used. During the design, the QCL is treated as an extended light source. To obtain the optimized collimation result, both surfaces of the collimator are aspheric. A 4-in-1 fiber combiner is fabricated using the AgI/Ag single-mode hollow-core fiber. The fiber has no end face reflection loss and low transmission loss. The experimental results show that the single-mode fiber coupling efficiency is 68%. After the fiber coupling, the beam propagation quality factor M² is 1.2. In addition, the power combining of four QCLs in the wavelength band of 7.6-7.8 μm is realized. When the input power is 2.27 W, the combined output power is 1.5 W. The beam combining efficiency is 66%. The transmission quality factors of the combined beams are MX2=2.67 and MY2=2.56. The low-loss working band of the fiber combiner ranges from 7 to 15.5 μm. The output optical power can be further increased by increasing the number of QCLs in the beam combining, which provides an effective way to expand the output power and wavelength range in the long-wave infrared wavelength band.