Fig. 1. Continuous-wave output powers for QCLs under room temperature reported in the wavelength range from 3.5µm to 10.7µm

Fig. 2. Longitudinal mode profile of the optical mode field with the lowest modal loss in the anti-reflection-anti-reflection coated device^[33]

Fig. 3. Schematic diagram of coherent beam combining^[37]

Fig. 4. MMI tree-like structure consisting of eight QCL units^[41]

Fig. 5. Schematic diagram of polarization beam combining^[45]

Fig. 6. Schematic diagrams of wavelength beam combining with an QCL array. (a) Direct type^[46]; (b) external-cavity type^[47]

Fig. 7. Schematic diagram of spectral beam combining^[48]

Fig. 8. Spatial beam combining setup^[55]. (a) Schematic diagram of the experimental setup; (b) optical field simulation of combined beam image

Fig. 9. Numerical and experimental far field patterns before and after beam shaping^[56]

Fig. 10. Schematic diagram of incoherent MPLC combine of four QCLs^[58]

Fig. 11. Single-mode hollow core fiber combiner^[64]. (a) Structure of hollow core fiber; (b) micrograph of output surface for single-mode hollow core 4×1 fiber combiner; (c) picture of hollow core fiber combiner

Fig. 12. Schematic diagram of a monolithic wavelength beam combined chip with farray waveguide grating integrated with five QCLs and AWG^[69]