Fig. 1. Energy band diagram of QCL

Fig. 2. Schematic of EC-QCL with Littrow structure

Fig. 3. Structure diagram of ultra-broad tunable external cavity mid-infrared laser (QCL chip 1 is a 4.0 μm waveband gain chip and QCL chip 2 is a 4.6 μm waveband gain chip)

Fig. 4. Diffraction efficiency of blazed grating (grating model is GR1325-30035, the size is 12.5 mm×25 mm×9.5 mm, the design wavelength is 3.5 μm, the grating constant is 300 lines/mm, and the blazed angle is 26.5°)

Fig. 5. 3-5 μm ultra-wide wavelength tuning spectra. The dotted line is the splitter transmittance curve and the solid lines are normalized intensity

Fig. 6. Side-mode suppression ratio of EC-QCL under different wave bands. (a) 4.0 μm; (b) 4.6 μm

Fig. 7. Power curves of two waveband lasers

Fig. 8. Curves of laser power changing with wavelength

Fig. 9. Spot shape obtained by spot analyzer at 20 cm

Fig. 10. Intensity characterization of spot shape in x and y directions (spot size at 1/e² has been marked). (a) x direction; (b) y direction