Lasers for the waveband of 3-5 μm are important in the recognition and detection of gas molecules, free space optical communication, and different frequency generation of terahertz (THz). The typical pollutants and greenhouse gases such as H₂S, SO₂, CO₂, and CO have strong absorption baseband in the waveband of 3-5 μm, so the development of lasers in this waveband is necessary.

In order to cover the waveband of 3-5 μm, the wavebands of 4.0 μm and 4.6 μm for two QCL gain chips are utilized as the gain medium. On the basis of the Littrow structure, the output beams of the two QCL gain chips are combined into one beam with a common aperture by using a low-pass and high-reflection beam splitter of 4.2 μm; the blazed grating of 300 lines/mm is used as the wavelength selector, the first-order diffraction light is fed back to the core of the gain chip to form an external cavity resonance, and the zero-order diffraction beam of the grating is used as the output light. By finely adjusting the position and the angle of the grating, the feedback light is returned to the core of the laser, and the laser wavelength is determined. The wavelength tuning gap of two QCL gain chips is used as the transition zone of the low-pass and high-reflection beam splitter, and the ultra-broad tuning range of the laser system can be achieved in the waveband of 3–5 μm.

Under temperature of 25 °C and injection current of 303 mA, the laser system operates from 3779 nm to 4836 nm (including a wavelength tuning gap of 179 nm) (Fig. 5) with a rotation angle of 34.54°-46.50° for blazed grating tuning. The maximum output power is 14.12 mW (Fig. 7), and the SMSR is 20 dB (Fig. 6). In the wavelength range of 3779-4076 nm, the QCL gain chip of 4.0 μm operates with a threshold current of 188 mA and maximum output power of 5.24 mW (Fig. 7), while in the wavelength range of 4255-4836 nm, the QCL gain chip of 4.6 μm operates with a threshold current of 166 mA and maximum output power is 14.12 mW. In the waveband of 3-5 μm, there is almost no water absorption, and gas molecules such as H₂S, SO₂, CO₂, and CO have strong absorption bands. In addition, the ability of the laser to have an ultra-broad wavelength tuning range makes it possible to simultaneously identify and detect these different molecules in the gas mixture.

In this paper, an ultra-broad tunable mid-infrared laser based on a beam combination of dual gain chips is designed. The laser system is built with QCL gain chips of 4.0 μm and 4.6 μm, a low-pass high-reflection beam splitter of 4.2 μm, and a blazed grating of 300 lines/mm. The experimental results show that the blazed grating angle for the QCL gain chip of 4.0 μm is 34.54°-37.69°, the maximum optical power is 5.24 mW, and the spectral tuning range is 297 nm. The blazed grating angle for the QCL gain chip of 4.6 μm is 39.67°-46.50°, the maximum optical power is 14.12 mW, and the spectral tuning range is 581 nm. The total tuning range of the laser is 3779-4836 nm (including a tuning gap of 179 nm), and the SMSR is 20 dB. The ultra-broad tunable mid-infrared laser can be used in gas molecule sensing, free space optical communication, and different frequency generation of THz in the waveband of 3-5 μm.