The mid-infrared waveband is the vibrational and rotational spectral region of molecules, of which 3-5 μm is the most important atmospheric window, making it an increasingly popular research topic. The wide-tuning characteristics of the external cavity quantum cascade laser in the mid-infrared waveband make it widely used in gas molecule sensing, difference-frequency THz generation, free-space optical communication, and other fields. We design a tunable quantum cascade laser with a 4-μm wavelength to realize these applications. The laser can achieve different light emission performances by replacing blazed grating, making it suitable for different conditions.
The experiment in this study is performed with the Littrow structure as the main body and quantum cascade gain chip with a central wavelength of 4 μm. During the experiment, the working temperature of the gain chip is kept at 25 ℃, and the quantum cascade laser gain chip is integrated with the thermoelectric cooler and even aspherical lens. Blazed gratings with groove spacings of 450 line/mm and 300 line/mm are selected as the feedback elements, and the zero-order diffraction light of the grating is selected as the output light; the first-order diffraction light is fed back to the active region of the gain chip to form an external cavity resonance. The feedback light is returned to the laser active region and the laser wavelength is selected by adjusting the grating pitch angle and rotation angle.
Based on the above results, the laser maximum power and spectral tuning range are 7.30 mW (Fig. 8) and 380 nm and the grating rotational angle is 11.06° when 450 line/mm gratings are used. With this configuration, the laser has a higher power value and wider spectral tuning characteristics, which is more suitable for applications requiring narrow linewidth and high-precision wavelength tuning, such as spectroscopy. When a 300 line/mm blazed grating is used, the highest power is 5.24 mW (Fig. 8), spectral tuning range is 297 nm (Fig. 5), and the rotation angle of the grating is 3.15°. This configuration is more suitable for space optical communication and other applications requiring high beam quality. The laser side-mode suppression ratio (SMSR) in both configurations is 20 dB (Fig. 6), which is suitable for practical use.
In this study, a widely tunable external cavity quantum cascade laser based on the Littrow structure is developed. A blazed grating is used as the feedback element for mode selection, and two gratings with different grating constants are selected for comparison. Experimental comparisons show that when a 450 line/mm blazed grating is used, a maximum power value of 7.30 W, tuning range of 3774-4154 nm, tuning width of 380 nm, and grating rotation angle of 11.06° are obtained. The laser has a higher power value and a wider spectral tuning range. When a 300 line/mm blazed grating is used, the laser beam quality is improved, with a maximum power value of 5.24 mW, tuning range of 3779-4154 nm, tuning width of 297 nm, and grating rotation angle of 3.15°. The 300 line/mm blazed grating configuration is more suitable for high beam quality applications, such as space optical communication. The performance of the laser obtained by using the 450 line/mm blazed grating configuration is more suitable for spectral applications requiring narrow linewidth and high-precision spectral tuning. An external cavity quantum cascade laser can achieve different performance indices by using different external cavity configurations and meet the use requirements of different application scenarios. It plays an important role in molecular gas sensing, difference-frequency terahertz generation, free-space optical communication, and other fields.