Objective Quantum cascade laser has the advantages of unipolarity and easy wavelength adjustment, which has become an important laser source for mid-wave infrared. To satisfy the demand for pulsed high-power quantum cascade lasers, wide-ridge waveguide technology is often used to obtain ultra-high pulse peak power, which will cause the transverse heat dissipation path of the active layer to become longer, make the heat accumulation of the core layer more serious, and reduce the performance of the device. In addition, increasing the ridge width leads to an increase in the number of intrinsic modes supported in the waveguide, which lases higher-order modes and eventually degrades the beam quality. Therefore, in this article, a longer cavity length and a narrower ridge structure are designed to significantly enhance the fundamental mode and increase the heat dissipation capacity simultaneously. Additionally, a long tapered structure is incorporated on the output surface to elevate the optical loss threshold. To a certain extent, high beam quality propagation is guaranteed and the optical power density of the device is reduced. It has important research significance.
Methods A new tapered waveguide structure model is established by COMSOL simulation software. The optical and thermal characteristics of a tapered high-power quantum cascade laser are simulated with an output of 60 W and a pulse frequency of 10 kHz in pulse mode. The optical field mode distribution under different ridge widths (Fig.3) and the influence of different geometric parameters on the port transmittance, the optical limiting factor of the waveguide (Fig.5-7), and the heat dissipation effect are mainly analyzed. The impact of different heat sink materials, varying heat sink temperatures (Fig.8), and diverse pulse widths (Fig.9) on the core region temperature of the fixed device structure under identical pulse mode is investigated.
Results and Discussions In the waveguide optical field mode analysis, for 4.6 μm wavelength, when the waveguide width is less than 5.6 μm, the luminescence mode of QCL laser is TM fundamental mode, and the higher order mode is restrained (Fig.4). In the analysis of output port characteristics, for 3 mm or longer cavity length, the waveguide loss is kept at a low level of 0.17 dB/cm, and the core area of the tapered waveguide laser is about 10 times larger than that of the strip ridge waveguide structure, which is more conducive to high-power output (Fig.7). In the heat dissipation analysis, when copper heat sink is used and the total cavity length is 4 mm, the entire core layer has a lower maximum internal temperature and a faster heat dissipation rate (Fig.9).
Conclusions The optical and thermal structures for the pulsed high-power quantum cascade laser which incorporates a novel tapered waveguide design are modeled and simulated by using COMSOL optical finite element simulation software. The impact of various geometric parameters on the mode distribution and transmission characteristics of waveguides is analyzed. In conclusion, at a wavelength of 4.6 μm, the ridge waveguide width is 5 μm, the taper Angle is 1.9°, and the ridge/taper length ratio is 1:3, which can ensure that the ridge waveguide will produce TM fundamental mode output, and the transmission, reflectivity, loss, and light limiting factor of the entire port can achieve the best effect. In addition, the structure is subjected to thermal simulation analysis under various pulse modes, which provides a useful reference for the selection of device operating mode and packaging mode. The relevant conclusions of this study can provide data support for the subsequent process design and experimental verification.
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