Fig. 1. Three-dimensional device structure diagram of coupled ridge dual-wavelength DFB QCL.

Fig. 2. (a) Electroluminescence spectrum of the materials. ΔE1 and ΔE2 represent the gain spectra from the upper energy level to the two lower energy levels, respectively. (b) Coupling coefficient varies with the grating duty cycle and grating depth for the two wavelengths. The upper insets show the calculated modal profiles of λ1 emission with σ=0.5 (left) and λ2 emission with σ=0.6 (right), respectively. (c) Schematic diagram of bi-period grating formed by different photolithographic stripes. (d) The propagation waves correspond to the gratings of the same color in Fig. 1(c).

Fig. 3. Dependences of supermode loss and loss difference on (a) ridge period and (b) array etching depth at λ1=7.5  μm. Fundamental supermode near-field profiles of the seven-element coupled ridge waveguide structure at (c) λ1=7.05  μm and (d) λ1=7.5  μm calculated using COMSOL finite element method software.

Fig. 4. (a) Cross-sectional photographs of the actual coupling ridge portions for parameter sets 1 (left) and 2 (right). (b) Cross-sectional photograph of actual device mounted epi-side down onto the graphite heat-sink.

Fig. 5. (a) L-I-V curves at Tsink from 10°C to 40°C for a 2-mm-long QCL with ridge width WS1=7.5  μm. The inset shows Jth as a function of T in pulsed mode. The dashed line is the fitting result obtained using the exponential function Jth=J0 exp(T/T0). (b) L-I-V curves at Tsink from 20°C to 40°C for a 2-mm-long QCL with ridge width WS2=8.9  μm.

Fig. 6. Far-field profile along the ridge width direction at room temperature for a device with ridge period P1=9.5  μm, ridge width WS1=7.5  μm, and depth D1=2.3  μm. (a) Before λ2 lasing. (b) Lasing at dual wavelengths. Data fittings performed using the Gaussian function are represented by solid lines.

Fig. 7. (a) Optical spectra measured by varying the current at room temperature (20°C). (b) Emission spectra of the device during operation when T was varied from −4°C to 60°C in increments of 4°C (measured at 1.05×Ith). The insets show the linearly fitted tuning characteristics of lasing frequency versus injection temperature. (c) The upper part, spectrum of the tuning range combined with temperature and current tuning. The insets are single-mode spectra of the λ1 and λ2 emissions at 20°C with SMSR of over 20 dB. The lower part, the absorption lines of gas molecules within the tuning range.