Fig. 1. Schematic conduction band diagram of one module of the four-quantum-wells active region under an applied electric field of 9.9  kV·cm−1.

Fig. 2. (a) Schematic diagram of the graded sampled grating DFB QCL. (b) Calculated mode loss versus wavelength for the QCL with graded sampled grating. (c) The envelope distribution of mode intensities (|Ey|2) inside lasers with three different grating structures. (d) The top view images of the DFB QCL structure and one-period sampled grating taken with a 3D stereomicroscope and one-period sampled grating.

Fig. 3. (a) Light peak output power-current-voltage (L-I-V) characteristics of the FP lasers of various dimensions at 8 K. (b) Temperature-dependent L-I-V characteristics in pulsed mode of the FP laser with dimensions 0.25×5  mm2. (c) Temperature-dependent L-I-V characteristics in CW mode of the FP QCL with dimensions 0.15×2  mm2. (d) Emission spectra of the device shown in (c) at various injection current densities.

Fig. 4. (a) CW L-I-V curves of DFB QCL with the dimensions 0.15×2  mm2 at various heat sink temperatures. (b) The corresponding emission spectra at various heat sink temperatures for an injection current of 0.95 A.

Fig. 5. (a) Output powers of coated and uncoated devices with uniform grating and device with graded sampled grating. (b) The optical powers and ratio between the front and the rear facets of 12 graded sampled DFB QCLs and five uniform grating DFB QCLs with the dimension of 0.15×2  mm2.

Table 1. Key Computed Parameters of the Active Region at a Temperature of 100 K^a