Fig. 1. （a）The band conductance diagram and the modulus square of the electron wave function of the optimized three-quantum-well module structure under the designed working voltage，（b）L-I-V curves of metal-metal waveguides made of gold and copper operating at different temperatures respectively. The upper illustration shows the change of threshold current density of the two waveguides as a function of temperature，and the lower illustration shows the spectrum of the copper waveguide at 8 K and 199.5 K. The pictures are quoted from Ref.［23］，（c）the subband wave function structure diagram of the double quantum well module structure，（d）the L-I curve of the device at different temperatures，and the figure shows the threshold current density at different temperatures. The pictures are quoted from Ref.［24］

Fig. 2. （a）Upper：principle of Third-order DFB；middle：k space wave vector diagram of shallow-etched grating，which cannot meet the emission conditions；lower：k space wave vector diagram of deep etching grating，which can be emitted when the conditions are met. The picture is quoted from Ref.［29］，（b）the schematic diagram of directional third-order DFB laser. The upper left is the photo of the device，the upper right is the SEM image of a single device，and the lower part is the unit structural parameters. The picture is quoted from Ref.［30］，（c）the schematic diagram of third-order DFB laser with π -phase locking. The device photo is on the upper left，and the design drawing of a ten-pair laser mold and SEM photo of the actual device are on the upper right. The SEM images of three units of a pair of π -phase locked lasers are shown in the lower left，and the SEM images of the adjacent positions of two laser antennas are shown in the lower right. The picture is quoted from Ref.［31］，（d）DFB laser separated by sinusoidal feedback grating and radiation hole array. The above is the design of separation structure，and the following is the SEM photo of the actual device. The picture is quoted from Ref.［32］

Fig. 3. （a）The upper shows the grating periodic structure decreasing to both sides in the GPH laser，while the bottom figure shows the corresponding photon band diagram in real space. A potential well has been formed. The pictures are quoted from Ref.［33］，（b）the difference between grating period gradient GPH（top）and filling rate gradient GPH（bottom），（c）the photon band diagram of grating period gradient GPH，（d）the change of the intrinsic frequency of the two boundary states with the change of filling rate，（e）the change of the corresponding photon band diagram. The pictures are quoted from Ref.［34］

Fig. 4. （a）The schematic diagram of the Terahertz MOPA device，（b）the SEM photo of the actual device，（c）the cavity laser mode and emission field distribution obtained by simulation. The pictures are quoted from Ref.［40］，（d）the schematic diagram of the optimized Terahertz MOPA，（e）the L-I-V curve of the laser at different temperatures，（f）the far-field distribution image of the laser. The pictures are quoted from Ref.［42］

Fig. 5. （a）The SEM photo of the second-order and fourth-order hybrid DFB lasers，（b）the L-I-V curves of the lasers at different temperatures. The illustration shows the spectrum of the lasers at different current densities at 62 K. The picture is quoted from Ref.［44］，（c）the SEM photo of QCL cavities array，（d）the L-I-V curve of the array at different temperatures，and the illustration is the device spectrum from the threshold current density to the peak current density. The picture is quoted from Ref.［45］

Fig. 6. （a）The schematic diagram of the "fishbone" grating. The picture is quoted from Ref.［50］，（b）The upper describes the cross section of laser polarization output using a "fishbone" grating array，and the lower is the SEM photo of the manufactured device. The picture is quoted from Ref.［51］，（c）the schematic diagram and active region supersurface structure diagram for quantum cascade vertical external-cavity surface-emitting laser. The picture is quoted from Ref.［52］，（d）the schematic diagram of the monolithic dynamic tunable polarization semiconductor laser and the polarization state of the emitted light under different pumping conditions. The picture is quoted from Ref.［53］，（e）the "fishbone" grating MOPA structure diagram，（f）and（g）one-way and orthogonal grating device SEM photos respectively，（h）the far-field distribution of orthogonal grating device，（I）the far field distribution at A point in（h）in the center（in blue）and the theory of actual measurement（red）power relationship with polaroid Angle. The picture is quoted from Ref.［54］

Fig. 7. （a）The external cavity structure of the second-order DFB laser and the metal mirror is shown on the left，and the intrinsic frequencies of the DFB in-cavity mode（red），the outer cavity mode（green）and the composite system（blue）vary as a function of the distance between the mirror and the cavity. The picture is quoted from Ref.［55］，（b）the schematic diagram of the sinusoidal feedback grating laser and the plunger structure，which is placed on the straight side of the waveguide. The following figure shows the laser frequency tuning range achieved by adjusting the distance between the cavity and plunger. The picture is quoted from Ref.［56］，（c）the schematic diagram of the structure and SEM photo of the metasurface structure of a frequency-tunable quantum cascade vertical external-cavity surface-emitting laser；the following figure shows the output frequencies of devices with different external cavity lengths LEC，showing the tuning range of the entire device. The picture is quoted from Ref.［57］