A mode-locked all-fiber laser based on nonlinear multimode interference effect of the graded index fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step index multimode fiber connected by a graded index multimode fiber. Compared with the theoretically proposed SMF-GIMF-SMF structure, the introduction of the step index multimode fiber (SIMF) eliminates the restriction on the length of GIMF and provides a more flexible method of making a SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulse width of ~446 fs, bandwidth of 4.48 nm at the fundamental repetition rate of 11.73 MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in large mode area double clad fiber laser make it highly attractive in ultrafast photonics.

The design of highly efficient fused silica transmission gratings with deep-etched triangular-shaped grooves in the $-1$st order diffraction is realized by the rigorous coupled wave analysis (RCWA). The antireflective effect of a subwavelength triangular-groove grating with gradient effective refractive index results in the higher diffraction efficiency ($>99.9\%$). The performance of the presented gratings is clearly better than traditional rectangular and blazed ones. The gratings are designed under Littrow mounting at a wavelength of 1064 nm to be used in high-power laser systems. A detailed fabrication tolerance, covering not only the errors in height but also the errors in the lateral dimension, is demonstrated. The physical process of the diffraction characteristics for such a triangular-groove grating can be well explained by the simplified modal method based on two-beam interference of the first two propagating modes excited by the incident wave. Based on the fact that the transmittance derived from the modal method is in good agreement with that calculated by the RCWA, the simplified modal method can be effectively utilized as an easily designed tool of the triangular-shaped gratings.

In this study, we investigated an absorber based on a center-aligned tandem nanopillar array for ultra-broadband solar energy harvesting theoretically. A high-efficiency, omnidirectional absorber was obtained by introducing the center-aligned tandem nanopillar array embedded in an Al₂O₃ dielectric layer. The multi-coupling modes at different wavelengths were interpreted. The strong absorption can be adjusted by changing the radii and heights of nanopillars. According to the simulation results, the average absorptance of the absorber exceeded 94% in the wavelength range from 300 nm to 2000 nm. In addition, the high-efficiency absorption was insensitive to the incident angle and polarization state. The research not only proposed an absorber which possesses a huge potential value for application areas, such as thermal photovoltaic systems, infrared detection, and isotropic absorption sensors, but also pointed out a new way to design an absorber with high efficiency in an ultrabroad wavelength range.