We experimentally demonstrate for the first time, to the best of our knowledge, an all-fiber passively mode-locked laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for saturable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode fiber. Using the nonlinear multimode interference structure with 48.8 mm length step-index multimode fiber, the modulation depth has been measured to be $\sim 5\%$. The passively mode-locked laser output pulses have a central wavelength of 1596.66 nm, bandwidth of 2.18 nm, pulsewidth of $\sim 625\mathrm{fs}$, and fundamental repetition rate of 8.726 MHz. Furthermore, the influence of total cavity dispersion on the optical spectrum, pulse width, and output power is investigated systematically by adding different lengths of single-mode fiber and dispersion compensation fiber in the laser cavity.

A 2.5nm-thickness molybdenum diselenide (MoSe₂) saturable absorber (SA) is prepared by electron beam evaporation (EBE) method. Applying the prepared MoSe₂ SA to an acousto-optic (AO) Q-switched fundamental laser, a dual-loss-modulated intra-cavity optical parametric oscillator (IOPO) has been experimentally realized. The signal-pulse train from this IOPO has 0.0053 standard deviation (SD) of pulse amplitude. When the MoSe₂ SA is applied to IOPO, the signal pulse is compressed by maximum 68%, the peak power increases by 274%, and the nonlinear conversion increases by 12.6%. To solve the established rate equation of IOPO, the ground-state and excited-state absorption cross section of MoSe₂ are rationally estimated to be 1.04×10⁻¹⁸cm⁻² and 6.25×10⁻¹⁹cm⁻² from the measured transmittance curve, and the excited-state lifetime is 275.6µs. The numerical solution of the equations fits the experimental data well.

Graphene-based surface plasmon waveguides (SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation (DFG) under near-infrared pumps. The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast $\mathrm{AlGaAs}/{\mathrm{AlO}}_x$ structure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an $\mathrm{AlGaAs}/{\mathrm{AlO}}_x$ waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.

We prepared ZrGeTe₄ nanosheets and investigated their nonlinear optical properties. ZrGeTe₄ nanosheets as a saturable absorber were applied in an Er-doped fiber laser for conventional soliton and harmonic mode-locking pulses generation.

Traditional laser diodes operating at 650 nm are more prone to high-order mode excitation, resulting in poorer beam quality. In this paper, we designed GaInP–AlGaInP laser diodes (LD) with a 650 nm range and a trench mode-modulation structure based on the structure of edge-emitting laser (EEL) diodes. The effect of the three-trench structure was investigated theoretically and experimentally. The right trench structure laser chips demonstrated good beam quality while maintaining a high power output. An electro-optical conversion efficiency of 56% was demonstrated with a slope efficiency of 1.32 W/A at a 40 mA current. The maximum optical output power reached 40.8 mW.