We propose a method to generate the high-order optical vortex in a few-mode fiber via cascaded acoustically driven vector mode conversion. Theoretical analysis showed that the vector mode conversion induced by the acoustically induced fiber grating (AIFG) could occur between two HE modes with adjacent azimuthal numbers. In the experiment conducted at 532 nm, two AIFGs were simultaneously induced in the same segment of the fiber by a radio frequency source containing two different frequency components. One AIFG was used to convert the left- and right-handed circular polarization fundamental modes to the $\pm 1$-order vortex modes, which were then further converted to the $\pm 2$-order vortex modes by the other AIFG. The topological charges of the vortex modes were verified using both coaxial and off-axial interference methods, showing typical signature patterns of spiral forms and forklike fringes, respectively.

We presented a method to actualize the optical vortex generation with wavelength tunability via an acoustically-induced fiber grating (AIFG) driven by a radio frequency source. The circular polarization fundamental mode could be converted to the first-order optical vortex through the AIFG, and its topological charges were verified by the spiral pattern of coaxial interference between the first-order optical vortex and a Gaussian-reference beam. A spectral tuning range from 1540 nm to 1560 nm was demonstrated with a wavelength tunability slope of 4.65 nm/kHz. The mode conversion efficiency was 95% within the whole tuning spectral range.

Theoretical analysis and experimental demonstration are presented for the generation of cylindrical vector beams (CVBs) via mode conversion in fiber from HE₁₁ mode to TM₀₁ and TE₀₁ modes, which have radial and azimuthal polarizations, respectively. Intermodal coupling is caused by an acoustic flexural wave applied on the fiber, whereas polarization control is necessary for the mode conversion, i.e. ${\text{HE}}_{11}^{\text{x}}\to {\text{TM}}_{0\text{1}}$ and ${\text{HE}}_{11}^{\text{y}}\to {\text{TE}}_{0\text{1}}$ for acoustic vibration along the x-axis. The frequency of the RF driving signal for actuating the acoustic wave is determined by the phase matching condition that the period of acoustic wave equals the beatlength of two coupled modes. With phase matching condition tunability, this approach can be used to generate different types of CVBs at the same wavelength over a broadband. Experimental demonstration was done in the visible and communication bands.

Cylindrical-vector beams (CVBs) with axial symmetry in polarization and field intensity are gathering increasing attention from fundamental research to practical applications. However, a majority of the CVBs are generated by modulating light beams in free space, and the temporal durations are far away from the ultrafast regime. Here, an ultrafast all-fiber based CVB laser is demonstrated via intermodal coupling in two mode fibers. In the temporal domain, chirp-free pulses are formed with combined actions of the ultrafast saturable absorption, self-phase modulation, and anomalous dispersion. In the spatial domain, the lateral offset splicing technique and a two mode fiber Bragg grating are adopted to excite and extract CVBs, respectively. The ultrafast CVB has an annular profile with a duration of 6.87 ps and a fundamental repetition rate of 13.16 MHz, and the output polarization status is switchable between radially and azimuthally polarized states. This all-fiber-based ultrafast CVB laser is a simple, low-cost source for diversified applications of nanoparticle manipulation, high-resolution imaging, material processing, spatiotemporal nonlinear optics, etc.

No existing model has completely explained the physics of the optical nonlinearity for epsilon-near-zero materials, which have recently emerged as candidates for design and fabrication of active nanophotonic devices. We propose to extend the Drude model by adopting a weighted electron effective mass, which takes into account the electron distribution in the nonparabolic conduction band. The extended Drude model is able to interpret the cause of the nonlinearity and provide a functional relationship between the refractive index and wavelength. Both the band nonparabolicity and the temperature-dependent mobility are responsible for the intraband-transition-induced optical nonlinearity. The spectrally resolved nonlinear refractive index and nonlinear susceptibilities are obtained from this model. The results can be applied to other transparent conducting oxides and will help in the design and modeling of spectral responses of nonlinear plasmonic devices.

Mode coupling from the fundamental vector mode ($H{E}_{11}^{\text{x}}$) to the cylindrical vector beams (CVBs) and optical vortex beams (OVBs) of a few-mode fiber excited by two acoustic flexural waves with orthogonal perturbations is achieved by using a composite acoustic transducer. The $H{E}_{11}^{\text{x}}$ mode is converted to TM₀₁ and TE₀₁ modes, which have radial and azimuthal polarizations, by using the lowest-order acoustic flexural modes of $F_{11}^{\text{x}}$ and $F_{11}^{\text{y}}$, respectively. Furthermore, $H{E}_{11}^{\text{x}}$ mode can also be converted to the ± 1-order OVBs of ${\text{HE}}_{21}^{even}\pm i{\text{HE}}_{21}^{odd}$ through the combined acoustic modes of ${\text{F}}_{11}^x\pm i{\text{F}}_{11}^y$. This technique provides a useful way of generating CVBs and OVBs in optical fiber with conveniently electrically-controlled mode conversion characteristics.