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We propose and numerically demonstrate a conversion and multicasting scheme of orbital angular momentum (OAM) states by using N-core supermode fiber (NCSF), where the topological charges of converted OAM states mainly depend on the injected OAM state and the number of fiber cores. The conversion efficiency (CE) of the converted OAM states could be optimized by properly designing the fiber structure. Take N = 6 as an example, ~37% CE could be achieved at telecom bands. Moreover, even for a fabricated NCSF, the CE could be dynamically changed by stretching the fiber or by adjusting the refractive index of the fiber cores through external control of the environmental conditions. Meanwhile, OAM multicasting could also be realized in the designed NCSF. The crosstalk between the multicasted OAM channels and their neighboring ones are assessed to be less than −30 dB. The proposed fiber-based OAM conversion and multicasting system is compatible with the existing optical fiber communication systems, showing potential applications in the future.

We observe the symmetrical dual-valley spectrum in periodically poled lithium niobate (PPLN) with a defect in the middle employing electro-optic effect. When the external electric fields are applied, light satisfying phase-matching condition (λ 0 ) will undergo polarization rotation and then turn back. However, the polarization states of wavelength at λ 0 ±Δλ will be changed at the end of the sample due to phase mismatching. A dual-valley transmission spectrum is thus obtained. Moreover, the depth of the valley could be tailored with different electric fields on the y-surface of the sample.

A tunable dual-wavelength filter is experimentally demonstrated in domain-engineered lithium niobate. Application of an electric field on the y-surfaces of the sample results in the optical axes rotating clockwise and anticlockwise, which makes selective polarization rotation. The quasi phase-matching wavelengths could be adjusted through suitable domain design. A unique dual valley spectrum is obtained in a periodically poled lithium niobate structure with a central defect if the sample is placed between two parallel polarizers. The expected bandwidth could be varied from ∼1 nm to ∼40 nm. Moreover, both the spectral response and group delay dispersion could be engineered.

The refractive indices of ordinary and extraordinary waves (no and ne) of congruent LiNbO3 (CLN) in the terahertz (THz) band are measured at different temperatures. Extended Cauchy equations are proposed to describe the temperature- and frequency-dependent refractive indices. With less than 0.5% deviation, both no and ne can be obtained accurately. The values of no and ne are ~7 and ~5.4, respectively, which are much larger than those in the visible band. The extremely high birefringence (~1.6) may lead to some interesting applications of linear, electro-optic, and nonlinear optical effects of CLN. As an example, true zero-order THz waveplates (WPs) could be obtained with considerable low loss based on thin CLN wafers. The working frequency can be tuned by changing the environmental temperature. Moreover, wide bandwidth achromatic THz WPs were also achievable by stacking four CLN wafers together with precise orientation control.