Modulation instability (MI) of cw states of a two-core fiber, incorporating the effects of coupling-coefficient dispersion (CCD), is studied by solving a pair of generalized, linearly coupled nonlinear Schrödinger equations. CCD refers to the property that the coupling coefficient depends on the optical wavelength, and earlier studies of MI do not account for this physics. CCD does not seriously affect the symmetric/antisymmetric cw, but can drastically modify the MI of the asymmetric state. Generally, new MI frequency bands are produced, and CCD reduces (enhances) the original MI band in the anomalous (normal) dispersion regime. Another remarkable result is the existence of a critical value for the CCD, where the MI gain spectrum undergoes an abrupt change. In the anomalous dispersion regime, a new low-frequency MI band is generated. In the normal dispersion regime, an MI band vanishes, reappears, and then moves up in frequency on crossing this critical value. In both dispersion regimes, the relative magnitude of the low-frequency band and the high-frequency band depends strongly on the total input power. It is possible to switch the dominant MI frequency between a low frequency and a high frequency by tuning the total input power, providing a promising scheme to manipulate MI-related nonlinear effects in two-core fibers. The MI bands are independent of the third-order dispersion, but can be shifted significantly by self-steepening at a sufficiently high total input power. The evolution of MI from a cw input is also demonstrated with a wave propagation study.

Three-dimensional (3D) glass chips are promising waveguide platforms for building hybrid 3D photonic circuits due to their 3D topological capabilities, large transparent windows, and low coupling dispersion. At present, the key challenge in scaling down a benchtop optical system to a glass chip is the lack of precise methods for controlling the mode field and optical coupling of 3D waveguide circuits. Here, we propose an overlap-controlled multi-scan (OCMS) method based on laser-direct lithography that allows customizing the refractive index profile of 3D waveguides with high spatial precision in a variety of glasses. On the basis of this method, we achieve variable mode-field distribution, robust and broadband coupling, and thereby demonstrate dispersionless LP₂₁-mode conversion of supercontinuum pulses with the largest deviation of <0.1 dB in coupling ratios on 210 nm broadband. This approach provides a route to achieve ultra-broadband and low-dispersion coupling in 3D photonic circuits, with overwhelming advantages over conventional planar waveguide-optic platforms for on-chip transmission and manipulation of ultrashort laser pulses and broadband supercontinuum.

We report a mode (de)multiplexer based on the structure of cascaded vertical waveguide directional couplers, where the spatial modes of a few-mode core are coupled to various single-mode cores placed above the few-core core. Using five cascaded directional couplers, we design and fabricate a six-mode (de)multiplexer with polymer material that can spatially combine or separate the E$_{11}$, E$_{21}$, E$_{12}$, E$_{22}$, E$_{31}$, and E$_{13}$ modes. A typical fabricated device, which has a length of 29 mm, provides coupling ratios varying from 62% to 90% and modal crosstalks from μ28.2 to μ11.6 dB in the wavelength range from 1530 to 1565 nm with weak polarization dependence. The proposed structure is simple and scalable and could be developed into a range of mode-controlling devices for mode-division-multiplexing applications based on few-mode fibers.

We propose a mode-conversion platform based on cascaded long-period waveguide gratings. We demonstrate two specific devices by cascading a surface-corrugated grating and a sidewall-corrugated grating. One device allows conversion among the ${\mathrm{LP}}_{01}$, ${\mathrm{LP}}_{11b}$, and ${\mathrm{LP}}_{11a}$ modes in a cyclic manner. The maximum conversion efficiency of a typical experimental device fabricated with polymer materials is about 94% at 1540 nm. The other device allows conversion between the ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{21a}$ modes. The maximum conversion efficiency of a typical experimental device is estimated to be higher than 90% at 1550 nm. The performance of both devices is polarization-insensitive. The proposed platform could be developed into various mode converters for mode-division-multiplexing systems and other mode-sensitive applications.

We compare three optical models of graphene, namely, the interface model, the isotropic model, and the anisotropic model, and verify them experimentally with two multimode slab waveguide samples operating at the wavelengths of 632.8 and 1536 nm. By comparing the calculated graphene-induced losses and the measurement data, we confirm that the interface model and the anisotropic model give correct results for both the transverse electric (TE) and transverse magnetic modes, while the isotropic model gives correct results only for the TE modes. With the experimental data, we also quantitatively verify the widely used expression for the surface conductivity of graphene in the optical regime. Our findings clarify the issue of modeling graphene in the analysis of graphene-incorporated waveguides and offer deeper insight into the optical properties of graphene for waveguide applications.

A robust graphene-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a pair of 3-dB long-period fiber gratings is proposed and demonstrated for ammonia (NH₃) gas sensing. The sensor operates on the principle of changing the phase of the cladding mode of a fiber through changing the conductivity of the graphene coating by adsorbed NH₃ molecules, which gives rise to a shift in the interference spectrum with an amount that depends on the gas concentration. Our experimental sensor shows a sensitivity of ~3 pm/ppm for a gas concentration from ~10 to ~180 ppm. The proposed in-fiber MZI can serve as a generic platform for the development of fiber sensors and devices that incorporate two-dimensional materials.