Finding the superior mode basis for mode-division multiplexing:a comparison of spatial modes in air-core fiber

Diverse spatial mode bases can be exploited in mode-division multiplexing(MDM)to sustain the capacity growth in fiber-optic communications,such as linearly polarized(LP)modes,vector modes,LP orbital angular momentum(LP-OAM)modes,and circularly polarized OAM(CP-OAM)modes.Nevertheless,which kind of mode bases is more appropriate to be utilized in fiber still remains unclear.Here,we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber(ACF).We first investigate the walk-off effect of four spatial mode bases over 1-km ACF,where LP and LP-OAM modes show intrinsic mode walk-off,while it is negligible for vector and CP-OAM modes.We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations,where degenerate even and odd vector modes suffer severe mode cross talk,while negligible for highorder degenerate CP-OAM modes based on the laws of angular momentum conservation.Moreover,we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations(bending,twisting,pressing,winding,and out-of-plane moving).The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing.Our findings may pave the way for robust shortreach MDM optical interconnects for data centers and high-performance computing.

Remote Measurement of the Angular Velocity Vector Based on Vectorial Doppler Effect Using Air-Core Optical Fiber

Rotational Doppler effect has made tremendous development in both theoretical and applied research over the last decade.Different from the inertial thinking of focusing on the scalar field dominated by helical phase light,we have revealed a vectorial Doppler effect in our previous work,which is based on the spatially variant polarized light fields to simultaneously acquire the speed and direction of a target.Here,further,we propose a method to construct a flexible and robust velocimeter based on that novel effect by employing an air-core fiber with kilometer-length scale for remotely measuring the vectorial information of angular velocity in situ.In addition,we experimentally substantiate that the measurement system still has commendable accuracy in determining the direction of movement even when the air-core fiber is interfered by the external environment.The demonstrations prove the potential of vectorial Doppler effect in practical scenarios and remote measurements.

Effect of Thermal Annealing on Mid-Infrared Transmission in Semiconductor Alloy-Core Glass-Cladded Fibers

We report a study investigating the effects of thermal annealing on the optical properties of Si-Ge alloy-core silica-cladded fibers.Low temperature fiber draw was performed with a laboratory-made draw tower at 1760°C that minimizes impurity diffusion from cladding to the core.As a post-drawing process,Si-Ge core fibers were annealed in a box furnace to alter the core structure.Microstructural and optical properties of fibers were investigated,and transmission losses were measured as 28 dB/cm at 6.1μm.Numerical studies were performed to analyze the experimental results and to find the optimum structure for low loss semiconductor-core glass-cladded fibers.

Dispersion Management with Higher Order Mode Fibers

Dispersion compensation with few-mode fibers is emerging as a promising technique that can provide full dispersion and dispersion-slope compensation. The inherent modal path diversity of these fibers allows implementation of static as well as tunable dispersion management schemes. In addition, the low non-linearity of this technology can improve system OSNR, leading to enhancements in transmission distances.

Intermodal group-velocity engineering for broadband nonlinear optics

Interest in the nonlinear properties of multi-mode optical waveguides has seen a recent resurgence on account of the large dimensionality afforded by the platform. The large volume of modes in these waveguides provides a new spatial degree of freedom for phase matching nonlinear optical processes. However, this spatial dimension is quantized, which narrows the conversion bandwidths of intermodal processes and constrains spectral and temporal tailoring of the light. Here we show that by engineering the relative group velocity within the spatial dimension, we can tailor the phase-matching bandwidth of intermodal parametric nonlinearities. We demonstrate group-velocity-tailored parametric nonlinear mixing between higher-order modes in a multi-mode fiber with gain bandwidths that are more than an order of magnitude larger than that previously thought possible for intermodal four-wave mixing. As evidence of the technological utility of this methodology, we seed this process to generate the first high-peak-power wavelength-tunable all-fiber quasi-CW laser in the Ti:sapphire wavelength regime.More generally, with the combination of intermodal interactions, which dramatically expand the phase-matching degrees of freedom for nonlinear optics, and intermodal group-velocity engineering, which enables tailoring of the bandwidth of such interactions, we showcase a platform for nonlinear optics that can be broadband while being wavelength agnostic.