All-fiber few-mode multicore photonic lantern mode multiplexer

Mode conversion between fibers with different refraction index distribution based on adiabatically tapered structures

Different fibers generally have different mode characteristics so their connections in many practical applications often require mode conversion. The feasibility of mode conversion between fibers with different refractive index distributions based on adiabatically tapered structures is theoretically analyzed. The first kind of mode conversion is between ring core fiber and convex core fiber; the second kind is between multicore fiber and single-core fiber. Three common tapered structures are investigated: tapered core, diffused core, and tapered cladding. The analysis results show that mode conversion by a tapered structure is not suitable for all the modes for a ring core fiber and a convex core fiber; however, it can be accomplished for multicore fiber and single-core fiber.

Programmable high-order mode control method based on acoustically induced fiber grating

We demonstrate a programmable high-order mode control method that can be implemented in high-power fiber lasers. 2 W average-power mode-locked pulses are obtained based on a mode-locked fiber laser working in dissipative soliton resonance regime. The fundamental mode (LP₀₁) is fully or partially converted to the high-order modes (LP_11a/b) via an acoustically-induced fiber grating. The mode-superposition fields are recorded using an optical 4f system, and mode components are subsequently analyzed by a mode decomposition algorithm. Our experiments suggest that the mode patterns are stable and dynamically switchable. The method is expected to possess good application value in optical tweezers, fiber communication, laser material processing and other research fields.

Prediction of the structure and refractive index profile of fused fiber-optic components: A numerical and experimental study

We report a generic theoretical framework for accurate simulation of the temporal and spatial evolution of fused fiber-optic components, fabricated by the "heat and pull" technique. The methodology is based on the solution of quasi-3D incompressible Navier-Stokes equations formulated for immiscible two-phase flow. The two-phase interface is resolved by employing an interface tracking approach combined with the immersed boundary method. The model facilitates accurate spatiotemporal prediction of the evolution of both the external shape of the optical component and the internal dopant concentration during fabrication. Validation of the model was obtained by extensive comparison to experimental results. The model was found to be a convenient theoretical tool that may reliably facilitate the design and fabrication process of a wide spectrum of optic components.

Recent progress in all-fiber ultrafast high-order mode lasers

Ultrafast high-order mode (HOM) lasers are a relatively new class of ultrafast optics. They play a significant role in the fieldsof scientific research and industrial applications due to the high peak power and unique properties of spatial intensity and polarization distribution. Generation of ultrafast HOM beams in all-fiber systems has become an important research direction. In this paper, all-fiber mode conversion techniques, pulsed HOM laser strategies, and few-mode/multi-mode fiber (FMF/MMF) lasers are reviewed. The main motivation of this review is to highlight recent advances in the field of all-fiber ultrafast HOM lasers, for example, generating different HOM pulses based on fiber mode converters and mode-locking in the FMF/MMF lasers. These results suggest that mode selective coupler can be used as a broad bandwidth mode converter with fast response and HOM can be directly oscillated in the FMF/MMF laser cavity with high stability. In addition, spatiotemporal mode-locking in the FMF/MMF is also involved. It is believed that the development of all-fiber ultrafast HOM lasers will continue to deepen, thus laying a good foundation for future applications.

Overcoming degradation in spatial multiplexing systems with stochastic nonlinear impairments

Single-mode optical fibres now underpin telecommunication systems and have allowed continuous increases in traffic volume and bandwidth demand whilst simultaneously reducing cost- and energy-per-bit over the last 40 years. However, it is now recognised that such systems are rapidly approaching the limits imposed by the nonlinear Kerr effect. To address this, recent research has been carried out into mitigating Kerr nonlinearities to increase the nonlinear threshold and into spatial multiplexing to offer additional spatial pathways. However, given the complexity associated with nonlinear transmission in spatial multiplexed systems subject to random inter-spatial-path nonlinearities it is widely believed that these technologies are mutually exclusive. By investigating the linear and nonlinear crosstalk in few-mode fibres based optical communications, we numerically demonstrate, for the first time, that even in the presence of significant random mixing of signals, substantial performance benefits are possible. To achieve this, the impact of linear mixing on the Kerr nonlinearities should be taken into account using different compensation strategies for different linear mixing regimes. For the optical communication systems studied, we demonstrate that the performance may be more than doubled with the appropriate selection of compensation method for fibre characteristics which match those presented in the literature.

Photonic lantern broadband orbital angular momentum mode multiplexer

Optical vortex beams that carry orbital angular momentum (OAM), also known as OAM modes, have attracted considerable interest in recent years as they can comprise an additional degree of freedom for a variety of advanced classical and quantum optical applications. While canonical methods of OAM mode generation are effective, a method that can simultaneously generate and multiplex OAM modes with low loss and over broad spectral range is still in great demand. Here, via novel design of an optical fiber device referred to as a photonic lantern, where the radial mode index ("m") is neglected, for the first time we demonstrate the simultaneous generation and multiplexing of OAM modes with low loss and over the broadest spectral range to date (550 nm). We further confirm the potential of this approach to preserve the quality of studied OAM modes by fusion splicing the end-facet of the fabricated device to a delivery ring-core fiber (RCF).

Scaling photonic lanterns for space-division multiplexing

We present a new technique allowing the fabrication of large modal count photonic lanterns for space-division multiplexing applications. We demonstrate mode-selective photonic lanterns supporting 10 and 15 spatial channels by using graded-index fibres and microstructured templates. These templates are a versatile approach to position the graded-index fibres in the required geometry for efficient mode sampling and conversion. Thus, providing an effective scalable method for large number of spatial modes in a repeatable manner. Further, we demonstrate the efficiency and functionality of our photonic lanterns for optical communications. Our results show low insertion and mode dependent losses, as well as enhanced mode selectivity when spliced to few mode transmission fibres. These photonic lantern mode multiplexers are an enabling technology for future ultra-high capacity optical transmission systems.

Tunable arbitrary unitary transformer based on multiple sections of multicore fibers with phase control

In this paper, we propose a novel tunable unitary transformer, which can achieve arbitrary discrete unitary transforms. The unitary transformer is composed of multiple sections of multi-core fibers with closely aligned coupled cores. Phase shifters are inserted before and after the sections to control the phases of the waves in the cores. A simple algorithm is proposed to find the optimal phase setup for the phase shifters to realize the desired unitary transforms. The proposed device is fiber based and is particularly suitable for the mode division multiplexing systems. A tunable mode MUX/DEMUX for a three-mode fiber is designed based on the proposed structure.