Most dual-core optical fibers are based on the mode coupling effect between the two cores of dual-core optical fibers. Through the connection of single-mode fiber and dual-core fiber, high-performance compact all-fiber coupling filter, compact all-fiber Mach-Zehnder interferometer and compact all-fiber Michelson interferometer can be made. The following focuses on the principle and application design of coupling filter based on non-coaxial dual-core fiber and Mach-Zehnder interferometer, as well as the application of dual-core fiber in optical connectors, optical amplifiers, optical add-drop multiplexers, optical switches, optical tweezers and optical fiber sensors

Coupled filter based on axisymmetric dual-core fiber

Researchers propose a coupling filter based on axisymmetric dual-core fiber. The filter fuses the cores of two ordinary single-mode fibers with the center of a section of two axisymmetric non-coaxial dual-core fibers with the same length L, and then connects the two single-mode fibers with broadband light source and spectrum analyzer respectively. The comb spectrum of the filter can be observed through the spectrum analyzer. The filter based on dual-core fiber coupling type is simple in structure and easy to manufacture, but the coupling efficiency and insertion loss of the filter will be affected due to the mode field mismatch between the axially symmetric dual-core fiber and the single-mode fiber

Improved Coupled Filter Based on Axisymmetric Double-core Fiber

In order to reduce insertion loss, researchers proposed an improved coupling filter based on axisymmetric dual-core fiber. Through the method of on-line monitoring the output power of the filter, the filter fuses the cores of two ordinary single-mode optical fibers with one of the cores of two axially symmetric coaxial dual-core optical fibers with the same length of L through dislocation. The insertion loss of this filter is small, but it needs to accurately align one core of single-mode fiber and dual-core fiber, which takes a long time and still has some difficulties

Coupled filter based on axial offset dual-core fiber

In order to solve the above problems, a coupled filter based on axial offset dual-core fiber is proposed here. The filter fuses the cores of two single-mode optical fibers with the central core of two axially offset dual-core optical fibers with the same length of L, respectively. Due to the waveguide symmetry of the central core of the single-mode fiber and the axial offset dual-core fiber, this filter is conducive to the alignment of the connection point between the dual-core fiber and the single-mode fiber core for mode field matching, and the fabrication time is short, which can reduce the insertion loss between the single-mode fiber and the dual-core fiber, thus improving the performance of the entire filter

Mach-Zehnder interferometer based on axisymmetric dual-core fiber

The interferometer fuses the cores of two ordinary single-mode optical fibers and the center of an axially symmetric non-coaxial dual-core optical fiber with different refractive index (i.e., mismatched dual-core optical fiber) of two cores of length L. The difference of effective refractive index between the two cores of a dual-core fiber causes the phase difference of the optical signal when the two cores transmit, and the interference occurs when the two optical signals are coupled to the single-mode fiber. The interferometer is simple in structure and easy to manufacture, but the insertion loss at the fusion of single-mode fiber and dual-core fiber is large due to the mode field mismatch

In order to solve the problem of insertion loss in the above-mentioned interferometer, researchers have proposed an improved Mach-Zehnder interferometer based on axisymmetric double-core fiber. The interferometer performs fused taper at the two fusion positions of single-mode fiber and dual-core fiber to achieve a good match between the mode fields of single-mode fiber and dual-core fiber, but the precise position of fused taper is not easy to determine by this method

Modified Mach-Zehnder interferometer based on axial offset dual-core fiber

Due to the problem that the shape symmetry of the drawn biconical region fiber can not be guaranteed in practical operation, an improved Mach-Zehnder interferometer based on axial offset biconical fiber is proposed. The interferometer fuses the cores of two ordinary single-mode optical fibers with the central core of non-coaxial dual-core optical fibers with offset axes, and selects two suitable positions on the dual-core optical fibers for melting taper. This filter is conducive to the alignment of the connection point between the core of the dual-core fiber and the single-mode fiber, and the fabrication time is shorter, which can reduce the insertion loss between the single-mode fiber and the dual-core fiber; At the same time, it can well control the energy coupling between the two cores, thus improving the performance of the whole filter

Optical connector

The main purpose of optical connector is to realize the connection of optical fiber. Researchers propose a new type of connector based on dual-core fiber. The axially symmetric non-coaxial dual-core fiber (TCF) is connected to the asymmetric dual-core fiber (DPCF) whose phase velocities of the two pairs of cores are inconsistent, and finally connected to the two single-mode fibers through the remote DPCF and the dual-core fiber. This optical connector has good application prospects because of its low loss and low signal crosstalk

Fiber amplifier

Erbium-doped fiber amplifier based on dual-core fiber can automatically provide channel power balance, ignoring loss changes between amplifiers and transient cross-saturation of signals. The device mainly utilizes the spatial separation of channels and the power-dependent saturation characteristics of erbium ions. In the dual-core erbium-doped fiber equalization amplifier, both cores are Er3+doped. Researchers propose a two-core erbium-doped fiber amplifier with flat gain. This amplifier can achieve output power balance to a certain extent, and is suitable for the requirements of multi-channel multistage amplification wavelength division multiplexing optical fiber systems and networks in the future communication development

Optical add-drop multiplexer

The function of optical add-drop multiplexer is to upload and download signals, and is an important part of long-distance trunk network and metropolitan area network. Researchers propose a dual-core fiber based add-drop multiplexer. After writing a fiber grating of equal length in the two fiber cores, the signal light of different wavelengths is input from the A1 end. The light enters the grating section after coupling with each other in the non-grating area. The optical signal of specific wavelength is reflected by the Bragg grating and output from the B1 end, while the other signals continue to transmit forward, thus achieving the function of downloading the signal. Similarly, reverse direction input can realize the signal upload function

All-optical switch

Optical switch is an optical device that performs mutual logical operation or conversion of optical signals in integrated optical path or transmission line. Researchers have proposed an all-optical switch based on dual-core fiber. Its principle is to use the coupling characteristics of dual-core fiber