High-order mode conversion in a few-mode fiber via laser-inscribed long-period gratings at 1.55 µm and 2 µm wavebands

Spatial mode conversion of single photons at the C-band using in fiber long-period gratings

The ability to convert the spatial mode of single photons opens up a promising path to enhancing quantum communication protocols by enabling high-dimensional encoding and efficient multiplexing. In this work, we demonstrate spatial mode conversion of single photons at 1550.6 nm using a fiber long-period grating (LPG). The fundamental [Formula: see text] mode was converted to higher-order modes [Formula: see text] and [Formula: see text], with quantum mode conversion efficiencies of 87.5 ± 1.4% and 96.1 ± 1.6%, respectively. The characterization of the converted single photons was carried out using a time-of-flight technique and coincidence measurements, by taking advantage of the differences in group velocity between the modes. We also performed loss measurements at the single-photon level and demonstrated mode re-conversion by using a second LPG to restore the photons back to the fundamental mode. These results highlight the potential of LPGs as a versatile tool for spatial mode manipulation at the single-photon level, with applications in high-dimensional quantum communication and nonlinear optical interactions.

High-efficiency broadband third-order OAM mode converter based on a multi-period preset-twist long-period fiber grating

All-fiber mode converters for generating orbital angular momentum (OAM) beams have many applications in optical communications, optical sensing and lasers. Currently, it is a great challenge to use a long-period fiber grating (LPFG) to broadband excite high-order OAM modes above the second-order. Here, we demonstrate a preset-twist LPFG fabrication method, which introduces asymmetry in the refractive index modulation area, for efficient generation of third-order modes. Through optimization, the generation of third-order OAM modes with 99.55% conversion efficiency, 0.81 dB insertion loss, and over 99% purity is achieved with only 40 pitch number. In addition, a multi-period preset-twist LPFG is proposed and demonstrated to achieve the excitation of broadband third-order mode with conversion efficiency of more than 99%, insertion loss of less than 1 dB, and mode purity of more than 90%. The 15 dB bandwidth (96.8% conversion efficiency) of the LPFG is 109 nm in the wavelength range from 1475 nm to 1584 nm, and the 20 dB bandwidth (99% conversion efficiency) of the LPFG is 92 nm from 1488 nm to 1580 nm. To the best of our knowledge, this is the first time to generate efficient and broadband third-order mode using a long-period fiber grating.

Femtosecond laser inscribed parallel long-period fiber gratings for multi-channel core mode conversion

We propose and demonstrate the inscription of parallel long-period gratings (LPGs) in a few-mode fiber (FMF) using femtosecond lasers. Mode conversion from the fundamental (LP₀₁) mode to high-order core modes, including LP₁₁, LP₂₁, LP₃₁, LP₀₂, and LP₁₂, is achieved by controlling the inscription period of the gratings. Taking advantage of the highly focused femtosecond laser, LPGs with different off-axis offsets were fabricated, and the resonance wavelength and the inscription efficiency of the gratings versus the offset were investigated. Based on the off-axis writing technique and using the femtosecond laser source, we wrote parallel LPGs that contain multi-gratings in a single FMF and achieved a multi-channel core mode converter in a single FMF with flexibility in terms of the resonant wavelength and mode conversion among different modes. This approach offers a new, to the best of our knowledge, option for implementation with high integration, and a multi-channel mode converter, which could find potential applications in FMF multi-wavelength laser systems, and wavelength/mode division multiplex communication systems. Furthermore, these microstructured LPGs integrated into an optical fiber can be used as a multifunctional sensor.

Singularities splitting phenomenon for the superposition of hybrid orders structured lights and the corresponding interference discrimination method

It is the basic characteristic of pure vortex light that there is a phase singularity at the origin. Such a singularity may be multiple degenerate, which determines the order of vortex light. Singularities splitting phenomenon means that singularities no longer concentrate at the origin but distribute around the space, usually occurring in impure vortex light. In this paper, we demonstrate the singularities splitting phenomenon and propose an analysis method, based on which one may rapidly estimate the modal components of impure vortex light. As two common singularity discrimination methods, the spiral and fork wire interference patterns are compared in distinguishing splitting singularities. The most widely used spiral interference pattern is revealed to be the worst form because of the low resolution. Instead, the fork wire interference pattern is with higher and easily adjusted resolution. 1‰ impurity is still able to be distinguished through fork wire interference patterns in the experiment.

Wavelength-Tunable, Ultra-Broadband, Biconical, Long-Period Fiber Grating Mode Converter Based on the Dual-Resonance Effect

We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO₂ laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP₀₁ to LP₁₁, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength.