OAM beam generation in space and its applications: A review

In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc.

Advances on Mode-Coupling Theories, Fabrication Techniques, and Applications of the Helical Long-Period Fiber Gratings: A Review

We study photopolymerization with high-order Bessel light beams with phase singularities on-axis. Self-trapping and self-focusing of propagation-invariant light beams in a photopolymer allow the fabrication of extended helical microfibers with a length scale of a centimeter, which is more than an order of magnitude larger than the propagation distance of the Bessel light beams. We show the evolution of microfibers rotating at a rate proportional to the incident optical power, while the periodicity of the helical structures remains constant, irrespective of the laser power. This suggests that optical momentum transfer plays a predominant role in the growth and rotation of such fiber structures.

Photopolymerization with high-order Bessel light beams.

Abstract 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.

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

A class of ultra-short chiral long period fiber gratings (CLPFGs) are prepared by writing a spiral curve on the surface of a six-mode fiber. The CLPFGs are applied to excite ±2nd- and ±3rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the CLPFG in these modes can be as high as 99%, when the length is only 0.5cm. The polarization characteristic of the excited higher-order OAM modes in CLPFGs was theoretically analyzed and experimentally investigated. Results show that the obtained ±2nd- and ±3rd-order OAM modes are polarization independent, as expected.

Excitation of high order orbital angular momentum modes in ultra-short chiral long period fiber gratings.

An all-fiber orbital angular momentum (OAM) mode generator enabling simultaneous generation of the second- and the third-order OAM modes with conversion efficiencies larger than 95% has been proposed and experimentally demonstrated, which is realized by using a high-order helical long-period fiber grating (HLPG) written in a thinned four-mode fiber. This is the first time, to the best of our knowledge, that two such OAM modes have been simultaneously obtained at wavelengths ranging from 1450 to 1620 nm by using only one fiber component, i.e., the HLPG. The proposed method provides a new way to simultaneously generate different orders of the OAM modes, which would further expand the OAM's applications to the fields of the optical tweezers, microscopy, and fiber communication, etc.

Simultaneous generation of the second- and third-order OAM modes by using a high-order helical long-period fiber grating.

Instead of inscribing the discrete phases into each local period of the seed HLPG, an equivalent approach to realize the phase-only sampled multichannel helical long-period fiber grating (HLPG) has been demonstrated both theoretically and experimentally which is realized by superposing the so-called sampling HLPGs with the seed HLPG in a same region of the fiber. The proposed approach can greatly facilitate the fabrication processes of the multichannel HLPG by using the direct CO2 laser writing technique, meanwhile it enables to provide much more design and fabrication flexibilities for high channel-count HLPG than the previous methods. As examples, a 3-channel and a 5-channel HLPG have been successfully demonstrated. Moreover, the orbital angular momentum (OAM) modes inherently existed at all channels of the fabricated HLPG have also been verified, which highlights that the proposed HLPGs could find application to multi-wavelength OAM mode converter as well.

The Superimposed Multi-Channel Helical Long-Period Fiber Grating and Its Application to Multi-Channel OAM Mode Generator

A simple method enabling the validation of the mode-selection rules obeyed in a single-helix helical long-period fiber grating (SHLPG) has been demonstrated both theoretically and experimentally, which is realized by investigating and analyzing the circular dichroism (CD) and the polarization dependence loss (PDL) spectra of the utilized SHLPG. It is confirmed for the first time, to the best of our knowledge, that in terms of the SHLPG’s helicity, the core mode ${{\rm HE}_{\textbf{11}}}$HE11 with either the left circular polarization or the right circular polarization will be selectively coupled into the higher HE or the TE/TM mode, respectively. As a result, the SHLPG would exhibit an enhanced CD, as well as an enhanced PDL near the resonant wavelengths of the HE and TE/TM modes, respectively. The results presented in this Letter will be helpful for efficiently designing and producing SHLPG-based circular polarizers and orbital-angular-momentum generators.

Taishu Detani

Papers

Simultaneous generation of the second- and third-order OAM modes by using a high-order helical long-period fiber grating.

The Superimposed Multi-Channel Helical Long-Period Fiber Grating and Its Application to Multi-Channel OAM Mode Generator

Demonstration of the mode-selection rules obeyed in a single-helix helical long-period fiber grating.