Excitation of cladding modes in photonic crystal fibers by flexural acoustic waves

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Generation of Orbital Angular Momentum Modes Using Fiber Systems

Orbital angular momentum (OAM) beams, characterized by the helical phase wavefront, have received significant interest in various areas of study. There are many methods to generate OAM beams, which can be roughly divided into two types: spatial methods and fiber methods. As a natural shaper of OAM beams, the fibers exhibit unique merits, namely, miniaturization and a low insertion loss. In this paper, we review the recent advances in fiber OAM mode generation systems, in both the interior and exterior of the beams. We introduce the basic concepts of fiber modes and the generation and detection theories of OAM modes. In addition, fiber systems based on different nuclear devices are introduced, including the long-period fiber grating, the mode-selective coupler, microstructural optical fiber, and the photonic lantern. Finally, the key challenges and prospects for fiber OAM mode systems are discussed.

Revised model of acousto-optic interaction in optical fibers endowed with a flexural wave

We propose a novel model of the acousto-optic interaction in circular fibers endowed with the lowest-order flexural acoustic wave that is based on the actual distribution of the acoustically induced displacement vector. The corresponding expression for the fiber's permittivity is derived and compared with the commonly used one. The resonance optical fiber modes and the propagation constants are found. It is predicted that the lengths of the well-known acoustically driven mode conversion LP_0,n→LP_1,n^' should be slightly different for the x- and y-polarized incident fundamental modes. Moreover, we unveil a new polarization-dependent mode conversion in which the azimuthal mode number ℓ, as well as the optical frequency of the generated standard fiber mode LP_ℓ,n^' , is governed by the linear polarization direction of the incident zero-order beam LP_0,n.

Acousto-optic interaction in polyimide coated optical fibers with flexural waves

Acousto-optic coupling in polyimide-coated single-mode optical fibers using flexural elastic waves is demonstrated. The effect of the polyimide coating on the acousto-optic interaction process is analyzed in detailed. Theoretical and experimental results are in good agreement. Although the elastic attenuation is significant, we show that acousto-optic coupling can be produced with a reasonably good efficiency. To our knowledge, it is the first experimental demonstration of acousto-optic coupling in optical fibers with robust protective coating.

Accurate mode characterization of two-mode optical fibers by in-fiber acousto-optics

Acousto-optic interaction in optical fibers is exploited for the accurate and broadband characterization of two-mode optical fibers. Coupling between LP₀₁ and LP_1m modes is produced in a broadband wavelength range. Difference in effective indices, group indices, and chromatic dispersions between the guided modes, are obtained from experimental measurements. Additionally, we show that the technique is suitable to investigate the fine modes structure of LP modes, and some other intriguing features related with modes' cut-off.

All-fiber polarization-dependent optical vortex beams generation via flexural acoustic wave

We report on a novel type of optical mode conversion in fiber acousto-optics. The all-fiber narrowband complete transformation of the fundamental mode into the frequency downshifted optical vortex beam of topological charge +1 or -1 via a lowest-order flexural acoustic wave is theoretically demonstrated. Moreover, such a process is found to be polarization dependent: both the topological charge and polarization state of the produced optical vortex are governed by the circular polarization handedness of the input mode. The possible applications of the established conversion for optical vortex manipulation are discussed.

Long-period gratings inscribed in photonic crystal fiber by symmetric CO2 laser irradiation

Long-period gratings (LPGs) inscribed in endlessly single mode (ESM) photonic crystal fibers (PCFs) with symmetric and asymmetric CO2 laser irradiation are investigated both numerically and experimentally. Parallel results from conventional single mode fibers (SMFs) are presented for comparison. Theoretical predictions, transmission measurements, and near-field imaging indicate that, regardless of the fiber type, symmetric index perturbation induced by laser irradiation with the aid of a 120° gold-coated reflecting mirror results in LP(0n) symmetric mode coupling, while asymmetric irradiation without using the mirror leads to LP(1n) asymmetric mode coupling. Our results show that, because of the azimuthally anisotropic hexagonal cladding structure, symmetric irradiation yields far more reproducible LPGs in PCFs than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPGs inscribed in SMFs due to the isotropy of its all-solid cladding structure.

Influence of acoustic waves on supercontinuum generation in photonic crystal fibers

In this work, the influence of applying acoustic frequencies to photonic crystal fibers (PCF) on the supercontinuum (SC) generation is presented. We will show numerically the strong influence of coupling modes in PCF in the latest stage of SC and simulations of different scenarios of phase math condition between the modes.

Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers

A novel light-controllable long-period fiber grating (LPFG) is demonstrated by making use of a PCF infiltrated with a photoresponsive liquid crystal (LC) mixture consisting of nematic LC molecules and light-sensitive 4-methoxyazobenzene (4MAB). With the aid of the photo-induced isomerization of 4MAB, the refractive index of the LC mixture can be modulated and the periodic index perturbation along the fiber can be achieved by exposing the PCF to a blue laser through a mask. The resonance wavelength and dip depth of the LPFG can be controlled by using different blue-laser irradiation time, numbers of period, and 4MAB concentrations. In addition, the photo-induced LPFG is erasable under green-laser illumination.

Experimental excitation and characterization of cladding modes in photonic crystal fiber

We extend the previous theoretical study on the effect of outer silica cladding to the analysis on real field profiles in a practical PCF. Clear field profiles for the first higher-order cladding modes are presented and discussed. The observed mode fields are reproduced by numerical calculations, and it turns out that they correspond to LP(16) mode groups. Optical properties of the observed modes such as lobe direction and polarization are also investigated. The results of this study will be useful in the design of the PCF-based optical devices utilizing cladding mode coupling.

Core-cladding mode coupling and recoupling in photonic crystal fiber for enhanced overlap of evanescent field using long-period gratings

Excitation of cladding modes has been achieved using long-period (LPGs) inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO(2) laser irradiation. Core-cladding mode coupling and recoupling has resulted in marked improvement in the evanescent field overlap throughout the cladding air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization of the PCF-LPG configuration can lead to a field power overlap as high as 22% with a confinement loss of less than 1 dB/m in the cladding mode.

Tunable gratings in a hollow-core photonic bandgap fiber based on acousto-optic interaction

We demonstrate a coherent intermodal coupling in a hollow-core photonic bandgap fibers (PBGF) using an acoustic grating. A theoretical study on the properties of acoustic and optical wave propagating the PBGF is performed indicating that the applied frequency higher than 10 MHz is required to obtain the acousto-optic interaction at the wavelength around 1550 nm. Wavelength-tunable coupling from fundamental mode to higher-order core modes is achieved with more than 90% coupling efficiency at the applied frequency of 11.54 MHz. The low dispersive properties and the mode filed distributions of the higher-order guided modes of the air-core PBGF in the experiment show reasonable agreement with the simulation results.

Tunable fiber gratings fabricated in photonic crystal fiber by use of mechanical pressure

The mode-coupling properties of tunable long-period fiber gratings (LPGs) formed in photonic crystal fibers (PCFs) are presented. The mode coupling from the fundamental core mode to a cladding mode of a PCF is obtained by use of periodic mechanical pressure. The strength and the wavelength of the resonant peak are tuned by adjusting the grating period and the pressure applied on the PCF. Contrary to the conventional fiber case, the resonant wavelength of the PCF LPG is decreased by increasing the periodicity.

Measurement of modal dispersion in optical fiber by means of acousto-optic coupling

We show that frequency-wavelength tuning characteristics of acousto-optic coupling can be used for measuring the difference of effective index, group index, and chromatic dispersion between core and cladding modes in single-mode fibers. Chromatic dispersion measurements of a 30-cm-long conventional single-mode fiber, a nonzero dispersion-shifted fiber, and a dispersion-compensating fiber with this new method are presented for the wavelength range 1500-1600 nm. Qualitative agreement with independently measured data is obtained.

Structural long-period gratings in photonic crystal fibers

We report what is believed to be the first example of structural long-period gratings written in pure silica photonic crystal fibers (PCFs). The gratings are realized by periodic collapse of the holes of the PCF by heat treatment with a CO(2) laser. The resulting periodic hole-size perturbation produces core-to-cladding-mode conversion. These results can lead to a new family of structural all-fiber devices that use the unique properties of PCFs.