Step-index Fiber Research Articles

Open boundary conditions for the simulation of leaky modes

We propose an open-boundary method for the simulation of the modes of confining dielectric structures. The technique is inclusive of normal modes, but is especially advantageous for the simulation of quasi-normal, or leaky, modes. The central idea is to utilize the asymptotic form of targeted solutions to eliminate the outer part of the computational domain and bring the numerical boundary close to the simulated structure. While a similar approach was previously demonstrated for scalar quantum models, here we put forward a generalization for fully vectorial fields. Accuracy in this new context is validated using step-index and tube-type hollow core fiber geometries. The method has broad applicability, as quasi-bound modes are intrinsic to many systems of interest in optics and photonics.

Advanced fiber in-coupling through nanoprinted axially symmetric structures

Here, we introduce and demonstrate nanoprinted all-dielectric nanostructures located on fiber end faces as a novel concept for the efficient coupling of light into optical fibers, especially at multiple incidence angles and across large angular intervals. Taking advantage of the unique properties of the nanoprinting technology, such as flexibly varying the width, height, and gap distance of each individual element, we realize different polymeric axial-symmetric structures, such as double-pitch gratings and aperiodic arrays, placed on the facet of commercial step-index fibers. Of particular note is the aperiodic geometry, enabling an unprecedentedly high average coupling efficiency across the entire angular range up to 80°, outperforming regular gratings and especially bare fibers by orders of magnitude. The excellent agreement between simulation and experiment clearly demonstrates the quality of the fabricated structures and the high accuracy of the nanoprinting process. Our approach enables realizing highly integrated and ready-to-use fiber devices, defining a new class of compact, flexible, and practically relevant all-fiber devices beyond the state-of-art. Applications can be found in a variety of cutting-edge fields that require highly efficient light collection over selected angular intervals, such as endoscopy or quantum technologies. Furthermore, fiber functionalization through nanoprinting represents a promising approach for interfacing highly complex functional photonic structures with optical fibers.

Room temperature mid-infrared fibre photoluminescence and lasing beyond 5 μm in Ce3+ -doped chalcogenide glass – INVITED

We review here our recent work in achieving mid-infrared (MIR) fibre lasing beyond 5 µm wavelength in Ce3+-doped selenide-chalcogenide fibre, as well as the observed photoluminescence in samples of the same composition but in particulate and bulk glass form as well as unstructured fibre and in the SIF (step index fibre) in which fibre lasing took place.

Soft, stretchable optical fibers via thermal drawing

Optical fibers that can sustain large elastic deformations are promising building blocks in soft robotics, medical and wearable devices, and advanced textiles. Thus far, however, the fabrication methods developed for soft optical fibers have remained unmatured. Here, we present thermal drawing as a materials and processing platform to fabricate 10s of meters-long soft, multi-material optical fibers with intriguing architectures. It offers unprecedented opportunities to realize step-index soft optical fibers, as well as photonic crystal fibers for transmission, reflection, and sensing.

Full-beam spatial coherence measurements of supercontinuum generation in multimode fibers

We study experimentally the spatial coherence across the full spatial beam profile of supercontinuum light generated in both in graded-index and step-index multimode fibers. We observe a decrease of the coherence area with an increase of the injected pump power. Numerical simulations based on linearly polarized modes and mode coupling theory are in good agreement with our experiments and indicate that the decrease of coherence area is strongly related to a change in the modal amplitude distribution.

Multimodal imaging using tellurite highly nonlinear multimode fibers

Since the development of the first fibroscope in the late nineteenth century, the use of innovating glass fibers has opened promising perspectives for medical applications particularly in the domain of multimodal imagery[1]. In this work, tellurite glasses have been used to design step-index optical fibers with a rectangular core-section for supercontinuum generation in the near infrared domain. The tellurite glasses selection and the fibers manufacturing by the stack-and-draw process will be presented. Characterizations performed on bulk samples and fibers will also be detailed[2]. The spectral broadening, which is obtained in this work in a short fiber sample, is used for imaging mouse kidney cells, labelled with three different fluorochromes, by means of two and three-photon absorption. Despite the multimode nature of the output beam, clear images of tubules, actin and nucleus are collected with a spatial resolution down to 1.2 µm[3].

Speckle backpropagation for compensation of nonlinear effects in few-mode optical fibers

We propose an alternative approach to compensation of intermodal interactions in few-mode optical fibers by means of digital backpropagation. Instead of solving the inverse generalized multimode nonlinear Schrödinger equation, we accomplish backpropagation of the multimode signals with help of their near-field intensity distributions captured by a camera. We demonstrate that this task can successfully be handled by a deep neural network and provide a proof of concept by training an autoencoder for backpropagation of six linearly polarized (LP) modes of a step-index fiber.

Design of high capacity 5.76 Tbits/s SDM-PDM-Nyquist superchannel WDM hybrid multiplexing in 3.1% Germania doped MMF

To dispatch the goal about walking towards the 4th Industrial Revolution, one of the main key materials that require alterations and enhancements is data communication and transmission. To keep up with the augmented rise in demand for data, fiber-optics communication and networks commence a significant role in the transfer of data at high speeds. This article exemplifies the expediency analysis of 5.76 Tbits/s SDM-PDM-Nyquist superchannel WDM hybrid multiplexing technique over a multimodal transmission link up to 10 km using C-band carrier frequencies. This system is designed to carry 48 channels of data that can be produced using 8 C-band carrier frequencies, 2 polarization states, and 3 LP modes through 3.1% Germania doped over pure silica step-index multimode fiber. The system exhibits a satisfactory performance (log BER -9.35, faithful Q-factor 6.09, extinction ratios 7.78, minimum OSNR 46.5 dB) up to a distance of 10 km. Each channel receives a satisfactory amount of power after the dual-stage amplification process in the transmission medium with an ultra-high spectral efficiency of 137% and a high bandwidth-distance product of 385 MHz.km

Point radiance calculation of GI-MMF and its application in a fiber connection.

Graded-index multimode fiber (GI-MMF) is advantageous for low modal dispersion over its counterpart step-index multimode fiber, which renders it highly suitable for high-speed data transmission in short-range data links. To date, several theories and calculation methods have been proposed for MMF transmission and connection, most of which are based on geometric optics. Although the basic principle is extremely simple, the manipulation of the modal power distribution (MPD) variation along the transmission line that considerably affects the channel bandwidth still poses several challenges. Currently, the radiance of a point on the emitting fiber is assumed to evaluate the MPD at fiber connections, as its measurement or calculation method has not been determined yet. Thus, this paper proposes a method to numerically estimate the point radiance of GI-MMF using the near-field pattern (NFP) and far-field pattern (FFP) of the fiber. The method used data based on analytic functions representing NFP and FFP and yielded accurate estimations for the point radiance of GI-MMF; the accuracy was verified by comparing the fiber NFP and FFP calculated from the derived point radiance with the NFP and FFP analytical functions. In addition, the numerical aperture of the points on the fiber end-face obtained from the point radiance was in accordance with the theoretical value. Subsequently, we substituted the point radiance function of the GI-MMF into the matrix model that was established to compute the MPD conversion at fiber connectors under generic misalignments, including lateral, longitudinal, and angular offsets. Accordingly, the influence of misalignments on the MPD in GI-MMF connectors was assessed, and the performance of the fiber channel linked by GI-MMF was evaluated.

Seeing the strong suppression of higher order modes in single trench fiber using the S2 technique.

The single trench fiber (STF) is a promising fiber design for mode area scaling and higher order mode (HOM) suppression. In this Letter, we experimentally demonstrate the strong HOM-suppression in a homemade STF using the spatially and spectrally resolved imaging (S2) technique. This STF has a 20-µm core and its performance is compared to a conventional step-index fiber with almost the same parameter. Results show that the bending loss of the HOM in STF is 8-times larger than conventional fiber at a bend radius of 7 cm. In addition, when severe coupling mismatch is introduced at the input end of the fiber, the STF can keep the fundamental-mode output while the conventional fiber cannot. To the best of our knowledge, this is the first time to experimentally analyze the HOM content in an STF and compare its performance with that of a conventional fiber. Our results indicate the great potential of the STF for filtering the HOM in fiber laser applications.

Optical fiber polarization-entangled photon pair source using intermodal spontaneous four-wave mixing in the visible spectral band

The theoretical and experimental investigation results of the generation of the polarization-entangled photon pairs operating at visible wavelengths are reported. The generation of polarization-entangled photon pairs was based on intermodal spontaneous four-wave mixing (IM-SFWM) using standard step-index few-mode fiber. It was shown theoretically that several combinations of IM-SFWM processes could occur depending on spatial modes of a pump beam. A polarization-entangled photon pair source based on Sagnac loop incorporating a segment of few-mode fiber were then experimentally created. A two-photon interference fringe visibility of our implemented photon pair source were 91.7% and 88.8% in H/V bases and D/A bases, respectively. A quantum state tomography was also conducted to reconstruct the density matrix of the generated state with a fidelity to a maximum entangled state of 93.1%.

A simple but accurate technique for prediction of confinement and normalized group delay parameters for propagation of first higher order mode in graded index fiber

Abstract This paper deals with expressions for simple but precise analyses of fractional modal power (FMP) inside the core of optical fibers, the excitation efficiency and the normalized group delay (NGD) for the first higher order (LP11) mode in step and parabolic index fibers both with and without Kerr type non linearity. To get the analytical results, we have employed simple power series Chebyshev expansion for the LP11 mode of the above mentioned fibers. At first, the analytical expression for linear case is found out and then by applying the method of iteration the propagation parameters are estimated when there is nonlinearity of the Kerr type. Here, some typical step and parabolic profile fibers have been used for our investigation. Our findings of confinement and group delay parameters perfectly match with the precise numerical findings made by the intricate finite element method (FEM). This implies the precision of our formalism. The study of nonlinear optical transmission systems will benefit from the findings.

Incoherent light in tapered graded-index fibre: A study of transmission and modal noise

We investigated the impact of taper length on light transmission through tapered graded-index fibres. We tested commercial fibres from Thorlabs and a custom graded-index fibre using both coherent and incoherent light sources. Our experimental results show optimum performance for taper transition lengths of 25 mm, although our simulations suggest further improvement may be possible for even shorter transition lengths. We also measured the modal noise power fluctuations caused by bending the fibre. Here, we observe that the custom fibre tapers have the highest transmission but suffer from the most modal noise. Accordingly, we find that the commercial graded-index fibre tapers promise practical usage as a beam mode-field converter, as they have lower power fluctuations but retain relatively high transmission if compared to commercial small core step-index fibre.

Fused optical fiber combiner based on indium fluoride glass: perspectives for mid-IR applications.

For the first time, to the best of our knowledge, the design and characterization of a 3 × 1 fused fiber combiner based on multimode step-index fluoroindate optical fibers (InF3) has been performed. Several efforts to develop a well-consolidated normalization procedure and a fabrication protocol have been required due to the low melting temperature and the mechanical properties of fluoroindates. Fabrication results demonstrate repeatability and absence of crystallization. Therefore, the described fabrication process paves the way for manufacturing fluoroindate devices. The electromagnetic design of the combiner is carried out through modal investigation and beam propagation method by computing the transmission efficiency. The experimental results agree with the simulation and demonstrate the device feasibility to operate in the mid-infrared spectral range.

The Generation of 1.2 μJ Pulses From a Mamyshev Oscillator Based on a High Concentration, Large-Mode-Area Yb-Doped Fiber

We demonstrate a high-energy and high-average power Mamyshev oscillator based on a high performance large-mode-area step-index germanosilicate-cladding Yb-doped fiber. The oscillation of higher-order modes is effectively suppressed by employing appropriate intracavity spatial mode filtering, allowing significant scaling of single pulse energy. The laser directly generates a nearly diffraction-limited beam with an average power of 17 W operating at the fundamental repetition rate of 14.01 MHz, corresponding to a pulse energy of 1.2 μJ. The pulses could be externally dechirped to ∼58 fs, leading to a peak power of ∼13 MW. To the best of our knowledge, this is the highest pulse energy with sub-100 fs duration ever obtained directly from a mode-locked fiber oscillator.

Modal dispersion characteristics of LP0m modes in a step-index multimode fiber.

A modal interferometer method (MIM) is applied to measure the differential mode delay (DMD) between the L P 0m modes traversing a step-index multimode fiber (SI-MMF). Only linearly polarized radial modes, i.e.,L P 0m modes, are excited and transmitted in the SI-MMF by using a single-mode-multimode-single-mode (SMS) fiber structure. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. The Fourier-transform-based MIM provides simultaneous measurements of the DMD between the L P 0m modes. The wavelength dependence of the DMD is estimated experimentally in both the 1260-1360nm and 1450-1625nm telecommunication bands. The normalized frequency dependence of the DMD is also investigated theoretically. The result suggests that the 1260-1360nm band is preferable to the 1450-1625nm band for a mode-division multiplexing (MDM) transmission employing an SI-MMF in terms of realizing a smaller DMD.

ANALYZING THE OPTIMUM LOSS AND DISPERSION OF DIFFERENT TYPES OF OPTICAL FIBERS

In this paper we studied the dispersion and loss characteristics of several types of optical fiber through a simulation study. The single mode step index optical fiber is the best road for a minimum dispersion. Using single mode step index fiber with proper profile the minimum dispersion and minimum loss is obtained at 1550 nm wavelength. Thus data with extremely high bandwidth can be transmitted to an enormous distance through it without any repeater or amplifier with minimum loss and dispersion.

Radial distribution of pump and signal intensities in step index EDFA for LP11 mode in Kerr nonlinear condition

Abstract In an all-optical communication system, an erbium-doped fiber amplifier performs a very significant role. The effectiveness of the operation of this kind of amplifier depends on different parameters of the amplifier. Variation of the intensities of pump and signal with distance along the radius of the fiber from the core axis is one such significant parameter. In our present case, we have studied the distribution of the intensities of both the pump and signal along the radius of the fiber in an erbium-doped dual-mode fiber amplifier for the LP11 mode. In the present case, some step-index fibers of different normalized frequencies have opted. Our study is an application of the Chebyshev technique expressing the LP11 modal field in the form of a power series. A little computation is required for the prediction of the concerned results by this technique. Results obtained from this study show an excellent match with those found by the rigorous finite element method establishing its accuracy. This study using such a user-friendly and accurate technique will be helpful to the optical engineers involved in this domain.

Mitigating Crosstalk of Vortex Beam Within an OAM-Mode Group Over an OAM Fiber by Reversing Transmission Matrix

For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC.

Spatially variant deblur and image enhancement in a single multimode fiber imaged by deep learning.

A single multimode fiber has been applied in minimally invasive endoscopy with wavefront shaping for biological research such as brain imaging. Most of the fibers, such as step-index and graded-index multimode fibers, give rise to spatially variant blur due to limits on the numerical aperture and collection efficiency. Routines to solve this problem are based on iterative algorithms, which are often slow and computer-intense. We developed a method to synthesize datasets for driving a deep learning network to deblur and denoise the spatially variant degraded image. This approach is fast (5 ms), up to three orders of magnitude faster than the iterative way. Furthermore, our method can be applied to different types of fiber endoscopy, and two types of fiber are tested here. The performance is verified on fluorescence beads and three kinds of biological tissue sections in the experiment, demonstrating effectiveness in image enhancement.