Step-index Multimode Optical Fiber Research Articles

Power over fiber using a multimode optical power with a core diameter of 50 µm

We report on the properties of the Power over Fiber (PoF) transmission link using a High-Power Laser Source operating at 976 nm and using three types of optical fiber with a core diameter of 50 µm. Two step-index profile multimode optical fibers and one fiber with a gradient index were used for optical power transmission. Optical light was converted to electricity using commercially available Photovoltaic Power Convertors (PPCs) with a maximal optical input power of 1.5 W and experimental PPCs with a maximal optical input power of 4.0 W. We experimentally proved optical power transmission up to a distance of 300 m. In the case of the commercially available working PPC and using the gradient index fiber we achieved a result of 0.534 W of electrical power and using the experimental PPC we achieved 0.645 W. In the case of the step-index optical fiber, the result was 1.3 W.

Investigation of mode coupling in strained and unstrained multimode step-index POFs using the Langevin equation

The Langevin equation (LE) is used to evaluate mode coupling in multimode step-index polymer optical fiber (SI POF) that is both unstrained and strained. The numerical solution of the LE matches the numerical solution of the power flow equation (PFE). Strain-induced mode coupling is noticeably stronger in strained fiber than in unstrained fiber of the same types. Therefore, compared to similar lengths for unstrained fibers, the coupling length of the equilibrium mode distribution (EMD) is attained and the length of fiber required to produce a steady-state distribution (SSD) are both much shorter for strained fibers. We have demonstrated that the mode coupling in strained and unstrained multimode SI POFs that comes from the random perturbations (RPs) of the fiber can be successfully treated by the LE. The study's findings can be used to improve communication and sensory systems that use multimode SI POFs under different bending circumstances. Additionally, it is crucial to be able to compute the modal distribution of the SI POFs used in the optical fiber sensory system at a specific length and under various bending scenarios.

Displacement Sensing of an Active String Actuator Using a Step-Index Multimode Optical Fiber Sensor.

A thin McKibben artificial muscle is a pneumatic actuator with an outer diameter of only 1.8 mm. We fabricated a string-shaped actuator called an “active string actuator,” which achieves a high contractile displacement by accumulating thin McKibben artificial muscles. To control the displacement, the length of the active string actuator should be estimated. However, this is difficult because bulky and rigid sensors are unsuitable for the sensor element of the active string actuator. Therefore, in this study, we propose a new sensing method for estimating the length of an active string actuator. The proposed sensing system is simple and comprises only three components: a step-index multimode optical fiber, a light emitter, and a light receiver. A step-index multimode optical fiber was combined with the active string actuator, and the length was estimated from the change in the amount of light propagating in the optical fiber when the active string actuator was driven. Fundamental experiments were conducted in this study, and the results demonstrated that the optical fiber sensor value changed with the actuator length. This suggests that it is possible to estimate the displacement of an active string actuator using an optical fiber sensor.

Design of Step-Index Multimode Optical Fiber

In this paper, a step-index fiber with core index 1.445 5 1 7 and cladding index 1.443 1 5 7 has been designed and studied. Multimode operation is achieved by using a fiber with core radius 25 μm operating at a wavelength of 1.3 μm. The mode parameters (effective refractive index, phase constant, fractional modal power in the core and cutoff wavelength) were calculated using RP fiber calculator (PRO version 2020). The shapes of the intensity and amplitude distribution of linearly polarized guided modes were shown.

Observation of strong mode coupling in multimode step index silica optical fibers with fluorinated thermoplastic polymer cladding

Using the power flow equation, the state of mode coupling in step index multimode silica optical fiber with fluorinated thermoplastic polymer cladding is investigated in this work. Coupling coefficient is determined by employing our previously reported calculation method. The silica optical fiber with fluorinated thermoplastic polymer cladding shows much stronger mode coupling than the all plastic-clad silica fibers investigated in earlier works. Consequently, it takes much shorter fiber length for the equilibrium mode distribution or steady state distribution to develop (faster launch beam narrowing), which is of great interest in employing silica optical fiber with fluorinated thermoplastic polymer as a part of telecommunication, medical and sensory system.

Investigation of the characteristics of a fiber-optic gas-liquid two-phase flow sensor.

A laboratory-prepared wedge-shaped fiber probe using step-index multimode plastic optical fiber was described and tested in a lab-scale gas-liquid flow generator. A three-dimensional model was established in order to fully simulate the process of bubble piercing by the optical fiber probe. A theoretical analysis of the luminous intensity distribution of the light transmission in the process of bubble piercing was undertaken under conditions of different relative positions between the fiber probe and the bubble axis. Using this analytical method, it was possible to accurately define the range of the central region of the bubble where the presignal appeared.

Deconvolution for multimode fiber imaging: modeling of spatially variant PSF.

Focusing light through a step-index multimode optical fiber (MMF) using wavefront control enables minimally-invasive endoscopy of biological tissue. The point spread function (PSF) of such an imaging system is spatially variant, and this variation limits compensation for blurring using most deconvolution algorithms as they require a uniform PSF. However, modeling the spatially variant PSF into a series of spatially invariant PSFs re-opens the possibility of deconvolution. To achieve this we developed svmPSF: an open-source Java-based framework compatible with ImageJ. The approach takes a series of point response measurements across the field-of-view (FOV) and applies principal component analysis to the measurements' co-variance matrix to generate a PSF model. By combining the svmPSF output with a modified Richardson-Lucy deconvolution algorithm, we were able to deblur and regularize fluorescence images of beads and live neurons acquired with a MMF, and thus effectively increasing the FOV.

Low-Attenuation Variable Mode Control Using Twist Processing for Step-Index Optical Fiber Loops

We propose a method of variable mode control that generates an equilibrium modal distribution (EMD) in step-index multimode fibers. The proposed method can control the modal power distribution variably from a low-order mode distribution to a high-order mode distribution (including EMD) by twisting fiber loops. The attenuation of the controller for EMD generation is five times smaller than those of conventional EMD generation methods. As a reference light source with high measurement reproducibility, the EMD light source obtained by the variable mode control method can be used to evaluate hard polymer cladding fiber link systems. The proposed method is applicable to all types of step-index multimode optical fibers, including all-glass fibers, hard polymer cladding fibers, and plastic optical fibers.

Focusing light in biological tissue through a multimode optical fiber: refractive index matching.

Controlling light propagation through a step-index multimode optical fiber (MMF) has several important applications, including biological imaging. However, little consideration has been given to the coupling of fiber and tissue optics. In this Letter, we characterized the effects of tissue-induced light distortions, in particular those arising from a mismatch in the refractive index of the pre-imaging calibration and biological media. By performing the calibration in a medium matching the refractive index of the brain, optimal focusing ability was achieved, as well as a gain in focus uniformity within the field-of-view. These changes in illumination resulted in a 30% improvement in spatial resolution and intensity in fluorescence images of beads and live brain tissue. Beyond refractive index matching, our results demonstrate that sample-induced aberrations can severely deteriorate images from MMF-based systems.

New method for calculating the coupling coefficient in graded index optical fibers

A simple method is proposed for determining the mode coupling coefficient D in graded index multimode optical fibers. It only requires observation of the output modal power distribution P(m, z) for one fiber length z as the Gaussian launching modal power distribution changes, with the Gaussian input light distribution centered along the graded index optical fiber axis (θ0=0) without radial offset (r0 = 0). A similar method we previously proposed for calculating the coupling coefficient D in a step-index multimode optical fibers where the output angular power distributions P(θ, z) for one fiber length z with the Gaussian input light distribution launched centrally along the step-index optical fiber axis (θ0=0) is needed to be known.

Theoretical modeling of a coupled plasmon waveguide resonance sensor based on multimode optical fiber

A coupled plasmon waveguide resonance (CPWR) sensor based on metal/dielectric-coated step index multimode optical fiber is proposed. Theoretical simulations using the four-layer Fresnel equations based on a bi-dimensional optical fiber model were implemented on four structures: Ag–ZnO, Au–ZnO, Ag–TiO2 and Au–TiO2. By controlling the thickness of dielectric layer, we managed to manipulate the CPWR resonance wavelengths. When a CPWR resonance dip is in the short wavelength region, it is insensitive to the change of surrounding refractive index (SRI) and can be used as a reference to improve the sensing accuracy of surface plasmon resonance (SPR) mode. With the increase of the thickness of the dielectric layer, the CPWR resonance dips shift to longer wavelength and the corresponding sensitivities increase. When the 1st CPWR resonance wavelength is near 1550 nm and SRI is around 1.333, the sensitivities of four structures reach 1360.61 nm/RIU, 1375.76 nm/RIU, 1048.48 nm/RIU and 1015.15 nm/RIU, respectively. The values are close to that of the conventional SPR optical fiber sensor while the spectral bandwidths of the optical fiber CPWR sensors are narrower.

Rayleigh backscattering from the fundamental mode in step-index multimode optical fibers.

The Rayleigh backscattering produced by an incident fundamental mode in a multimode step-index optical fiber was analyzed using a recently developed diffraction technique. The complete set of backward propagating modes, both radial and azimuthal, was determined and regarded. For this type of fiber, normalized mode functions were constructed in an explicit form, thus providing a unified power scale to characterize the relationships between various excited modes. The dependencies of the mode excitation efficiencies on the technical parameters of the fiber and the frequency of the launched radiation were studied. A comparison of the mode excitation efficiencies was performed with those in the fiber with quadratic refractive index profiles.

Study of Wavelength-Dependent Bend Loss in Step-Index Multimode Fiber-Optic Microbend Displacement Sensor

In this article, the wavelength dependence of bend loss in a step-index multimode optical fiber (100 µm core diameter; fused silica) was investigated for fiber bend radii ranging between 2.0 and 4.5 mm using six excitation wavelengths, namely, 337.1, 470, 590, 632.8, 750 and 810 nm. The results obtained from fitting the bend loss measurements to Kao's model and utilizing MATLAB indicate that bend loss is wavelength dependent and transmission loss in multimode optical fibers increases with the decrease in the fiber bend radius. Furthermore, the response of a microbend fiber optic displacement sensor was characterized at 337.1, 470, 632.8, 750 and 810 nm. Measurements obtained from the microbend sensor indicate that the sensor output power is linear with the applied displacement and the sensor output is wavelength dependent.

Результаты экспериментальных исследований маломодовых режимов волоконных брэгговских решеток на многомодовых световодах

Nowadays fiber Bragg gratings (FBGs) are widely used in various devices and applications of fiber optics. They become a basic component for fiber optic sensor networks. The most of those both commercial and prototype fiber optic sensor systems are based on well-researched either singlemode or multimode regime of signal transmission over optical fibers. However some works are concerned with researches of FBGs written on silica multimode optical fibers (MMF) for various photonics and fiber optics applications. Thus, FBGs written on graded-index MMFs are used in optical filters of fiber optic data transmission systems 10G designed for local area networks and radio-over-fiber systems covering micro- and pico-cells. Some works are focused on FBGs written on step-index MMFs. Slanted or tilted multimode FBGs are also widely used in vibration, temperature, deformation etc. fiber optic sensors. Another group of publications is devoted to conventional FBGs on silica telecommunication graded-index MMFs 50/125 as well as 62.5/125 and experimental approbation of various fiber optic sensors based on them. However, the most part of mentioned papers are focused on just analysis of FBG spectral response during laser-excited optical signal propagation that corresponds to a few-mode regime. Few-mode regime adds a new another one dimension to parameter space that is associated with guided modes of particular order which limited number (from two to a few dozens) transfer the most part of optical signal power over tested optical fiber. Optical fiber operating in a few-mode regime of data transmission is interferometer that in combination with FBG can be considered as an alternative basis for design of elements with new unique properties. This work is concerned with experimental researches of few-mode regimes of FBG operating during transferring of laser based optical signals. We tested FBGs written on silica graded-index multimode fibers 50/125 Cat. OM2+/OM3 with Bragg wavelength 1550 nm by using them in a set of developed experimental schemes based on excitation of multimode fibers by corresponding laser sources. The researches were also focused on analysis of both spectral and pulse responses under changing of selected mode mixing and power diffusion processes due to tension and/or stress local and distributed action to FBG or sensor fiber. Results of spectral and pulse response measurements at the output of schemes with installed described FBGs are represented.

Encircled Angular Flux Representation of the Modal Power Distribution and Its Behavior in a Step Index Multimode Fiber

The optical properties of inline optical components inserted at the input to a multimode optical fiber (MMF) strongly affect the propagating modal power distribution (MPD). To realize stable systems with predictable reproducible performance and to encourage widespread use of MMF systems, improvements to the system design process and to the characterization process need to be made and new interface standards need to be defined. To this end, we have developed a new reproducible MPD measurement and representation together with its theoretical definition. By modifying the encircled flux (EF) representation, which is based on the near-field pattern of a graded-index multimode optical fiber (GI-MMF), we define the encircled angular flux (EAF) for step-index multimode optical fibers (SI-MMFs) based on their far-field patterns (FFPs). Using a SI-MMF, which is used for low cost short distance interconnects, as an example, we show the changes in the MPD along the fiber, reveal an unusual insertion loss-increasing phenomenon due to the evanescent tails of the core modes extending into the cladding, and characterize the equilibrium mode distribution (EMD) in the fiber. The EAF representation enables these phenomena to be quantified. We also propose an EAF template that consists of the target EMD and its tolerance. If device system designers use the EAF template to set the launch conditions, they can perform a fair assessment of the components, and they can design the system performance even if some components are replaced with others from a different supplier manufactured by a different method. We call this concept “Total MPD Management.”

Skew ray tracing in a step-index optical fiber using geometric algebra

We used geometric algebra to compute the paths of skew rays in a cylindrical, step-index multimode optical fiber. To do this, we used the vector addition form for the law of propagation, the exponential of an imaginary vector form for the law of refraction, and the juxtaposed vector product form for the law of reflection. In particular, the exponential forms of the vector rotations enables us to take advantage of the addition or subtraction of exponential arguments of two rotated vectors in the derivation of the ray tracing invariants in cylindrical and spherical coordinates. We showed that the light rays inside the optical fiber trace a polygonal helical path characterized by three invariants that relate successive reflections inside the fiber: the ray path distance, the difference in axial distances, and the difference in the azimuthal angles. We also rederived the known generalized formula for the numerical aperture for skew rays, which simplifies to the standard form for meridional rays.

Spatial coherence at the output of multimode optical fibers.

The modulus of the complex degree of coherence is directly measured at the output of a step-index multimode optical fiber using lateral-sheering, delay-dithering Mach-Zehnder interferometer. Pumping the multimode fiber with monochromatic light always results in spatially-coherent output, whereas for the broadband pumping the modal dispersion of the fiber leads to a partially coherent output. While the coherence radius is a function of the numerical aperture only, the residual coherence outside the main peak is an interesting function of two dimensionless parameters: the number of non-degenerate modes and the ratio of the modal dispersion to the coherence time of the source. We develop a simple model describing this residual coherence and verify its predictions experimentally.

Bent induced refractive index profile variation and mode field distribution of step-index multimode optical fiber

The effect of bending of step-index optical fiber on its refractive index profile and the mode field distribution were investigated. An enhanced slab model is suggested in this investigation. A qualitative study has been done on a bent step-index optical fiber. A very small radius of bending curvature (R) has been reached, practically R is 9.25mm. In this case a dramatic change of the refractive index profile has been observed with an induced birefringence. The refractive index profile is recovered from the interferograms which were generated by Mach–Zehnder interferometer. The interferogram has been analyzed using advanced image analyses software. We have proposed another approach to calculate the refractive index profile of bent optical fiber. In this approach the fiber is divided into layers and slabs, simultaneously. The induced refractive index profile variation of the bent optical fiber, for parallel and perpendicular components of the light beam, is calculated considering the refraction of the light beam traversing the fiber. The mode field distribution and mode numbers in these two directions of polarizations are determined for both straight and bent fibers.

Mode couplings and elasto-optic effects study in a proposed mechanical microperturbed multimode optical fiber sensor

A step index multimode optical fiber with a perturbation on a micrometer scale, inducing a periodic deformation of the fiber section along its propagation axis, is theoretically investigated. The studied microperturbation is mechanically achieved using two microstructured jaws squeezing the straight fiber. As opposed to optical fiber microbend sensors, the optical axis of the proposed transducer is not bended; only the optical fiber section is deformed. Further, the strain applied on the fiber produces a periodical elliptical modification of the core and a modulation of the index of refraction. As a consequence of the micrometer scale perturbation period, the resulting mode coupling occurs directly between guided and radiated modes. To simulate the transmission induced by these kinds of perturbations, simplified models considering only total mode couplings are often used. In order to investigate the range of validity of this approximation, results are compared to the electromagnetic mode couplings rigorously computed for the first time, to our knowledge, with a large multimode fiber (more than 6000 linear polarized modes) using the Marcuse model. In addition, in order to have a more complete modeling of the proposed transducer, the anisotropic elasto-optic effects in the stressed multimode fiber are considered. In this way, the transmission of the microperturbed optical fiber transmission and, therefore, the behavior of the transducer are physically explained and its applications as a future stretching sensor are discussed.

Mode coupling in chalcogenide-glass optical fibers

Using the time-independent power flow equation, we have examined the mode coupling caused by intrinsic perturbation effects in step-index As2Se3 chalcogenide-glass multimode optical fiber in the mid-infrared region. Results show that the coupling length where the equilibrium mode distribution is achieved depends on the wavelength and is of the order of kilometers.