Step-index Optical Fiber Research Articles

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.

Novel Measurement-Based Efficient Computational Approach to Modeling Optical Power Transmission in Step-Index Polymer Optical Fiber

Polymer optical fibers (POFs) are playing an important role in industrial applications nowadays due to their ease of handling and resilience to bending and environmental effects. A POF can tolerate a bending radius of less than 20 mm, it can work in environments with temperatures ranging from −55 °C to +105 °C, and its lifetime is around 20 years. In this paper, we propose a novel, rigorous, and efficient computational model to estimate the most important parameters that determine the characteristics of light propagation through a step-index polymer optical fiber (SI-POF). The model uses attenuation, diffusion, and mode group delay as functions of the propagation angle to characterize the optical power transmission in the SI-POF. Taking into consideration the mode group delay allows us to generalize the computational model to be applicable to POFs with different index profiles. In particular, we use experimental measurements of spatial distributions and frequency responses to derive accurate parameters for our SI-POF simulation model. The experimental data were measured at different fiber lengths according to the cut-back method. This method consists of taking several measurements such as frequency responses, angular intensity distributions, and optical power measurements over a long length of fiber (>100 m), then cutting back the fiber while maintaining the same launching conditions and repeating the measurements on the shorter lengths of fiber. The model derivation uses an objective function to minimize the differences between the experimental measurements and the simulated results. The use of the matrix exponential method (MEM) to implement the SI-POF model results in a computationally efficient model that is suitable for POF-based system-level studies. The efficiency gain is due to the independence of the calculation time with respect to the fiber length, in contrast to the classic analytical solutions of the time-dependent power flow equation. The robustness of the proposed model is validated by calculating the goodness-of-fit of the model predictions relative to experimental data.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses.

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.

Treatment of Mode Coupling in Step-Index Multimode Microstructured Polymer Optical Fibers by the Langevin Equation.

By solving the Langevin equation, mode coupling in a multimode step-index microstructured polymer optical fibers (SI mPOF) with a solid core was investigated. The numerical integration of the Langevin equation was based on the computer-simulated Langevin force. The numerical solution of the Langevin equation corresponded to the previously reported theoretical data. We demonstrated that by solving the Langevin equation (stochastic differential equation), one can successfully treat a mode coupling in multimode SI mPOF as a stochastic process, since it is caused by its intrinsic random perturbations. Thus, the Langevin equation allowed for a stochastic mathematical description of mode coupling in SI mPOF. Regarding the efficiency and execution speed, the Langevin equation was more favorable than the power flow equation. Such knowledge is useful for the use of multimode SI mPOFs for potential sensing and communication applications.

Probing the dispersive properties of optical fibers with an array of femtosecond-written fiber Bragg gratings

We propose an efficient method to determine the effective refractive index of step-index optical fibers from the visible to the mid-IR and thus allowing to infer their dispersive properties over a broad spectral range. The validity of the method, based on the writing of an array of fiber Bragg gratings (FBGs) with known periods using the fs scanning phase mask technique, is first confirmed with a standard silica fiber, then applied to various fluoride glass fibers to determine their effective refractive index and dispersion over more than three octaves, i.e. from 550 to 4800 nm.

Investigation of mode coupling in step-index air-clad silica optical fibers

This paper analyzes the state of mode coupling in a high numerical aperture (NA) step-index (SI) air-clad silica optical fiber using the power flow equation. The coupling coefficient of this equation was fine-tuned at first to allow it to correctly reconstruct previously reported experimental intensity distributions of output far-field (FF) patterns. At a length of 2100 m, the equilibrium mode distribution in the investigated fiber was obtained to be established. In our best knowledge for the first time, this paper reports on the strength of mode coupling and output beam quality of SI air-clad silica optical fiber. The results obtained can be exploited to employ SI air-clad fibers in broadband high-NA astro-photonics systems.

Theoretical Investigation of the Capacity of Space Division Multiplexing with Multimode Step-Index Air-Clad Silica Optical Fibers

We studied the effect of mode coupling on the space division multiplexing (SDM) capabilities of multimode step-index (SI) air-clad silica optical fibers by numerically solving the power flow equation. Mode coupling considerably reduces the length of these fibers at which space division multiplexing may be achieved with minimal crosstalk between neighboring optical channels, according to the findings. Up to 120 m and 30 m, respectively, the two and three spatially multiplexed channels in the investigated multimode step-index silica optical fibers can be used with low crosstalk. When building a space division multiplexing-based optical fiber transmission system, such characterization of optical fibers should be taken into account.

General Step-Index Optical Fiber Modal Characteristic and Sensitivity Analysis

An analytical approach about the general type of step-index optical fiber modes and their variations is presented in this paper. The theoretical model is solved through our analytical method. With the boundary conditions, a composite equation set is established and solved. The precise results of all modes are achieved with a high-precision numerical solution. In this paper, the key modal qualities contain electromagnetic fields' distributions, the effective refractive index, and its tendencies. And the modal sensitivity is based on the partial derivative of the effective refractive index (<inline-formula><tex-math notation="LaTeX">$\partial$</tex-math></inline-formula>n_eff&#x002F;<inline-formula><tex-math notation="LaTeX">$\partial$</tex-math></inline-formula>n_i and <inline-formula><tex-math notation="LaTeX">$\partial$</tex-math></inline-formula>n_eff&#x002F;<inline-formula><tex-math notation="LaTeX">$\partial$</tex-math></inline-formula>r_i). The derivative rule of special function allows analyzing the sensitivity of modes. Furthermore, four qualitative deductions are derived during the process, including the qualitative analysis of modes and their variation, several of which are supported with previous experiments. Our work is beneficial for exploiting new fiber and researching relative fiber sensors.

Femtosecond optical parametric chirped-pulse amplification in birefringent step-index fiber.

We demonstrate an optical parametric chirped-pulse amplifier (OPCPA) that uses birefringence phase matching in a step-index single-mode optical fiber. The OPCPA is pumped with chirped pulses that can be compressed to sub-30-fs duration. The signal (idler) pulses are generated at 905 nm (1270 nm), have 26 nJ (20 nJ) pulse energy, and are compressible to 70 fs duration. The short compressed signal and idler pulse durations are enabled by the broad bandwidth of the pump pulses. Numerical simulations guiding the design are consistent with the experimental results and predict that scaling to higher pulse energies will be possible. Forgoing a photonic crystal fiber for phase-matching offers practical advantages, including allowing energy scaling with mode area.

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers.

In this paper, the inscription of 2-mm-long fiber Bragg gratings (FBGs) on benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a femtosecond laser and a point-by-point FBG inscription technique is reported. The highest reflectivity of approximately 99% is obtained with a pulse energy of 68.5 nJ, showing a large refractive index modulation amplitude of 7.2 × 10-4. Afterwards, grating stabilities at room and higher temperatures of up to 80°C are investigated.

Calculation of Fundamental Mode Properties for Single-Mode Fibers

In this research, properties for the fundamental mode of single-mode step-index optical fibers with core diameters 9.8–15.6 µm, core refractive index and cladding refractive index are calculated at a wavelength of 1.55 µm by using RP Fiber Calculator and then compared with the results obtained from equations. It is shown that there is a good agreement for all properties. These results can be useful for designing practical fibers

Maintaining chromatic dispersion and signal gain performances in a chalcogenide buffer step-index optical fiber.

Aiming at maintaining the chromatic dispersion properties and fiber optical parametric amplification (FOPA) performance when fiber core fluctuation occurs, we propose a buffer step-index optical fiber. The AsSe2 chalcogenide glass is employed as the core material due to its high nonlinearity and broad transmission spectrum. The calculated results in this study show that the chromatic dispersion variation due to the change of core diameter can be greatly suppressed and a continuous and very broad FOPA signal gain spectrum can be obtained and maintained by carefully controlling the core, buffer and cladding properties such as refractive index and diameters. The calculated results in this study showed that by using the proposed 3-cm-long fiber pumped at 5.02 µm, a broad signal gain bandwidth from 3 to 14 µm at about 15 dB is attainable although the fiber core diameter Dc drastically fluctuated from 2 to 5 µm and the buffer diameter Db varies from 8.9 to 9.3 µm. Moreover, when Dc varies in smaller range from 3 to 4 µm, the FOPA signal gain spectra calculated at different fixed values of Db in the range from 8.9 to 9.3 µm are highly maintained. When Db is kept at 9.0 µm and Dc varies from 3 to 4 µm, the calculated FOPA signal gain spectra at different pump wavelengths from 4.98 to 5.02 µm are also nearly identical in the wavelength range from 3 up to 13 µm.

Experimental Assessment of the Transmission Performance of Step Index Polymer Optical Fibers Using a Green Laser Diode.

Large-core polymer optical fiber (POF) links have limitations in capacity and reach due to the fibers’ high modal dispersion and attenuation. Most of these links use red laser diodes, even though the attenuation spectrum of poly(methyl methacrylate) (PMMA), the basic polymer used to manufacture these fibers, has a lower minimum in the green region. Therefore, we set out to explore the potential use of green light in transmission systems, comparing the performances of three step-index polymer optical fibers (SI-POFs) with different numerical apertures. We obtained measurements of intensity distribution, frequency response and bit error rate (BER), as functions of fiber length. We have also compared the fibers’ frequency responses with red and green light for a few selected lengths. Our results confirm that SI-POFs attenuate less in response to green light, which can increase their length. This advantage is partially counterbalanced by a slightly higher dispersion that limits the capacity of the high-aperture fibers, particularly at relatively short lengths. Our conclusions are critical to understanding SI-POF behavior and to designing thorough SI-POF models that can aid the design of POF-based links for different scenarios.

The response of optical fibres to gravitational waves

The response of optical fibre modes to plane gravitational waves (GW’s) of low frequency is computed. By solving perturbatively the Maxwell equations for step-index optical fibres in a GW background and implementing appropriate boundary conditions to describe single-mode fibres, explicit formulae for the perturbations of the phase and the polarisation of the fibre modes are obtained.

Large refractive index modulation based on a BDK-doped step-index PMMA optical fiber for highly reflective Bragg grating inscription.

We experimentally report high reflectivity on the poly(methyl methacrylate) (PMMA)-based polymer optical fiber Bragg gratings by means of a 266 nm pulsed laser and phase mask technique. In the first recipe, fiber Bragg gratings (FBGs) were manufactured with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2×10-4 was obtained. In the second recipe, FBGs were inscribed by 22 pulses with a lower pulse energy of 1.4 mJ, showing a stable refractive index change of 6.2×10-4. Both behaviors may mainly be attributed to the movement of initiating radicals arising from benzyl dimethyl ketal (BDK) under UV irradiation. The high refractive index change in step-index fibers paves the way to tilted FBG manufacturing with large tilt angles potentially for biomedical applications.

Nested compound negative curvature hollow-core fiber for single-mode operation in the infrared region

In this study, a novel tubular hollow-core fiber design with extended cladding structures aiming low transmission losses and dominant single-mode guidance in the infrared region is proposed. The fiber parameters are optimized to achieve low-loss operation at the target wavelength of 2μm, at which the emission spectrum of thulium-doped fiber lasers are centered. The confinement loss of the fundamental mode is found to be less than 0.02 dB/km in the near- and mid-infrared region, which makes the proposed design a strong alternative for conventional step-index silica optical fibers. Strong suppression of the higher order modes, and low bending sensitivity are also observed in the proposed design. The proposed fiber shows promising results to achieve low-loss transmission in the infrared spectrum.

Characterization of Mach Zehnder interferometer plastic optical fiber for intensity-based temperature sensor

This paper presents characterization of optical temperature sensor using step-index plastic optical fiber (SI-POF) in intrinsic Mach Zehnder interferometer (MZI) structure. The MZI was developed by forming two tapers at several distance. By exploiting the wavelength dependency of spectrum intensity, measurement was done by using intensity based measurement at two different wavelengths. Characterizations were done to obtain the sensor performance which were sensitivity and hysteresis. The characterization was carried out by launching white LED to the sensor while the sensor was in temperature controlled-oven. The output spectrum was observed at the other sensor tip using spectrometer at various temperature which are 35°C to 85°C with increment of 10°C. It was found that the sensor has sensitivity and hysteresis of 1×10−3/°C and 1.97×10−2, respectively.

Nonlinear spatial reshaping of pulsed beam in a step-index few-mode optical fiber.

In this paper we demonstrate the spatial and spectral dynamics of pulse propagation in a step-index few-mode optical fiber, through an experimental and numerical analysis. The Kerr induced spatial self-cleaning is demonstrated by coupling a sub-nanosecond pulsed laser at 532nm into the fiber supporting above 10 modes. A bell-shaped and approximately single mode beam can be obtained for peak powers above 6kW and it remained relatively unchanged up to 25kW. But at significantly higher input peak powers, the spatial contents of spectral sidebands change dramatically, because of intermodal four wave mixing effect.

Langevin Equation Approach to Mode Coupling in Heated Step-Index Plastic Optical Fibers

The Langevin equation is used to investigate the state of mode coupling in step-index plastic optical fibers under varied temperature. The numerical integration of the Langevin equation is based on the computer-simulated Langevin force. The solution matches the experimental data reported previously. We find that elevated temperatures of plastic optical fibers are strengthening mode coupling. We show that, solving the Langevin equation (stochastic differential equation), one can treat a mode coupling in heated multimode step-index plastic optical fibers, which is a result of fiber’s intrinsic random perturbations.

Dynamic Characterization of Macrobending Loss Optical Fiber-Based Load Sensor

The weight of vehicles passing through the road greatly affects road damage, so it is necessary to have a non-stop weighing system or Weight in Motion (WIM). In this study, the dynamic characterization of the WIM sensor was carried out based on the principle of optical fiber macrobending. In this study, a single-mode step-index optical fiber was used as the sensor material and a laser diode with a power of 5 mW and a wavelength of 1,550 nm as a light source. Characterization was carried out by running over the sensor using a motor with three variations of speed, namely 10 km/hour, 15 km/hour, and 20 km/hour. Two different conditions were also carried out, namely, the sensor was directly crushed and the sensor was reinforced in the form of a half-cylinder wooden beam. The test was carried out with three different types of sensors. From the observations, data shows that the addition of a beam can increase the accuracy of the reading as seen from the smaller the difference in the output voltage reading for the same type of sensor and vehicle speed. Besides that, there is a strengthening of the sensor resistance up to 10 times which is known from the sensor output voltage where the voltage at the addition of the beam is 1/10 of the reading without the beam. This is due to an increase in the sensor area exposed to the load.