Step-index Multimode Fiber Research Articles

Fiber-Optic Liquid Level Sensor

A fiber-optic liquid level sensor based on multimode interference (MMI) effects is proposed and demonstrated. We show that MMI and self-image effects can be effectively applied for multiplexed liquid level sensing, because the natural response as a band-pass filter for each sensor is clearly distinct from each other, in the case for several sensors working at the same time. Using a standard 105/125 step-index multimode fiber (MMF) a simple discrete level sensor was fabricated, that can also discriminate the refractive index (RI) of the liquid during the level measurement. The MMI liquid level sensors are not only inexpensive, but their fabrication is simple.

Attenuation and modal dispersion models for spatially multiplexed co-propagating helical optical channels in step index fibers: Comment

We comment on a recent paper [J Opt Laser Technol 43 (2011) 430–436] in which the authors introduce dispersion and attenuation models of spatially multiplexed channels in step index multimode fibers. We consider the theoretical explanation of the model given in the paper incorrect.

Evaluation of the performance of high phosphorous with germanium codoped multimode optical fiber for use as a radiation sensor at low dose rates

We propose a GeO2–P2O5-codoped step index multimode (SIMM) fiber having a core diameter of around 50 μm with numerical aperture of around 0.21–0.22. The proposed SIMM fiber shows excellent linear radiation response behavior with sensitivity of around 0.69–0.97 dB/m/100 rad at a 505 nm wavelength within the dose rate range of 10–100 rad/h, as well as very low recovery at room temperature using a Co60 gamma radiation source. This enables its practical application in fiber optic personal dosimeters for measurement of low dose gamma radiation.

Monitoring of Curing Temperature of Early-Age Cement Paste Using Biconical Tapered Fiber Sensor

In this paper, a biconical tapered fiber sensor is applied to monitor the temperature of cement paste during curing process. A simple sensing principle of the proposed sensor is presented by analyzing the V-number of leaky rays. An experimental is carried out by tapering the plastic multi-mode step-index fiber to measure the temperature of the cement paste. The numerical result shows that the V-number of leaky ray is increasing with the raised in temperature. The experimental results are in agreement with the results from the theoretical analysis.

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.

Telluride Glass Step Index Fiber for the far Infrared

Nulling interferometry is an important technique under development for the DARWIN planet finding mission which enables the detection of the weak infrared emission lines of an orbiting planet. This technique requires the use of single mode optical fibers transmitting light as far as possible in the infrared. Tellurium based glasses transmit infrared light far beyond the second atmospheric window but are usually not stable enough toward crystallisation to be drawn into optical fiber. It was recently shown that the GeTe4 initial glass composition could be stabilized by the introduction of few percents of Se. In this paper, we report the development and the optical characterisation of far-infrared transmitting step index optical. The evolution of the refractive index of the glass versus the ratio Se/Te was measured in order to design a multi mode step index fiber. Its core diameter was 200 μm therefore permitting to easily characterise its optical properties. This multimode fiber transmitted light from 4 to about 15 μ m with losses near 20 dB.m-1. Despite these significant losses, the fiber was efficiently used to detect the broad absorption band of the CO2 located at 15 μm. These results are very promising in the context of the DARWIN mission of the European Space Agency.

Investigation of optical fibers for coherent anti-Stokes Raman scattering (CARS) spectroscopy in reacting flows

The objective of this work is to investigate the feasibility of intense laser-beam propagation through optical fibers for temperature and species concentration measurements in gas-phase reacting flows using coherent anti-Stokes Raman scattering (CARS) spectroscopy. In particular, damage thresholds of fibers, nonlinear effects during beam propagation, and beam quality at the output of the fibers are studied for the propagation of nanosecond (ns) and picosecond (ps) laser beams. It is observed that ps pulses are better suited for fiber-based nonlinear optical spectroscopic techniques, which generally depend on laser irradiance rather than fluence. A ps fiber-coupled CARS system using multimode step-index fibers is developed. Temperature measurements using this system are demonstrated in an atmospheric pressure, near-adiabatic laboratory flame. Proof-of-concept measurements show significant promise for fiber-based CARS spectroscopy in harsh combustion environments. Furthermore, since ps-CARS spectroscopy allows the suppression of non-resonant background, this technique could be utilized for improving the sensitivity and accuracy of CARS thermometry in high-pressure hydrocarbon-fueled combustors.

Attenuation and modal dispersion models for spatially multiplexed co-propagating helical optical channels in step index fibers

Spatial reuse of optical frequencies has been shown to be possible through a novel spatial domain multiplexing (SDM) technique that uses spatial multiplexer at the input end to launch multiple channels of the same wavelength inside a single strand of carrier fiber and then employs spatial filtering methods to de-multiplex the different optical channels at the output end. The individual SDM channels are confined to dedicated spatial locations inside the fiber while traversing through it owing to helical propagation of light. This presents attenuation and dispersion models of such a system. Experimentally obtained beam profile and resultant crosstalk of two such spatially multiplexed co-propagating SDM channels of the same wavelength over standard step index multimode optical fibers are also presented.

Methods of modulation of light wave propagated in optical fiber using ultrasonic wave

Glass optical waveguides are a promising medium for mixed laser-ultrasonic applications. Initial analysis of the possibilities of compensating for the flaws associated with laser and ultrasonic technologies, e.g. in the process of cutting biological tissues, showed that such a mixed technology could be more effective. The measurements of ultrasonic wave transmission ware performed for three representative types of optical fibers: a multimode step-index fiber, a single mode step-index fiber and multimode gradient-index fiber. The material the optical fiber is made of enables simultaneous transmission of light and ultrasonic wave. This study presents amplitude characteristics of the measured output voltages of the capacitive sensor for the types of optical fibers in question. The work also presents methods of modulation of light wave propagated in optical fiber by means of ultrasonic wave, which is performed with the use of radial vibrations of piezoelectric disk and longitudinal vibrations generated by a sandwich ultrasonic transducer with a wave velocity transformer. Experiments were performed using a Mach-Zehnder interferometer.

Power Coupling Efficiency Enhancement in Multimode Step-Index Fiber Using Refractive and Diffractive Microlenses

A novel study has been undertaken for the coupling efficiency enhancement on planar (refractive and diffractive) microlenses as coupling elements with various geometrical shapes in fibre optics communication. When the dispersion of the planar diffractive microlens is expressed in terms of an Abbe number, it is easy to solve for the refractive and diffractive power coupling efficiency that result in a single material. Coupling loss is reduced with microlens used as coupler even when increasing wavelength of the Gaussian beam. A slight difference of power coupling efficiency is observed with the different direction of electric polarisation for a Gaussian beam of large spot size at its waist due to coupling property has been discussed and shown to agree well with theoretical model.

Nonchemical-vapor-deposition process for fabrication of highly efficient Yb-doped large core fibers

A new fabrication process of active optical silica glass based on direct sand vitrification is proposed. This method, an alternative to chemical vapor deposition (CVD), allows the fabrication of homogeneous and highly Yb(3+)-doped rods that are ten times larger in diameter than those produced by CVD. For large-mode-area fibers fabricated by the stack-and-draw method, this is a tremendous technical breakthrough that could offer great flexibility in fiber design. As a proof of concept, we focused here on the fabrication and characterization of active core material surrounded by pure silica. Consequently, we draw a simple multimode step-index fiber. The index ripple in the core that matches our objectives is approximately 2.2x10(-4). For this first demonstration, the core material is codoped with Yb(2)O(3) (3600 parts in 10(6)(ppm) by weight) and Al(2)O(3), yielding a 180 dB m(-1) absorption at a wavelength of 975 nm, whereas the background loss is around 0.8 dB m(-1). The continuous-wave laser obtained with this fiber exhibits 74% slope efficiency.

Equilibrium Mode Distribution and Steady State Distribution in Step-Index Glass Optical Fibers

Using the power flow equation, we have examined mode coupling in a step-index multimode glass optical fiber. As a result, the coupling length at which the equilibrium mode distribution is achieved and the length of fiber required for achieving the steady-state mode distribution are obtained. These lengths are much longer for glass fiber than they are for plastic optical fibers. Our results are in good agreement with experimental results reported earlier.

Inscription and characterization of Bragg gratings in single-crystal sapphire optical fibres for high-temperature sensor applications

We report the fabrication and high-temperature characteristics of Bragg gratings in single-crystal sapphire fibres inscribed by infrared femtosecond laser pulses. For stable read-out of the multimode Bragg reflection signal, the sapphire fibre was illuminated by broadband light from a superluminescent diode applying a long section of a step-index multimode silica fibre. This evenly distributed excitation of many guided modes in the sapphire fibre resulted in the polychromator measurement of stable reflection peak shapes and allowed for multiplexed temperature sensing up to 1745 °C at remote read-out with repeatability better than 1 °C. During the temperature tests an annealing process of the sapphire grating was observed at temperatures above 1400 °C. Possibilities for further improvement of sensors performance are discussed with a view to avoiding instabilities from the lack of a fibre cladding that shields the sapphire waveguide from the environment.

Calculation of the coupling coefficient in step index glass optical fibers

A method for calculating the coupling coefficient in step-index multimode optical fibers is verified for glass fibers by comparison to published data and to an analytical solution for the steady-state mode distribution. The coefficient that the method calculates is used to determine the state of mode coupling along the fiber, including the coupling length for achieving the equilibrium mode distribution when measurement of fiber characteristics (such as linear attenuation or bandwidth) becomes meaningful.

Radiation response behavior of high phosphorous doped step-index multimode optical fibers under low dose gamma irradiation

Abstract This paper revealed the role of various parameters on the radiation response behavior of P 2 O 5 doped step-index multimode (SIMM) fibers having different content of P 2 O 5 (12–6 mol%) at room temperature. Their suitability for use as a radiation sensitive fiber in fiber optic dosimeter was studied under 60 Co-gamma irradiation at different dose rates 0.01–0 Gy/h. The suitable wavelength region is found to be 500–600 nm where the fibers shows the maximum radiation sensitivity due to formation of phosphorous–oxygen hole centers identified from their radiation induced absorption spectra. The influence of each parameter like the doping levels of P 2 O 5 , dose rates, doping region and energy of the radiation source on their sensitivities was examined. The radiation sensitivities recorded at 502, 540 and 560 nm depends strongly on P 2 O 5 content of the fiber at low dose rates within 0.01–1 Gy/h. However, the fibers shows almost dose rate independent behavior at high dose rates (>1.0 Gy/h) under 60 Co-gamma radiation source of energy 1.25 MeV with respect to all the three wavelengths. The fiber shows almost linear relation to the total dose up to saturation levels of 4.0 Gy at all dose rates. Their sensitivities could be explained through analyses of such type of glass as well as behavior of the radiation induced P-related defects generated in the light guiding core region. At low dose rates the fibers becomes more radiation sensitive compared to the high dose rates due to conversion of POHC centers to P 1 (phosphorous E′) defect centers as observed from their induced loss curves during gamma irradiation. At low dose rates 0.1–1 Gy/h under 502 nm transmission wavelength the fibers shows an excellent linear relation with respect to Cs-137 radiation source of energy 0.662 MeV having average sensitivity of 1.0452 ± 0.0346 dB/m/Gy and very low fading behavior at room temperature. The results suggest that P-doped SIMM fiber (40 μm core diameter) containing 16 mol% P 2 O 5 exhibit an excellent linear radiation response property of high sensitivity around 0.89 ± 0.09 dB/m/Gy at 502 nm wavelength with very low recovery and little dose rate dependence within 0.5–10 Gy/h region that makes them a very promising candidate as radiation sensor for use in fiber optic personal dosimeter to detect low dose gamma radiation of 0.002 Gy for human safety purpose.

Beam quality output of a few-modes fiber seeded by an off-center single-mode fiber source

We study the beam quality at the output of a multimode step-index optical fiber with a small number of modes following injection from a single-mode input fiber. It is shown that the output beam quality is optimized when the injecting fiber is offset from the center of the receiving core. Theoretical results demonstrating this phenomenon are in good agreement with an experimental demonstration. These results may have practical consequences in beam-coupling schemes and in applications emphasizing the importance of beam quality.

Beam quality factor of mixed modes emerging from a multimode step-index fiber

The beam quality factors (or M 2 parameter) of coherent and incoherent superposition of the several lower-order LP modes emerging from a step-index fiber have been calculated by using the second-order moment method. The results indicate that, for an individual LP mode, the M 2 parameter takes its maximum value when the normalized frequency V of the step-index fiber approaches the cut-off frequency, and it gradually becomes constant as V increases. In the case of incoherent superposition, the larger the fraction intensity carried by the higher-order mode, the larger the beam quality factor M 2. Under certain circumstances, the value of the M 2 parameter of the mixed mode that comprises several LP-modes contents may become even smaller and closer to the ideal Gaussian beam than that of the fundamental mode in a step-index fiber. However, in the case of the coherent superposition, the value of the M 2 parameter of the mixed mode may be greater than that of the higher-order constituent mode. The results reported here could be helpful for the application of the high-power fiber laser systems.

Specklegram in a grapefruit fiber and its response to external mechanical disturbance in a single-multiple-single mode fiber structure

Specklegram in multimode fiber has successfully been used as a sensor for detecting mechanical disturbance. Speckles in a multimode pure silica grapefruit fiber are observed and compared to that of a step-index multimode fiber, showing different features between them. The sensitivities to external disturbance of two kinds of fiber were measured, based on single-multiple-single mode (SMS) fiber structure. Experimental results show that the grapefruit fiber shows higher sensitivity than does the step-index multimode fiber. The transmission spectrum of the grapefruit fiber was measured as well, showing some oscillation features that are significantly different from that of a step-index multimode fiber. The experiments may provide suggestions to understand the mechanisms of light propagation in grapefruit fibers.

Degradation of beam quality and depolarization of the laser beam in a step-index multimode optical fiber

The characteristics of a laser beam are altered during propagating through large-core multimode optical fibers. The distribution of modes excited by the input laser beam is modified by means of mode coupling on transmission through the fiber, leading to the degradation of beam quality and the depolarization of the delivered beam. The relationship between the beam quality factor ( M 2) of output beam from a large-core multimode fiber and the fiber length, as well as the relationship between the degree of polarization ( V) of output beam from such a fiber and the fiber length, are introduced in this paper. When a laser beam was well launched into a large-core step-index multimode fiber, M 2 of the output beam was a compound tanh function of the fiber length. A linear polarization beam that well launched into such fiber suffered depolarization. The V of the output beam was an exponent function of fiber length. And the misalignment between beam axis and fiber axis made the beam quality degrade faster but made no difference of the utmost M 2 in the aligned and misaligned conditions. Also, the misalignment condition made the polarization of output beam degrade faster.

All fiber on-axis coupling scheme between single mode fiber and GRIN fiber

In this paper we present a simple and novel method to maximize on-axis coupling efficiency to radially symmetric fibers without the need for extra free space optical elements. The method is based on inserting a segment of step-index multimode fiber (MMF), cleaved to a particular length, between the input fiber and the output fiber (OF). The MMF segment modifies the input field to match the guided modes in the OF. Using this technique we show that, by inserting an appropriate length MMF segment, it is theoretically possible to obtain a coupling coefficient as high as −0.8 dB between a single mode fiber and a graded index ring-shaped fiber and −0.32 dB for a multi-shell fiber. Our experimental measurements showed good agreement with theoretical predictions for the ring fiber.