Showing papers on "Step-index profile published in 2012"

Bending-resistant large core diameter high numerical aperture multimode fiber

Abstract: A bending-resistant large core diameter high numerical aperture multimode fiber includes a core and a cladding surrounding the core. The core has a radius R1 in a range of 28 to 50 microns, a refractive index profile of a parabola shape with α being in a range of 1.9 to 2.2, and a maximum relative refractive index difference Δ1% max being in a range of 1.9% to 2.5%. The cladding includes an inner cladding and/or a trench cladding, and an outer cladding disposed from the inner to the outer in sequence. The radius R2 of the inner cladding is in a range of 28 to 55 microns, and the relative refractive index difference Δ2% is −0.1% to 0.1%. The radius R3 of the trench cladding is in a range of 28 to 60 microns, and the relative refractive index difference Δ3% is in a range of −0.15% to −0.8%.

Ultrahigh Sensitivity Refractive Index Sensor Based on Optical Microfiber

Abstract: We demonstrate a nonadiabatic microfiber sensor with a taper diameter of few micrometers. The modal interference caused by the abrupt taper results in a sinusoidal spectral response. The wavelength shift arising because of the changes in the external refractive index is found to be significant, achieving a maximum sensitivity of 18681.82 nm/RIU. The measured results show good agreement with the theoretical predictions. The high sensitivity and the simplicity offer the sensor the potential for many real applications.

Multimode interference tapered fiber refractive index sensors

Abstract: Real-time monitoring of the fabrication process of tapering down a multimode-interference-based fiber structure is presented. The device is composed of a pure silica multimode fiber (MMF) with an initial 125 μm diameter spliced between two single-mode fibers. The process allows a thin MMF with adjustable parameters to obtain a high signal transmittance, arising from constructive interference among the guided modes at the output end of the MMF. Tapered structures with waist diameters as low as 55 μm were easily fabricated without the limitation of fragile splices or difficulty in controlling lateral fiber alignments. The sensing device is shown to be sensitive to the external environment, and a maximum sensitivity of 2946 nm/refractive index unit in the refractive index range of 1.42–1.43 was attained.

A reflective optical fiber refractometer based on multimode interference

Abstract: A reflective multimode interference based-fiber optic sensor is described, aimed at measuring refractive index variations by means of intensity variation based on the fiber tip-interaction concept. The sensing head is a section of a multimode fiber, spliced to a single-mode fiber at the input end; its characteristics are studied when two multimode fibers, with distinct core diameters (viz. 50 and 105 μm), are considered. The multimode fiber end face is placed in contact with the liquid sample, so as to provide a refractive index measurement via variation in amplitude of the interference spectral peaks – which is essentially insensitive to temperature changes. Therefore, the proposed configuration permits measurement of refractive index variations free from system temperature cross-sensitivity effects. Resolutions of 2.2 × 10 −4 RIU and 3.8 × 10 −4 RIU, for multimode fiber tips with core diameters of 50 μm and 105 μm, respectively, could be achieved.

Few mode optical fibers for mode division multiplexing

Abstract: An optical fiber comprising: (i) a core (20) having a refractive index profile; (ii) an annular cladding (50) surrounding the core; (iii)a primary coating (62) contacting and surrounding the cladding, the primary coating having an in situ modulus of less than 0.35 MPa and an in situ glass transition temperature of less than -35° C; and (iv) a secondary coating (64) surrounding the primary coating, the secondary coating having an in situ modulus of greater than 1200 MPa; wherein the refractive index profile of said core is constructed to provide few mode transmission with an LP 11 theoretical cutoff wavelength greater than 2.0 μιη and an effective area greater than 110 microns for the LP01 mode at 1550 nm.

In-line fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature based on thinned fiber

Abstract: In this letter, we propose and demonstrate a novel and simple method for simultaneous measurement of refractive index and temperature using all-fiber in-line single-mode-multimode-thinned-single-mode (SMTS) fiber structures based on a small core and cladding diameters thinned fiber Mach–Zehnder interferometer (MZI). The principle of operation relies on the strong effect of the core and cladding mode interference of the thinned fiber through the employing of a multimode fiber. By measuring the shifts of the attenuation peak wavelengths of the two arbitrarily selected interference orders in the transmission spectrum, we can simultaneously measure the refractive index and temperature surrounding the sensor. Results indicate that the selected two interference orders have sensitivities of −2.85 × 10 −3 RIU with 3.49 × 10 −1 °C, and −1.12 × 10 −3 RIU with 4.07 × 10 −1 °C respectively at RI range from 1.3325 to 1.3715 and temperature range from 24 °C to 84 °C. The fabrication is easy, safe and cost effective, and includes only the fusion splicing, making the device properly attractive for physical, biological and chemical sensing in practical applications.

Fabry–Pérot Cavity Based on a High-Birefringent Fiber Bragg Grating for Refractive Index and Temperature Measurement

Abstract: A fiber-optic sensor for simultaneous measurement of refractive index and temperature is described. The refractive index measurement is based on the visibility variations of a Fabry-Perot interferometer. It is formed with the interfering waves generated from a low reflectivity Bragg grating inscribed on a Panda fiber and from the fiber end tip (Fresnel reflection) in contact with the liquid. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed. The temperature is determined by the wavelength shift of the FBG peaks. Results show the feasibility of simultaneous measurement of refractive index and temperature and also the possibility of adjusting fringe visibility via polarization control.

Nonadiabatic fiber taper-based Mach-Zehnder interferometer for refractive index sensing

Abstract: A Mach-Zehnder interferometer based on a nonadiabatic fiber taper is proposed for refractive index (RI) sensing Only the first order cladding guided mode is excited in the down-taper The radius of the fiber core in the interference region is too thin (<164 μm) to confine the fundamental mode in the fiber core, the fundamental core guided mode is converted to the fundamental cladding guided mode and interference occurs between it and the first order cladding guided mode The sensitivity of the sensor is 980 nm per refractive index unit for the surrounding refractive index ranging from 1332 to 1392

A polarization-maintaining fiber loop mirror based sensor for liquid refractive index absolute measurement

Abstract: A fiber sensor based on polarization-maintaining fiber loop mirror (FLM) for liquid refractive index absolute measurement is described. Two sections of polarization maintaining fibers (PMFs) with different length were inserted into the FLM. One section of PMF was chemical etched for liquid refractive index measurement and the other one for reference. Liquid refractive index absolute measurement can be obtained by detecting the interference wavelength difference corresponding to the two sections of PMFs. High sensitivity of 63.82 ± 0.04 nm/RIU (refractive index unit) and a resolution of 3.14 × 10−4 RIU were obtained for the proposed liquid refractive index sensor.

Method for modifying the refractive index of an optical material and resulting optical vision component

Abstract: A method for modifying the refractive index of an optical polymeric material. The method comprises continuously irradiating predetermined regions of an optical, polymeric material with femtosecond laser pulses to form a gradient index refractive structure within the material. An optical device includes an optical, polymeric lens material having an anterior surface and posterior surface and an optical axis intersecting the surfaces and at least one laser-modified, GRIN layer disposed between the anterior surface and the posterior surface and arranged along a first axis 45° to 90° to the optical axis and further characterized by a variation in index of refraction across at least one of at least a portion of the adjacent segments and along each segment.

High sensitivity Mach–Zehnder interferometer sensors based on concatenated ultra-abrupt tapers on thinned fibers

Abstract: This study proposes a simple, cost - effective method to fabricate fiber-based Mach–Zehnder interferometer (MZI) sensors by concatenating two ultra - abrupt fiber tapers together using a fusion splicer. By concatenating, the taper diameter and length ratio is 1:1 that is much greater than that (1:10) by stretching. The refractive index sensitivity is comparable to the MZI sensors based on long-period fiber grating pairs or stretched fiber taper pairs. The MZI fiber claddings are etched to improve the sensitivity of refractive index measurements. The sensitivity is 664.57 nm/RIU (refractive index unit) for the refractive index ranging from 1.3348 to 1.3558, which is 2–6 times greater than those measured by long period fiber gratings (LPFGs) after sensitivity enhancement.

Refractive index profile changes caused by arc discharge in long-period fiber gratings fabricated by a point-by-point method

Abstract: Long-period fiber gratings were inscribed in a commercial silica fiber by a point-by-point arc discharge technique with different discharge conditions. The refractive index (RI) profile change induced by arc discharge was measured using the quantitative phase microscopy for the first time to our knowledge. The causes of the transmission variations induced by different arc discharges and the mechanisms of the RI profile change were investigated based on the measured phase profiles. The RI in the core and the cladding has clearly changed due to arc discharge. The central dip in the core profile diminished very much, and the index gradient became gradual. The resonance wavelengths have fluctuated by discharge current and time owing to variations of the reduction of the core-cladding RI difference and the extent of the RI change region.

Bragg waveguides with low-index liquid cores.

Abstract: The spectral properties of light confined to low-index media by binary layered structures is discussed. A novel phase-based model with a simple analytical form is derived for the approximation of the center of arbitrary bandgaps of binary layered structures operating at arbitrary effective indices. An analytical approximation to the sensitivity of the bandgap center to changes in the core refractive index is thus derived. Experimentally, significant shifting of the fundamental bandgap of a hollow-core Bragg fiber with a large cladding layer refractive index contrast is demonstrated by filling the core with liquids of various refractive indices. Confirmation of these results against theory is shown, including the new analytical model, highlighting the importance of considering material dispersion. The work demonstrates the broad and sensitive tunability of Bragg structures and includes discussions on refractive index sensing.

Water density and polarizability deduced from the refractive index determined by interferometric measurements up to 250 MPa

Abstract: The refractive index of water is precisely determined in the visible light range as a function of the pressure until 250 MPa by means of a new measurement device that uses a special pipe tee included in an interferometer set. This technique allows revisiting the Bradley-Tait and Sellmeier equations to make them dependent on the wavelength and the pressure, respectively. The Bradley-Tait equation for the pressure dependence of the water refractive index is completed by a wavelength-dependent factor. Also, in the considered pressure and wavelength ranges, it is shown that the Sellmeier coefficients can be straightforwardly linked to the pressure, allowing the determination of the refractive index of water for either any wavelength or pressure. A new simple model allows the determination of the density of water as a function of the measured refractive index. Finally, the polarizability of water as function of pressure and wavelength is calculated by means of the Lorentz-Lorenz equation.

Tapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

Abstract: A Mach–Zehnder interferometer formed in single mode fiber is implemented. The interferometer is built by two mechanically-induced long-period gratings. In addition, a fiber taper in the middle section is inserted. The spectral properties of the whole system are analyzed. Visibility of the interference fringes up to 0.80 (the higher ever reported using mechanically-induced long-period gratings) with fringe spacing in the 4.1 to 0.86 nm range are experimentally demonstrated. The proposed device allows reducing the fiber diameter of the section between gratings with a minimal effect in the interference fringe spacing. The sensitivity of the interferometer to external refractive index changes was also studied. It is experimentally shown that, due to the nature of the cladding mode excited, it is necessary to taper the fiber to improve the system sensitivity to external refractive index. Fiber tapers with different diameter, inserted between the long-period gratings pair were fabricated and tested for measuring external refractive index changes. A maximum resolution of 2.3×10−4 RIU in a refractive index range from 1.36 to 1.402 is achieved.

Tapered-fiber-based refractive index sensor at an air/solution interface

Abstract: An approach to achieve refractive index sensing at an air and aqueous glycerol solution interface is proposed using a tapered-fiber-based microfiber Mach–Zehnder interferometer (MFMZI). Compared to a surrounding uniform medium of air or solutions, the spectral interference visibility of the MFMZI at the air/solution interface is significantly reduced due to a weak coupling between the fundamental cladding mode and high-order asymmetric cladding modes, which are extremely sensitive to the external refractive index. The MFMZI is experimentally demonstrated as an evanescent wave refractive index sensor to measure concentrations of glycerol solutions by monitoring average power attenuation of the tapered fiber.

Simultaneous measurement of refractive index and temperature by integrating an external Fabry-Perot cavity with a fiber Bragg grating

Abstract: A fiber sensor for simultaneous measurements of refractive index and temperature based on the integration of a fiber Bragg grating (FBG) with an external Fabry-Perot (F-P) cavity is presented. The fringe contrast of the interference spectrum generated by the F-P cavity is used to determine the external refractive index, while the wavelength shift of the FBG is used to measure temperature. The result showed that the refractive index and temperature sensitivity for the integrated sensor is 8.1 × 10(-6) and 0.01006 nm/°C, respectively.

High-Bandwidth, Radiation-Resistant Multimode Optical Fiber

Abstract: A multimode optical fiber includes a central core and an outer cladding (e.g., an outer optical cladding). Typically, the optical fiber's central core is a depressed, central core having an alpha-index profile (i.e., a graded-index profile), an outer radius r1, and a maximum refractive index difference Δn1 with respect to the outer cladding. The central core's alpha-index profile has a minimum refractive index at the central core's outer radius r1 that corresponds to a refractive index difference Δnend with respect to the outer cladding. Exemplary optical-fiber embodiments may include an inner cladding having an outer radius r2 and a width w2. Exemplary optical-fiber embodiments may include a buried trench having a width w3 and an outer radius r3. Furthermore, exemplary optical-fiber embodiments may include an intermediate cladding having an outer radius r4 and a width w4.

Experimental Analysis of Optical Fiber Multimode Interference Structure and its Application to Refractive Index Measurement

Abstract: We investigated a fiber-based multimode interference phenomenon in the wavelength domain by using a white light source and an optical spectrum analyzer. This phenomenon was produced by a larger-core optical fiber joined at both ends with smaller-core optical fibers. We examined the variation of interference wavelength with changes in the length of the larger-core fiber. The interference wavelengths were blue-shifted and the interference signals were sharpened with an increase in the length of the larger-core fiber. The calculated results agreed well with the measured results. Next, we investigated how the input and output fibers with a small core influence the interference signal characteristics. By comparing the amplitude differences of the interference signal we find the conditions of input and output (I/O) fibers for higher sensitivity. In addition, an interference-signal shift was observed by changing the medium surrounding an multimode interference (MMI) structure. The amount of shift increased at a longer wavelength. This leads to the sensitive detection of the refractive index. Finally, a demonstration of the optical fiber refractometer with a multimode interference structure was given by refractive-index measurements of ethanol/water solutions.

Front light module

Abstract: A front light module, including: a light source; a light guide plate, having a first refractive index and having a side face neighboring the light source; a medium layer, placed over the light guide plate and having a second refractive index, wherein the second refractive index is smaller than the first refractive index; a transparent material layer, placed over the medium layer; and a transparent glue layer, placed under the light guide plate and having a third refractive index, wherein the third refractive index is larger than the refractive index of air and smaller than or equal to the first refractive index.

Broad-bandwidth optical fiber

Abstract: The present invention embraces an optical fiber that includes a central core having an alpha refractive index profile with respect to an outer optical cladding. The optical fiber also includes an inner cladding and a buried trench. The central core includes a core matrix doped with at least fluorine and a dopant element that increases refractive index. The optical fiber typically has reduced bending losses and cladding effect as well as a high bandwidth at the wavelengths of 850 nanometers and 1300 nanometers for high-data-rate applications.

Refractive index profiling of an optical waveguide from the determination of the effective index with measured differential fields.

Abstract: The evanescent tails of a guiding mode as well as its first and second derivatives were measured by a modified end-fire coupling method. The effective index of the waveguide can be obtained by simultaneously fitting these three fields using single parameter. Combined with an inverse calculation algorithm, the fields with fitted evanescent tails showed great improvement in the refractive index profiling of the optical waveguide, especially at the substrate region. Single-mode optical fibers and planar waveguides of proton-exchanged (PE) and titanium-indiffusion (Ti:LiNbO3) on lithium niobate substrates with different refractive index profiles were measured for the demonstration.

Laser Diode to Single Mode Circular Core Graded Index Fiber Excitation via Hemispherical Microlens on the Fiber Tip: Identification of Suitable Refractive Index Profile for Maximum Efficiency with Consideration for Allowable Aperture

Abstract: Abstract Based on the prescribed ABCD matrix for refraction by hemispherical lens under paraxial approximation, we formulate an analytical expression of coupling efficiency of a laser diode to a single mode circular core graded index fiber via a hemispherical lens on the tip of the fiber. The analysis involves Gaussian field distribution for both the source and the fiber and consideration of allowable aperture provided by the hemispherical lens. In order to find out the suitable refractive index profile for maximum coupling efficiency, we investigate the coupling optics for different profile exponents in graded index refractive index profile. In this context two different laser diodes emitting wavelengths 1.3 μm and 1.5 μm are employed for the concerned study. It is found that the graded index fiber having triangular refractive index profile is the most efficient one for the said two wavelengths of practical importance. Our formalism is simple and executable with little computation. The results found should be of tremendous importance to the designer of such coupling in the field of optimum launch optics.

Step-index few-mode fiber designs for spatial multiplexing

Abstract: A few-mode optical fiber comprises a core surrounded by a cladding, having a step index profile that is structured to support propagation of a plurality of desired signal-carrying modes, while suppressing undesired modes. The core and cladding are configured such that the undesired modes have respective effective indices that are close to, or less than, the cladding index such that the undesired modes are leaky modes. The index spacing between the desired mode having the lowest effective index and the leaky mode with the highest effective index is sufficiently large so as to substantially prevent coupling therebetween.

Highly Sensitive Refractive Index Sensor Based on a Cladding-Etched Thin-Core Fiber Sandwiched between Two Single-Mode Fibers

Abstract: A refractive index (RI) sensor based on a cladding-etched thin-core single-mode fiber (TCSMF) sandwiched between two single-mode fibers is demonstrated. The experimental results show that the sensitivity, within the RI range of 1.333–1.340, is enhanced at least 6 times by etching. It increases with the surrounding RI and reaches 857.5 nm/RIU at RI of 1.3684, and it can be expected to be higher with the decrease of the cladding diameter of the TCSMF.

High refractive index without absorption in a rare-earth-ion-doped optical fiber

Abstract: We demonstrate a new scheme for realizing the high refractive index with zero absorption in an Er3+-doped ZrF4–BaF2–LaF3–AlF3–NaF optical fiber. It is found that the refraction index of the probe laser can easily be controlled via adjusting properly the parameters of the corresponding system. Our scheme may provide some new possibilities for technological applications in optical-fiber communication.