Showing papers on "Step-index profile published in 2005"

Negative index of refraction in optical metamaterials.

Abstract: A double-periodic array of pairs of parallel gold nanorods is shown to have a negative refractive index in the optical range. Such behavior results from the plasmon resonance in the pairs of nanorods for both the electric and the magnetic components of light. The refractive index is retrieved from direct phase and amplitude measurements for transmission and reflection, which are all in excellent agreement with simulations. Both experiments and simulations demonstrate that a negative refractive index n???0.3 is achieved at the optical communication wavelength of 1.5??m using the array of nanorods. The retrieved refractive index critically depends on the phase of the transmitted wave, which emphasizes the importance of phase measurements in finding n?.

Highly sensitive fiber Bragg grating refractive index sensors

Abstract: We combine fiber Bragg grating (FBG) technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3 μm, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Perot interferometer (FFPI) with a radius of ~1.5 μm and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4 X 10(-5). Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors.

Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition.

Abstract: The deposition of an overlay of higher refractive index than the cladding in a Long Period Fiber Grating (LPFG) permits to improve the sensitivity to ambient refractive index changes in a great manner. When the overlay is thick enough, one of the cladding modes is guided by the overlay. This causes important shifts in the effective index values of the cladding modes, and henceforward fast shifts of the resonance wavelength of the attenuations bands in the transmission spectrum. This could be applied for improving the sensitivity of LPFG sensors. The problem is analysed with a numerical method based on LP mode approximation and coupled mode theory, which agrees with so far published experimental results.

Design concept for optical fibers with enhanced SBS threshold

Abstract: We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

Thinned fiber Bragg gratings as refractive index sensors

Abstract: In this work, highly sensitive refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating (FBG) sensors. When the cladding diameter is reduced, significant changes in the effective refractive index occur due to surrounding medium refractive index modifications, leading to Bragg wavelength shifts. Uniformly thinned FBGs have been obtained by using wet chemical etching in hydrofluoric acid solutions. In order to prove sensor sensitivity, experimental tests have been carried out by using glycerine solutions with well-known refractive indices. Obtained results agree well with the numerical analysis carried out by using the three-layer fiber model. If the cladding layer is completely removed, resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for the outer refractive index around 1.450 and 1.333, respectively, are possible. Finally, a novel approach based on the selective etching along the grating region has been analyzed, leading to high-sensitivity refractive index sensors based on intensity measurements.

Refractive index sensor based on microstructured fiber Bragg grating

Abstract: In this work, an all-fiber high-resolution refractive index sensor based on a microstructured fiber Bragg grating is presented. The proposed structure relies on a partial and localized etching of the cladding layer along a standard grating. The main spectral changes of the structured grating are the increasing of the stopband and the formation of a narrow allowed band strongly dependent on the etching features and the surrounding refractive index. A sensor prototype has been fabricated by using wet etching and a proper masking procedure; experimental results reveal the possibility to carry out low-cost refractive index measurements with a resolution of 4/spl middot/10/sup -5/.

Demodulation technique for weakly tilted fiber Bragg grating refractometer

Abstract: In this letter, a demodulation technique is presented in order to measure the surrounding refractive index in the range 1-1.45 by means of a weakly tilted fiber Bragg grating. This technique is based on the global monitoring of the cladding modes in the transmitted spectrum and on the computation of two statistical parameters. We report a resolution of 210/sup -4/ in terms of the refractive index as well as a temperature-insensitive behavior.

Analysis of optical response of long period fiber gratings to nm-thick thin-film coating.

Abstract: We have theoretically and experimentally demonstrated that the resonant wavelength of long period fiber gratings (LPG) can be shifted by a large magnitude by coating with only a nm-thick thin-film that has a refractive index higher than that of the glass cladding. The resonant wavelength shift can result from either the variation of the thickness of the film and/or the variation of its refractive index. These results demonstrate the sensitivity of LPG-based sensors can be enhanced by using a film of nm-thickness and refractive index greater than silica. This coating schematic offers an efficient platform for achieving high-performance index-modulating fiber devices and high-performance index/thickness-sensing LPG-based fiber sensors for detecting optical property variations of the thin-film coating.

Holey fiber tapers with resonance transmission for high-resolution refractive index sensing.

Abstract: The use of large-mode-area tapered holey fibers with collapsed air holes for refractive index sensing is demonstrated. The collapsing of the holes is achieved by tapering the fibers with a “slow-and-hot” method. This non adiabatic process makes the core mode to couple to multiple modes of the solid taper waist. Owing to the beating between the modes the transmission spectra of the tapered holey fibers exhibit several interference peaks. They shift remarkable to longer wavelengths as the external index increases. The multiple peaks, combined with a fitting algorithm, may allow high-accuracy refractometric measurements which can be used for diverse applications.

Simultaneous measurement of refractive index and temperature using a single ultralong-period fiber grating

Abstract: Novel ultralong-period fiber gratings (ULPFGs) with periods of up to several millimeters are reported here, which are fabricated by the high-frequency CO/sub 2/ laser pulses exposure method. The experimental results show that the different resonant peaks of the ULPFG have different temperature and refractive index sensitivities. In addition, it is found that different resonant peaks of the ULPFG have quite different sensitivities to refractive index. Hence, this offers the potential for realizing simultaneous measurement of refractive index and temperature.

Modification of the refractive index response of long period gratings using thin film overlays

Abstract: The response of a fibre optic long period grating (LPG) to the surrounding refractive index is shown to be modified by the deposition of a thin film of thickness of order 100 nm onto the cladding. The LPG becomes sensitive to surrounding refractive indices greater than that of silica, and exhibits an enhanced sensitivity to external refractive indices lower than that of silica.

Brillouin gain analysis for fibers with different refractive indices.

Abstract: We studied an anomalous result in the measured stimulated Brillouin scattering threshold for single-mode optical fibers with different refractive index profiles and effective areas. It was found that, in addition to the optical effective area, the refractive-index-dependent interaction between the optical and the acoustic fields must be considered in order to account for the differences in the stimulated Brillouin scattering threshold between fiber types. A detailed analytical formalism describing the acousto-optic interaction was derived to accommodate the influence of the index profile design on Brillouin scattering.

Refractive index profiling of axially symmetric optical fibers: a new technique

Abstract: We present a new technique for determining the refractive index profiles of axially symmetric optical fibers based on imaging phase gradients introduced into a transmitted optical field by a fiber sample. An image of the phase gradients within the field is obtained using a new non-interferometric technique based on bright field microscopy. This provides sufficient information to reconstruct the refractive index profile using the inverse Abel transform. The technique is robust, rapid and possesses high spatial resolution and we demonstrate its application to the reconstruction of the refractive index profiles of a single-mode and a multimode optical fiber.

Measurement of the refractive index of a magnetic fluid by the retroreflection on the fiber-optic end face

Abstract: A simple method based on the retroreflection on the fiber-optic end face is developed to measure the refractive index of a magnetic fluid in this letter. The measuring principle, accuracy, and sensitivity of this method are analyzed theoretically, and high precision and resolution can be achieved in principle. Experimental measurements are done to investigate the concentration and temperature dependent refractive index of the magnetic fluid. The linear dependence relation is obtained for both cases. The thermo-optical coefficient of the magnetic fluid is measured to be around −2.4×10−4°C−1.

Second mode transition in microstructured optical fibers: determination of the critical geometrical parameter and study of the matrix refractive index and effects of cladding size.

Abstract: We carried out a numerical study of the second mode transition in finite-sized, microstructured optical fibers (MOFs) for several values of the matrix refractive index. We determined a unique critical geometrical parameter for the second mode cutoff that is valid for all the matrix refractive indices studied. Finite size effects and extrapolated results for infinite structures are described. Using scaling laws, we provide a generalized phase diagram for solid-core MOFs that is valid for all refractive indices, including those of the promising chalcogenide MOFs.

Effective parameters of resonant negative refractive index metamaterials : interpretation and validity

Abstract: We present a numerical study of resonant negative refractive index (NRI) metamaterials in which their effective parameters are calculated. For a periodic media consisting of split-ring resonators and metallic wires the effective refractive index is compared by means of two different methods. The first one is an inversion procedure in which the effective refractive index is calculated from the reflection and transmission coefficients of a finite structure and the second one consists its calculation from the phase velocity issued from the dispersion diagram. A significant difference between the two cases is highlighted in the frequency interval of interest (NRI regime) and for the finite media, counterintuitive observations are made. These anomalous features are observed in a frequency range in which there is a non-negligible contribution of the higher-order modes to propagation inside the periodic metamaterial. Hence in this particular frequency interval, the media cannot be described by an effective refractive index in a Fresnel sense. In other frequency regions, both methods converge and it will be shown that the metamaterial exhibits a negative refractive index.

Optical fiber with specialized index profile to compensate for bend-induced distortions

Abstract: An optical fiber that exhibits reduced mode distortions as the fiber is bent is formed by properly defining its refractive index profile during fabrication. The as-fabricated profile is defined as a “pre-distorted” profile that takes into account the gradient introduced by bending the fiber. A parabolic index profile is one exemplary bend-resistant profile that exhibits a quadratic form. A raised-cone index is another profile that may be used as the “as-fabricated” profile. In any properly configured form, factors such as bend loss and mode distortion are significantly reduced, since the profile undergoes a shift of essentially constant gradient as a bend is introduced. The resultant effective area of the inventive fiber is substantially improved over state-of-the-art fiber that is subjected to bending during installation. The as-fabricated profile may be incorporated into various types of fibers (birefringent, photonic bandgap, etc.), and is particularly well-suited for use in a fiber amplifier arrangement.

Enhancement of sensitivity in long-period fiber gratings with deposition of low-refractive-index materials.

Abstract: It was proved [Opt Lett30, 720 (2005) ] that the deposition of an overlay of higher refractive index than that of the cladding on a long-period fiber grating (LPFG) causes large shifts in the attenuation bands induced by the grating The result is an enhancement of the sensitivity of the LPFG to variations in the ambient and overlay refractive indices or the overlay thickness The limitation of the previous design to materials with higher refractive indices than that of the cladding of the LPFG is overcome with a five-layer model To this purpose, a first overlay of higher refractive index than that of the cladding of the LPFG will enhance the sensitivity of the device to variations in the refractive index of a second overlay of lower refractive index than that of the cladding of the LPFG Moreover, it is proved that, if the second overlay is thick enough, its behavior resembles that of an infinite layer

Modeling of stimulated Brillouin scattering in optical fibers with arbitrary radial index profile

Abstract: Stimulated Brillouin scattering (SBS) is an impairment seen in narrowband transmission over optical telecommunications fiber at high laser power. Accurate modeling is necessary to predict the strength of this phenomenon. One method of SBS control is the optimal design of the fiber radial index of refraction profile. Previous work has been limited to modeling optical fibers with one or more radial step changes in index. Here, the author derives and solves a general set of differential equations that allow the numerical solution of SBS spectral gain for an arbitrary radial index profile. Simulated and measured spectra are compared for several fibers with GeO/sub 2/-doped cores. It is found that radial material displacement plays a significant role in the SBS interaction in fibers with a complex radial index profile.

Refractive indices of cubic-phase MgxZn1-xO thin-film alloys

Abstract: Refractive indices for cubic-phase MgxZn1−xO (0.55⩽x⩽1) thin-film alloys were determined from transmission measurements and Manifacier envelope method. The refractive index of cubic MgxZn1−xO decreases with the Mg fraction increase, such as at the wavelength of 400nm the refractive index decreases from 1.89 to 1.73 as x increases from 0.57 to 1.0. The refractive index dispersion relation of cubic MgxZn1−xO in the visible-light region (400–800nm) follows the first-order Sellmeier equation and decreases with the increase of Mg fraction.

Long-period fiber gratings with overlay of variable refractive index

Abstract: A theoretical analysis is presented of a long-period fiber grating (LPFG) with an overlay of variable refractive index. The highest sensitivity of the resonance wavelengths to variations in the refractive index of the overlay can be optimized. There are two key points for a good design: the selection of an overlay refractive index close to that of the cladding of the LPFG and the overlay thickness. The problem is analyzed with a numerical method based on coupled-mode theory.

Refractive index variable element and method of varying refractive index

Abstract: A refractive index variable element includes a structure including quantum dots having discrete energy levels and a dielectric matrix surrounding the quantum dots, and an electron injector injecting an electron into the quantum dots through the dielectric matrix.

Optical film and image display

Abstract: An optical film of the invention comprises: a complex type scattering-dichroic absorbing polarizer including a film that has a structure having a minute domain dispersed in a matrix formed of an optically-transparent water-soluble resin including an iodine based light absorbing material; and a retardation layer including a transparent layer that has a thickness of at most 10 μm and exhibits refractive index anisotropy characterized by nx≈ny>nz, where a thickness direction of the transparent layer is defined as Z-axis and refractive index in Z-axial direction is defined as nz, an in-plane refractive index of the transparent layer perpendicular to Z-axis gives a maximum is defined as X-axis and refractive index in X-axial direction is defined as nx, and a refractive index of the transparent layer perpendicular to Z- and X axes is defined as X-axis and refractive index in Y-axial direction is defined as ny. The optical film has a high transmittance and a high degree of polarization, can produce high contrast in a wide viewing angle range, and can suppress unevenness in transmittance during black viewing.

Fabrication of light-induced self-written waveguides with a W-shaped refractive index profile

Abstract: This paper proposes a novel technique for automatic waveguide formation by means of the self-trapping effect of optical fiber irradiation into a photopolymerizing resin. We investigate experimentally the phenomenon of thin cladding layer formation surrounding the core following the core creation. In the proposed technique, a counterdiffusion effect involving polymerizing monomers via the core/cladding interface causes enrichment of a low refractive index monomer, and a resultant "W-shaped" refractive index profile is realized. The measured propagation loss of the fabricated waveguide is 1.7 dB/cm at 0.68 /spl mu/m wavelength. This technology is appropriate for the fabrication of large-core optical waveguides of greater than 0.5 mm in diameter and is useful for automating the optical fiber connection and packaging process by virtue of being an all-passive optically induced process.

Micrometer-scale particle sizing by laser diffraction: critical impact of the imaginary component of refractive index.

Abstract: Purpose. This study evaluated the effect of the imaginary component of the refractive index on laser diffraction particle size data for pharmaceutical samples.

Method for compensating modal dispersion in multimode optical fiber transmission path

Abstract: In an optical transmission path including multimode optical fibers, modal dispersion is reduced so that signal light can be transmitted at high speed and across a broad band, at low-cost and over a long distance. To reduce modal dispersion, when the transmission path is constructed by coupling a plurality of multimode optical fibers, a length ratio for the multimode optical fibers that obtains the maximum band of the optical transmission path is determined, and the multimode optical fibers are coupled according to this length ratio. The multimode optical fibers that are used have specific refractive index profiles as mode dispersion-compensating fibers. The compensated fiber and the mode dispersion-compensating fiber are coupled with specific lengths.

Broadband optical fiber

Abstract: An optical waveguide fiber having a multi-segmented core surrounded by a cladding, the core having a central segment and an annular segment surrounding the central segment. The central segment has a positive relative refractive index profile, and the annular segment has a negative relative refractive index profile. The broadband optical fiber has a bandwidth of at least 2 GHz-km for one or more wavelengths between 775 and 1100 nm.

Rare earth doped single polarization double clad optical fiber and a method for making such fiber

Abstract: An optical fiber (10), comprising: (i) a rare earth doped silica based elongated core (12) with a first refractive index (n1 with an aspect ratio of 1.5 to 10; (ii) a silica based moat (13) abutting and at least substantially surrounding the core, the moat having a refractive index n2, wherein n2 n3; and n3>n2; (iv) a silica based outer cladding (16) surrounding said inner cladding, the outer cladding having a fourth refractive index (n4), such that n4< n3; the optical fiber exhibits single polarization at the operating wavelength band.