Core-cladding Index Difference Research Articles

Dispersion properties of single-mode optical fibers in telecommunication region: poly (methyl methacrylate) (PMMA) versus silica

Abstract In this paper, the dispersion characteristics of two standard single-mode optical fibers (SMFs), fabricated with silica and poly (methyl methacrylate) (PMMA) are studied in telecommunication spectral regions. The effect of structural parameters, such as the radius of the fiber core and the relative core-cladding index difference, is numerically investigated. It is found that over whole spectral range, the PMMA-based SMF shows lower dispersion than the silica SMF. Also, the zero-dispersion wavelength (ZDW) of PMMA-based SMF is longer than that of silica fiber. The results may be of practical importance for the telecommunication applications.

Investigation of evanescent scattering for low-distortion submicron vibration sensing using ferromagnetic cantilevers.

In this study, we investigate the dynamic performance of a previously reported evanescent-scattering platform for submicron vibration sensing with low distortions. The platform consists of self-assembled ferromagnetic cantilevers located above a liquid-cladded optical waveguide. Theoretical analyses show enhancement of sensitivity and dynamic sensing range by reducing the waveguide core-cladding index difference. Moreover, a careful tradeoff between sensitivity and linearity is required, which is determined by the bias position of the cantilever tip. Experimental results confirm that our platform can offer low total-harmonic-distortions (THD) of < 3.00% with a submicron displacement of 0.40 µm over the frequency range from 80 Hz to 750 Hz. The measured THD value is very close to our theoretical prediction. Thus, our platform can be employed in submicron vibration sensing with high-precision requirements.

IMPACT OF FLUCTUATIONS OF GRADED-INDEX OPTICAL FIBER PARAMETERS ON PROPAGATION OF GAUSSIAN BEAMS

In this paper, the impact of fluctuations of graded-index (GRIN) optical fiber parameters on propagation of Gaussian beams is investigated by using ABCD matrix approach and a stochastic process. Since GRIN optical fibers have many applications in the optical communications, variations of fiber parameters such as core refractive index, core-cladding index difference and core radius affect the propagation of optical signals. Results show that the variations of the mentioned parameters change the beam spot size as well as the periodicity of the beam. Also, the fluctuations of such fiber parameters increase the spot size of the output Gaussian beam in comparison with the case when the fluctuations are ignored.

Compact three-core fibers with ultra-low differential group delays for broadband mode-division multiplexing.

An approximate explicit condition for the achievement of zero differential group delay (DGD) in a homogeneous multicore fiber (MCF) is presented and verified numerically for a step-index three-core fiber. This condition is explored for the study of compact three-core fibers with low DGDs. To achieve an ultra-low DGD in the C-band, a three-core fiber with a central refractive-index dip in each core is proposed and analyzed in detail. A specific design with three touching cores and a core-cladding index difference of 0.3% yields a maximum DGD smaller than 3.2 ps/km in the C-band. The fiber is suitable for broadband mode-division multiplexing applications and the design approach could be applied to MCFs with more cores.

Acoustooptic Generation and Characterization of the Higher Order Modes in a Four-Mode Fiber for Mode-Division Multiplexed Transmission

We demonstrate a highly efficient acoustooptic mode converter for selective excitation of the higher-order modes in a four-mode fiber which guides the LP01, LP11, LP21, and LP02 modes at telecommunication wavelengths. The coupling efficiency and the mode extinction ratio are measured by analyzing the output far-field patterns. The acoustic resonance frequencies and the operating bandwidths are compared with the theoretical results, by which the modal characteristics, the core diameter, and the core-cladding index difference are estimated.

Multi-spectral bands optical coherence tomography assisted by ultrawide band photonic crystal fiber splitter

We report fabrication and performance of the 2×2 photonic crystal fiber (PCF) splitter that enables multi-spectral bands operation of an optical coherence tomography (OCT) system. The PCF splitter was made by coupling PCFs to a planar lightwave circuit (PLC) splitter chip, which worked as a single mode splitter from visible to near infrared wavelengths. The core size of the PLC splitter chip was about 3μm×3μm and the core-cladding index difference was about 0.25%, which gave the cutoff wavelength at 630nm. The implemented PCF-PLC splitter showed a small excess loss of 1.2∼3.3dB and a low polarization-dependent loss of 0.15∼0.19dB over a wide wavelength range from 640nm to 1000nm. With the splitter, OCT images of a human finger and a nail have been successfully obtained at several spectral bands; 680nm, 840nm, and 930nm center wavelengths.

Bragg grating based biochemical sensor using submicron Si/SiO_2 waveguides for lab-on-a-chip applications: a novel design

A novel biochemical sensor based on a submicrometer size, high core-cladding index difference, silica core Si-SiO(2) waveguide with a Bragg grating written in its cladding region is proposed and analyzed. Waveguide parameters are optimized to obtain maximum sensitivity, and for lower refractive index samples, an optimum core width is found to exist for both the TE and the TM mode configurations. Owing to the high index contrast at the Si-SiO(2) interface, the structure is much more sensitive while operating in the TM mode configuration, showing extremely high sensitivity [200-740 nm refractive index units (RIU)] for the ambient refractive indices between 1.33 and 1.63, which is of the order of most surface plasmon polariton (SPP) based biosensors. Further, unlike SPP based sensors, the proposed structure is free from any metallic layer or bulky prism and hence easy to realize. Owing to its simple structure and small dimensions, the proposed device could be easily integrated with planar lightwave circuits and could be used for lab-on-a-chip applications.

Liquid-core, liquid-cladding photonic crystal fibers

We experimentally demonstrate a simple and novel technique to simultaneously insert a liquid into the core of a hollow-core photonic crystal fiber (PCF) and a different liquid into its cladding. The result is a liquid-core, liquid-cladding waveguide in which the two liquids can be selected to yield specific guidance characteristics. As an example, we tuned the core-cladding index difference by proper choice of the inserted liquids to obtain control over the number of guided modes. Single-mode guidance was achieved for a particular choice of liquids. We also experimentally and theoretically investigated the nature of light confinement and observed the transition from photonic bandgap to total internal reflection guidance both with the core-cladding index contrast and with the PCF length.

A High Index Contrast Silicon Oxynitride Materials Platform for Er-doped Microphotonic Amplifiers

AbstractEr-based optical amplification continues to be the ideal low noise, WDM crosstalk free, broadband candidate for waveguide amplifiers. Design analysis of the applicability of Er-Doped Waveguide Amplifiers (EDWAs) for micron-scale integrated photonics in a planar lightwave circuit concludes: (i) an &gt;80× increase in gain efficiency, and (ii) a &gt;40×increase in device shrink can be realized, for a high index contrast EDWA (with a core-cladding index difference of δn=0.1↔0.7), compared to a conventional Er-doped fiber amplifier. The materials challenge now is to establish a robust materials system which meets this high index difference design requirement while simultaneously leveraging the capability of silicon (Si) processing: a host platform for EDWAs must be found which can integrate with Si Microphotonics. Silicon nitride (Si3N4), silicon oxide (SiO2) and a miscible silicon oxynitride alloy (SiON) of the two meet this materials challenge. We present the results of reactive and conventional magnetron sputtering based materials characterization for this high index host system. Room temperature and 4 K photo-luminescence studies for annealed samples show the reduction of non-radiative de-excitation centers while maintaining an amorphous host structure. Atomic force microscopy shows less than 1 nm peak-to-peak roughness in deposited films. Prism coupler measurements show a reliable reproducibility of host index of refraction with waveguide scattering loss &lt;2 dB/cm. We conclude that the SiON host system forms an optimal waveguide core for an SiO2-clad EDWA. Initial gain measurements show a gain coefficient of approximately 3.9 dB/cm.

Laser direct writing polymeric single-mode waveguide devices with a rib structure.

A focused argon-ion laser beam is used on a spin-coated polymeric thin-film deposited upon a SiO2/Si substrate to polymerize the core for fabrication of Gaussian profile optical channel waveguides. A rib structure that allows only the fundamental mode to propagate even with its higher dimension and high-index contrast between the core and the cladding was fabricated. When the thickness of the core-index region outside the rib section decreases, the waveguide produces higher-order modes at the output. The waveguide reported here has cross-sectional dimensions and numerical apertures that match the single-mode fibers for efficient coupling. I used a mixture of two intermiscible acrylate monomers for the cladding and the core of the waveguides. The polymerization process and its dependent dwell time or scan speed and the laser power intensity are shown. I present the operational characteristics of directional couplers using a rib waveguide structure with a core-cladding index difference.

Studies on a few-mode fiber-optic strain sensor based on LP/sub 01/-LP/sub 02/ mode interference

We studied a fiber-optic strain sensor based on interference between LP/sub 01/-LP/sub 02/ modes of a circularly symmetric few-mode fiber (FMF). A simple theoretical analysis for predicting the performance characteristics of such a sensor is developed. Fiber elongations producing differential phase shift of 2/spl pi/ (/spl delta/l/sub 2/spl pi//) between the two modes are measured, which is found to be independent of sensing length of the fiber as predicted by theory. A good agreement between the experimental and theoretical values of /spl delta/l/sub 2/spl pi// is observed. Theoretical calculations predict that the sensitivity of such a sensor is maximum for fibers with V=4.82 and increases with the increase in the core cladding index difference /spl Delta/n.

Physical properties of optical fiber sidetap grating filters: free-space model

Presents theoretical and experimental results on the wavelength properties of sidetap grating filters written in single-mode optical fibers. A simple theoretical model based on Fraunhofer diffraction and antenna theory is used, modified to take into account the longitudinal variation of the incident field. Expressions are derived for the filter attenuation and bandwidth, and good agreement is demonstrated with experiment. The results show that the shape of the attenuation spectrum does not change with the filter length or index modulation, but that the magnitude of the attenuation increases with both parameters. The results also show that the effect of decreasing the fiber core-cladding index difference is to increase the peak attenuation and reduce the filter bandwidth. The existence of a strict longitudinal phase-matching condition is shown to cause asymmetry in the shape of the spectrum. Comparison with the properties of bulk sidetap gratings shows there to be a shift in the peak wavelength and apodization of the filter sidelobes. A simple diagram for the design of sidetap filters is presented for the first time.

A Vectorial Galerkin Method Based on H Fields

A vectorial Galerkin method based on the transverse magnetic field (H formulation) is developed and presented. The general results are applied to a simple step index fiber with a large core-cladding index difference. A comparison with the exact values and other numerical results is made, the results are found to be in reasonable agreement. In addition, we also demonstrate the difference of the effective refractive index in optical waveguide systems between two different formulations, that is, the vectorial Galerkin method based on E field and H field with Hermit-Gaussian basis functions

An approximate solution of the vector wave equation for optical waveguides using galerkin's method with hermite—gaussian basis functions

Abstract A vector Galerkin's method based on the transverse electric fields (E formulation) is formulated and presented. The general results are applied to a simple step index rectangular waveguides with large core-cladding index difference.

Solution of the vector wave equation for general dielectric waveguides by the Galerkin method

Numerical guided mode solutions for vector waves of general dielectric waveguides are obtained by expanding the unknown field vectors in a double series consisting only of sine functions. The authors formulate solutions by series expansion for the electric and magnetic vector wave equations that yield different matrix equations but identical eigenvalues. The general results are applied to a simple step-index fiber with a large core-cladding index difference and to a fiber core in proximity to a semiinfinite dielectric half space. >

Low-loss criterion for fused polarisation-maintaining fibre couplers

A simple low-loss criterion for polarisation-maintaining directional couplers made of fused tapered stress-induced birefringent fibres is proposed and used to explain the loss characteristics of some experimental couplers. It is shown that the very low excess loss of some experimental couplers is due to the combined use of index-matched stress-applying regions and a high core-cladding index difference.

Dopant diffusion loss mechanism in high-birefringent-fiber fused couplers

The observation of dopant diffusion during the tapering of high-birefringent fibers that increases with initial modal birefringence is reported. The resulting reduction in the core-cladding index difference is postulated to result in an increased taper transmission loss and is shown to correspond to increased 3-dB fused-coupler insertion loss.

Design criterion for low-loss optical coupler tapers

A criterion for the local taper angle of a low-loss single-mode fibre coupler is established by considering the dominant mode coupling in the coupler taper. Effects due to structure parameters, such as cladding/core thickness ratio, corecladding index difference and core separation, are investigated. It is found that the criterion for the coupler taper can be well approximated by that for the single-mode fibre taper with the same structure parameters.

Dielectric-waveguide strain gauge: a theoretical study

We present a theoretical analysis of the strain dependence of bend loss from a multimode slab dielectric waveguide formed in the shape of an arc of a circle. This dependence suggests its use as a strain gauge. We calculate the strain sensitivity of this gauge as a function of the core–cladding index difference, bend radius, and arc angle. The method that we use is a geometrical-optics analysis modified to account for the curvature of the boundary between the core and the cladding.

Configuration of the optical transmission line using stimulated Raman scattering for signal light amplification

Expansion of a repeater spacing of optical transmission line is investigated using forward stimulated Raman scattering generated by a 1.34-μm Nd:YAG laser to amplify the signal light. Achievable repeater spacing at the 1.42-μm wavelength transmission is numerically estimated on the basis of coupled power equations and experimentally obtained Raman gain for various core-cladding index difference. A transmission distance of more than 200 km is predicted for an input signal light power of 100 μw and a pump power of 200 mW using a fiber with a relative index difference of 1 percent.