Design of Low Dispersion Flattened Optical Fiber

Dispersion of the transmitted optical signal causes distortion for both digital and anal og transmission along optical fibers. When c onsidering th e major implementation of optical fiber t ransmission which i nvolves s ome form of di gital modulation, then dispersion mechanisms within the fiber cause broade ning of the transmitted light pulses as they travel along the channel. This paper demonstrates that non-zero dispersion flatting optical fiber is achievable with dispersion extended over more than spectral ra nge of 300nm. A total dis persion of (0.0398444 ps/nm.km) is obtained due to a design of an optical fiber with triple-clad at wavelength of 1551nm. Also the flatting optical fiber is tested using computer simulation a t two dis tances at 50km and 100km


Introduction
The possibility of low dispersion over extended range of wavelength was presented by Kawakami and Nishida in 1974 [1], and studied extensively thereafter by [2,3].By manipulating the index profile of a fiber, total dispersion can be to go to zero at two or three different wavelengths, and remain close to zero in between.Dispersion flattening occurs by partial cancellation of waveguide dispersion by material dispersion in the wavelength range of operation.In some applications such as wavelength division multiplexing, where a number of signals with different wavelengths are carried by one fiber, it is desired to design the fiber optic system such that all optical signals experience relatively the same low distortion.
The information capacity of fiberoptic systems using dispersion flattened fiber and wavelength division multiplexing (WDM) schemes can be increased many

Material Dispersion
The same physical processes which introduce fiber attenuation also produce a refractive index which varies with wavelength.This intrinsic or material dispersion is primarily a property of the glass used in the core, although the dispersion of the cladding will influence the fiber in proportion to the fraction of guided energy which actually resides outside the core.

Waveguide dispersion
The energy distribution in singlemode fiber is a consequence of the boundary conditions at the corecladding interface, and is therefore a function of optical frequency.The waveguide dispersion occurs because a singlemode fiber only confines about 80 percent of the optical power to the core.Dispersion thus arises, since the 20 percent of the light propagating in the cladding travels faster than the light confined to the core.The amount of waveguide dispersion depends on the fiber design.

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The effect of waveguide dispersion W D on pulse spreading can be approximated in assuming that the refractive index of the material is independent of the wavelength, where [8] Where, V is the normalized frequency The contribution of W D to the total dispersion parameter D, is given by

Dispersion flattened fiber design
Dispersion-flattened fibers are special fibers that provide small dispersion over an extended range of wavelengths.The fiber presented in this paper is four layer circular dielectric waveguides, consisting of a core and three claddings as shown in fig.
(1) A unified formulation is developed which is applicable to all possible four layer fibers with stepindex profiles.Furthermore, by reducing one or two layers to zero, triple-clad, and double-clad geometries are also covered by this formation.
The sensitivity of the total dispersion of fiber due to changes in the structural parameters ( ) are shown in fig.
(1), twelve different fiber material types of silica base and different doping concentration of dopants are used as core and/or cladding.These materials and their Sellmeir's coefficients are tabulated in Table (1).To calculate the total dispersion (D ), both the material dispersion and waveguide dispersion must be calculated.The material dispersion depends mainly on the refractive index.By using equation ( 8), the refractive index of each layer can be determined.
Where, J λ is resonance wavelength and J B is the oscillator strength.
These constants have been tabulated for several kinds of fibers in table (1).The first and second derivatives of the Sellmeir's equation must be calculated, these derivatives of the refractive index as a function of wavelength can then be used to obtain the material dispersion factor  5) and are given by: Where Total dispersion can be calculated by :

Simulation and Results
The triple-clad fiber profile illustrates in fig.
(2) Is tested by using simulation software (optifiber software) include material compositions and radii of various layers of the waveguide, and the wavelength.A listing of silica based materials, commonly used in optical fiber fabrication, is provided in  wavelength moves toward smaller wavelength with a highly increasing dispersion value.Until the certain value then dispersion value will highly decrease.Therefore the selected width of this clad is very important and the value of this index is also very important.These values can be considered as important design parameters.The increase width of the third clad effects the dispersion curve and moves it toward larger wavelength.Also it causes decrease in the waveguide dispersion.
The experiment setup shown in fig.
(5) Describes the optical communication system with using proposed dispersion flatting fiber (DFF).This system consists of optical transmitter with output power of 5dBm at wavelength of 1550nm.
Optical transmitter produces the optical digital signal at bit rate of 40Gbit /s with none return-to-zero (NRZ) code as modulation type.This signal travels over the standard single mode optical fiber.Also, the proposed optical fiber (DFF) with length of 100km and total dispersion equal to 0.0398444 ps/nm.km is used.

Conclusions
The present paper concerned with a flatting dispersion optical fiber (DFF), and an optical transmission line incorporating it.The research studied and optimized this fiber geometry.The total dispersion (D) value achieved in this paper is very low with respect to the previous studies.The proposed DFF is designed with waveguide dispersion of (-13.1272ps/nm.km),material dispersion of (13.0551 ps/nm.km),and the total dispersion of 0.0398444 ps/nm.kmat 1551nm.The proposed DFF has a quality factor of (6.05) and BER of (   PDF created with pdfFactory Pro trial version www.pdffactory.com are optimized in designing this fiber.Thereby, it is expected to obtain three waveguide dispersion factors for the three cladding regions.The waveguide dispersion factors for triple-clad are the extension of equation ( index, first clad index, second clad index and third clad index respectively are used in the design and simulation of the dispersion flattening fiber.Also the refractive indices are calculated the wavelength are demonstrated in Fig. (4).By utilizing the optifiber software, the values of the total dispersion (D) equal to 0.0398444 ps/nm.km it can be achieved at 1fig.(4) Shows the dispersion of the DFF with triple clad.When the thickness of first clad increased, the phase matching PDF created with pdfFactory Pro trial version www.pdffactory.com shows the eye diagram of the received signal with using proposed DFF with length of 50 km, it is very good ,very clear and it has no disturbance .While the eye diagram of received signal with proposed DFF with length of 100km is shown in Fig.(6b).It is also very good, clear with a little disturbance.The international accepted value for bit error rate (BER for the designed fiber greater than the above value, therefore, the design is very bad.But in our case the BER for the proposed DFF with length of

Figure ( 3 )Figure
Figure (3) Index profiles of proposed four layer fiber structure

Table ( 1) Sellmeier's coefficients for material compositions of silica-based glasses
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