The Effect of Core Diameter on Raman Scattering Loss

Abstract

This paper highlights the effect of core diameter of different types of optical fiber cables on Raman scattering loss of an optical fiber communication system. This paper reports the numerical analysis of Raman scattering losses at three windows of operating wavelength of a laser for four types of optical fiber cables namely- Multi-Mode Step Index, Multi-Mode Graded Index silica fiber and plastic fibers.

This loss characteristic of the mentioned types of optical fiber cable has been analyzed through numerical approach. From the numerical analysis of the present research work it is strongly revealed that although the Raman scattering loss is affected by the operating wavelength, it is also strongly governed by the core diameter and the type of the cable. From the rigorous investigation of the numerical analysis it is found that the Raman scattering loss declines with the application of Multi-Mode Graded Index silica fiber. And for Plastic fiber application Step Index fiber offers better performance.

Introduction

Optical fiber communication is a method of transmitting information from one place to another by guiding pulses of light through optical fiber [12, 13]. Optical communication system faces problems like dispersion, attenuation and non-linear effects. Among them dispersion affects the system the most. Dispersion is a pulse spreading in an optical fiber which increases along the fiber length.

For the transmitted optical signals dispersion which create distortion for digital and analog transmission towards optical fibers.at the time of consideration of main execution of optical fiber transmission that include various kinds of digital modulation, after dispersion working into the cause of fiber amplifying of the dispatched light pulses like they passage towards the channel.

The categories of dispersion include Modal dispersion which is Pulse spreading caused by time delay, Chromatic dispersion which is Pulse spreading caused by different wavelength of light propagate by different velocities, Material dispersion which is Wavelength dependency on index of refracting of glass, Waveguide dispersion which is Due to physical structure of the waveguide and Polarization mode dispersion which occurs due to Birefringence [1, 3].

Dispersion compensation is the most important feature required in optical fiber communication system because absence of it leads to pulse spreading that causes the output pulses to overlap [2, 3]. The Nonlinear effects of optical fibers, for example self‐phase modulation (SPM) and four wave mixing (FWM), be able to menifest at high powers of optical in a fiber-based system. In SPM, the incorporeal and temporal modifications to optical pulses be possible to ascend. The another one is FWM, it is among the strongest parametric wave mixing nonlinearities.

The nonlinear effects in optical fiber occur either due to intensity dependence of refractive index of the medium or due to inelastic-scattering phenomenon. Various types of nonlinear effects based on first effect such as self-phase modulation, cross-phase modulation and four-wave mixing. Their thresholds, managements and applications are also discussed and comparative study of these effects is presented. Transmission of signal information through optical fibers rapidly improved due to quality of transmission and broad bandwidth. This contribution covers modulation techniques employed in the optical transmission medium. The focus is on negative influences of the optical environment.

The optical fibers, which are commonly use, may be divided into two groups depend on their modal properties, which are single-mode fibers, and multimode fibers. The Single-mode fibers are step-index. Multimode fibers are seperated into two groups step-index, and graded-index. Step-index or graded-index indicate the variation of the index of refraction with radial space from the fiber orbit. These three forms of fibers namely step-index multimode, graded-index, and single-mode.

Theirs fibers formed by a core that surrounded by a cladding. In core of the higher index of refraction likened to the cladding create total inner reflection at the core cladding interface in step-index fibers. From graded-index fibers, the step by step decrease in the index of refraction with rang from the fiber orbit create light cord to turning back unto the axis as they extend. Multimode guides are determined by different propagation ways for cord.

A modal definition of multimode fibers indicates multiple propagation speeds for multiple modes. Consequently, when a short pulse energy enter into the fiber combine into a multiple of modes, will come at the acceptance end of the fiber distributed over a time period. The spreading out in time of the accepted pulse is because of the multiple propagation suspension of the multiple modes

Among the three (0.89 um, 1.3 μm & 1.55 μm) communication windows 1.55 μm offers lowest attenuation, greater repeater spacing and higher bit rate. These phenomena made it possible to use coherent optical sources compatible with the standard silicon fibers used in optical fiber communication [3].

Therefore most of the recent work on light sources and detectors has been concentrated. Semiconductor sources and detector with Group-III-V compounds in active layers have been studied extensively and used almost exclusively for the present light-wave communication systems in these wavelength regions [5-9]. Efforts have been made to enhance the laser and photo-detector performances.

However the losses due to Raman scattering increases at the rate of 2 with increasing wavelength the future generation of optical fibers, light sources, and detectors may well be operating at still longer wavelengths [10]. It has been reported that to reduce Stimulated Raman scattering loss of optical fibers electronic filters are applied widely [17]. Conversely, to utilize the enormous potential, the prospect to take into consideration for optical fiber communication system that offers minimum loss over an enormous potential bandwidth still required to be investigated effectively [11].

Mathematical Modeling

Stimulated Raman Scattering

Stimulated Raman scattering in an optical fiber has been the subject of intense research for over two decades; since SRS processes are relevant to many aspects of optical communication systems, optical data processing systems, optical amplifiers etc [14]. Low loss optical fibers are currently being considered as transmission media for optical communication systems. At low power densities the losses of an optical fiber will be determined by spontaneous Ram’In the case of forward stimulated Raman scattering the principal effect is a frequency shift of the radiation transmitted by the fiber to lower frequencies.

Sufficiently large frequency shifts could produce amplitude distortion at the receiver if the detector is intrinsically frequency sensitive or if narrow band filters are used. On the other hand backward-wave stimulated scattering processes (either Raman or Brillouin) will result in a severe attenuation of the forward traveling, information carrying wave, due to the transfer of energy to the stimulated backward wave

Stimulated Raman scattering is a nonlinear response of glass fibers to the optical intensity of light. This is caused by vibrations of the crystal (or glass) lattice. Stimulated Raman scattering produces a high-frequency optical phonon, as compared to Brillouin scattering, which produces a low-frequency acoustical phonon and a scattered photon.

Simulated Raman Scattering.

When two laser beams with different wavelengths (and normally with the same polarization direction) propagate together through a Raman-active medium, the longer wavelength beam acknowledges optical amplification at the expense of the shorter wavelength beam. This phenomenon has been used for Raman amplifiers and Raman lasers. At high power level SRS leads to scattering of pump photons to first Stokes photons, then first Stokes acting as a pump generates second Stokes and so on. For many applications the suppression of higher-order Stokes or even first Stokes is desired. Various techniques to perform this have been used; among them are methods based on dual-frequency pumping or four-wave mixing [15,16].

In Stimulated Raman Scattering, the scattering is predominately in the forward direction, hence the power is not lost to the receiver. Stimulated Raman Scattering also requires optical power to be higher than a threshold to happen. The formula below gives the threshold [1]:

where,

PR = Stimulated Raman Scattering Optical Power

Level Threshold (watts)

d = Fiber radius (um)

λ = Light source wavelength (µm)

α = Fiber loss (dB/km)

Discussion

The current analytical research work show that the performance improvement of optical fiber network is possible by reducing the diameter of the optical fiber cable at any operating wavelength of laser.

From the ranges it is clear that Multi-mode graded index Silica fiber offers lowest Stimulated Raman Scattering loss. The numerical findings of this research work can be a doorway for the researchers to facilitate the design of optical fiber cable that offers lowest Stimulated Raman Scattering loss after the experimental validation.

Conclusion

A comparative analysis of effect of diameter of optical fiber cable on Stimulated Raman Scattering loss in Optical Fiber Communication System has been presented in this paper. The diameter dependence of Raman scattering loss at the three windows of Optical Fiber Communication system has been analyzed considering Multi-Mode Step Index, Multi-Mode Graded Index silica fiber and plastic fibers as the transmission media. From the outcome of the comparative analysis through numerical approach it is it ascertained that the lowest Raman scattering loss has been reported for Multi-Mode Graded Index silica fiber. This research work reports that the Raman scattering loss is affected by the core diameter and the type of the cable.

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