Dispersion” Submitted by Muhammad Abul Khayer Azad, Roll No: M10053103P, Session: October 2005 has been accepted as satisfactory in partial fulfillment of the requirements for the degree of Master of Science in Information and Communication Technology on March 9, 2010. I express my sincere thanks to him for his continuous guidance, suggestions and wholehearted help throughout the course of the work. On the other hand, group velocity dispersion (GVD) is one of the linear effects in the optical fiber that also limits bit rates.
In this research work, an analytical model for a WDM system is developed that includes the combined effects of XPM, first-order and second-order GVD. The spectral characteristics are highly dependent on the channel spacing and GVD of the fiber, i.e. it is found that at a modulation frequency of 10 GHz and a channel spacing of 0.8 nm over a distance of 100 km DSF incurs an XPM crosstalk penalty of 23 dB and 16 dB more than SSMF for the first order GVD and second order GVD respectively.
The crosstalk effect due to the second-order GVD is insignificant in the presence of the first-order GVD in a single-span system, but for a high bit rate and long-distance compensated first-order GVD system, the second-order GVD effect becomes significant and plays a key role in limiting system performance. The simulation results show that the system suffers the maximum penalty from the XPM effect at zero dispersion coefficient and that the XPM disturbance depends linearly on the optical power of the injected signals over a very large range of system parameters.
Introduction
- Communication system 1
- Evolution of optical communication 2
- Review of previous works 4
- Objectives of the thesis 8
- Thesis organization 9
Therefore, it is essential to study the impact of XPM and GVD to accurately predict the performance limitation or system failure due to these effects in the WDM transmission system. In the next section, we describe some of the research works carried out to evaluate the impact of XPM in WDM optical transmission system. This is due to the effect of the baseband filter in the optical receivers.
2006) investigated the performance optimization in SCM-WDM Passive Optical Networks in the presence of XPM and GVD both experimentally and theoretically [21]. 2006) have theoretically shown the effect of XPM on the performance of a WDM optical transmission system with short time dispersion - managed fiber (SPDMF) [22]. They analyzed the performance of 2-channel WDM system using SPDMF to evaluate the impact of XPM on BER.
No work has yet been reported on the impact of XPM crosstalk considering the presence of first- and second-order GVD together in a WDM system. Thus, it is important to study the effect of XPM and in the presence of first- and second-order GVD to accurately predict the performance limitation or system outage due to XPM in the WDM system.
Optical Communication and Fiber Nonlinearities
Principles of optical communication 10
- Optical transmitter 11
- Optical receiver 11
- Optical fiber 12
The main function of an optical transmitter is to convert the electrical signal into an optical signal using a light source. The most important consideration when selecting a source is its ability to deliver maximum power in the appropriate wavelength to the optical fiber. The coupler is typically a microlens that focuses the optical signal onto the input plane of an optical fiber with the greatest possible efficiency.
The amount of power output is an important factor in the design of an optical communication system. Optical fibers are the transmission medium of a fiber optic communication system that bridges the distance between an optical transmitter and an optical receiver. In order to ensure that the transmitted signal is propagated to the receiver with acceptable levels of attenuation and distortion, the main concern in fiber design is that the same information can be received by the receiver with minimal error.
With the development in the field of optical fiber communication, the attenuation of the signal could be reduced to 0.2 dB/km. But many aspects of non-linear properties of the fiber still remained as the limitation of optical fibers.
Optical fiber characteristics 12
- Linear characteristics 12
- Attenuation 13
- Intrinsic attenuation 13
- Extrinsic attenuation 15
- Chromatic dispersion 15
- Group velocity dispersion 17
Factors that contributed to this loss parameter reduction were improved fiber design technique, low loss fiber window, dispersion compensation, etc. Intrinsic attenuation is caused by substances present in the fiber, while external attenuation is caused by external forces such as bending. When a light signal hits an impurity in the fiber, one of two things happens: It is scattered or absorbed.
The OH symbols indicate that at wavelengths 850 nm, 1310 nm and 1550 nm the presence of hydroxyl radicals in the cable material causes an increase in attenuation. These radicals are due to the presence of residual water that penetrates the fiber optic cable material through either a chemical reaction in the manufacturing process or as moisture in the environment. This phenomenon causes a light signal to be absorbed by natural impurities in the glass and converted into vibrational energy or another form of energy such as heat.
As the light travels into the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fiber. If the light is scattered at an angle that does not support continuous forward motion, however, the light is deflected from the nucleus and attenuation occurs.
Depending on the angle of incidence, part of the light propagates forward and the rest deviates from the propagation path and escapes from the fiber core. The same principle applies to analyzing losses associated with localized events in the fiber, such as splices. Bending stress affects the refractive index and the critical angle of the light beam in that specific area.
Microbending is caused by imperfections in the cylindrical geometry of fibers during the manufacturing process. Where D (λ) is the chromatic dispersion parameter of the fiber and is the spectral width of the light source, as usual. However, chromatic dispersion is an important phenomenon in the propagation of short pulses in optical fibers.
Fiber losses 18
The Kerr effect, also called the quadratic electro-optic effect (QEO effect), is a change in the refractive index of a material in response to an applied electric field. The Kerr effect was discovered in 1875 by John Kerr, a Scottish physicist. In a multichannel system, the nonlinear phase shift of the signal at the central wavelength i is described by [25]. However, due to the chromatic dispersion of the fiber, the baud rate for a single optical channel eventually reached its limit.
A post-amplifier is used to counteract the insertion loss of the multiplexer at the transmitter. It is also customary to include an in-line amplifier to account for the attenuation of the fiber. The only significant non-linear contribution is from (3). The nonlinear polarization is then reduced to
The first two terms on the right-hand side of Eq. 3.11) are the non-linearity factors that arise due to the non-linear refractive index of the fiber. The XPM effect is modeled in the same way as the SPM effect by introducing another term on the non-linear part of the NLS equation. The effective non-linear refractive index n2 of the XPM effect is twice that of the SPM effect.
If we consider only the dispersion and the source term of the phase disturbance at z=z', the Fourier transform of equation (3.13) becomes. 3.16) Where the Kronecker delta (zz') is introduced to account for the fact that the source term exists only in the infinitesimal fiber section at z=z'. In this chapter we calculate the BER of the system at the receiver in the presence of amplifier noise. At the output of the fiber, the probe signal undergoes an XPM effect and the waveform at the output becomes distorted.
At the output of the trapezoidal filter, a photodiode converts the optical signal into an electrical signal. It has been found that the spectral characteristics of the fiber depend on the channel spacing and as a result at higher modulation frequency and lower channel spacing the amount of crosstalk increases. Again, while only second-order GVD is considered (Fig. 4.6), the amount of cross-talk (for the same modulation frequency and channel separation) is only -80 dB.
The optical input and output spectrum of the pump and probe are shown in Fig. Fig. 4.27 shows the eye diagram of the 4 channels at the input and output of the transmission system. It is observed (Fig. 4.28) that the
Of the three fiber types, the XPM effect is the smallest for SSMF and the largest for DSF.
