Optical Signal to Noise Ratio in Software

Creation QR in Software Optical Signal to Noise Ratio

12 Clifford Headley and Govin P Agrawal, Raman Amplification in Fiber Optical Communication Systems, San Diego, CA: Elsevier Academic Press, 2005, p 9 13 ITU-T G9591, Optical transport network physical layer interfaces, International Telecommunications Union, March 2006 14 Papoulis, A, Probability, Random Variables and Stochastic Processes, New York, NY: McGraw-Hill, 1984 15 Govin P Agrawal, Fiber Optic Communication Systems, 2nd ed, New York, NY: John Wiley & Sons, Inc, 1997, pp 170 172 16 ITU-T G supplement 41, Design guidelines for optical fiber submarine cable systems, International Telecommunications Union, 2007 17 Ivan Kaminow and Tingye Li, Optical Fiber Telecommunications IVB Systems and Impairments, San Diego, CA: Academic Press, 2002, p 174 18 Agrawal, Fiber Optic Communication Systems, 2nd ed, New York, NY: John Wiley & Sons, Inc, 1997, p 177 19 ITU-T G977, Characteristics of optically amplified optical fibre submarine cable systems, International Telecommunications Union, December 2006 20 Kaminow and Li, Optical Fiber Telecommunications IVB Systems and Impairments, San Diego, CA: Academic Press, 2002, p 177 21 ITU-T G supplement 41, Design guidelines for optical fibre submarine cable systems, International Telecommunications Union, June 2007 22 ITU-T G supplement 41, Design guidelines for optical fibre submarine cable systems, International Telecommunications Union, June 2007 23 ITU-T G975, Forward error correction for submarine systems, International Telecommunications Union, October 2000 24 ITU-T G9751, Forward error correction for high bit rate DWDM submarine systems, International Telecommunications Union, February 2004

Chromatic dispersion (CD) is a property of optical fiber (or optical component) that causes different wavelengths of light to propagate at different velocities Since all light sources consist of a narrow spectrum of light (comprising of many wavelengths), all fiber transmissions are affected by chromatic dispersion to some degree In addition, any signal modulating a light source results in its spectral broadening and hence exacerbating the chromatic dispersion effect Since each wavelength of a signal pulse propagates in a fiber at a slightly different velocity, each wavelength arrives at the fiber end at a different time This results in signal pulse spreading, which leads two intersymbol interference between pulses and increases bit errors, see Fig 41a For example, assume an optical pulse of width Tb and spectral width can be broken into a group of seven monochromatic spectral components with wavelengths 1 to 7 and launched into a fiber of length L Then in the presence of chromatic dispersion, each spectral component will propagate in the fiber at different velocities v1 to v7 and will arrive at the fiber end at different group delay times of 1 to 7 This results in pulse spreading by the amount of called relative group delay, see Fig 41b and Eq (41) and Eq (42) Total pulse width at the end of the fiber is Tb + x = L vx

(41) (42)

= 7 1 where L = fiber length, m x = propagation time of the x th spectral component, s = relative group delay of the pulse, s vx = phase velocity of the x th spectral component, m/s

Input pulse { 1 7 group}

1 2 3 4 5 6 7

1 2 3 4 5 6 7

This example is not realistic because an optical pulse cannot be broken into its monochromatic components But it is useful in helping to explain the effect All optical sources will always emit a range of wavelengths referred to as its spectral width Plotting the delays of various spectral components in a fiber will result in a relative group delay curve similar to Fig 42 Chromatic dispersion can be defined as the relative group delay of a range of spectral components divided by the range of the component wavelengths,