 that is on the optical fiber dispersion. So these are the learning outcomes. At end of this session students will be able to illustrate the dispersion losses in optical fiber. Now before starting to this lecture just recall how many types of losses we have studied in the optical fiber here. So we have studied almost two to three losses which affects the signal while transmitting in the fiber optical cable here. The first loss what we have seen was the absorption loss here. So the absorption loss was the imperfection of due to the atomic structure of the fiber materials. So in which it can be an eccentric and the eccentric here. So in eccentric we have seen that is the transition of an electrons from the lower energy to the higher energy state here. So due to which there is a loss of signal in the optical fiber. An eccentric in which the water molecules gets affected in the signal. So that is known as the eccentric losses. So after that we have seen the scattering losses. In the scattering losses we have seen the release and my scattering. The light gets scattered when the operating wavelength is less than or greater than the operating wavelength in release and my scattering respectively. And we have seen the bending losses also that is the micro bending and the micro bending that is the manufacturing losses. So we can overcome that losses while we have to make the new optical fiber. The dispersion loss is a little bit different as compared to the absorption the scattering and the bending losses here. So we will study now regarding to the dispersion. So let us understand first what do you mean by the dispersion here. So this is the fiber optical cable. This is the core inside which the light is going to be transmitted and this is the cladding here. So when the input pulse we are going to send that is 1 0 1 0 1 0 that is on off of the LED or the laser as a source here. So we are going to transmit the signal. So over a period of time that is when the signal is being transmitted inside the fiber optics cable here. So there is an overlapping of two signals that is we have seen that is the signal 1 0 and 1. So inside this due to the refractive index of the core there is an overlapping of a signal here. When the output is received at the receiver side here so the overlap of the pulses will takes place. It is indistinguishable pulse says we cannot distinguish between the 1 and 0. So such case are also known as an interference ISI that is inter-symbol interference. So we will not be able to receive the signal properly at the receiver side here. So in short we can say that when the optical signal or the pulse guided into the fiber optics the pulse gets spread and broaden as it transmit through the fiber here. Such type of occurrence is called as a dispersion. So what is the effect of the dispersion? The spreading of an optical pulse limits the information carrying capacity of the fiber. Means your fiber optics will not transmit as the data should be transmitted here. So how we are going to overcome this the dispersion losses here. So there are we will see now there are the two types of different types of dispersion in the optical fibers that is the intra-model dispersion and the inter-model dispersion here. That is the intra-model is also known as the chromatic dispersion which occurs in the all types of fiber means the it occurs in the single mode as well as in the multi-mode here and intra-model dispersion occurs only in the multi-mode it doesn't takes place into the single mode because only it transmits the fundamental mode here. So now in detail we are going to study about the intra-model and the inter-model dispersion here. So the intra-model dispersion is nothing but it primarily depends on the fiber material. So the fiber material used for the manufacturing are the silica and the OH bond like this okay the SiO2. So in that depending on the fiber material again there are two types of intra-model dispersion that is the material dispersion and the waveguide dispersion here. So now we'll see the in detail what do you mean by the material dispersion here. So the material dispersion takes place due to the different wavelength traveling at the different speed inside the fiber here. So the different wavelength traveling at the different speed inside the fiber it depends on the the velocity of a light traveling inside the core here. As you know that in the step index and the graded index that is the refractive index is going to be changed in the graded index here. So the different wavelength will be traveling at the different speed inside the fibers here. So now we'll see the material dispersion that is we have seen that is the wavelength 1 and wavelength 2 is shown here. Same like this is core and this is the cladding here. So how we are going to minimize the material dispersion is that we can minimize by using the wavelength such as 8, 17 nanometers, 13 nanometers and 15, 15 nanometers wavelength which is also known as the zero dispersion wavelength that is ZDW here. So if you see only calculate the dispersion losses or the attenuation due to the dispersion so it will be minimum almost it is zero at 8, 70, 1300 nanometers and 1500 nanometers. But every time we cannot send the data through the light that is which is wavelength of 8, 70 or 1300 because there the attenuation losses due to the absorptions and the scattering is more at the wavelength if you use at the lower wavelength here. So that's why we have shifted that that is known as the dispersion shifted fibers here that is will be 1550. So where the dispersion losses are zero means the broadening of pulse will be less at that time and the attenuation due to the other effects also will be less here. So that's why it can be minimized by using such a wavelength here. When the wavelength is less than ZDW that is zero dispersion wavelength it travels slower and when it is a higher than the ZW it travels faster here. Thus the speed is changed and it is used in such a way that all the waves passing through the fiber will move with the constant speed and hence the material dispersion is minimized here. And this dispersion will not occur in the single mode fiber here it will happen in only the multimode fiber that is the material dispersion here. So now we will see the waveguide dispersion. The waveguide dispersion is caused due to the difference in the index of the refraction between the core and cladding. The index of the refraction means at which the angle is going to be refracted here. Means whenever you're going to incident the light that is a different wavelength that is lambda 1 and lambda 2. So it has been incident at the acceptance cone and it is going to be incident at phi 1 and this is incident at the phi 2. So there is a difference in the index of the refraction between core and cladding here. So waveguide means which will travels from transmitter to the receiver here which results in the drag effect between the core and cladding portion of the power here. So such type of dispersions are known as the waveguide dispersion here. So now we will see the intermodal dispersion. So now we have seen the intramodal dispersion. So the intramodal dispersion occurs only in the multimode fiber. So we will see in detail now that is the intramodal dispersion. When more than one mode is transmitting through the fiber they will have the different wavelength and will have to take different time to propagate through the fiber here which leads to the intramodal dispersion. So this is the lambda 1, this is the lambda 2 which will be received at the different speed and the different time at the receiver side. Then each mode will be different wavelength with a different speed. Hence they reach at the end of the fiber with a different time here. This results in the plers broadening here. Thus this can be a distorted pulse. Such phenomena is called as an intramodal dispersion here. So the refractive index for the single mode only the fundamental mode is transmitted. So there is no problem of receiving the signal at the receiver side here. But in the multimode whenever there is a more than one mode is being transmitted here. So at that time the whatever the signal is being sent here. So due to the refractive index of an core and the angle of incidence. So this lambda 2 will travel with a different speed and lambda 1 will travel with a different speed if it use at the graded index also and the step index and it will reach at the receiver side with a different time and with a different speed here. So what will happen? It will be the overlapping of the two signals will takes place here and which can be a pulse broadening. So such type of things are known as the intramodal dispersion here. So as we have seen due to the spreading of light or the broadening of a light in the optical fiber, there is information carrying capacity becomes less here. So if you want to calculate that how much is the usable bandwidth. So this is an example we have taken. So it can be calculated based on the bandwidth distance product and this value is also expressed in Hertz kilometer. If the fiber is having the bandwidth distance product of 250 megahertz per kilometer, and the length of the link is five kilometer that is the length of an optical fiber, then the usable bandwidth over the this link can be calculated that is usable bandwidth is equal to 250 megahertz per kilometer that is divided by five kilometers it becomes an 50 megahertz. So this is the usable bandwidth of an the information carrying capacity of an the fiber optics here. So these are my references. Thank you.