 the lecture that is on transmission characteristics in optical fiber. So, these are the learning outcomes at end of this session students will be able to classify the different types of attenuation in optical signal. So, now we will understand the basic of attenuation as shown in the figure this is the input optical pulse train that is being transmitted through an optical fiber which is having the length L and this is an output optical pulse train which has been obtained at the end of the optical fiber. So, first we will understand what do you mean by attenuation. So, why there is a reduction in amplitude of the signal at the input which is being given as to the optical fiber and the optical pulse train at the output and there is an overlapping of the signal also. So, such type of things are known as an attenuation and this is measured with respect to the distance travelled by the light rays in the optical cable and attenuation is always expressed in dB. So, in this lecture we are going to see what occurs to optical pulse when they propagate through an optical fiber. So, we will start with this transmission characteristics of an optical fiber in order to understand the attenuation of the signals and the do calculation particular with the dB and dBm units are very useful. So, let us look at the propagation of the optical pulses through an optical fiber. So, if I have my encoded data in the pulse train that is your optical pulse trains of the sequence of zeros and one and when want to send this pulse through the train through an optical fiber then at the output end of the shape of this pulse becomes very something like this. So, what I notice is that the there is a reduction in the amplitude of the pulses and the power level of this pulses also goes down. At the same time the duration of the pulse also increases. So, such type of attenuation are known as the dispersion. So, when I send this pulses through the optical fiber the pulse becomes very like this. So, then I have to pay attention that I have to take care that this pulse do not overlap here, overlap. Also the power level of the pulses should remain same such as the level that the optical detector should be able to detect them otherwise I will lose the information. So, before these pulses are overlapped or their power levels goes down beyond certain limit I will have to reshape this pulse. I will have to bring them to the original form and this is done the devices known as end repeaters. So, in the communication link you send the pulses through an optical fiber and then to the certain distance you will have to reshape the pulses and bring them to the original form and then you again send it to the next segment or then to the next segment and so on. If the L is a repeater less than the link then the repeater less length of the link for the given data rate depends on the two things. One is how much the pulse have been attenuated and what is the spread of these pulses, what is the broadening of these pulses so that they do not overlap. So, overlapping and the reduction in the amplitude is known as the attenuation. So, now we will see the next how the attenuation has been calculated. So, the attenuation is being calculated through the loss coefficient and the in dB if you want to calculate that is loss dB that is log to the base P in upon P out. This is the power input whatever the pulse you are going to give and this is the length of optical fiber and this is the P out how much the power is received at the optical power at the output. If you want to calculate the loss coefficient that is alpha of length L so that is 10 divided by L log to the P in upon P out where the L is in kilometers. So, what is an attenuation is that the power at the input end which I have sent through the optical fiber if it is P in the power that I have received through the output end is the P out. Then if I define the loss of the link losses of the fiber through length L in dB 10 times the log to the base 10 in over the P out. So, this is the loss in dB through L length of an optical fiber if it is an often convenient to represent the loss in terms of loss coefficient which is the loss in dB per kilometer length of the fiber. So, I divide this thing by the length L and I get the loss coefficient alpha that is this in dB per kilometers where L is in kilometers. So, now just recall what will be the effect of attenuation on single mode fiber and the multi mode fiber. So, now we will see the different types of attenuation mechanism that is the material absorption scattering bending losses and dispersion. So, these are the major four losses occurs when you are going to send the data through P in and P out. So, the material absorption is due to the in centric and eccentric materials and the scattering are of again two types that is the linear and non-linear. So, bending loss are again the micro bends and the micro bends and the dispersion is nothing but that is an overlapping of two signals that we have to remove. So, now let us see in this lecture the details regarding according to the material absorption that is in centric and eccentric. So, this is known as the material absorption. So, the absorption is a major cause of the signal loss in an optical fiber. The absorption is defined as the portion of attenuation resulting from the conversion of an optical power into the another energy form such as heat. The absorption in the optical fiber is explained by these three factors that is imperfections in the atomic structure of the fiber material the in centric or the basic fiber material properties and third one is the eccentric that is the presence of impurities that is fiber material properties. So, we will see what do you mean by the imperfections in the atomic structures of an fiber material. The imperfections in the atomic structures induced by absorptions by the presence of missing molecules or the oxygen defects. The absorption is also induced by the diffusion of an hydrogen molecules into the glass fiber since the eccentric and in centric materials properties are the main cause of absorptions as they were discussed. We are going to discuss. So, we will see now the material absorption it is due to the vibration frequency of an atomic bonds in the IR region that is infrared region and it is caused due to the electronic absorptions band in the ultraviolet regions also. So, we will see the details now. So, this is the graph showing for the fiber losses due to the absorption loss. The eccentric absorption is caused by the basic fiber material properties. If an optical fibers were absolutely pure with no imperfections or impurities then all absorption would be an eccentric. Incentric absorption set the minimum level of the absorption. In the fiber optics the pure glass fibers are used most of the time. The silica fibers are used because of their low eccentric material absorptions at the wavelength of operation. So, as shown in this figure the wavelength of operation ranges from 700 nanometers to 1600 nanometers. The figure shows the level of attenuation wavelength of operation. This wavelength of operation is between the two eccentric absorption regions. The first region is the ultraviolet region. This is the ultraviolet region and this is the infrared region. So, in the ultraviolet region it is caused due to the electronic absorption bands. Basically, the absorption occurs when the light particles photons interact with an electrons and which gets excited to the higher energy level. So, this is the tail of the ultraviolet absorption region. And the main cause of the eccentric absorption is in the infrared is the characteristics of vibrations frequency that is vibration frequency of an atomic bonds. In the silica glass the absorption is caused due to the vibrations of silicon oxygen that is SiO bonds and the eccentric between the vibrating bonds and the electromagnetic field of the optical signal caused by the eccentric absorptions. The light energy is transferred from the electromagnetic field to the bond. So, now we will see next that is the material absorption that is an eccentric absorption. The eccentric absorptions is caused by the impurity introduced in the fiber materials. The metal impurities such as iron, nickel and chromium are introduced into the fiber during the fabrications. The eccentric absorption is caused by the electronic transitions of this metal ions from one energy level to the another. The eccentric absorptions also occurs when hydroxyl ions that is OH bonds are introduced into the fiber. The water in the silica forms and the silicon hydroxyl SiOH bonds. These bonds are fundamentally absorptions at 2700 nanometers. However, the harmonics of overtones of this fundamental absorption occurs in the region of operation. As shown in this figure this is the OH absorption band near 1000 nanometers 1200 and the 1400 this is the highest. See absorption peaks defines the three regions as shown now here that is window preferred. The first window is at 850 and second one is at the 1300 and the third one is the 1500 nanometers. The fiber optic systems are operated at this wavelength is defined by one of these windows. So, the amount of water impurities present in the fiber would be less than the few parts per billion that is why the fiber attenuation caused by the eccentric absorption is affected by the level of impurities OH bond present in the fibers. If the amount of impurities in the fiber is reduced then the fiber attenuation is reduced. So, these are my references. We have used the reference books such as and the John senior. Thank you.