Hello and welcome back to this channel. Dear students, this topic is measurement of dispersion rather intermodal dispersion of the optical cable. Then measurement of NA that is numerical aperture and we will also discuss the I pattern. First part is measurement of dispersion. As I said, we'll be discussing the technique to measure intermodel dispersion in case of fiber optic cable. So this is the uh block schematic which is used to measure the intermodel dispersion in optical cable. We are making use of pulse laser source. As the name indicates this is a laser which generates different pulses of light. So this is the pulse uh laser source whose output is as shown by these uh red arrows. This is lens one. Lens one output of this part lens one is applied to the beam splitter. Now one portion as the name indicates this beam splitter splits the beam into two portion. One portion is like this. That means it is coming out from this beam splitter. And one more lens we are using that is lens five. And then this lens five is used to focus the light beam on the APD. APD stands for avalanche photo detector. We are making use of photo diode which is APD. So this incoming light which is generated from the pulse laser is applied to lens one. Then through the beam splitter splitting of this beam takes place. It is applied to the lens five. Lens 5 focuses that uh light beam on APD avalanche photo detector. We know the function of photo detector is to convert optical signal into electrical signal. Then this is given to the sampling oscilloscope. This sampling oscilloscope that is sampling CRO is used and this output of APD avalanch photo detector is used as a trigger signal for this sampling oscilloscope. So we can say at this point you are taking out the input uh that means the signal light ray which is coming from the pulse laser source as it is without passing through the fiber optic cable. So this is one input to this sampling oscilloscope. Now the second part from the beam splitter is applied to lens two. Again lens two is used to focus the light beam. Then this particular portion of the light beam is propagating through the FOC that is fiber optic cable. Whenever this light propagates passes through the fiber optic cable, broadening of pulse takes place. That's what we want to measure. And then again we are making use of lens three and lens four and the output is applied to avalanche photo detector that is avalanche photo diode. The output of this avalanche photo diode which is electrical quantity rather as we discussed it converts optical signal into electrical signal. But keep in mind this optical signal which which is applied at the at the input of this APD is related to the light ray which is propagating through the fiber optic cable. Whereas the output I mean the input to this APD is not the light ray which is apply which is propagating through the fiber optic cable. So this APD converts the output light from the uh propagating through the fiber optic cable into electrical signal and it is applied to the another input of sampling oscilloscope. Now this pulse broadening is denoted by 3 dB rather it is 3 dB bandwidth. Uh so this is T3 dB which is T03 dB. T0 is the pulse width at the output at output of optical cable and it is in terms of 3dB minus TI² 3 dB. So TI is the pulse width at the input upon L. L is the length of optical cable. So this we are taking square root of this numerator. That's why it is bracket to 1/2. Then once you'll get this pulse broadening which is denoted by T uh T3 dB. we can calculate the bandwidth. So bandwidth is given by 44 upon 3 dB. The next part is measurement of NA that is numerical aperture of an optical cable. If we know the refractive index of the media then uh suppose we have the values of n_sub_1 and n_sub_2 then this numerical aperture na can be directly calculated using the formula square root of n_sub_1² minus n_sub_2² this is possible if you are knowing the refractive indexes of the material. Apart from this, this setup gives us how to gives us an idea how to measure the numerical aperture of an optical cable. At the input side, we are making use of a light source. We know that it can be LED or a laser. Usually, LEDs are preferred. Then its output is propagated through FOC. That is it is propagated through fiber optic cable. This is applied to rotating fiber mount. As the name indicates one end of the optical fiber is connected to this and we are rotating it. Then this light is applied is passing through the small aperture and then it is applied to the photo detector. We know that usually APDS that is avalanche photo detectors are preferred. It converts optical signal into electrical signal and its output is applied to the power meter. Every time this rotation of one end of the optical cable in is done in such a in such a way that you are getting some significant output at the of the power meter. That means you have to measure all the angles where you are getting some readings from the power meter. These readings of angles after rotating this one end of the optical cable gives us the value of acceptance angle. Then numerical aperture is calculated using the formula n0 sin theta max. This n0 is the refractive index of the medium apart from fiberoptic cable. That means you can say n0 is the refractive index of the medium outside the fiberoptic cable. If the light rays are coming entering from the air medium then we know that for air refractive index is ideally one. So n0 will be one. In that case n a will be n0 that is 1 sin theta max where theta max is the maximum angle which is given as 12 acceptance angle. We we just now discussed how to measure this acceptance angle that is by rotating this fiber mount. So this is the way to measure the numerical aperture of an optical cable. Now the last part of this session is eye pattern. From the exam point of view, you may expect the question like this. draw the I pattern and uh explain the various information that can be generated by making use of I pattern or draw and explain the typical setup which is used for the generation of I pattern. Basically I pattern is superimposition of logic zeros and logic one on a particular waveform. This diagram shows the typical setup which is used to generate the I pattern. So we are making use of random bit pattern which generates the random bits logic uh ones and logic zeros. It is given to the transmitter. Then this data is applied propagating through the OFC that is optical fiber cable. At other end of OFC we are using RX that is receiver session that is a photo detector and then it is applied to one input of the CRO. Ideally we are making use of two plates of the oscilloscope that is X plate horizontal plate and vertical plate that is Y plate to the Y plate we are applying received signal that means we can say the signal which is coming out from the optical cable and the standard sawtooth signal is applied to the X plate. This sawtooth signal is generated at the rate 1 by TB where TB is the symbol duration. We are making use of pattern bit generation. So one signal which is coming out from the optical cable is applied to one plate of CRO. Another standard sawtooth wave signal or random bit pattern is applied to the X plate. Whenever uh these two signals are applied there is a superimposition or overlapping of this logic zeros and logic one on the on the sawtooth bit pattern and this generate a waveform which is similar to the I. So it is called I pattern. Now this type of wave form is displayed which is called the I pattern. Different informations can be obtained from this I pattern. This particular instant, this particular time period is called best sampling time or best instant at which we can perform the sampling of uh signals. Then this height represents the distortions at the sampling. I have written few important points related to this diagram. So width of eye opening width means this this particular portion indicates the eye opening. So width of this eye opening that means from this point to this point this particular portion represents width of the eye opening. So this width of the eye opening gives us the value of sampling interval that is the sampling time period. Then the height of I opening this particular portion uh this we can say this is the horizontal line which is the reference line. If you measure the height from this point up to the opening of I this part is opening of I. So this height gives us the value of margin over noise. So I have written in the diagram as well. This is margin over noise. Then rate of I closing. This diagram shows the opening of I. As the diagram goes on shrinking you can say the I is closed because these two ends will appear very close to each other. So rate at which the closing of I takes place gives the sensitivity to the timing errors. That means it gives the value of sensitivity to the timing errors as shown in this diagram. This is a straight line. So if you measure this slope this slope of of this straight line of the eye opening gives us sensitivity of the timing error. When I is completely closed, as I said, this is the diagram which shows opening of I. When I is completely show closed, it indicates that there is a effect of maximum inter symbol interference that is maximum ISI. Then this particular height represents the distortions of the zero crossing. So all these informations can be obtained uh if this eye pattern is generated is displayed on the oscilloscope. So this is about the generation of the eye pattern and what are the different informations that can be obtained by observing this eye pattern. So dear students that's it for this session and that's it for this series. So thank you. Thanks a lot for watching this series.