Advanced Optical Communications Prof. R.K Shevgaonkar Department of Electrical Engineering Indian Institute of Technology, Bombay Lecture No. # 40 Laboratory Experiment 2 Let us now see a demonstration on a very important instrument which is used, what is called optical time domain reflective meter or OTDR. The OTDR is used to monitor the condition of optical fiber, even after, it is commission in the system. (Refer Slide Time: 00:42) You see, it is a very compact device, in which the light is launched and the scattered light is received; and that is displayed as a function of time or as a function of distance. So, OTDR essentially receive the reflected light from different locations on optical fiber and by that it tells you the status of the optical fiber. So, this is the display which you see, it can work in different modes. These are some advance of OTDR features. You can choose any of this three for operations. So, let us go to the basic OTDR mode. This is the start button and after the fiber is connected, you can start. That is the typical display which you see on the OTDR screen. So, here the horizontal axis gives you re distance and the vertical axis is in terms of DBs.
These are the different locations. What is the status? That is what actually displayed here. So, it shows you location; we can also see the events which are different locations. What is the condition of the fiber? These for recording the data, you can have different settings; you can put distance in meters or kilometers; you can decide the pulse length; you can choose the wave length at which you are calculating the condition. So, you have 1310 nanometer or 1550 nanometers. These are various parameters which the OTDR uses for estimating the condition of the optical fiber. These are the source settings, because as we mention, two wave lengths can be used. That is the modulation rate and pulse rate which is used for sending pulses. (No audio 04:54 to7:20) Let us now conduct the experiment by using OTDR. So, you have this fibers pool here and we want to find out the loss parameters of this optical fiber. So, you see this is the connectorized fiber and this connectorized fiber can be connected to OTDR from this jacket. So, now, the fiber is connected to OTDR. Now we can start the OTDR mode, you can choose any of the two wave lengths. So, let us say, we choose 1310 nanometer wave length. Then we can start with start button and this shows that now the light source is on. So, the optical pulses are launched inside the optical fiber. And, the reflected light is integrated and measured. As you know, see the scattering of light is very weak inside the optical fiber. One has to integrate the reflected signal to improve the signal to noise ratio. So, before things are displayed, many pulses are transmitted inside the optical fiber; or data is collected now and it is displayed. So, you see, this is the trace of the OTDR. So, as we saw, the x-axis is the distance and y axis is in terms of the DBs which measure the loss. So, one can see here, the length of the fiber is about four point four kilometer that is what is shown here. So, you see, your having the scattered light reduces continuously and difference between this and this essentially tells you the loss inside the optical fiber. And at this end, there is abrupt cut-off we got a fiber is ended. So, beyond this, there is no trace of the scattered light. So, you see, sloping line here tells you the loss on the optical fiber. (No audio 11:00 to 11:42)
So, the data shows here, you see we are having one reflection which is minus 47.6; another reflection is from other end of the optical fiber which is minus 49.7. The distance of the fiber is 4.4 kilometers and the total loss on the fiber is 1.422 db. So, these are the parameters which are used, the pulse width is 30 nanoseconds; the time taken is 45 seconds; the span length or the length of the fiber is four point four kilometers and the total loss on the optical fiber is 1.42 db. So, that gives you attenuation constant on the optical fiber which is 0.322 db per kilometers. Let us now conduct another experiment; this is another fiber. The fiber is connected to OTDR, we go through the same process; go to OTDR mode. (No audio 14.40 to 15:10) Let us through the experiment now on 1550 nanometer. Laser is on now; data is required. You see the time remaining here. The data has been acquired and now is displayed on the screen. So, you see something interesting here; you are seeing a sharp peak at this location. This is another peak at this location and then signal drops rapidly. This is the enlarge version of the trace. So, you can see from here to here, this is the fiber; loss is not really very large, but when you are having these two peaks here which are called the panel reflections from the two ends of the optical fiber - one the near end; one from the further end. And, beyond this, there is no optical fiber or this is a very lousy medium here. So, slowly the light intensity dies down. So, this is the one kilometer long spool of optical fiber. We can again see the analysis, these are the reflections which are seen from different locations and that is the loss; you see here, 0.343db which is over distance of this 1 kilometer. Actually, we have here the another fiber which is the very bad quality fiber which is connected. So, this is a good fiber from here to here, this is the joint. And, there is another very bad quality fiber which goes almost up to this point which is two kilometers. So, you see here the losses is not much, but on this fiber which is not a good quality fiber in bad state. So, you have significant loss on this one kilometer. And, again you see here, a final reflection which is from other end of optical fiber.
Now, what we can do is, we can take this two spools or optical fiber and connect it. And, we would like to see from OTDR, the signature of this two fibers plus also the location of the joint (No audio 19:46 to 20:29) Distance, we have to increase now, because or one spool of fiber which was four kilometers; the other one was two kilometers. You can set the pulse width which essentially decides the resolution of the faults on optical fiber. That shows the acquisition time; you have chosen the time to be 60 seconds. You press the start button now and data acquisition starts. So, you see initially, you see lot of noise. And, after more and more data is acquired, because of integration, the single noise ratio improves and the noise goes down. (No audio 21:30 to 22:00) 14 seconds left; the acquisition is complete now and we get final display. We see here now, this is the fiber which the first fiber which we are tested; as the second fiber which we are tested. And, at the joint, you have substantial loss. So, you see, certainly the power drops at this location. And then, this is the fiber loss in the second fiber. This peaks which you see here, this peak these are the ones which comes because of the final reflection. So, that is the 4.4 kilometer length of the fiber. So, you can see here, because of the connecter the loss is significant here. So, it is not a good connector. So, these are the parameters for the first fiber 4.4 kilometer distance, 1.4 DB loss. (No audio 23:52 to 24:31) In this experiment, we will do time domain measurement of material and intermodal dispersion. This is the experimental setup; we will be using two fiber reels. (No audio 24:58 to 25:37) This is a two channel CRO. (No audio 25:39 to 26:09).
This is a wave form modulator. This is the laser transmitter and photo receiver. A wave form generator; this is a LED transmitter and laser transmitter, and wave photo receiver. And, photo receiver takes the optical input and gives the signal output to the CRO. The output of the wave form generator, the r f input of the laser transmitter and optical output. The output of the wave form generator is fed to the r f input. And, the output of the wave form generator is fed to the input of result transmitter, and the optical output of laser transmitter is fed to the photo receiver. The received r f signal is seen on a CRO. Channel two is the received signal; channel one of CRO is connected to the output of wave form generator. Here, you can see the channel one that is the wave form generator on the CRO. (no audio 28:47 to 29:29) This is the output of the wave form generator on channel one. Frequency of the wave form can be adjusted. Channel two shows the wave form of the received signal after passing through the small patch of fiber. Here, we will first estimate the rise time of laser transmitter. You can clearly see, there is rise time of the received signal is higher. You can see that, the rise time of the received signal after transmitting it; fiber is higher that is, we decrease the amplitude of the laser; the amplitude of the received signal also decreases. You will now measure the rise time for laser. Cursor one is at the ten percent level, it shows minus ten nanoseconds. Now, we will adjust cursor two to 90 percent. Cursor two is at three nanoseconds. Now, switch off the laser transmitter ; now we will connect wave form generator, LED transmitter and photo receiver. Now, output of LED transmitter is connected to the photo receiver and output of wave form generator is given as r f input to the LED transmitter. The output of photo receiver is seen on channel two of the CRO. The channel one; the channel two. Channel two is the r f output of the photo receiver. Here also, now, you measure the rise time of LED to adjust the cursor one to 10 percent of the maximum amplitude; cursor one is at minus 6.89 nanoseconds. Now, we adjust cursor two to 90 percent of the maximum amplitude; cursor two is at 8.8 nanoseconds. We will now connect grated index multimode fiber between the transmitter and receiver; these are the fiber specifications. We are connecting two spools wire; this connector is ST to ST type of connectors used to connect the two spools. We are now measured the rise time of LED and laser. Now, we will connect the fiber between the transmitter and the receiver and we will see the difference in the rise time, because of
dispersion. Then we will subtract the effect of the rise time of the source that is LED and laser. And, we will get the rise time because of dispersion. (No audio 36:34 to 37:03) The output of the LED is connected to the fibers. In the other end of the fiber is connected to the photo receiver. So, now light will travel thirty three hundred meter from the transmitter to the receiver. Again, we see the r f output on the CRO. There, we can see the rise time of the received signal is much higher than the (( )) of the transmitter. Channel one is the output of waveform generator. We will now, measure the rise time of LED transmitter. Again, we just the cursor one to 10 percent of the maximum, which is minus 15 nanoseconds; cursor one is at minus 15 nanoseconds; cursor two is at 12 nanoseconds. Difference between the reading of cursor two and cursor one will give us the rise time. Now, we will connect the tools spools of fiber between the laser transmitter and the photo receiver. The output of laser transmitter is connected to the fiber and the other end of the fiber is connected to photo receiver; the r f output is seen on channel two of the CRO. So, as we increase the power of laser transmitter, after a certain value lasing starts and we get a sharp increase. Below the lasing current, received signal was very less. Here also, we can see an increase in the rise time. You will now measure the rise time of the received signal. Now, we adjust it cursor one to ten percent of the amplitude. Similarly, we will adjust cursor two to 90 percent of the amplitude. Cursor two is at 9 nanoseconds and cursor one is at minus 10 nanoseconds. Difference between these two readings will give us rise time of the laser plus the reel 1 and 2.
(Refer Slide Time: 42:52) We will now, measure the dispersion in the fiber. For that, we will first calculate fiber rise time for laser source and LED sources. The rise time for laser plus receiver system is calculated as follows. From over readings, cursor one was at minus ten nanoseconds and cursor two was at three nanoseconds. So, the rise time of the laser plus receiver system comes out to be 13 nanoseconds. Similarly, the rise time of the laser plus fiber plus receiver system comes out to be nineteen nanoseconds. Now, these total 19 nanoseconds have effect of the rise time for laser and receivers system as well. So, to calculate the rise time of the only fiber for laser source, we subtract eight from the total rise time. So, we get the fiber rise time for laser source as 13.384 nanoseconds. Now, similarly, we will calculate the fiber rise time for LED source. The rise time for LED plus receiver system from our readings comes out to be 15.6 nanoseconds; and the rise time of the LED plus fiber plus receiver system come out to be 27 nanoseconds. So, as in the previous case, we will subtract the effect of rise time of LED plus receiver system from the total rise time to get the fiber rise time for LED source. And the fiber rise time for LED source comes out to be 22.037 nanoseconds.
(Refer Slide Time: 44:22) Now, we know that, the spectral width of the laser source that we used is one nanometer. Whereas, the spectral width of LED source is 13 nanometers. Therefore, we can assume, that the laser source an experiences negligible material dispersion as compare to inter model dispersion because of its narrow spectrum width. So, pulse dispersion in fiber with laser source is assumed entirely due to intermodal dispersion. Whereas, dispersion in fiber with LED as contribution of material as well as intermodal dispersion. (Refer Slide Time: 44:57)
We will now use this information to calculate the intermodal dispersion and material dispersion in the fiber. From over previous argument, the pulse broadening due to intermodal dispersion is captured by the rise time of fiber for laser source. So, we take the pulse broadening due to intermodal dispersion as a rise time of laser source which is 13.38 for nanoseconds. From this, we calculate the intermodal dispersion as the pulse broadening due to intermodal dispersion, tau, I am divided by the length of the fiber which is 3.374 kilometers. So, we get intermodal dispersion as 3.97 nanoseconds per kilometer. Now, to calculate the pulse broadening due to material dispersion, we subtract the effect of intermodal dispersion from the total fiber rise time for LED source; which is done in this equation. So, we get the pulse broadening we have to material dispersion as 17.5 nanoseconds. Now, finally, we calculate the material dispersion as d mat equal to the pulse broadening due to material dispersion tau mat, divided by the spectral width of the source which is thirty nanometers in this case and the length of the fiber which is 3.374 kilometers; and we get material dispersion as 173 picoseconds per nanometer kilometer. Here, we can see that material dispersion is negligible for sources of narrow spectral widths. Whereas, for sources whose spectral width is large, the material dispersion is comparable to the intermodal dispersion. (Refer Slide Time: 46:43)
In this experiment, we will measure chromatic dispersion with a WDM system and a ring resonator. Figure shows the schematic diagram of the experimental set up for demonstrating chromatic dispersion in wave length division multiplexing system. We have a bit generator, whose r f output is connected to the two lasers. The output of the lasers sources is combined in a wave length division multiplexer; the output from the one of the port is taken further. Now, this bit generator generates pulses which modulate the output of this 1550 nanometer laser and 1310 nanometer laser. Because of chromatic dispersion, the pulses of different wave lengths travel at different velocities. So, as they travel further and further, the separation between the two pulses increases. One of the methods of achieving long fiber lengths in user ring resonator. So, here we have a WDM output which we give to a 50 50 coupler. So, half of the power of port one goes to port three and other half goes to port four. Now, the power coupled to port four goes to four kilometer long fiber and then again comes into the 50 50 coupler. Now, again half of the power goes to port three and half of the power goes to port four. So, at port three, we will get pulses; the first pulse will be coming directly from the input port one; the second pulse will be coming from the port two after travelling four kilometer length, then the third pulse will be coming from the fiber which has travelled in this resonator twice. So, it would have travelled eight kilometers and we will get pulses which have travelled larger lengths. Now, output of four three is collected by a photo receiver and the r f output is seen on CRO. So, we will see the dispersion on CRO. This is the experimental setup; these are the various components. This is the data generator which we are using. We set the output of bit generator to 0.01 mega bits per second. We connect the output of data generator to 1550 and 1310 nanometer lasers. This is a 1550 nanometer laser. We have fifteen fifty nanometer laser and thirteen ten nanometer laser, both of which are filled with the output of bit generator. Output of fifteen fifty nanometer is connected to the port 3 of WDM and the output of thirteen ten laser is effect to port four. This is thirteen ten, fifteen fifty multiplexer. Multiplex signal comes out from port one and port two; you use a signal of port one. So, the multiplex signal of port one, you spread to the port 1 of 1550 coupler. The port 2 of the 5050 coupler is connected to four kilometer fiber. The output is taken from port three, and port four is connected to port 2 via 4 kilometer long fiber. So, it forms ring
resonator. The output of port three is taken to the photo receiver and you see the output in the CRO. Port two is connected to port four where four kilometer long fiber reels; this is a four kilometer long fiber of single mode. This is a photo receiver r f output; this is the output of the photo receiver. Channel one that is yellow line this is the transmitted data and channel two is the received data. Here, you can see that, there are distinct peaks which correspond to the data rate which we had said. Now, in channel two, we can see that there are multiple peaks each with diminishing amplitude; this is because after each pass of the signal in of the 4 kilometer fiber we get some of the power at the receiver and some is again coupled to the fiber. So, you can see the first peak on channel two, it is a bit elongated as compared to that of channel one. Now, we will see the second peak at the receiver; this second peak at the receiver is after the light travelled 4 kilometer in the fiber, after one lap in the fiber. This peak corresponds to the first pass in the ring resonator. Here, we can clearly see that, it is much more elongated as compared to the first peak. You will see the third peak; this third peak has travelled (( )) kilometers in the fiber. Here, we can clearly see this, splitting of the pulse in two. Here, we can clearly see the effect of chromatic dispersion since the 1550 nanometer signal and 1310 nanometer signal travels at different velocities. So, we get a splitting of the pulse. Now, we will see the forth pass. This signal has travelled 12 kilometers in the fiber. Here, we can see, the original pulses split clearly in two different pulses corresponding one, corresponding two to 1550 nanometer, as are corresponding to 1310 nanometer. (No audio 59:04 to 59:55) As you see, one further peak you can see they are separated even via higher distance; you can see that we are separated with the increase time length; you can clearly see the difference at the peaks.