P11: Optical Communications Basics Chair for Communications Prof. Dr.-Ing. Werner Rosenkranz 1 Basics 2 1.1 Preface .............................................................................................................................. 2 1.2 Fundamentals of Optical Transmission ............................................................................ 2 1.3 External Modulation Using the Mach-Zehnder Modulator .............................................. 3 2 Homework 4 3 Exercise 5 3.1 Generation of the Simulation Model ................................................................................ 5 3.2 Working with the Simulation Model ................................................................................ 9 3.3 Simulation with Noise .................................................................................................... 11 I 3.2. Working with the Simulation Model 1 Basics 1.1 Preface The purpose of this lab project is twofold: On one hand, the basics of optical communications as given in the lecture ”Digital Communications” are clarified and considered in more detail by means of computer simulations. On the other hand, the basics of MATLAB learned in previous lab projects are applied to the important field of numerical simulation. This handout is structured as follows: The current chapter explains the basics of optical communications. Understanding of these basics is essential and can be double-checked individually by means of the questions given in chapter 2. These questions should be prepared in written style. Each participant is supposed to be able to answer these questions during oral discussion of the project right before the start the project itself. The execution of the project is given step-by-step in chapter 3. 1.2 Fundamentals of Optical Transmission The fundamentals of optical transmission are given in the lecture “Digital Communications” in section 3.3.3 and are not repeated in this handout. The corresponding questions to this section are given in chapter 2 as questions 1 to 6. 3.2. Working with the Simulation Model 1.3 External Modulation Using the Mach-Zehnder Modulator The most versatile device for optical modulation is the so-called Mach-Zehnder Modulator (MZM). Its general structure is depicted in Fig. 1.1. As can be seen, the MZM has an optical input given by the corresponding electrical field Ein , an optical output Eout , and two electrical inputs given by the voltages U1 and U 2 . Figure 1.1: General structure of Mach-Zehnder modulator (MZM). In our application, Ein (t ) is an optical wave with constant envelope emitted from an infrared laser in continuous-wave operation. It is split into two branches. Using the voltages U1 and U 2 , the electrical field in each branch may be delayed. Finally, the two branches are recombined resulting in superposition of the two signals. Depending on the particular delay that is determined by the two voltages, constructive or destructive interference may be realized as well as phase shifting. In case of U1 U 2 U , which is called push-pull mode, the MZM operates as pure intensity modulator. Input power Pin and output power Pout are related by 2 U Pout Pin cos 2 , π Uπ (0.1) where U π is a characteristic value of each particular MZM. Obviously, in push-pull mode the MZM has only one independent electrical input. If this electrical input voltage depends on time (i.e. U (t ) ) and carries the binary digital data, the MZM can be used to transfer the data from amplitude modulation in electrical baseband to intensity modulation of an optical band-pass signal. 3.2. Working with the Simulation Model 2 Homework 1. Is optical transmission baseband transmission or band pass transmission? 2. Typically, the wavelength for optical transmission is in the range of 1550 nm. Compute the corresponding frequency. 3. Explain the difference between direct and external modulation. 4. Explain “on-off-keying”. 5. The fiber channel may be modeled by a linear system with transfer function H(f). Which signals does this linear system relate? 6. What is the relation between the complex envelope of the electrical field inside an optical fiber and its instantaneous power? 7. Make a sketch of the characteristic of the MZM that relates the voltage U on the horizontal axis to the ratio Pout Pin on the vertical axis. Mark the voltage Uπ. 8. Have a look at the block diagram of an optical transmission system in the lecture notes. Identify possible sources of noise. 3.2. Working with the Simulation Model 3 Exercise 3.1 Generation of the Simulation Model Before we can start simulating, we have to setup up our simulation environment MOVE-IT. Therefore, we use the numerical computing environment MATLAB. To establish a new simulation environment you have to perform the following steps: 1. Start MATLAB. 2. Run the command "move_it" in MATLAB workspace. Now you should see theMOVE-IT environment. 3. Open the predefined (but empty) simulation model “communications_lab_p11”. Click the open-icon and choose the folder “communications_lab_p11”. Since the pre-defined model is empty, you will receive a message that you should confirm. Then, your window should look as follows The empty simulation model opens and should look like this: . 3.2. Working with the Simulation Model We start the model with a digital source. In the library on the left hand side of the window, click the button with the pictogram and drag the box that appears into the empty area on the right hand side of the library. Click on the box to drop it from the mouse. This is a first but very simple simulation model: a generator for digital data. The structure of this model is represented by MATLAB-code, which you should examine by selecting “Edit Model - Show Model File”. To start this small simulation model, press the simulation button, which looks as follows: The time to run this simulation is very short, and since so far no output has been specified, there will not be any display of simulation results. However, the simulation result is available as a variable in the MATLAB workspace. You can examine it by typing datagen_out in the command window. Now, right-click on the box of the data generator and select “Help”. This will open a file that gives you more information on the properties of this specific module. Then, double click on the data generator to open a box that lets you edit the parameters that you can use to modify how exactly the module behaves. You are not supposed to understand each detail, but you should play around with the most important parameters like “length” or “level” and watch the impact on the output vector datagen_out when you repeat the simulation. Question: What is impact of the parameters “length” and “level” on the output variable? Answer: 3.2. Working with the Simulation Model Next, extend your simulation model by means of a pulse shape filter, which is denoted by the pictogram . Check out the model file to follow the changes. Connect the two modules by first clicking on the output button of the data generator and then clicking on the input button of the pulse shape filter. Again, check out the model file and see how the connection is realized. Question: How is the connection realized in the MATLAB-Code? Answer: Simulate again. Examine the output of the pulse shape filter by typing into the command window figure; plot(pulsef_out); . Question: What is the main difference of the variables datagen_out and pulsef_out? Answer: 3.2. Working with the Simulation Model The simulation model should now be completed by adding the following elements: a bias-tee for tuning the bias point of the following Mach-Zehnder modulator, a Mach-Zehnder modulator for external modulation (first input is for optical signal, second input is for electrical signal), a continuous wave laser source, an optical fiber, a photo diode, a receiver filter, the sampler, and finally a slicer that at the same time determines the bit error probability. Moreover, a module for examination of the eye diagram should be included: Now connect all the modules. For the photo diode, the second output does not need to be connected. For the eye diagram module, the input should be connected with the output of the receiver filter. Question: Why does the slicer have two inputs and how should they be connected? Answer: 3.2. Working with the Simulation Model Start the simulation. Although the eye diagram shows that a clear distinction between “0” and “1” is possible, the slicer measures a bit error ratio around 0.5, which means that the detector does not work properly. We need to tune it. First, we need to set the clock phase properly. In real systems, for this purpose a clock recovery circuit is required. For simplicity, in the simulation we set the clock phase manually. This is done by setting the parameter delay in the sampling module to the value of 13 instead of the default value of 1. This means that we delay the sampling clock by 12 signal samples, which is equal to 12/32 of the bit period since every bit is sampled by 32 samples on the time axis. Moreover, at the slicer we need to synchronize the bit patterns of transmitted bits and reference bits by setting the parameter delay to 1. Finally, in the slicer we need to set the decision threshold properly. For this, you should have a look at the eye diagram to find a suitable value. If you have done so, repeat the simulation. You should observe error-free transmission. 3.2 Working with the Simulation Model In this exercise, we will have a closer look on the simulation model. We will change some of the properties of the system setup and have a look on the impact on the simulation result. Double-click on the optical fiber. As you see, the default values for the fiber suggest a length of 100 km and a nonlinear model. Throughout this project, we restrict to a linear model, so you need to set the corresponding parameter to “false”. Moreover, in a first step we only want to examine transmitter and receiver without any fiber. Therefore, you need to set the length of the fiber to zero. Repeat the simulation. Most probably, you will see a bit error ratio that is larger than zero. Question: Why do errors occur? Which parameter of the receiver needs to be adapted to observe again error-free transmission? Answer: 3.2. Working with the Simulation Model Examine the value of the switching voltage Uπ of the MZM. Plot the electrical input signal into the MZM using figure;plot(bias_t_out);. Make a drawing of the MZM characteristic (according to question number 7 in chapter 2) and mark the section of the characteristic where the MZM is driven. From the parameters of the pulse shaping filter and the bias-tee, understand how the input signal into the MZM is generated. Play around with the bias value of the bias tee to vary the point of operation on the characteristic. In particular, set the bias value such that the signal is logically inverted. Question: Which bias value needs to be set for the bias tee to achieve logical inversion? What is the value of the bit error ratio in this special case? Answer: Now reset the bias settings to the default values. In the next step, we want to examine the impact of fiber loss and fiber dispersion. From the parameter “f_sym” of the pulse shaping filter, you can observe that the data rate is set to 10 Gb/s. Question: What is the maximum length of the fiber for which the simulation results in errorfree transmission? Answer: 3.2. Working with the Simulation Model 3.3 Simulation with Noise All the simulations so far did not include any noise. Since in the setup, optical amplifiers are not used, the main source of noise is thermal noise in the receiver electronics. The noise can be considered in the simulation by giving the parameter “PSD therm” of the photo diode, which denotes the power spectrum density N0, a certain value greater than zero. A realistic value that is used from now on is N0 5 1022 A Hz 2 . The impact of noise is to be investigated for a fiber of length L =100 km. To have better statistics, increase the number of simulated bits. The values you select should be powers of two (e.g. 128, which is the default value) such that the fast Fourier transform (FFT) can be used in case of filtering (e.g. in the fiber). Select a value that you find acceptable in terms of the time that the computer requires to carry out the simulation. If you carry out the simulation, due to noise you will hardly see any eye diagram. This is because the signal-to-noise ratio is too low. To improve it, you should increase signal power. By default, the laser emits a signal of power PL=1 mW, which is equal to 0 dBm. This value can be modified using the parameter ”power” of the laser. Increase the laser power until you see a clear eye diagram. Adapt the receiver to achieve error-free transmission. In the next step, the bit error ratio as function of laser power is to be determined. Decrease the laser power in steps of 1 dB and adapt the receiver for each new value of laser power. As soon as error-free transmission is not possible any more, try to optimize your receiver to achieve minimum bit error ratio. Sketch the bit error ratio as function of laser power such that you create a waterfall curve.