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At the beginning of an optical fiber is information that needs to be transmitted, along with a laser just ready to do the job. It is the role of modulation to bridge the gap between these two entities, and generate the optical information that is to travel through an optical network. This tutorial aims to demystify the world of modulation – covering exactly what it is and how it can be achieved. Welcome to Modulation An optical network is digital, meaning that data is represented by 1s and 0s. In electrical networks a high electrical current or voltage can represent a 1, while a low electrical current or voltage corresponds to a 0. To create these “pulses” from a constant source of electricity, a very fast electrical switch can be used in much the same way as you can create electrical pulses by switching a wall outlet on and off repeatedly (only many times faster!). In the optical domain, we have a laser as our signal source. In regular operation, this laser is giving out light all of the time. What is needed is some kind of switching capability in order to alter this constant flow into a stream of high and low power levels to represent the digital information. This is the role of “modulation,” and we say that the light needs to be “modulated.” Modulation Formats Before deciding how to actually perform this modulation, we need to first choose what is known as a modulation “format” or “scheme.” This essentially describes how successive bits are to be transmitted – in most cases, whether or not the light level falls back to its low point (0) before the next pulse (1) is transmitted. In the non-return-to-zero (NRZ) modulation format, a sequence of 1s is transmitted without the light level falling back to its zero level in between each bit. Return-to-zero (RZ) does exactly what it says – with the light level always falling back to its zero level in between bits, even in a succession of 1s. The diagram below illustrates the difference between NRZ and RZ through a short data stream. In almost all optical networks in place today NRZ is the modulation format in use, primarily due to its simplicity and ease of implementation. RZ is being increasingly considered for more modern network designs, as it can provide certain advantages, particularly over long distances where it is less susceptible to Chromatic Dispersion and Polarization Mode Dispersion (PMD). More elaborate modulation formats are always the subject of a great deal of research, both in industry and academia. Examples of possible optical modulation formats of the future include “duo-binary” that has 3 levels (-1, 0, and 1) and the “quaternary” format with its 4-level system. Direct or External Modulation Once we know the modulation format we are going to use, we now have the decision of how we are going to modulate the laser light. There are two ways to modulate – either “directly” or “externally.” You can think of direct modulation as switching the laser on and off, whereas in external modulation the laser’s output is constant and sent through a shutter that can open or close to effectively switch the light output on and off. The simplest method is direct modulation. Our Laser Basicstutorial showed how an electrical current passing through a laser causes light to be emitetd. A data stream of 1s and 0s can be represented by high and low levels of electrical current, which can then be directed through the laser chip itself. The result is that a 1 gives out light, whereas a 0 gives out little or no light. Direct modulation is very cheap to implement, as it requires few additional components, and it also allows reasonably high power signals to be generated (on the order of 10 dBm of output power – see the Optical Units Referencefor an explanation of optical powers). On the down side it is very susceptible to “chirp,” where the exact wavelength of the light emitted varies throughout each pulse. This happens because of small changes in the laser material’s refractive index due to the varying electrical current being applied to it. Chirp is undesirable, as a pulse with a broad range of wavelengths is susceptible to dispersion through the transmission optical fiber (see the Dispersion tutorial). Direct modulation also becomes problematic at bit rates above a few GHz, due to the increased dispersion and problems with the laser’s so-called “relaxation oscillations.” These are rapid variations in light output that occur just after a laser has been switched on, and they take time to settle down. This causes difficulties at high bit-rates. Another disadvantage to direct modulation is that there are implications for the reliability of lasers that are switched rapidly on and off in this way. Despite the drawbacks, however, directly modulated lasers will often be used for relatively low bit-rate signals over relatively short distances. For example, a directly modulated Fabry-Perot laser can be used for rates of 565 Mbit/s over 20 km with acceptable dispersion, or for higher rates over shorter distances. Distributed Feedback (DFB) Lasersunder direct modulation could cover up to 100 km or so at 2.5 Gbit/s, but at 10 Gbit/s would be limited to only a few kilometers (the narrower range of wavelengths from DFB lasers improves their dispersion performance). The alternative to direct modulation is external modulation. In this method, the electrical current applied to the laser is kept constant. Immediately this removes issues with direct modulation such as excessive chirp (and therefore dispersion), relaxation oscillations, and laser reliability. The laser will either give a constant level of output optical power, or give a stream of light pulses at the desired bit-rate (lasers can be designed to do this without having to switch them on and off rapidly). An external modulator is a device that is placed between the laser and the output fiber. It is designed to let light through when it needs to transmit a 1, and to block light from passing through it to represent a 0. Signals from externally modulated DFB lasers can travel around 10 times farther (in terms of their dispersion performance) than their directly modulated counterparts. This extra performance does come at an increased price, however, due to the additional technology required. Types of External Modulator There are various methods available for external modulation. The main types used in optical networks are Electro-Optic and Electro-Absorption modulators. Electro-Optic ModulatorsIn electro-optic modulators, the refractive index of a material can be altered through an applied electric field. This change in refractive index can cause a change in what is known as the “phase” of a lightwave passing through it. “Phase” describes the relative location of the peaks of the light waves. If two light waves are in phase then their peaks occur at the same location and the waves add together to give a higher intensity of light. If two waves are out of phase by half a wavelength then when they combine they cancel each other out to give no light. To make this usable as a modulator, the light from the laser arrives at the electro-optic modulator and is then split equally into two different paths. An electric field can be applied to each of these paths in order to shift the phase of the waves so that they arrive at the far end either in phase to give a pulse of light (a 1) or half a wavelength out of phase to give no light (a 0). You may also hear these devices referred to as Mach-Zehnder modulators. Lithium niobate is the material most commonly used in such a device. Electro-Absorption (EA) ModulatorsElectro-absorption modulators are made of similar materials to semiconductor lasers, allowing them to be integrated with the lasers they are to be used with. This provides an advantage over electro-optic modulators, the material for which is incompatible with that of the laser and does not allow such neat integration into small packages. Light is generated in a laser by electric current being passed through a semiconductor material. The design of the device means that the direction in which the electric current passes makes a difference to its behavior. Applying current in the conventional direction that results in light output is known as “forward bias.” If current is passed in the opposite direction the material is said to be under “reverse bias,” and the device now prevents light from building up and passing through. An electro-absorption modulator is therefore very similar to a laser, but when it wishes to prevent light passing through it (to give a 0) it needs a high reverse bias to give it these light-absorbing properties. To represent a 1 it needs to allow light through, and so no current is applied and it behaves transparently to the laser light. Key Points Laser light needs to be modulated to represent 1s and 0s. The non-return-to-zero (NRZ) modulation format is in common use and does not return to zero light output when transmitting successive 1s. Return-to-zero (RZ) formatting may have advantages for long-distance applications. Direct modulation involves switching the laser on and off rapidly, with an electrical current representing the digital data. Advantages of direct modulation include its simplicity and cost effectiveness; disadvantages include poor chirp performance (leading to dispersion) and poor performance at high bit-rates. External modulators act as a shutter to transmit or block light from a laser that is operated by a constant electrical current. Advantages of external modulation include improved chirp performance and suitability for very high bit-rates; disadvantages include complexity and cost. Electro-optic modulators split laser light into two paths and then cause phase shifts to either cancel out the two waves (to give a 0) or combine them (to give a 1). Electro-absorption modulators are reverse-biased lasers that can absorb or transmit light and can be integrated with the transmission laser in a small package.