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Optical Sources • Optical Transmitter Coverts Electrical Optical Fiber Communication Deepak Khadka Chapter 4: Optical Sources • Optical transmitter coverts electrical input signal into corresponding optical signal. The optical signal is then launched into the fiber. Optical source is the major component in an optical transmitter. • LED (Light Emitting Diode) and LASER (Light Amplification by Stimulated Emission of Radiation) are the devices that are used widely as optical sources. Characteristics (Properties) of Light Source of Communication To be useful in an optical link, a light source needs the following characteristics: • It must be possible to operate the device continuously at a variety of temperatures for many years. • Must have compatible size and configuration to effectively launch light into an optical fiber. • Emit light at wavelength where fiber has low losses and low dispersion. • Must have high intensity light output. • Their light must be nearly monochromatic as much as possible. • Allow direct modulation over wide bandwidth. • For fiber links, the wavelength of the output should coincide with one of transmission windows for the fiber type used. • The power requirement for its operation must be low. • High coupling efficiency. • High optical output power. • High reliability. • Low weight and low cost. 4.1 LED( Light Emitting Diode) A light-emitting diode (LED) is a semiconductor device that emits incoherent light, through spontaneous emission, when a current is passed through it. Typically LEDs for the 850-nm region are fabricated using GaAs and AlGaAs. LEDs for the 1300-nm and 1550-nm regions are fabricated using InGaAsP and InP. The basic LED types used for fiber optic communication systems are the surface-emitting LED (SLED), the edge-emitting LED (ELED), and the superluminescent diode (SLD). LED performance differences help link designers decide which device is appropriate for the intended application. For short- distance (0 to 3 km), low-data-rate fiber optic systems, SLEDs and ELEDs are the preferred optical source. Typically, SLEDs operate efficiently for bit rates up to 250 megabits per second (Mb/s). Because SLEDs emit light over a wide area (wide far-field angle), they are almost exclusively used in multimode systems. For medium-distance, medium-data-rate systems, ELEDs are preferred. 1 Optical Fiber Communication Deepak Khadka ELEDs may be modulated at rates up to 400 Mb/s. ELEDs may be used for both single mode and multimode fiber systems. Both SLDs and ELEDs are used in long-distance, high-data-rate systems. SLDs are ELED-based diodes designed to operate in the superluminescence mode. A further discussion on superluminescence is provided later in this chapter. SLDs may be modulated at bit rates of over 400 Mb/s. • A light-emitting diode (LED) is a semiconductor light source. Modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. Electronic symbol Pin configuration anode and cathode The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon. Note : The light emitting region of both leds 2 Optical Fiber Communication Deepak Khadka The recombination of excess minority carriers is the mechanism by which optical radiation is generated. LED configurations At present there are two main types of LED used in optical fiber links – 1. Surface emitting LED. 2. Edge emitting LED. Both devices used a DH structure to constrain the carriers and the light to an active layer. Surface Emitting LEDs • In surface emitting LEDs the plane of active light emitting region is oriented perpendicularly to the axis of the fiber 3 Optical Fiber Communication Deepak Khadka • A circular well is etched through the substrate of the device. A fiber is then connected to accept the emitted light. • In addition, the epoxy resin that binds the optical fiber to the SLED reduces the refractive index mismatch, increasing coupling efficiency • At the back of device is a heat sink. The current flows through the p-type material and forms the small circular active region resulting in the intense beam of light. The circular active area in practical surface emitters is nominally 50 µm in diameter and upto 2.5 µm thick. • The emission pattern is essentially isotropic with a 120 degree half-power beamwidth • The isotropic emission pattern from surface emitting LED is of Lambartian pattern. In Lambartian pattern, the emitting surface is uniformly bright, but its projected area diminishes as cos θ, where θ is the angle between the viewing direction and the normal to the surface as shown in Fig. 3.1.3. The beam intensity is maximum along the normal. • The power is reduced to 50% of its peak when θ = 60 therefore the total half-power beamwidth is 120degree. The radiation pattern decides the coupling efficiency of LED. Edge-Emitting LED: 4 Optical Fiber Communication Deepak Khadka Fig: Structure of Edge Emitting. DH, strip Contact LED The demand for optical sources for longer distance, higher bandwidth systems operating at longer wavelengths led to the development of edge-emitting LEDs. Figure shows a typical ELED structure. • In order to reduce the losses caused by absorption in the active layer and to make the beam more directional, the light is collected from the edge of the LED. Such a device is known as edge emitting LED or ELED. • It consists of an active junction region which is the source of incoherent light and two guiding layers. The refractive index of guiding layers is lower than active region but higher than outer surrounding material. Thus a waveguide channel is form and optical radiation is directed into the fiber. • Edge emitter‘s emission pattern is more concentrated (directional) providing improved coupling efficiency. The beam is Lambartian in the plane parallel to the junction but diverges more slowly in the plane perpendicular to the junction. In this plane, the beam divergence is limited. In the parallel plane, there is no beam confinement and the radiation is Lambartian. To maximize the useful output power, a reflector may be placed at the end of the diode opposite the emitting edge. Fig. 3.1.5 shows radiation from ELED 5 Optical Fiber Communication Deepak Khadka ELEDs emit light in a narrow emission angle allowing for better source-to-fiber coupling. They couple more power into small NA fibers than SLEDs. ELEDs can couple enough power into single mode fibers for some applications. ELEDs emit power over a narrower spectral range than SLEDs. However, ELEDs typically are more sensitive to temperature fluctuations than SLEDs. Examples of SLED structure: Examples of ELED structure: 6 Optical Fiber Communication Deepak Khadka 4.2 Lasers A laser is a device that produces optical radiation by the process of stimulated emission. It is necessary to contain photons produced by stimulated emission within the laser active region. Figure 3 shows an optical cavity formed to contain the emitted photons by placing one reflecting mirror at each end of an amplifying medium. One mirror is made partially reflecting so that some radiation can escape from the cavity for coupling to an optical fiber. Figure 3. - Optical cavity for producing lasing. 7 Optical Fiber Communication Deepak Khadka Only a portion of the optical radiation is amplified. For a particular laser structure, there are only certain wavelengths that will be amplified by that laser. Amplification occurs when selected wavelengths, also called laser modes, reflect back and forth through the cavity. For lasing to occur, the optical gain of the selected modes must exceed the optical loss during one round-trip through the cavity. This process is referred to as optical feedback. The lasing threshold is the lowest drive current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission. Figure 4 illustrates the transition from spontaneous emission to stimulated emission by plotting the relative optical output power and input drive current of a semiconductor laser diode. The lowest current at which stimulated emission exceeds spontaneous emission is the threshold current. Before the threshold current is reached, the optical output power increases only slightly with small increases in drive current. However, after the threshold current is reached, the optical output power increases significantly with small changes in drive currents. Figure 4. - The optical output power as a function of input drive current of a semiconductor laser diode. 8 Optical Fiber Communication Deepak Khadka Many types of materials including gas, liquid, and semiconductors can form the lasing medium. However, in this chapter we only discuss semiconductor laser diodes. Semiconductor laser diodes are the primary lasers used in fiber optics. A laser diode emits light that is highly monochromatic and very directional. This means that the LD's output has a narrow spectral width and small output beam angle. A semiconductor LD's geometry is similar to an ELED with light-guiding regions surrounding the active region. Optical feedback is established by making the front facet partially reflective. This chapter provides no diagram detailing LD structures because they are similar to ELEDs in design. The rear facet is typically coated with a reflective layer so that all of the light striking the facet is reflected back into the active region.
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