Modern Communication Systems Optical Fibre Communication Systems EEE440 Modern Communication Systems Optical Fibre Communication Systems En. Mohd Nazri Mahmud MPhil (Cambridge, UK) BEng (Essex, UK) nazriee@eng.usm.my Room 2.14 Semester 1 2011-2012

Test on mobile and wireless Announcement EEE440 Test on mobile and wireless Tuesday 6/12/2011;11am to 1pm DK1 Semester 1 2011-2012

System elements Semester 1 2011-2012 Key elements: Transmitter – consists of a light source and drive circuitry Cable – contains the optical fibres Receiver – consists of a photodetecter, Amplifier and signal restoring circuitry Additional elements: Optical amplifier Connectors Splices Couplers and regenerators Semester 1 2011-2012

System elements Semester 1 2011-2012

Light sources Semiconductor light-emitting diodes (LEDs) and laser diodes are suitable Major differences between LED and Laser LED has an incoherent optical output whereas Laser produces highly coherent, monochromatic and directional output because a cavity exist for wavelength selectivity LED Generally used for multimode fibre For optical communications requiring bit rates less than 100-200 Mb/s Best for high-speed local applications which needs many wavelengths on the same fibre Laser diodes the best light source for long-hauled fibre-optic links due to brightness, narrow spectral width and coherence Semester 1 2011-2012

Light source - LASER LASER stands for Light Amplification by Stimulated Emission of Radiation Principle of operation Semiconductor material can generate light when current is injected directly into it due to the stimulated emission of photons in the material The stimulated emission of photons occur when an external photon impinges on an excited laser material The direct injection of current causes the particles of the laser materials to undergo the process of excitation whereby the particles move from a lower energy level (or ground state) to a higher energy level (or excited state) To initiate the lasing action, the number of particles in the excited state must be made greater than the number of particles in the ground state (ie. Population inversion) Semester 1 2011-2012

Light source - LASER Principle of operation The excited particles in the population inversion state are unstable and can return to the stable ground state again and spontaneously emit photon The photons from the spontaneous emission trigger stimulated emission of other photons resulting in a cascade of stimulated emission (ie lasing action that generate optical signal) A source entity transmits a frame. After the destination entity receives the frame, it indicates its willingness to accept another frame by sending back an acknowledgment to the frame just received. The source must wait until it receives the acknowledgment before sending the next frame. The destination can thus stop the flow of data simply by withholding acknowledgment. This procedure works fine and, indeed, can hardly be improved upon when a message is sent in a few large frames. However, it is often the case that a source will break up a large block of data into smaller blocks and transmit the data in many frames (because of limited buffer size, errors detected sooner with less to resend, to prevent media hogging). With the use of multiple frames for a single message, the stop-and-wait procedure may be inadequate, mainly since only one frame at a time can be in transit. Semester 1 2011-2012

LASER Laser diode: principle of operation: (a) Stimulated emission; (b) light amplification and positive feedback; (c) pumping to create population inversion Semester 1 2011-2012

LASER Lasing effect: (a) Gain and loss; (b) input-output characteristic; (c) setup to measure input-output characteristic Semester 1 2011-2012

Light source - LASER There are many semiconductor laser types Fabry-Perot laser Distributed Feedback (DFB) laser Distributed Bragg Reflector (DBR) laser Distributed Reflector (DR) Laser Semester 1 2011-2012

Fabry-Perot Laser Consists of a heterojunction-structured semiconductor laser: 2 adjoining semiconductor materials with different band-gap energies A pair of flat, partially reflecting mirrors are directed toward each other to enclose the cavity When the junction is forward bias, electrons and holes are injected into the p and n regions These can recombine and release a photon energy, hv . Semester 1 2011-2012

Fabry-Perot Laser The two mirrors and the active medium between them form a laser Mirrors provide positive feedback: the return of stimulated photons to an active medium to stimulate more photons The two mirrors form a resonator with length L Let an arbitrary wave travel from the left-hand mirror to the right-hand one At the right-hand mirror, the wave experiences a 180° phase shift and continues to propagate. At the left-hand mirror, this wave again has the same phase shift and continues to travel yielding a stable pattern called a standing wave . Semester 1 2011-2012

Fabry-Perot Laser The only difference between the two waves shown in Figures 9.13(b) and 9.13(c) is their wavelengths. Thus, a resonator can support only a wave with a certain wavelength, the wave that forms a standing-wave pattern The resonator supports a wavelength where 2L/N = 1300.8 nm. But this resonator also supports wavelengths equal to 2L/(N ± 1), 2L/(N ± 2), 2L/(N ± 3), and so forth. Many wavelengths may exist. Wavelengths selected by a resonator are called longitudinal modes. When the length of a resonator increases or decreases, the laser switches from one longitudinal mode to another. This is called mode hop. Semester 1 2011-2012

Fabry-Perot Laser However, the active medium provides gain within only a small range of wavelengths. Since a laser is formed by a resonator and an active medium and since radiation is the result of their interaction, only several resonant wavelengths that fall within the gain curve might be radiated. Light generation starts only when gain exceeds loss. Thus, eventually only those resonant wavelengths that are within the gain-over-loss curve will actually be radiated. Waves With N, N±1, and N±2 might be radiated, but only waves with N and N±1 will be the actual laser output. Modes N±2, depicted in black, are not generated. To make this explanation more specific, let's introduce spacing between two adjacent longitudinal modes, N – N+1. Indeed, from Formula 9.13 we can obtain Thus, for a resonator whose L = 0.4 mm and that works at around 1300 nm, we can compute N – N+1 2.1 nm. Assuming the line width of a gain curve is equal to 7 nm, we find that this active medium can support three longitudinal modes. Semester 1 2011-2012

DFB Laser To reduce the spectral width, we need to make a laser diode merely radiate only one longitudinal mode with distributed-feedback (DFB) laser diodes Has the Bragg grating incorporated into its heterostructure in the vicinity of an active region. The Bragg grating works like a mirror, selectively reflecting only one wavelength, B This wavelength can be found from the Bragg condition given in Formula 6.10 and repeated here: where is the period of grating, neff = n sin , and n is the refractive index of the medium. All these quantities, including angle , are illustrated in Figure 9.15(b). As for the meaning of the term distributed feedback laser diode, the word feedback emphasizes that we have the means to return stimulated photons to an active medium. This is done by reflecting a portion of the light at each slope of the grating, as Figure 9.15(b) shows for one beam. All the portions reflected at each slope of this corrugated structure then combine so that most of the light will be reflected back, provided of course that the condition shown in Formula 9.15 is maintained. The word distributed implies that reflection occurs not at a single point—a mirror, say, as in the Fabry-Perot laser—but at many points dispersed along the active region. The net result of this arrangement is that the B laser radiates only one wave, with the wavelength equaling B. Thus, its radiation contains only a single longitudinal mode and, as a result, the spectral width of this radiation is extremely narrow. (See Figure 9.15[c].) Actually, we distinguish between the spectral width of the entire output of light—Figure 9.14(e)—and the linewidth of each mode composing this light—Figure 9.15(c). In a singlemode operation, these two widths coincide. Semester 1 2011-2012