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EEE440

EEE440. Modern Communication Systems Optical Fibre Communication Systems. Announcement. EEE440 Test on mobile and wireless Tuesday 6/12/2011;11am to 1pm DK1. System elements. System elements. Light sources. Semiconductor light-emitting diodes (LEDs) and laser diodes are suitable

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EEE440

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  1. EEE440 Modern Communication Systems Optical Fibre Communication Systems

  2. Announcement EEE440 Test on mobile and wireless Tuesday 6/12/2011;11am to 1pm DK1

  3. System elements

  4. System elements

  5. 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

  6. 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)

  7. 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)

  8. LASER Laser diode: principle of operation: (a) Stimulated  emission; (b) light amplification and positive feedback; (c) pumping to create population inversion

  9. LASER Lasing effect: (a) Gain and loss; (b) input-output characteristic; (c) setup to measure input-output characteristic

  10. 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

  11. 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

  12. 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

  13. 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.

  14. 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.

  15. 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

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