Lec-8 Optical Amplifiers and Detectors(03-Mar-2011)

Lec-8 Optical Amplifiers and Detectors(03-Mar-2011) -...

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1 Photonic Network By Dr. M H Zaidi LASERS Revisited 1 Photonic Network By Dr. M H Zaidi Review of previous Lecture The light emitter is an important element because it is often the most costly element in the system, and its characteristics often strongly influence the final performance limits of a given link. L aser diodes are typically constructed of for short-wavelength devices. Long-wavelength devices generally incorporate InGaAsP Lasers have faster rise and fall times than LEDs. Fabry-Perot lasers are the most economical, but they are generally noisy, slower devices. Directly modulated DFBs exhibit positive frequency Chirp which is the biggest drawback of them. 2
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2 Photonic Network By Dr. M H Zaidi Laser is essentially an optical amplifier enclosed within a reflective cavity that causes it to oscillate via positive feedback. Semiconductor lasers use semiconductor as the gain medium, whereas fiber lasers typically use erbium-doped fiber as the gain medium. WDM sources typically have output powers between 0 and 10 dBm. Lasers 3 Photonic Network By Dr. M H Zaidi Fabry-perot lasers A Fabry-Perot filter consists of the cavity formed by two highly reflective mirrors placed parallel to each other This filter is also called a Fabry-Perot interferometer or etalon. Fabry-Perot filters have been used for WDM applications in several optical network testbeds There are better filters today, such as the thin-film resonant multicavity filter Fabry-Perot cavity is also used in lasers Compact Fabry-Perot filters are commercially available components Their main advantage over some of the other devices is that they can be tuned to select different channels in a WDM system 4
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3 Photonic Network By Dr. M H Zaidi Principle of Operation Principle of operation is explained in the previous figure For those wavelengths for which the cavity length is an integral multiple of half the wavelength in the cavity – so that a round trip through the cavity is an integral multiple of the wavelength –all the light waves transmitted through the right facet add in phase. Such wavelengths are called the resonant wavelength of the cavity. In WDM systems, the separation between two adjacent wavelengths must be at least a FWHM in order to minimize crosstalk. 5 Photonic Network By Dr. M H Zaidi Longitudinal Modes Two conditions must be satisfied. First, the wavelength must be within the bandwidth of the gain medium that is used. Second condition is that the length of the cavity must be an integral multiple of half the wavelength in the cavity. Multiple-longitudinal mode (MLM) laser. MLM lasers have large spectral widths, typically around 10 nm. Likewise, a narrow spectral width is also needed to minimize crosstalk in WDM systems.
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Lec-8 Optical Amplifiers and Detectors(03-Mar-2011) -...

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