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7. Optical Fiber Communication Systems. Inter-Continental Optical Fiber Communications. Optical Fiber Communication Systems and Fiber Sensors ( Example of Measuring the Gap of Bridge ). Attenuation and Dispersion in Silica Fiber. Erbium Doped Fiber Amplifier (EDFA).
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Optical Fiber Communication Systems and Fiber Sensors (Example of Measuring the Gap of Bridge)
Erbium Doped Fiber Amplifier (EDFA) • Er3+ ions doped into the core region of an optical fiber to achieve light amplification need (stimulated emission > absorption) population inversion required (N2> N1) • Optical gain, Gop=K(N2 –N1), where K is a constant depends on pumping intensity • Er3+-doped fiber is usually inserted into the fiber communication line by splicing • Gain efficiency : 8-10 dB/mW
Optical Fiber Splice and Mismatches in a Fiber-to-fiber Connection
Digital Signals—RZ and NRZ Formats • A return-to-zero (RZ) format is the signal returns to zero between each pulse. The signal is self-clocking. • A non-return-to-zero (NRZ) format does not have a rest state. NRZ is not inherently a self-synchronizing code, thus some additional synchronization technique must be used for avoiding bit slip.
Some Digital Modulations in Communication • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK) • Quadrature Amplitude Modulation (QAM)
Quadrature PSK (QPSK) • In the example shown in the right figure, 00 is represented by sin(2πf0t), 10 is represented by sin(2πf0t+π), 01 is represented by sin(2πf0t+π/2), and 11 is represented by sin(2πf0t+3π/2).
Analog Quadrature Amplitude Modulation (AQAM) A low-pass filter removes the high frequency terms containing cos(4πf0t), leaving only the I(t) term.
Example of Digital 8-QAM • In this 8-QAM example, eight different modulation states using 4 phase angles and 2 amplitudes: one at 50% modulation; the other at 100%. (4 phases X 2 amplitudes = 8 modulation states). • In QAM, the constellation diagram is a useful representation. The constellation points are usually arranged in a square grid.