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) Er 3+ ions doped into the core region of an optical fiber to achieve light amplification need (stimulated emission > absorption) population inversion required (N 2 > N 1 ) Optical gain, G op =K(N 2 –N 1 ), where K is a constant depends on pumping intensity Er 3+ -doped fiber is usually inserted into the fiber communication line by splicing Gain efficiency : 8-10 dB/mW

Classification of Optical Fibers

Theory of Step-index Fiber

Theory of Step-index Fiber (Cont’)

Single-mode Operation of Step- index Fiber

Coupling from LD to Fiber

Fiber Lens to Improve Coupling Efficiency

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)

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Quadrature PSK (QPSK) In the example shown in the right figure, 00 is represented by sin(2πf 0 t), 10 is represented by sin(2πf 0 t+π), 01 is represented by sin(2πf 0 t+π/2), and 11 is represented by sin(2πf 0 t+3π/2).

Analog Quadrature Amplitude Modulation (AQAM) A low-pass filter removes the high frequency terms containing cos(4πf 0 t), 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.

Example of Constellation Diagram in 16-QAM

Examples of Constellation Diagrams in QAMs

Example of 8-QAM Optical Fiber Communication System

Eye Diagram

Eye Diagram (Cont’)