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)

• 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

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(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.

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’)