High Capacity Wavelength Division Multiplexed (WDM)

Optical Fibre Communication

Yijun Shan

Supervisor: Dr. Robert KilleySecond Assessor: Prof Polina Bayvel

OUTLINE

• Introduction

• Amplitude-shift Keying (ASK) Transmission

• Binary Phase-shift Keying (BPSK) Transmission

• Quadrature Phase-shift Keying (QPSK) Transmission

• Techniques to Achieve near Nyquist WDM Channel Spacing

WDM?

Motivation?

Wavelength Division Multiplexing

[1] Govind P. Agrawal (2002). Fiber-Optic Communications Systems. 3rd ed. New York: John Wiley & Sons, Inc.. 332.

[1]

WDM-Amplitude shift Keying (ASK)

Message Signal: 10Gbits/s

By using erbium-doped fibre amplifiers (EDFA) and dispersion compensation fibres (DCF) to periodically compensate the fibre loss and signal distortion

Carrier Signal:1550nm (193.41THz) Gaussian Filters

Linear Amplitude Modulator

Channel spacing•Too large – Wasted bandwidth and fibre capacity•Too small – Overlapping channels

Filter Bandwidth•Too large – Crosstalk introduced •Too small – Intersymbol Inference (ISI) introduced

Transmission Impairments

• Fibre losses– The use of erbium-doped fibre amplifiers (EDFA) allows

overcoming this limitation.

• Dispersion– It is usually minimized by optimizing the dispersion compensation

with a proper management. In this project, dispersion compensation fibre (DCF) was used to compensate the signal distortion.

• Nonlinear fibre effects– Kerr Effects: Raman Scattering and Brillouin Scattering– Scattering Effects: Self Phase Modulation, Cross Phase

Modulation and Four Wave Mixing

ASKChannel Spacing/GHz 12

Filter Bandwidth/GHz 11

Back-to-Back Bit Error Rate 1.64E-09

Spectral Efficiency/bit/s/Hz 0.83

N-Channels 3 5 7Distance/km 3072 2688 1728

• Amplitude shift keying method is simply and easy to apply, but its sensitivity to bit rates is said to be low.

• Binary Phase shift Keying (BPSK)– 2 phase states, 1 bit per symbol

• Quadrature Phase shift Keying (QPSK)– 4 phase states, 2 bits per symbol

• 8-Phase shift Keying (8-PSK)– 8 phase states, 3 bits per symbol

Phase shift Keying Modulation

Instead of using direct detection, coherent detection is introduced to match the phase shifted during the transmission.

WDM-BPSK

Optical Phase Modulator

Two BPSK signals are added together for transmission. However, the second BPSK signal’s carrier is phase shifted by 90 degree before being modulated

WDM-QPSK

ASK BPSK QPSKChannel Spacing/GHz 12 11.5 14

Filter Bandwidth/GHz 11 12 14

Back-to-Back Bit Error Rate 1.64E-09 8.07E-09 3.07E-09

Spectral Efficiency/bit/s/Hz 0.83 0.86 1.43

Maximum Reachable Fibre Distance/kmNumber of channels ASK BPSK QPSK

1 7296 12672 6912

3 3072 4992 3072

5 2688 4608 2304

Nyquist?

Nyquist and Raised-Cosine Filters• The Nyquist bandwidth is the minimum bandwidth that can

be used to represent a signal.• It is important to limit the spectral occupancy of the signal

to improve bandwidth efficiency and remove adjacent channel interference.

• Raised Cosine Filters allow an approximation to this minimum bandwidth.

3-Channel ASK Gaussian Filter Raised Cosine FilterChannel Spacing/GHz 12 10

Filter Bandwidth/GHz 11 10

Back-to-Back Bit Error Rate 1.64E-09 1.50E-16

Spectral Efficiency/bit/s/Hz 0.83 1

Maximum Reachable Distance/km 3072 2304

3-Channel QPSK Gaussian Filter Raised Cosine FilterChannel Spacing/GHz 14 12.5

Filter Bandwidth/GHz 14 12.5

Back-to-Back Bit Error Rate 3.07E-09 4.56E-08

Spectral Efficiency/bit/s/Hz 1.43 1.6

Maximum Reachable Distance/km 3072 3072

Thank you!

Dispersion Compensating Fibre

Erbium-Doped Fibre Amplifiers Achieved by stimulated

emission of photons from dopant ionising the doped fibre.

Primary source of noise is ASE. Electrons in the upper energy level can also decay by spontaneous emission, which occurs at random. Some emitted photons interact with others

Gain saturation: to achieve optimum noise performance of DFAS are operated under a significant amount of gain compression. Since that reduces the rate of spontaneous emission., thereby reducing ASE

Forward Error Correction• A mathematical method by which data is encoded

such away that upon receipt, all the errors are detected and possibly corrected.

• Extra information is included along with original data, to provide redundancy for correcting up to a specified amount of bit errors.

• FEC allows the receiver to “fix” errors based on the additional information transmitted within the original data

Four Wave Mixing and Cross Phase Modulation• To produce the new optical waves, the signal

power is reduced. This is known as power depletion

• In multi-channel system, several channels are transmitted with several equally spaced frequencies. So some of these mixing products will occur at or near the operating frequencies. These new optical wave interfere destructively with the signal and degrade the performance