Chapter 10: WDM

WDM: A powerful aspect of an optical communication link is that many different wavelengths can be sent along a single fiber simultaneously in to 1300-to-1600-nm spectral band. The technology of combining a number of wavelengths onto the same fiber is known as wavelength-division multiplexing. Conceptually the WDM scheme is the same as frequency–division multiplexing used in microwave radio and satellite system. The key system features of WDM are as follows:

Operational Prinicple of WDM:

Since an optical source has a narrow linewidth, the point-to-point links of a single fiber make use of only a very narrow portion of the transmission bandwidth capacity of a fiber. E.g. the modulated output of a DFB laser has a frequency spectrum of 10-50 MHz, which is equivalent to a nominal spectral linewidth of 10-3 nm. When using such a source, a guard band of 0.4-0.8nm is typically employed. This is done to take into account possible drifts of the peak wave length due to aging or temperature effects and to give both the manufacturer and the users some leeway in specifying and choosing the precise peak emission wavelength.

Fig. 3-1: Optical fiber attenuation

To find the optical bandwidth corresponding to a particular spectral width in this region, we use the relationship c = λν, which relates the wavelength λ to the carrier frequency ν

The implementation of WDM networks requires a variety of passive and/or active devices to combine, distribute, isolate, and amplify optical power at different wavelengths.

Fig. 10-2: Typical WDM Network

Passive Components:

Fig. 10.3 : Basic star coupler

10.2.6 Fiber Grating Filters

Fig. 10-15: Reflecting grating

The following figure describes how the white light splits up into its spectral components when passes through a transmitting grating.

Construction of grating:

Fig. 10-16: Bragg grating formation

Fiber:

Fig. 10-17: Simple demux function

Fig. 11-1: Applications of optical amplifiers

Fig. 11-2: Generic optical amplifier

The signal to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with the doping ions. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fiber. The pump laser excites ions into a higher energy from where they can decay viastimulated emission of a photon at the signal wavelength back to a lower energy level.

Features of SOA

1. Works in both the 1300-nm and the 1550-nm low attenuation windows

2. Can easily be integrated on the same substrate as other optical devices

3. Consume less power

4. Has fewer components so compact

5. More rapid gain response (1ps to .1 ns)

Features of DFA

1. Can pump the device at several different wavelengths

2. Low coupling loss to the compatible fiber transmission medium,

3. Very low dependence of gain on light polarization.

4. Lightly transparent to signal format and bit rate.

5. Slow gain response (0.1 -10ms )