Page 220 - Optical Communications Essentials

Wavelength Division Multiplexing

210 Chapter Twelve

Incident light Reflection
grating
λ 1 , λ 2 , λ 3 , λ 4 ...
λ 1

• λ 2
•

λ N
Optical
fibers
Figure 12.8. The angle at which reflected
light leaves a reflection grating depends
on its wavelength.

d
Structural
support Λ
layers
Optical
fibers
α λ 1

λ 2
•
λ 1 , λ 2 , λ 3 , λ 4 ...
•
Incident light λ
N
Transmission
grating
Figure 12.9. Each wavelength emerges at a
slightly different angle after passing through a
transmission grating.

where λ is the wavelength, d is the thickness of the grating, n g is the refractive
index of the material, Λ is the grating period, and α is the incident angle, as
shown in Fig. 12.9. The phase grating is called thin for Q 1 and thick for
Q 10. After a spectrum of wavelength channels passes through the grating,
each wavelength emerges at a slightly different angle and can be focused into a
receiving fiber.

12.2.5. Interleavers
Another wavelength multiplexing component is an interleaver, which is a
passive, low-dispersion device that can increase the channel density in a WDM
system. This device can combine or separate very high-density channels with a
spacing as low as 3.125GHz. A unique feature is that it can be custom-designed
to route or drop a group of channels while allowing all other wavelengths to
pass straight through (which commonly are referred to as the express channels).

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.