412
10 Wavelength Division Multiplexing (WDM)
Fig. 10.18 Operational
concept of a three-port
circulator
Table 10.4 Typical
parameter values of
commercially available
optical circulators
Parameter
Unit
Value
Wavelength band
nm
C-band: 1525–1565
L-band: 1570–1610
Insertion loss
dB
<0.6
Channel isolation
dB
>40
Optical return loss
dB
>50
Operating power
mW
<500
Polarization-dependent loss
dB
<0.1
Polarization-mode dispersion
ps
<0.1
Size (diameter × length)
mm
5.5 × 50
10.4 WDM Devices Based on Grating Principles
A grating is an important element in WDM systems for combining and separating
individual wavelengths. Basically, a grating is a periodic structure or perturbation in
a material. This variation in the material has the property of reflecting or transmitting light in a certain direction depending on the wavelength. Thus, gratings can be
categorized as either reflecting or transmitting gratings.
10.4.1 Grating Basics
Figure 10.19 defines various parameters for a reflection grating. Here, θ i is the incident angle of the light, θ d is the diffracted angle, and is the period of the grating
(the periodicity of the structural variation in the material). In a transmission grating