Page 159 - Optical Communications Essentials
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Passive Optical Components
Passive Optical Components 149
ting loss plus the excess loss in each path through the star. The splitting loss is given
in decibels by
1
Splitting loss =−10 log = 10 log N (9.5)
N
Similar to Eq. (9.2), for a single input power P in and N output powers, the excess loss
in decibels is given by
P
Fiber star excess loss = 10 log in (9.6)
N P
= i 1 out,i
The insertion loss and crosstalk can be found from Eqs. (9.3) and (9.4), respectively.
9.1.5. Mach-Zehnder interferometer
Wavelength-dependent multiplexers can be made by using Mach-Zehnder inter-
ferometry techniques. These devices can be either active or passive. Here we
first look at passive multiplexers. Figure 9.4 illustrates the constituents of an
individual Mach-Zehnder interferometer (MZI). This 2 2 MZI consists of
three stages: an initial 3-dB directional coupler which splits the input signals
equally into two separate paths, a central section where one of the waveguides
is longer by ∆L to give a wavelength-dependent phase shift between the two
arms, and another 3-dB coupler which recombines the signals at the output.
The function of this arrangement is that, by splitting the input beams and
introducing a specific phase shift in one of the paths, the recombined signals
will interfere constructively at one output and destructively at the other. Thus,
if the input to port 1 is at λ 1 and the input to port 2 is at λ 2 , then both signals
finally emerge from only one output port.
Reciprocally, if independent signals at wavelengths λ 1 and λ 2 enter the bot-
tom port on the right-hand side of Fig. 9.4 (this time going from right to left
through the device), then the signal at λ 1 will exit the device from the top port
on the left-hand side, and the signal at λ 2 will exit the MZI coupler from the
bottom port.
Figure 9.4. Layout of a basic 2 2 Mach-Zehnder
interferometer.
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