Page 160 - Optical Communications Essentials

Passive Optical Components

150 Chapter Nine

MZI Equation To have input signals at wavelengths λ 1 and λ 2 that enter ports 1 and 2,
respectively, emerge from only one output port, the length difference in the interfer-
ometer arms should be

  1 1   −1 c
∆L =  2 n eff  −   = (9.7)
   λ 1 λ    2 n eff ∆ν
2
where n eff is the effective refractive index of the MZI material and ∆ν is the frequency
separation of the two wavelengths.

Example Assume that the input wavelengths of a 2 2 silicon MZI are separated by
100GHz (or equivalently, 0.75nm at 1550nm). With n eff 1.5 in a silicon waveguide,
we know from Eq. (9.7) that the waveguide length difference must be

8
3 10 m/s
∆L = = 1 mm
11
21 5 ( . ) 10 /s
n
By using basic 2 2 MZI modules, any size N N multiplexers (with N 2 )
can be constructed.

9.2. Isolators and Circulators
In a number of applications it is desirable to have a passive optical device that
is nonreciprocal; that is, it works differently when its inputs and outputs are
reversed. Two examples of such a device are isolators and circulators. To under-
stand the operation of these devices, we need to recall some facts about polar-
ization and polarization-sensitive components from Chap. 3.

■ Light can be represented as a combination of a parallel vibration and a per-
pendicular vibration, which are called the two orthogonal plane polarization
states of a light wave.
■ A polarizer is a material or device that transmits only one polarization com-
ponent and blocks the other.
■ A Faraday rotator is a device that rotates the state of polarization (SOP) of
light passing through it by a specific angular amount.
■ A device made from birefringent materials (called a walk-off polarizer) splits
the light signal entering it into two orthogonally (perpendicularly) polarized
beams, which then follow different paths through the material.
■ A half-wave plate rotates the SOP of a lightwave by 90°; for example, it converts
right circularly polarized light to left circularly polarized light.

9.2.1. Optical isolators
Optical isolators are devices that allow light to pass through them in only one
direction. This is important in a number of instances to prevent scattered or

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