Page 161 - Optical Communications Essentials

Passive Optical Components

Passive Optical Components 151

reflected light from traveling in the reverse direction. One common application
of an optical isolator is to keep such light from entering a laser diode and pos-
sibly causing instabilities in the optical output.
Many design configurations of varying complexity exist for optical isolators.
The simple ones depend on the state of polarization of the input light. However,
such a design results in a 3-dB loss (one-half the power) when unpolarized light
is passed through it, since it blocks one-half of the input signal. In practice, the
optical isolator should be independent of the SOP since light in an optical link
normally is not polarized.
Figure 9.5 shows a design for a polarization-independent isolator that is made
of three miniature optical components. The core of the device consists of a
45° Faraday rotator that is placed between two wedge-shaped birefringent plates
or walk-off polarizers. These plates could consist of a material such as YVO 4 or
TiO 2 , as described in Chap. 3. Light traveling in the forward direction (left to
right in Fig. 9.5) is separated into ordinary and extraordinary rays by the first
birefringent plate. The Faraday rotator then rotates the polarization plane of
each ray by 45°. After exiting the Faraday rotator the two rays pass through the
second birefringent plate. The axis of this polarizer plate is oriented in such a way
that the relationship between the two types of rays is maintained. Thus when
they exit the polarizer, they both are refracted in an identical parallel direction.
Going in the reverse direction (right to left), the relationship of the ordinary and
extraordinary rays is reversed when exiting the Faraday rotator due to the non-
reciprocity of the Faraday rotation. Consequently, the rays diverge when they exit
the left-hand birefringent plate and are not coupled to the fiber anymore.
Table 9.3 lists some operational characteristics of commercially available isola-
tors. The packages have similar configurations to the tap coupler shown in Fig. 9.2.

Figure 9.5. Design and operation of a polarization-independent
isolator made of three miniature optical components.

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