Page 162 - Optical Communications Essentials

Passive Optical Components

152 Chapter Nine

TABLE 9.3. Typical Parameter Values of Commercially Available Isolators
Parameter Unit Value
nm 1310, 1550
Central wavelength λ c
Peak isolation dB 40
Isolation at λ c 20nm dB 30
Insertion loss dB 0.5
Polarization-dependent loss dB 0.1
Polarization mode dispersion ps 0.25
Size (diameter length) mm 6 35

Figure 9.6. Operational con-
cept of a three-port circulator.

9.2.2. Optical circulators
An optical circulator is a nonreciprocal multiport passive device that directs
light sequentially from port to port in only one direction. The operation of a cir-
culator is similar to that of an isolator except that its construction is more com-
plex. Typically it consists of a number of walk-off polarizers, half-wave plates,
and Faraday rotators and has three or four ports, as shown in Fig. 9.6. To see
how it works, consider the three-port circulator. Here an input on port 1 is sent
out on port 2, an input on port 2 is sent out on port 3, and an input on port 3 is
sent out on port 1.
Similarly, in a four-port device, ideally one could have four inputs and four
outputs. We say ideally since that description assumes the circulator is perfectly
symmetric. In practice, in many applications it is not necessary to have four
inputs and four outputs. Furthermore, such a perfectly symmetric circulator is
rather complex to fabricate. Therefore, in a four-port circulator it is common to
have three input ports and three output ports by making port 1 an input-only
port, ports 2 and 3 input and output ports, and port 4 an output-only port.
A variety of circulators are available commercially. These devices have low
insertion loss, high isolation over a wide wavelength range, minimal polarization-
dependent loss (PDL), and low polarization mode dispersion (PMD). Table 9.4
lists some operational characteristics of commercially available circulators.

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.