10.3 Nonreciprocal Isolators and Circulators
411
Table 10.3 Typical
parameter values of
commercially available
optical isolators
Parameter
Unit
Value
Central wavelength λ c
nm
1310, 1550
Peak isolation
dB
40
Isolation at λ c ± 20 nm
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
reversed when exiting the Faraday rotator due to the nonreciprocity of the Faraday
rotation. Consequently, the rays diverge when they exit the left-hand birefringent
plate and are not coupled to the left-hand fiber.
Table 10.3 lists some operational characteristics of commercially available
isolators. The packages have similar configurations as the tap coupler shown in
Fig. 10.7.
10.3.2 Characteristics of Optical Circulators
An optical circulator is a nonreciprocal multiport passive device that directs light
sequentially from port to port in only one direction. This device is used in optical
amplifiers, add/drop multiplexers, and dispersion compensation modules. The operation of a circulator is similar to that of an isolator except that its construction is
more complex. Typically it consists of a number of walk-off polarizers, half-wave
plates, and Faraday rotators and has three or four input /output ports. To see how it
functions, consider the three-port circulator, as shown in Fig. 10.18. 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
if the circulator is perfectly symmetrical. However, in actual applications it usually
is not necessary to have four inputs and four outputs. Furthermore, such a perfectly
symmetrical circulator is rather tedious to fabricate. Therefore in a four-port circulator it is common to have three input ports and three output ports, making port 1 be
an input-only port, 2 and 3 being input and output ports, and port 4 be 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 polarizationdependent loss (PDL), and low polarization-mode dispersion (PMD). Table 10.4 lists
some operational characteristics of commercially available circulators.
411
Table 10.3 Typical
parameter values of
commercially available
optical isolators
Parameter
Unit
Value
Central wavelength λ c
nm
1310, 1550
Peak isolation
dB
40
Isolation at λ c ± 20 nm
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
reversed when exiting the Faraday rotator due to the nonreciprocity of the Faraday
rotation. Consequently, the rays diverge when they exit the left-hand birefringent
plate and are not coupled to the left-hand fiber.
Table 10.3 lists some operational characteristics of commercially available
isolators. The packages have similar configurations as the tap coupler shown in
Fig. 10.7.
10.3.2 Characteristics of Optical Circulators
An optical circulator is a nonreciprocal multiport passive device that directs light
sequentially from port to port in only one direction. This device is used in optical
amplifiers, add/drop multiplexers, and dispersion compensation modules. The operation of a circulator is similar to that of an isolator except that its construction is
more complex. Typically it consists of a number of walk-off polarizers, half-wave
plates, and Faraday rotators and has three or four input /output ports. To see how it
functions, consider the three-port circulator, as shown in Fig. 10.18. 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
if the circulator is perfectly symmetrical. However, in actual applications it usually
is not necessary to have four inputs and four outputs. Furthermore, such a perfectly
symmetrical circulator is rather tedious to fabricate. Therefore in a four-port circulator it is common to have three input ports and three output ports, making port 1 be
an input-only port, 2 and 3 being input and output ports, and port 4 be 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 polarizationdependent loss (PDL), and low polarization-mode dispersion (PMD). Table 10.4 lists
some operational characteristics of commercially available circulators.