What Are Fiber Optic Circulators?
What is an optical circulator?
The optical circulator has similar function and design as the optical isolator. An
optical circulator is an nonreciprocal passive device that directs light sequentially from
port 1 to port 2, from port 2 to port 3, and so on in only one direction. The operation of
a circulator is similar to that of an isolator except its constructions is more complex.
Like the isolator, its uses polarization to do its job.
The typical construction of an circulator consists of a number of walk-off polarizers,
half-wave plates and Faraday rotators. Typically an optical ciruclator has three or four ports.
A variety of circulators are available commercially. They have low insertion loss, high
isolation over a wide wavelength range, minimal polarization dependent loss, and low
polarization mode dispersion.
The typical insertion loss of an isolator is about 0.6dB, channel isolation is over
40dB, optical return loss is over 50dB and polarization dependent loss is lower than 0.1dB.
The applications of optical ciruclators
In advanced optical communication systems, circulators are used for bi-directional
transmissions, WDM networks, fiber amplifier systems, and for optical time domain
reflectometer (OTDR) measurements.
Optical circulators are essential compoents of optical communication systems. They
enable the routing of light from one optical fiber or waveguide to another based upon the
direction of light propagation.
Optical ciruculators extend the basic idea behind an optical isolator and add more
functionality to the device. A circulator does not disgard the backward propagating light,
as an isolator does, but directs it to another port, thus resulting in a three-port device
in the simplest configuration. More ports can be added if one wants to redirect light
coming from the third port to a fourth port. Even six ports circulators exist which direct
light to different ports in a circular fashion depending on which port light enters.
You may guess that with the increasing of ports, the design becomes increasingly
complex. You are absolutely correct on that guess. A second layer of complexity is added
for polarization-independent circulators because they must split the incoming light from
any port into its orthogonally polarized components and process each component separately.
In general, a circulator requires a large number of parts. The most important component
in a polarization independent circulator is the beam displacer. Beam displacer is made
from a strongly birefringent medium such that it displaces the orthogonally polarized
components spatially by different amounts.
In spite of their complexity, optical circulators are available commercially in a
relatively compact size with fiber pigtails on each end. Insertion losses are also very
acceptable for such complex devices.
Optical circulators have found many applications in designing lightwave systems. A
example could be a three-port circulator used with a fiber grating to realize a narrowband
bandpass filter working in transmission. The circulator coverts the device into a
transmission filter for all practical purposes.
Without a circulator, many applications have to be replaced by a 3dB fiber coupler
which will introduce 3dB losses. The use of circulator allows to realize the same goal
with less than -1dB insertion losses.
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