PM Optical Circulators: Technology used and Categorization

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 Since 1990, polarization maintaining
optical circulator has become one of the essential components in advanced
optical communication systems. Nowadays, its applications have expanded widely
not only in telecommunication industry but also in medical and imaging fields.
Here, we will discuss this indispensable component in detail. So, let’s start
with the basics.

 

PM Optical Circulators

What is a PM Optical Circulator?

PM
optical circulator is a three or four port non
reciprocal passive component which functions similar to an isolator. It
transmits the light wave from one port to next port with maximum intensity
while maintaining polarization and blocking any light transmission from one
port to the previous port. Thus, it is also featured as a unidirectional
circulator.

 

Technology Used in PM Optical Circulators

Polarization Maintaining Optical
Circulators are designed on the basis of nonreciprocal polarization rotation of
the Faraday effect. As the working of optical circulators is based on several
components such as Faraday rotator, birefringent crystal, waveplate and beam
displacer, let’s take a quick look at each of them.

 

Faraday Effect:

It is a magneto-optical effect which
explains the phenomenon in which polarization plane of electromagnetic wave (or
light wave) is rotated inside a material under magnetic field applied in
parallel to the direction of wave propagation. The unique aspect of this effect
is that the direction of rotation is independent of the propagation direction
of light wave, which implies that rotation is non-reciprocal.

 

Light Propagation in Birefringent Crystal:

Birefringent crystal is a common material
used in the designing of optical circulators. The crystals used in optical
circulators are typically anisotropic uniaxial which means they have two
refractive indices with one optical axis. The function of this birefringent
crystal depends on the propagation direction of light and its optic axis
orientation (crystal cutting). The crystals which are generally used include
quartz, rutile, YVO4, etc.

 

Waveplate:

Also called retardation plate, a waveplate
is one of the applications of birefringent crystal. It is made by cutting a
crystal into a particular orientation. Due to small birefringence, crystal
quartz is broadly used for making waveplates.

 

Beam Displacer:

A birefringent crystal based beam displacer
is used to split an incoming light beam into two beams with orthogonal
polarization states.

 

Categorization of Optical Circulators

Optical circulators are mainly divided into
two categories:

Polarization-dependent optical circulator

Polarization-independent optical circulator

 

The former type is only functional for a
light wave with a particular polarization state and is only used in a few
applications such as free space communications between satellites and crystal
sensing.

 

On the other hand, the latter type is
functionally independent of the polarization state of light. While in ordinary
circulators, the polarization is not maintained but there are polarization
maintaining optical circulators available in the market also. They are used in
a large variety of applications.

 

According to their functionality, optical
circulators can also be divided into two groups.

Full Circulator – light passes through all
ports in a full circle

Quasi Circulator – light passes through all
ports but the light from last port is lost

 

Fortunately, due to the advancement in
technology, you can avail highly reliable and efficient Polarization
Maintaining Optical circulators not only in standard specifications but in
customized specifications too.