WHAT IS OPTICAL CIRCULATOR AND ITS APPLICATIONS?

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 An
optical circulator is a multi-port (minimum three ports) nonreciprocal passive
component.

 

The
function of an optical circulator is similar to that of a microwave circulator—to
transmit a lightwave from one port to the next sequential port with a maximum
intensity, but at the same time to block any light transmission from one port
to the previous port. Optical circulators are based on the nonreciprocal
polarization rotation of the Faraday effect.

 

Starting
from the 1990s optical circulators has become one of the indispensable elements
in advanced optical communication systems, especially WDM systems. The
applications of the optical circulator expanded within the telecommunications
industry (together with erbium-doped fiber amplifiers and fiber Bragg
gratings), but also expanded into the medical and imaging fields.

 

Since
optical
circulators are based on several components, including Faraday rotator,
birefringent crystal, waveplate, and beam displacer, we will have to explain
these technologies before jumping into the detail of circulator.

 

1.
Faraday Effect

 

The
Faraday effect is a magneto-optic effect discovered by Michael Faraday in 1845.
It is a phenomenon in which the polarization plane of an electromagnetic
(light) wave is rotated in a material under a magnetic field applied parallel
to the propagation direction of the lightwave. A unique feature of the Faraday
effect is that the direction of the rotation is independent of the propagation
direction of the light, that is, the rotation is nonreciprocal.

 

The
Verdet constant is a measure of the strength of the Faraday effect in a
particular material, and a large Verdet constant indicates that the material
has a strong Faraday effect. The Verdet constant normally varies with
wavelength and temperature. Therefore, an optical circulator is typically only
functional within a specific wavelength band and its performance typically
varies with temperature. Depending on the operating wavelength range, different
Faraday materials are used in the optical circulator.

 

Rare-earth-doped
glasses and garnet crystals are the common Faraday materials used in optical
circulators for optical communication applications due to their large Verdet
constant at 1310 nm and 1550 nm wavelength windows. Yttrium Iron Garnet and
Bismuth-substituted Iron Garnets are the most common materials.

 

The
Verdet constant of the BIG is typically more than 5 times larger the YIG, so a
compact device can be made using the BIG crystals. All these materials usually
need an external magnet to be functional as a Faraday rotator. Recently,
however, a pre-magnetized garnet (also call latching garnet) crystal has been
developed that eliminates the use of an external magnet, providing further
potential benefit in reducing overall size.

 

Faraday
rotators in optical circulators are mostly used under a saturated magnetic
field, and the rotation angle increases almost linearly with the thickness of
the rotator in a given wavelength (typically 40 nm) range. The temperature and
wavelength dependence of the Faraday rotation angle of the typical BIG crystals
at wavelength of 1550 nm is 0.04-0.07 deg/°C and 0.04-0.06 deg/nm,
respectively.

 

Another
common material used in the construction of optical circulators is the
birefringent crystal. Birefringent crystals used in optical circulators are
typically anisotropic uniaxial crystals (having two refractive indices with one
optical axis). In an anisotropic medium, the phase velocity of the light
depends on the direction of the propagation in the medium and the polarization
state of the light. Therefore, depending on the polarization state of the light
beam and the relative orientation of the crystal, the polarization of the beam
can be changed or the beam can be split into two beams with orthogonal
polarization states.

 

The
refractive index ellipsoid for a uniaxial crystal is shown in the above figure.
When the direction of the propagation is along the z-axis (optic axis), the
intersection of the plane through the origin and normal to the propagation
direction So is a circle; therefore, the refractive index is a constant and independent
of the polarization of the light. When the direction of the propagation S forms
an angle θ with the optic axis, the intersection of the plane through the
origin and normal to S becomes an ellipse. In this case, for the light with the
polarization direction perpendicular to the plane defined by the optic axis and
S, the refractive index, is called the ordinary refractive index no, is given
by the radius ro and independent of the angle θ. This light is called ordinary
ray and it propagates in the birefringent material as if in an isotropic medium
and follows the Snell’s law at the boundary.

 

On
the other hand, for light with the polarization direction along the plane
defined by the optic axis and S, the refractive index is determined by the
radius re and varies with the angle θ. This light is called the extraordinary
ray and the corresponding refractive index is called the extraordinary
refractive index ne. In this case ne is a function of θ and can be expressed as

 

The
ne varies from no to ne depending on the direction of propagation. A
birefringent crystal with no < ne is called a positive crystal, and one with
no > ne is called a negative crystal.

 

Therefore,
the function of a birefringent crystal depends on its optic axis orientation
(crystal cutting) and the direction of the propagation of a light. Birefringent
crystals commonly used in optical circulators are quartz, rutile, calcite, and
YVO4.

 

HOW
OPTICAL CIRCULATOR WORKS

Optical
circulators can be divided into two categories.

 

polarization-dependent
optical circulator, which is only functional for a light with a particular
polarization state. The polarization-dependent circulators are only used in
limited applications such as free-space communications between satellites, and
optical sensing.

 

polarization-independent
optical circulator, which is functional independent of the polarization state
of a light. It is known that the state of polarization of a light is not
maintained and varies during the propagation in a standard optical fiber due to
the birefringence caused by the imperfection of the fiber. Therefore, the
majority of optical circulators used in fiber optic communication systems are
designed for polarization-independent operation.

 

Optical
circulators can be divided into two groups based on their functionality.

 

Full
circulator, in which light passes through all ports in a complete circle (i.e.,
light from the last port is transmitted back to the first port). In the case of
a full three-port circulator, light passes through from port 1 to port 2, port
2 to port 3, and port 3 back to port 1.

Quasi-circulator,
in which light passes through all ports sequentially but light from the last
port is lost and cannot be transmitted back to the first port. In a
quasi-three-port circulator, light passes through from port 1 to port 2 and
port 2 to port 3, but any light from port 3 is lost and cannot be propagated
back to port 1. In most applications only a quasi-circulator is required.

The
operation of optical circulators is based on two main principles.

 

Polarization
splitting and recombining together with nonreciprocal polarization rotation.

Asymmetric
field conversion with nonreciprocal phase shift.