Isolators & Circulators
Isolator,Definition
? An isolator is a device that allows
light to pass along a fibre in one
direction but not in the opposite
direction,
Application
? coupling a laser to a fibre,Reflected
light can cause instability in the laser
and cause many undesirable effects,
? high speed communication system
? optical amplifiers
? fibre ring cavity
The Faraday Effect
? Light travelling within the material has
its plane of polarisation (electric and
magnetic field vectors) rotated by an
amount depending on the length and
the strength of the magnetic field,
? the most important aspect is that the
effect is asymmetric,
? materials, YIG (Yttrium-Iron-Garnet)
Simple Isolator
Problem
? The isolator is plarization-dependent
? incoming light randomly polarised will
lose 50% of its power!
? If the signal is polarised and the
orientation is wrong then we can lose
all of the signal,
? if the polarisation varies with time the
variations will be translated into
variations in attenuation:serious noise!
Polarisation Independent
Isolator
? separating the incoming ray into its
two orthogonal polarisations
? processing them separately
? and then re-combining them at the
output
Practical configuration
Circulator,Definition
? Light entering at any
particular port (say
port 1) travels around
the circulator and
exits at the next port
(say port 2),Light
entering at port 2
leaves at port 3 and
so on,
Application
Principle of Operation I
Polarising Beam Splitter Cube
Birefringent,Walk-off” Block(YVO4)
Faraday Rotator and Phase Plate
? A ray input on Port 1 is split into two
separate rays of orthogonal
polarisations by first birefringent walk-off
block (block A),
? Both rays are rotated through 90°,
? The two rays meet another birefringent
walk-off block (block B) identical with
the first.The light is refracted and re-
combined
Principle of Operation II
? Light entering from Port 3 is split in
block B,
? Travelling in the reverse direction the
polarisation of both rays is unchanged,
? Birefringent block A now passes the
upper ray unchanged but shifts the
lower one further away,
? The two rays are then re-combined
using the reflector prism and the
polarising beamsplitter cube,
Polarisation Control
Definition
? What we are aiming at is to build a device
that receives light in any polarisation state
and can convert it without loss into any other
polarisation,
? To do this you need a device that is
birefringent but which can have its axes of
birefringence rotated,
Fibre Loop Polarisation Controller
Configuration
A typical loop diameter,about 750 mm,
Principle
? This rotation of the birefringent axes has
the effect of phase retardation of one
component of the signal light in relation
to the other,Thus it changes the
polarisation of the signal,By rotating the
fibre loops you can adjust the device to
produce light at any desired olarisation,
Fibre Squeezer Polarisation
Controller
Isolator,Definition
? An isolator is a device that allows
light to pass along a fibre in one
direction but not in the opposite
direction,
Application
? coupling a laser to a fibre,Reflected
light can cause instability in the laser
and cause many undesirable effects,
? high speed communication system
? optical amplifiers
? fibre ring cavity
The Faraday Effect
? Light travelling within the material has
its plane of polarisation (electric and
magnetic field vectors) rotated by an
amount depending on the length and
the strength of the magnetic field,
? the most important aspect is that the
effect is asymmetric,
? materials, YIG (Yttrium-Iron-Garnet)
Simple Isolator
Problem
? The isolator is plarization-dependent
? incoming light randomly polarised will
lose 50% of its power!
? If the signal is polarised and the
orientation is wrong then we can lose
all of the signal,
? if the polarisation varies with time the
variations will be translated into
variations in attenuation:serious noise!
Polarisation Independent
Isolator
? separating the incoming ray into its
two orthogonal polarisations
? processing them separately
? and then re-combining them at the
output
Practical configuration
Circulator,Definition
? Light entering at any
particular port (say
port 1) travels around
the circulator and
exits at the next port
(say port 2),Light
entering at port 2
leaves at port 3 and
so on,
Application
Principle of Operation I
Polarising Beam Splitter Cube
Birefringent,Walk-off” Block(YVO4)
Faraday Rotator and Phase Plate
? A ray input on Port 1 is split into two
separate rays of orthogonal
polarisations by first birefringent walk-off
block (block A),
? Both rays are rotated through 90°,
? The two rays meet another birefringent
walk-off block (block B) identical with
the first.The light is refracted and re-
combined
Principle of Operation II
? Light entering from Port 3 is split in
block B,
? Travelling in the reverse direction the
polarisation of both rays is unchanged,
? Birefringent block A now passes the
upper ray unchanged but shifts the
lower one further away,
? The two rays are then re-combined
using the reflector prism and the
polarising beamsplitter cube,
Polarisation Control
Definition
? What we are aiming at is to build a device
that receives light in any polarisation state
and can convert it without loss into any other
polarisation,
? To do this you need a device that is
birefringent but which can have its axes of
birefringence rotated,
Fibre Loop Polarisation Controller
Configuration
A typical loop diameter,about 750 mm,
Principle
? This rotation of the birefringent axes has
the effect of phase retardation of one
component of the signal light in relation
to the other,Thus it changes the
polarisation of the signal,By rotating the
fibre loops you can adjust the device to
produce light at any desired olarisation,
Fibre Squeezer Polarisation
Controller
