Optical isolator
Encyclopedia
An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback
into an optical oscillator, such as a laser cavity. The operation of the device depends on the Faraday effect
(which in turn is produced by magneto-optic effect
), which is used in the main component, the Faraday rotator
.
, applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday effect. The angle of rotation, 
, is given by,
,
where,
is the Verdet constant of the material (amorphous or crystalline; solid, liquid, or gaseous) of which the rotator is made, and 
is the length of the rotator. This is shown in Figure 2. Specifically for an optical isolator, the values are chosen to give a rotation of 45 degrees.
The polarization dependent isolator, or Faraday isolator, is made of three parts, an input polarizer (polarized vertically), a Faraday rotator, and an output polarizer, called an analyser (polarized at 45 degrees)
Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45 degrees. The analyser then enables the light to be transmitted through the isolator.
Light traveling in the backward direction becomes polarized at 45 degrees by the analyser. The Faraday rotator will again rotate the polarization by 45 degrees. This means the light is polarized horizontally (the rotation is sensitive to direction of propagation). Since the polarizer is vertically aligned, the light will be extinguished.
Figure 2 shows a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser,
, is set to 45 degrees. The Faraday rotator is chosen to give a 45 degree rotation.
Polarization dependent isolators are typically used in free space optical systems. This is because the polarization of the source is typically maintained by the system. In optical fibre systems, the polarization direction is typically dispersed in non polarization maintaining systems. Hence the angle of polarization will lead to a loss.
The polarization independent isolator is made of three parts, an input birefringent
wedge (with its ordinary polarization direction vertical and its extraordinary polarization direction horizontal), a Faraday rotator, and an output birefringent wedge (with its ordinary polarization direction at 45 degrees, and its extraordinary polarization direction at -45 degrees).
Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0 degrees) and horizontal (90 degrees) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45 degrees. This means the o-ray is now at 45 degrees, and the e-ray is at -45 degrees. The output birefringent wedge then recombines the two components.
Light traveling in the backward direction is separated into the o-ray at 45 degrees, and the e-ray at -45 degrees by the birefringent wedge. The Faraday Rotator again rotates both the rays by 45 degrees. Now the o-ray is at 90 degrees, and the e-ray is at 0 degrees. Instead of being focused by the second birefringent wedge, the rays diverge.
Typically collimator
s are used on either side of the isolator. In the transmitted direction the beam is split and then combined and focused into the output collimator. In the isolated direction the beam is split, and then diverged, so it does not focus at the collimator.
Figure 3 shows the propagation of light through a polarization independent isolator. The forward travelling light is shown in blue, and the backward propagating light is shown in red. The rays were traced using an ordinary refractive index of 2, and an extraordinary refractive index of 3. The wedge angle is 7 degrees.
, low absorption coefficient, low non-linear refractive index
and high damage threshold. Also, to prevent self-focusing
and other thermal related effects, the optic should be as short as possible. The two most commonly used materials for the 700-1100 nm range are terbium doped borosilicate glass and terbium gallium garnet crystal (TGG). For long distance fibre communication, typically at 1310 nm or 1550 nm, yttrium iron garnet
crystals are used (YIG). Commercial YIG based Faraday isolators reach isolations higher than 30 dB
.
Optical isolators are different from 1/4 wave plate
based isolators because the Faraday rotator provides non-reciprocal rotation while maintaining linear polarization
. That is, the polarization rotation due to the Faraday rotator is always in the same relative direction. So in the forward direction, the rotation is positive 45 degrees. In the reverse direction, the rotation is negative 45 degrees. This is due to the change in the relative magnetic field direction, positive one way, negative the other. This then adds to a total of 90 degrees when the light travels in the forward direction and then the negative direction. This allows the higher isolation to be achieved.
and the second law of thermodynamics
, by allowing light energy to flow from a cold object to a hot object and blocking it in the other direction, but the violation is avoided because the isolator must absorb (not reflect) the light from the hot object and will eventually reradiate it to the cold one.
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
into an optical oscillator, such as a laser cavity. The operation of the device depends on the Faraday effect
Faraday effect
In physics, the Faraday effect or Faraday rotation is a Magneto-optical phenomenon, that is, an interaction between light and a magnetic field in a medium...
(which in turn is produced by magneto-optic effect
Magneto-optic effect
A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field...
), which is used in the main component, the Faraday rotator
Faraday rotator
A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magneto-optic effect....
.
Theory
The main component of the optical isolator is the Faraday rotator. The magnetic field,, applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday effect. The angle of rotation, , is given by,,where,
is the Verdet constant of the material (amorphous or crystalline; solid, liquid, or gaseous) of which the rotator is made, and is the length of the rotator. This is shown in Figure 2. Specifically for an optical isolator, the values are chosen to give a rotation of 45 degrees.Polarization dependent isolator
Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45 degrees. The analyser then enables the light to be transmitted through the isolator.
Light traveling in the backward direction becomes polarized at 45 degrees by the analyser. The Faraday rotator will again rotate the polarization by 45 degrees. This means the light is polarized horizontally (the rotation is sensitive to direction of propagation). Since the polarizer is vertically aligned, the light will be extinguished.
Figure 2 shows a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser,
, is set to 45 degrees. The Faraday rotator is chosen to give a 45 degree rotation.Polarization dependent isolators are typically used in free space optical systems. This is because the polarization of the source is typically maintained by the system. In optical fibre systems, the polarization direction is typically dispersed in non polarization maintaining systems. Hence the angle of polarization will lead to a loss.
Polarization independent isolator
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
wedge (with its ordinary polarization direction vertical and its extraordinary polarization direction horizontal), a Faraday rotator, and an output birefringent wedge (with its ordinary polarization direction at 45 degrees, and its extraordinary polarization direction at -45 degrees).
Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0 degrees) and horizontal (90 degrees) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45 degrees. This means the o-ray is now at 45 degrees, and the e-ray is at -45 degrees. The output birefringent wedge then recombines the two components.
Light traveling in the backward direction is separated into the o-ray at 45 degrees, and the e-ray at -45 degrees by the birefringent wedge. The Faraday Rotator again rotates both the rays by 45 degrees. Now the o-ray is at 90 degrees, and the e-ray is at 0 degrees. Instead of being focused by the second birefringent wedge, the rays diverge.
Typically collimator
Collimator
A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.- Optical collimators :In optics, a collimator may...
s are used on either side of the isolator. In the transmitted direction the beam is split and then combined and focused into the output collimator. In the isolated direction the beam is split, and then diverged, so it does not focus at the collimator.
Figure 3 shows the propagation of light through a polarization independent isolator. The forward travelling light is shown in blue, and the backward propagating light is shown in red. The rays were traced using an ordinary refractive index of 2, and an extraordinary refractive index of 3. The wedge angle is 7 degrees.
The Faraday rotator
The most important optical element in an isolator is the Faraday rotator. The characteristics that one looks for in a Faraday rotator optic include a high Verdet constantVerdet constant
The Verdet constant is an optical "constant" that describes the strength of the Faraday effect for a particular material.The Verdet constant for most materials is extremely small and is wavelength dependent. It is strongest in substances containing paramagnetic ions such as terbium...
, low absorption coefficient, low non-linear refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
and high damage threshold. Also, to prevent self-focusing
Self-focusing
Self-focusing is a non-linear optical process induced by the change in refractive index of materials exposed to intense electromagnetic radiation. A medium whose refractive index increases with the electric field intensity acts as a focusing lens for an electromagnetic wave characterised by an...
and other thermal related effects, the optic should be as short as possible. The two most commonly used materials for the 700-1100 nm range are terbium doped borosilicate glass and terbium gallium garnet crystal (TGG). For long distance fibre communication, typically at 1310 nm or 1550 nm, yttrium iron garnet
Yttrium iron garnet
Yttrium iron garnet is a kind of synthetic garnet, with chemical composition 323, or Y3Fe5O12. It is a ferrimagnetic material with a Curie temperature of 550 K....
crystals are used (YIG). Commercial YIG based Faraday isolators reach isolations higher than 30 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
.
Optical isolators are different from 1/4 wave plate
Wave plate
A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it.- Operation :A wave plate works by shifting the phase between two perpendicular polarization components of the light wave. A typical wave plate is simply a birefringent crystal...
based isolators because the Faraday rotator provides non-reciprocal rotation while maintaining linear polarization
Linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...
. That is, the polarization rotation due to the Faraday rotator is always in the same relative direction. So in the forward direction, the rotation is positive 45 degrees. In the reverse direction, the rotation is negative 45 degrees. This is due to the change in the relative magnetic field direction, positive one way, negative the other. This then adds to a total of 90 degrees when the light travels in the forward direction and then the negative direction. This allows the higher isolation to be achieved.
Optical isolators and thermodynamics
It might seem at first glance that a device that allows light to flow in only one direction would violate Kirchhoff's lawKirchhoff's law of thermal radiation
In thermodynamics, Kirchhoff's law of thermal radiation, or Kirchhoff's law for short, is a general statement equating emission and absorption in heated objects, proposed by Gustav Kirchhoff in 1859, following from general considerations of thermodynamic equilibrium and detailed balance.An object...
and the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...
, by allowing light energy to flow from a cold object to a hot object and blocking it in the other direction, but the violation is avoided because the isolator must absorb (not reflect) the light from the hot object and will eventually reradiate it to the cold one.