2.2 Basic Laws and Definitions of Optics
47
Fig. 2.12 A Faraday rotator
is a device that rotates the
state of polarization, for
example, clockwise by 45°
or a quarter of a wavelength
Polarized input
45° polarization
rotation
45° Faraday rotator
Output
maintained after the rotation. For example, if the input light to a 45° Faraday rotator
is linearly polarized in a vertical direction, then the rotated light exiting the crystal
also is linearly polarized at a 45° angle. The Faraday rotator material usually is an
asymmetric crystal such as yttrium iron garnet (YIG), and the degree of angular
rotation is proportional to the thickness of the device.
Birefringent or double-refractive crystals have a property called double refraction.
This means that the indices of refraction are slightly different along two perpendicular
axes of the crystal as shown in Fig. 2.13. A device made from such materials is known
as a spatial walk-off polarizer (SWP). The SWP splits the light signal entering it into
two orthogonally (perpendicularly) polarized beams. One of the beams is called an
ordinary ray or o-ray because it obeys Snell’s law of refraction at the crystal surface.
The second beam is called the extraordinary ray or e-ray because it refracts at an
angle that deviates from the prediction of the standard form of Snell’s law. Each
of the two orthogonal polarization components thus is refracted at a different angle
as shown in Fig. 2.13. For example, if the incident unpolarized light arrives at an
angle perpendicular to the surface of the device, the o-ray can pass straight through
the device whereas the e-ray component is deflected at a slight angle so it follows a
different path through the material.
Birefringent
material
Ordinary ray
Unpolarized
incident ray
Extraordinary
ray
Extraordinary ray polarization
Ordinary ray polarization
Fig. 2.13 A birefringent crystal splits the light signal entering it into two perpendicularly polarized
beams
47
Fig. 2.12 A Faraday rotator
is a device that rotates the
state of polarization, for
example, clockwise by 45°
or a quarter of a wavelength
Polarized input
45° polarization
rotation
45° Faraday rotator
Output
maintained after the rotation. For example, if the input light to a 45° Faraday rotator
is linearly polarized in a vertical direction, then the rotated light exiting the crystal
also is linearly polarized at a 45° angle. The Faraday rotator material usually is an
asymmetric crystal such as yttrium iron garnet (YIG), and the degree of angular
rotation is proportional to the thickness of the device.
Birefringent or double-refractive crystals have a property called double refraction.
This means that the indices of refraction are slightly different along two perpendicular
axes of the crystal as shown in Fig. 2.13. A device made from such materials is known
as a spatial walk-off polarizer (SWP). The SWP splits the light signal entering it into
two orthogonally (perpendicularly) polarized beams. One of the beams is called an
ordinary ray or o-ray because it obeys Snell’s law of refraction at the crystal surface.
The second beam is called the extraordinary ray or e-ray because it refracts at an
angle that deviates from the prediction of the standard form of Snell’s law. Each
of the two orthogonal polarization components thus is refracted at a different angle
as shown in Fig. 2.13. For example, if the incident unpolarized light arrives at an
angle perpendicular to the surface of the device, the o-ray can pass straight through
the device whereas the e-ray component is deflected at a slight angle so it follows a
different path through the material.
Birefringent
material
Ordinary ray
Unpolarized
incident ray
Extraordinary
ray
Extraordinary ray polarization
Ordinary ray polarization
Fig. 2.13 A birefringent crystal splits the light signal entering it into two perpendicularly polarized
beams