Optical axis gratings
Encyclopedia
Optical axis gratings are gratings
of optical axis
of a birefringent
material. In OAGs, the birefringence of the material is constant, while the direction of optical axis is periodically modulated in a fixed direction. In this way they are different from the regular phase gratings, in which the refractive index
is modulated and the direction of the optical axis is constant.
The optical axis in OAGs can be modulated in either transverse or the longitudinal direction, which causes it to act as a diffractive or a reflective component. Numerous modulation profiles allow variation in the optical properties of the OAGs.
-thick layer
. The cycloidal OAGs have already been proven to be very efficient in beam steering
and optical switching.
In another type of OAG, the optical axis is modulated in the direction of light propagation (Fig. 2) with a modulation period equal to a fraction of the wavelength (200-3000 nm). This modulation prevents these frequencies from propagating within the grating, acting as a band-stop filter
. As a result, any light with frequency within the matching range will be reflected from the OAG. However, unlike cholesterics which reflect only one of two circular polarizations of incident light, this OAG reflects any polarization.
gratings. This new type of grating has broad potential in imaging, liquid crystal display
, communication, and numerous military applications.
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
of optical axis
Optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...
of a birefringent
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...
material. In OAGs, the birefringence of the material is constant, while the direction of optical axis is periodically modulated in a fixed direction. In this way they are different from the regular phase gratings, in which the 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....
is modulated and the direction of the optical axis is constant.
The optical axis in OAGs can be modulated in either transverse or the longitudinal direction, which causes it to act as a diffractive or a reflective component. Numerous modulation profiles allow variation in the optical properties of the OAGs.
Examples of OAGs
An example of a transverse OAG, the so-called cycloidal OAG, is shown in Fig. 1. The optical axis in this grating is monotonously modulated in transverse direction. This grating is capable of diffracting all incident light into either +1st or −1st order in a micrometerMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
-thick layer
. The cycloidal OAGs have already been proven to be very efficient in beam steering
Beam steering
Beam steering is about changing the direction of the main lobe of a radiation pattern.In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the RF signals driving the elements.In optical systems, beam steering may be accomplished by...
and optical switching.
In another type of OAG, the optical axis is modulated in the direction of light propagation (Fig. 2) with a modulation period equal to a fraction of the wavelength (200-3000 nm). This modulation prevents these frequencies from propagating within the grating, acting as a band-stop filter
Band-stop filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter...
. As a result, any light with frequency within the matching range will be reflected from the OAG. However, unlike cholesterics which reflect only one of two circular polarizations of incident light, this OAG reflects any polarization.
Applications
Optical axis gratings can be implemented in various materials, including liquid crystals, polymers, birefringent crystals, magnetic crystals and subwavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
gratings. This new type of grating has broad potential in imaging, liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
, communication, and numerous military applications.