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Debbie Foster's List: Polarization
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Absorption Filters
Absorption filters work by reducing the incident light through absorption of specific wavelengths. Absorption filters are commonly made from pigmented gelatin or dyed glass. The spectral performance of an absorption filter is a function of the quantity of the dye present in the glass or gelatin matrix and the physical thickness of the filter itself.
Absorption filters are used to generate special effects in numerous photography applications and are extensively used in the cinema industry. Absorption filters are also found in traffic signals and on boats, aircrafts and vehicles as directional signals.
Dichroic Filters
Dichroic filters are far more capable and precise in their ability to obstruct unwanted wavelengths when compared to glass and gel absorption filters. Multi-layered thin film coatings are used for the manufacture of dichroic filters. These coatings are built up onto optical-grade glass using vacuum deposition.
Dichroic filters are widely used in a number of applications such as specific filtration for photography and optical microscopy. Dichroic filters are used instead of absorption filters for high quality color enlarges to fine tune the light color transmitted through the color transparency or negative.
Interference Filters
Interference filters differs from absorption filters. Rather than absorbing, interference filters reflect and destructively interfere with unwanted wavelength.
Modern interference filters are formed after the Fabry-Perot interferometer designed in the late 1800s by Alfred Perot and Charles Fabry. Interference filters are manufactured with a number of layers of thin films applied to a flat optically transparent glass surface.
Successive layers of dielectric materials are used to produce modern interference filters. The thickness of the dielectric materials ranges from 1/4 to 1/2 of the targeted wavelength. The dielectric materials are coated onto a flat optical glass of a polymer surface under vacuum conditions.
Light which is incident on the multilayered dielectric surface is either passes through the filter with constructive reinforcement or reflected and decreases in magnitude by destructive interference.
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Apr 01, 10
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Polarization of an Optical Wave through Polarizers and Wave Plates
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Birefringence of crystals can modify the Polarization State of light which is very useful in many applications. This type of optical components are called birefringent wave plates or retardation plates (or just wave plates or retarders for short).
The velocities of the extraordinary and ordinary rays through the birefringent materials vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. In the case of an incident linearly polarized beam this is given by a=2pd(ne-no)/l(a-phase difference; d-thickness of waveplate; ne, no-refractive indices of extraordinary and ordinary rays respectively; l-wavelength). At any specific wavelength the phase difference is governed by the thickness of the waveplate.
Red Optronics provides the following waveplates: octadic-wave (l/8), quarter-wave (l/4), half-wave (l/2) and full-wave (l) plates.
Half Wave Plate
The half wave plate can be used to rotate the polarization state of a plane polarized light as shown in Figure 1.
Suppose a plane-polarized wave is normally incident on a wave plate, and the plane of polarization is at an angle q with respect to the fast axis, as shown. After passing through the plate, the original plane wave has been rotated through an angle 2q. A half-wave plate is very handy in rotating the plane of polarization from a polarized laser to any other desired plane (especially if the laser is too large to rotate). Most large ion lasers are vertically polarized. To obtain horizontal polarization, simply place a half-wave plate in the beam with its fast (or slow) axis 45° to the vertical. The l/2 plates can also change left circularly polarized light into right circularly polarized light or vice versa. The thickness of half waveplate is such that the phase difference is 1/2 wavelength (l/2, Zero order) or certain multiple of 1/2-wavelength [(2n+1)l/2, multiple order].
Quarter Wave Plate
Quarter wave plate are used to turn plane-polarized light into circularly
polarized light and vice versa. To do this, we must orient the wave plate so that equal amounts of fast and slow waves are excited. We may do this by orienting an incident plane-polarized wave at 45° to the fast (or slow) axis, as shown in Figure 2. When a l/4 plate is double passed, i.e., by mirror reflection, it acts as a l/2 plate and rotates the plane of polarization to a certain angle, i.e., 90°. This scheme is widely used in isolators, Q-switches, etc.The thickness of the quarter waveplate is such that the phase difference is 1/4 wavelength (l/4, Zero order) or certain multiple of 1/4-wavelength [(2n+1)l/4, multiple order].
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One of the rays travels with the same velocity in every direction through the crystal and is termed the ordinary ray. The other ray travels with a velocity that is dependent upon the propagation direction within the crystal. This light ray is termed the extraordinary ray. The distance of separation between the ordinary and extraordinary ray increases with increasing crystal thickness. The two independent refractive indices of anisotropic crystals are quantified in terms of their birefringence, a measure of the difference in refractive index. Thus, the birefringence (B, often termed d, or D) of a crystal is defined as:
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