Optics >>> Waveplate

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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