Waveplate (Phase Retarder)
Waveplates (retardation plates or phase shifters) are made from materials which exhibit birefringence. The velocities of the extraordinary and ordinary rays through the birefringent material varies inversely with their refractive indices. This 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 α=2πd(n en o)/λ, where α is phase difference; d is thickness of waveplate; n e,n o are refractive indices of extraordinary and ordinary rays respectively; λ is wavelength. At any specific wavelength the phase difference is governed by the thickness of the retarder. |
λ/2 Waveplate
The thickness of a half waveplate is such that the phase difference is 1/2-wavelength (λ/2,true zero order) or certain multiple of 1/2-wavelength [(2n+1)λ/2,multiple order].
A linearly polarized beam incident on a half waveplate emerges as a linearly polarized beam but rotated such that its angle to the optical axis is twice that of the incident beam. Therefore, half-waveplates can be used as continuously adjustable polarization rotators. Half-waveplates are used to rotate the plane of polarization, electro-optic modulation and as a variable ratio beamsplitter when used in conjunction with a polarization cube. |
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λ/4 Waveplate
The thickness of the quarter waveplate is such that the phase difference is 1/4 wavelength (λ/ 4,true zero order) or certain multiple of 1/4 wavelength [(2n+1)λ/4,multiple order]. If the angle θ (between the electric field vector of the incident linearly polarized beam and the retarder principal plane) of the quarter-waveplate is 45°, the emergent beam is circularly polarized. When a quarter waveplate is double passed, i.e. by mirror reflection, it acts as a half waveplates and rotates the plane of polarization to a certain angle. Quarter waveplate are used in creating circular polarization from linear or linear polarization from circular, ellipsometry, optical pumping, suppressing unwanted reflection and optical isolation |
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| Waveplate |
Illustration |
Properties and Application |
| Multi-order Waveplate |
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Please refer to Low-Order Waveplate. |
| Low-Order Waveplate |
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Its properties are much better than the multi-order waveplate because of its thinner thickness (less than 0.5mm). Better temperature (380C), wavelength (1.5nm) and incident angle (4.50) bandwidth and high damage threshold make it widely used in common application. Also, it is economical. |
| Zero-Order Waveplate |
Cemented And Optically Contracted |
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It is constructed of two multi-order waveplates with their axis crossed. Thus, it performs as a zero-order waveplate because of the effect of two plates counteracting each other. It has wide temperature bandwidth and wavelength bandwidth properties. Because it is cemented, the damage threshold must be considered (about 10MW/cm2) when used. |
| Air-spaced |
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Its performance is as good as cemented zero-order waveplate. Additionally, the air-spaced construction enables it is suitable for the high power laser application. The damage threshold is more than 500MW/cm2 . |
| True Zero-order Waveplate |
Cemented |
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The true zero-order waveplate means that the thickness of waveplate is very thin (less than 0.1mm) which make the true zero-order waveplate excellent in temperature, wavelength and incident angle (about 20º) bandwidth. Therefore, it is excellent choice for the highly accurate application. It is cemented with a block of glass which is limited to low and medium power application. |
| Single Plate |
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In order to enable the waveplate suitible for high damage threshold (more than 1GW/cm2) application: Lasertec provides a single plate of true zero-order waveplate * Provide mounts upon request. *The thickness of this waveplate means handling can be difficult, we can provide mounts upon request. |
AchromaticWaveplate
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Achromatic waveplate is similar to Zero-order waveplate except that the two plates are made from different materials, such as crystal quartz and magnesium fluoride. Since the dispersion of the birefringence can be different for the two materials, it is possible to specify the retardation values at a wavelength range. Hence, the retardation of the resulting waveplate can be made to low sensitivity to wavelength change. |
Double Wavelength Waveplate
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In fact, it is a low order or multi-order waveplate, but which is suitable for two different wavelengths with different retardation. The dual-wavelength waveplate is often applied in laser harmonic generation. |
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A kind of achromatic waveplate that based on the phase change different between the S-polarization and P-polarization of a beam when the beam is reflected by TIR at a special designed incident angle. By this method, the little relationship between phase retardation and wavelength can be got. Keeping the reflective surfaces clean is essential for its application |
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