Operating Principal of Wave Plates
Optical wave plates are constructed from birefringent material that introduces a phase difference between the fast and slow principal axis of the wave plate. The birefringent properties of the material create a difference in refractive index between the two axis. This in return creates a difference in the velocity between the two orthogonal components. The fast principal axis of the wave plate has a lower refractive index resulting in faster wave velocity. The slow axis has a higher refractive index with slower velocity. The actual phase shift created depends on the properties of the material, the thickness of the wave plate and the wavelength of the signal, and can be described as:
where n1 is the refractive index of the principal plane, n2 is the refractive index of the orthogonal plane and d is the thickness of the wave plate.
Using a Wave Plate
Wave plates are typically available as λ/4 or λ/2 meaning a phase shift of quarter of a wavelength or half a wavelength (respectively) is created.
As described above a wave plate has two principal axis, fast and slow each with a different refractive index and hence a different wave velocity. When applying a linearly polarized beam to a half-wave plate the beam will emerge from the wave plate as a linearly polarized light but with a polarization plane that is rotated with respect to the polarization of the input beam. The rotation of the polarization plane is such that the angle between the output polarization and the wave plate’s axis is twice the angle between the input polarization and the wave plate’s axis. When applying a circularly polarized beam, a clockwise circular polarization will transform into a counter-clockwise circular polarization and vice versa.
λ/2 wave plates are typically used as polarization rotators. Mounted on a Rotation Stage, a λ/2 can be used as a continuously adjustable polarization rotator. Additionally, when used in conjunction with a Polarizing Beamsplitter the pair can be used as a variable ratio beamsplitter.
A quarter-wave plate is designed such that the phase shift created between the fast and slow axis is quarter of a wavelength (λ/4) or a multiple of λ/4. When applying a linearly polarized beam with the polarization plane aligned at 45° to the wave plate’s principal plane, the output beam will be circularly polarized. Similarly when applying a circularly polarized beam to a λ/4 wave plate the output beam will be linearly polarized. Accordingly quarter wave plates are used in Optical Isolators, Optical pumps and EO modulators.
Choosing a Wave Plate
Thorlabs offers achromatic, zero-order (both unmounted wave plates and mounted wave plates) and multi-order wave plates (single wavelength and dual wavelength) with either λ/4 or λ/2 phase shift.
While our Achromatic Wave Plates provide phase retardance over a large spectral range, zero-order and multi-order wave plates provide a phase shift that is wavelength dependent. Our achromatic wave plates are available with four AR coatings: 260-410 nm, 400-800 nm, 690-1200 nm, and 1100-2000 nm.
|Round Zero-Order Wave Plate Comparison|
|Sizes||Ø1/2" and Ø1"||Ø1"|
|Retardances Available||1/4 λ and 1/2 λ||1/4 λ|
|Surface Quality||20-10 Scratch-Dig||60-40 Scratch-Dig|
|Coating||V Coat||Broadband AR|
|0.25%||0.5% Average Over Specified Coating Range|
Zero-order waveplates are designed such that the phase shift created is exactly one quarter or one half of a wave. They offer substantially lower dependence on temperature and wavelength than multi-order wave plates. Our Zero-Order Quartz Wave Plates are composed of two wave plates stacked together with the fast axis of one aligned to the slow axis of the other to achieve zero-order performance. Thorlabs' zero-order wave plates are available for a number of discrete wavelengths ranging from 266 nm to 2020 nm. Our Economy Zero-Order Quarter-Wave Plates consist of a thin layer of liquid crystal polymer retarding material sandwiched between two glass plates and are available at discrete wavelengths between 405 nm and 1053 nm. Our quartz zero-order wave plates provide better retardance accuracy and lower reflectance (see table), while our LCP zero-order wave plates produce a smaller decrease in retardance at larger AOIs. In addition, Thorlabs also offers unmounted true Zero-Order Telecom Wave Plates for WDM applications.
Multi-Order Wave Plates are made such that the retardance of a light path will undergo a certain number of full wavelength shifts (also referred to as the order, or m) in addition to the fractional design retardance. Compared to their zero-order counterparts, the retardance of multi-order wave plates is more sensitive to wavelength and temperature changes. Multi-order wave plates are, however, a more economical solution for many applications where increased sensitivities are not an issue. Our multi-order wave plates are available for a number of discrete wavelengths ranging from 266 nm to 1550 nm. Thorlabs also offers Dual-Wavelength Multi-Order Wave Plates designed for use at both 532 nm and 1064 nm.