Waveplates, also known as retardationplates or phase shifters, are flat optical components designed to manipulatethe polarization state of light waves. They consist of a birefringent material,which has different refractive indices for different polarization directions.

When a polarized light wave enters awaveplate, it splits into two components, called the ordinary ray and theextraordinary ray, each of which travels at a different speed through thebirefringent material. As a result, the two rays accumulate a relative phaseshift that depends on the thickness and birefringence of the waveplate. Themost common types of waveplates are the quarter-waveplate and the half-waveplate.

A quarter-waveplate introduces a relativephase shift of π/2between the two polarization components, which converts linearly polarizedlight into circularly polarized light or vice versa.

A half-waveplate introduces a relativephase shift of π, which rotates the polarization direction of linearlypolarized light by a certain angle.

Waveplates are widely used in variousoptical applications, such as polarization control, interferometry, microscopy,and spectroscopy. They can also be combined with other optical elements, suchas polarizers, lenses, and mirrors, to create more complex optical systems.

Multiple order and zero order waveplatesare two types of waveplates that differ in their performance characteristics.

Zero Order: Azero order waveplate is a waveplate that is designed to introduce a specificphase shift (e.g., π/2 for a quarter-waveplate or π for a half-waveplate) betweenthe two polarization components of incident light, without introducing anyadditional phase shifts. This means that the output polarization state ispurely a function of the input polarization state and the waveplate's thicknessand birefringence. Zero order waveplates are often preferred in applicationswhere high polarization purity and accuracy are required, such as inpolarization-sensitive interferometry and polarimetry.

Standard Zero Order: A standard zero order waveplate is made by taking a birefringentmaterial (such as quartz or magnesium fluoride) and cutting it to a specificthickness based on the desired phase shift (e.g., π/2 for a quarter-waveplate).

The thickness is chosen to ensure that thephase difference between the two polarization components of the incident lightis exactly the desired value, and the resulting waveplate is called a zeroorder waveplate. However, due to small variations in the birefringent material,there can still be a small amount of unwanted phase shift introduced, which canlead to some degree of polarization distortion in the output light. This isreferred to as residual birefringence.

True Zero Order: A true zero order waveplate, on the other hand, is made by using amore precise manufacturing process that minimizes the residual birefringence.One method to achieve this is to use a birefringent material with a very smallbirefringence, such as crystalline quartz, and to cut the material at aspecific angle to the crystal axis. This produces a waveplate with a muchsmaller residual birefringence, resulting in a higher degree of polarizationpurity in the output light.

Multiple Order: A multiple order waveplate, on the other hand, is a waveplate thatintroduces multiple phase shifts (i.e., integer multiples of the designed phaseshift) between the two polarization components of incident light. This occursbecause the thickness of the waveplate is not an exact multiple of the wavelengthof light, which causes the different wavelength components of the incidentlight to accumulate different phase shifts.

Multiple order waveplates are lessexpensive to manufacture than zero order waveplates and can be used in manypolarization control applications. However, they have some drawbacks, includinga lower polarization purity and a higher sensitivity to wavelength andtemperature changes.

Achromatic – Achromatic waveplates consist of two different materials thatpractically eliminate chromatic dispersion. Standard achromatic lenses are madefrom two types of glass, which are matched to achieve a desired focal lengthwhile minimizing or removing chromatic aberration. Achromatic waveplatesoperate on the same basic principle. For example, Achromatic Waveplates are made from crystalquartz and magnesium fluoride to achieve nearly constant retardation across abroad spectral band.

Applications:

LasersApplications:

1.    Wavelength combining and separation:Waveplates are used to manipulate the polarization of light in laser systems tocombine and separate different wavelengths of light. This is useful inapplications such as spectroscopy, where different wavelengths need to beseparated and detected separately, or in telecommunications where differentwavelengths are combined for high-speed data transmission.

2.    Q-switching: Waveplates can beused in Q-switching, a technique used to achieve extremely high pulse powers inlasers. Q-switching involves rapidly switching the laser cavity from a low-Qstate to a high-Q state, which causes the stored energy to be released in a shortpulse. Waveplates can be used to control the polarization of light in thecavity, which affects the Q-switching performance and pulse characteristics.

3.    Destructive feedback quenching: In lasersystems, destructive feedback can occur when reflected light interferes withthe laser beam and causes damage to the laser. Waveplates can be used in tocreate and Optical Isolator to control the polarization of the reflectedlight and prevent destructive feedback.

4.    Circular polarization in industriallaser cutting: Circularly polarized light can be used in industriallaser cutting systems to achieve cleaner and more uniform cuts. This is becausecircularly polarized light produces a more symmetric beam profile and reducesthe effects of beam distortion and scattering. Waveplates can be used toconvert linearly polarized light to circularly polarized light in thesesystems.

Variable  Beamplitter: Avariable beamsplitter is a device that divides a beam of light into twoseparate beams, with the amount of light split between them being adjustable.It uses a polarizing beamsplitter cube and a half waveplate to control thetransmission (P polarized light) and reflection (S-polarized light) inside thecube. By rotating the half waveplate, the polarization angle of the incominglight can be modified, which adjusts the amount of light transmitted throughthe beamsplitter versus the amount that is reflected. This provides precisecontrol over the output ratio of the beamsplitter.

If the polarization of the two separated beams needs to bealigned on the same plane, a half waveplate can be inserted into the path ofone of the output beams. By adjusting the orientation of the waveplate, bothbeams can be polarized in the same direction. This is crucial in applicationswhere the beams need to form an interference pattern, such as in recording ahologram or writing a holographic diffraction grating.

Polarization Cleanup: In some optical systems, multiple reflections from mirrors cancause changes in the polarization state of light. When the plane ofpolarization of a beam is aligned with or perpendicular to the plane of amirror, there is usually no change in polarization upon reflection. However, ifthe polarization direction makes an angle with the plane of incidence,reflections from mirrors, can cause small phase shifts between the parallel andperpendicular polarization components. This can result in the reflected wavebeing slightly elliptically polarized, which can be observed as degradedextinction when a polarizer is inserted and rotated. To correct for this, afull waveplate can be inserted and tilted slightly about either its fast orslow axes to adjust the retardation and cancel out the ellipticity of thereflected wave.

Optical Isolator - An optical isolator is a device used to eliminate undesiredreflections in optical systems. It typically consists of a quarter-waveplateand a linear polarizer or polarizing beamsplitter cube. The incoming light beamis first linearly polarized by the beamsplitter and then converted to circularpolarization by the quarter-waveplate. Any outgoing beam that is reflected backinto the isolator will be converted by the quarter-waveplate into a beam thatis linearly polarized perpendicular to the input beam. This beam will beblocked by the beamsplitter cube and reflected in a direction other than theinput direction.

Definitions andTerminology:

Fast Axis:The fast axis is the axis along which light travels with the higher velocity orlower refractive index in a birefringence material. It is also known as the"extraordinary axis" or "e-axis." Light polarized along thefast axis will experience less delay or phase shift compared to light polarizedalong the slow axis.

Slow Axis:The slow axis is the axis along which light travels with the lower velocity orhigher refractive index in a birefringence material. It is also known as the"ordinary axis" or "o-axis." Light polarized along the slowaxis will experience more delay or phase shift compared to light polarizedalong the fast axis.

Birefringence: Birefringence is a phenomenon in optics where a material exhibitsdifferent refractive indices for light waves of different polarization states.This differential refractive index causes light waves of different polarizationstates to refract or bend at different angles as they pass through thematerial. As a result, a single incident light wave can be split into two ormore waves that propagate with different velocities and directions, leading todouble refraction or the splitting of a light wave into two or more rays withdifferent paths.

When unpolarized light passes through abirefringent material, the different refractive indices for differentpolarization states cause the incident light to be separated into its paralleland orthogonal components, leading to the phenomenon known as birefringent orbirefringent interference. This can result in the generation of interferencepatterns, or the polarization of light being modified as it propagates throughthe birefringent material.

Retardation:Retardation refers to the phase shift that occurs between the polarizationcomponent projected along the fast axis and the component projected along theslow axis of a waveplate. It can be specified in units of degrees, waves, ornanometers, with one full wave of retardation being equivalent to 360°, halfwave being equivalent to 180° and a quarter being equivalent to wave 90°. Inwaves it would be, 1λ, λ/2 and λ/4. In nanometers, λ represents the wavelengthof light in nanometers. For example, if the half waveplate is designed for usewith light of wavelength 532 nm, the retardation would be 0.5 * 532 nm = 266nm.

Retardation values for most waveplates areλ/4, λ/2, and 1λ. However, other values can be useful a phase shift may occurbetween optical components caused by an internal reflection in one of thecomponents. A compensating waveplate can be used to restore the desiredpolarization.