Waveplate Tutorial

Waveplates, also known as retardation plates or phase shifters, are flat optical components designed to manipulate the 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 a waveplate, it splits into two components, called the ordinary ray and the extraordinary ray, each of which travels at a different speed through the birefringent material. As a result, the two rays accumulate a relative phase shift 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 relative phase shift of π/2between the two polarization components, which converts linearly polarized light into circularly polarized light or vice versa.

A half-waveplate introduces a relative phase shift of π, which rotates the polarization direction of linearly polarized light by a certain angle.

Waveplates are widely used in various optical applications, such as polarization control, interferometry, microscopy,and spectroscopy. They can also be combined with other optical elements, such as 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 specific phase shift (e.g., π/2 for a quarter-waveplate or π for a half-waveplate) between the two polarization components of incident light, without introducing any additional phase shifts. This means that the output polarization state is purely a function of the input polarization state and the waveplate's thickness and birefringence. Zero order waveplates are often preferred in applications where 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 birefringent material (such as quartz or magnesium fluoride) and cutting it to a specific thickness based on the desired phase shift (e.g., π/2 for a quarter-waveplate).

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

 

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

 

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

 

Multiple order waveplates are less expensive to manufacture than zero order waveplates and can be used in many polarization control applications. However, they have some drawbacks, including a lower polarization purity and a higher sensitivity to wavelength and temperature changes.

 

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

Applications:

Lasers Applications:

 

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

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

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

4. Circular polarization in industrial laser cutting: Circularly polarized light can be used in industrial laser cutting systems to achieve cleaner and more uniform cuts. This is because circularly polarized light produces a more symmetric beam profile and reduces the effects of beam distortion and scattering. Waveplates can be used to convert linearly polarized light to circularly polarized light in these systems.

 

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

If the polarization of the two separated beams needs to be aligned 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, both beams can be polarized in the same direction. This is crucial in applications where the beams need to form an interference pattern, such as in recording a hologram or writing a holographic diffraction grating.

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

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

Definitions and Terminology:

 

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

 

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

 

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

 

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

 

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

 

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