polarization techniques

This application note provides an overview of polarized light, delay, and the most important tools for manipulating the polarization state. The description of polarization techniques covers the operation and application of common light manipulation devices such as optical isolators, light attenuators, polarization rotators, and variable beam splitters.

light polarization

In classical physics, light of a single color is described by an electromagnetic field in which electric and magnetic fields oscillate at a frequency ( ν ) that is related to the wavelength ( Λ ). This is expressed by the formula C=Λ νC = Λ νC=Λ νC, where c is the speed of light. Visible light has wavelengths between 400 and 750 nm.

This formula demonstrates how polarization techniques can be applied specifically to influence the properties of light in optical systems. A key concept involved is the polarization state, which is determined by the orientation of the electric field and can be modified or amplified depending on the application. This targeted control is a central aspect of polarization techniques.

Circularly polarized light is produced when a linear component of the electric field is phase shifted by λ/4. Elliptically polarized light occurs when the phase shift between the field components is arbitrary.

We generate linearly polarized light by passing unpolarized light through a polarizing medium. A polarizer blocks components that do not align with its axis. Vertically polarized light that is sent through a polarizer at 45° reduces its amplitude by the factor 1/√2 and halves the intensity. A horizontally oriented polarizer does not allow any vertically polarized light to pass through.

delay

A phase shifter can extend the optical path length for polarization. This changes the polarization state and enables targeted applications.

Lambda/4 plate:

A Lambda/4 plate generates a phase shift and converts elliptically polarized light into linearly polarized light. With birefringent materials, this delay is caused by the fast and slow axis of the plate. A linear polarizer and a mirror are used for alignment.

Lambda/2 plate

The Lambda/2 plate (half-wave plate) is another important element in polarization techniques. It produces a phase shift of 180 degrees (λ/2) between the orthogonal components of the electric field. This special delay causes a rotation of the polarization plane of linearly polarized light. When the fast axis of the Lambda/2 plate is placed at a specific angle relative to the polarization plane of the incident light, the polarization rotates at twice the angle of the plate's orientation.

In circularly polarized light, the Lambda/2 plate also causes a reversal of polarization – it changes the so-called „polarity“ of the light from right-to-left circularly or vice versa. This property is particularly valuable in optical signal processing, where precise control of the polarization state is required.

Difference between optical isolators and attenuators

Optical isolators

Optical isolators are important components in laser technology and telecommunications. They prevent the light from reflecting back into a light source in order to maintain its stability and avoid damage to sensitive components. An optical isolator combines a linear polarizer and a lambda/4 plate (quarter-wave plate). First, the linear polarizer polarizes the incident light in a fixed plane. The lambda/4 plate then converts the linearly polarized light into circularly polarized light. When this light is reflected and passes through the isolator, the circularity of the polarized light is reversed. Re-passing through the lambda/4 plate rotates the light into a polarization plane perpendicular to the original direction. The polarizer then blocks this returning light and prevents any feedback.

Optical Attenuators

Optical attenuators work in a similar way, but with a different purpose. A half-wave plate (Lambda/2 plate) is inserted between two crossed polarizers so that no light passes through the system without a delay. However, the half-wave plate can rotate the incident light by a controlled angle, allowing a certain amount of light to pass through. Depending on the orientation of the fast axis of the half-wave plate relative to the polarization of the light, the transmission through the polarizers can be adjusted continuously. Optical attenuators are used in optical measurement systems and in laser technology to precisely control the light intensity without affecting the light source itself.

polarization rotator

A polarization rotator uses the properties of a lambda/2 plate to rotate the polarization plane of linearly polarized light. The light is guided through the lambda/2 plate, with the polarization rotated by the double angle of the plate's orientation relative to the polarization plane. For example, a 22.5° orientation of the fast axis of the lambda/2 plate results in a rotation of the polarization by 45°. This ability to precisely rotate the polarization plane is useful in optical systems and experiments where precise control of light polarization is crucial.

Variable beam splitter

A variable beam splitter uses a combination of a lambda/2 plate and a polarizing beam splitter to control the amount of light that passes through and is reflected. The lambda/2 plate rotates the polarization of the incident light so that at a specific polarization level it either lets more light through the beam splitter or reflects it. If the fast axis of the lambda/2 plate is aligned parallel to the input polarization, it reflects all the light; if it is rotated 45°, it lets most of the light through. In modern systems, a variable delayer replaces the half-wave plate, enabling continuous light control without mechanical movement. This configuration enables precise control of the light distribution and is used in optical networks, imaging systems, and precision measurement instruments.

Conclusion

In this application note, we have provided a basic description of light polarization and some of the tools for controlling the polarization state of light. Delayers and Polarizers were used in simple devices that enable some of the common manipulations that are required wherever light is measured.

See all our polarizers here: