Tutorials

[Commonly used optical components] Principles and selection of polarization components

March 16, 2026 Shopify API WaveQuanta Technical Resources

[Commonly used optical components] Principles and selection of polarizers

Abstract: In optical experiments, the position of polarizers is crucial. In the traditional field of laser amplification, polarization-selective components can be used with Faraday rotators to form an optical isolation system, or with electro-optical crystals to form electro-optical switches for solid-state lasers. In the emerging field of pump detection, polarization-selective components can be used to detect changes in the polarization state of the signal light by the detected sample, and then carry out ultrafast dynamics research on magnetic materials, etc.

1. Principles and types of polarization selection components

There are two types of polarization selection components: absorption type and spectroscopic type [1]. Absorptive elements absorb very little light in a certain polarization direction, but strongly absorb light perpendicular to the polarization direction, so that the light passing through the element is light in a single polarization direction. Spectroscopic elements are the most commonly used polarization elements, and their working principles are generally based on birefringence or polarization splitting films. Common ones include Glan prisms, thin film polarizers (TFP) and polarization beam splitters. , PBS). The former is based on the birefringence effect, and the latter two are based on polarizing light splitting films. Next, we will introduce them one by one.

1. Glan prism

A Glan prism consists of two identical right-angled prisms (the material is birefringent crystals such as calcite), as shown in the figure. Two prisms are glued together with an air gap in between. The direction of the optical axis of the crystal material is parallel to the paper surface. If the polarization direction of the incident laser is in the same direction as the optical axis, it is e-ray (extraordinary ray, e-ray), and if it is perpendicular to the optical axis, it is o-ray (ordinary ray, o-ray). The refractive index of the two beams of light in the crystal is different. On the surface of the air gap, the o-light is totally reflected (the incident angle is the Brewster's angle of the o-light), and a small part of the e-light is also reflected. Most of the e-light is transmitted through the air gap and another part of the crystal and then output.

[Commonly used optical components] Princ - Figure 2

Figure 1 Glan prism

2. Thin film polarizer (TFP)

Thin film polarizer is a film system based on interference effect plated on a glass substrate. There are generally two types: those placed at Brewster's angle and those placed at 45 degrees. The former has little reflection of transmitted light, but due to the placement of Brewster's angle, if its reflected light is used, it is generally necessary to use a pair of TFPs to select the s-polarized light and adjust it to be parallel to the incident light.

[Commonly used optical components] Princ - Figure 3

TFP placed at 45 degrees facilitates the direct use of reflected s-polarized light (the incident angle of a conventional reflector is 45 degrees or 0 degrees). It has a certain proportion of reflection of transmitted light. It needs to be coated with an anti-reflection coating on the back surface to improve the transmittance of transmitted light. The film system design of the front surface is more complex [2], and the cost is higher than that of TFP placed at Brewster's angle.

Figure 2 Thin film polarizer (TFP)

3. Polarization beam splitter (PBS)

The principle of the polarization beam splitter is similar to that of TFP. It is made of two right-angle prisms bonded together. The internal reflection surface is coated with a special film system to separate s-polarization and p-polarization.

[Commonly used optical components] Princ - Figure 4

Figure 3 Polarization beam splitter (PBS)

2. Selection of polarization selecting components

When using polarization selecting components, the most important parameter is the extinction ratio. The extinction ratio refers to the ratio of the transmittance of the polarizing element for p-polarized light and the transmittance of s-polarized light (different merchants will have different definitions of "extinction ratio", so you need to consult the merchant's definition before purchasing). Due to different usage requirements in different occasions, here we define the use of polarization components as transmission type and reflection type, that is, the light that ultimately needs to be used is transmitted light or reflected light. In actual use, the extinction ratio of Glan prism is relatively high, and it can generally achieve an extinction ratio of Tp/Ts > 1000:1, and is generally used as a transmission type (it will have a small amount of reflection for p polarization, so the extinction ratio of the reflection type is not high); for TFP, it can be used as a transmission type or a reflection type, and generally can achieve an extinction ratio of greater than 200:1. The same is true for PBS.

TFP with Glan prism and Brewster angle placement can work under wide spectrum conditions and is often used in the field of broadband femtosecond lasers (Glan prism as a block material will introduce a certain amount of dispersion, and its impact on the femtosecond pulse width needs to be considered when polarizing femtosecond lasers). For example, a Glan prism can maintain an extinction ratio of 1000:1 in the range of 700-900 nm; a TFP placed at the Brewster angle can achieve an extinction ratio of 200:1 in a bandwidth range of 40 nm [3]. However, PBS and TFP designed to be placed at 45 degrees can only maintain an extinction ratio of 200:1 within a narrow spectrum range.

Glan prism and TFP have higher damage thresholds. Using a 1064 nm nanosecond laser for testing, the damage thresholds can reach more than 5 J/cm2; while the damage threshold of PBS is relatively low, 1 J/cm2. Due to the limited size of bulk materials, TFP is more suitable for use in high-energy laser systems with large beam apertures.

In addition, there is one more thing to pay special attention to when using TFP and PBS. When using TFP, there will be a ghost image problem (Ghost reflections), that is, due to the reflection of the back surface, there is a parallel weak light next to the main beam (this problem mainly occurs when using the reflective type). The influence of this beam must be eliminated during laser amplification; PBS will also have this problem, but the degree will be weaker.