The difference of beamsplitter, polarizing beamsplitter and non-polarizing beamsplitter

Optical spectroscopic technology is an indispensable and important part of modern optical systems, and the beam splitter prism, as a key optical component to achieve this function, plays an important role in various optical instruments and equipment. According to the different spectroscopic principles and application requirements, the beam splitter prism is mainly divided into three categories: ordinary beam splitter prism, polarizing beam splitter prism and non-polarizing beam splitter prism. These three types of beam splitters have their own characteristics in structural design, working principle and application scenarios. Understanding their differences is crucial for the correct selection and use of beam splitters.

Basic Principles and Characteristics of General Beamsplitter

General beamsplitters are the most basic beam-splitting elements, whose main function is to split the incident beam into two or more beams of outgoing light according to a certain ratio, regardless ofthe polarization state of the light. The working principle of this type of beamsplitter is mainly based on the laws of reflection and refraction of light.

From a structural point of view, ordinary beam splitters are usually made of two right-angle prisms glued together, with a beam splitter film coated on the glued surface. This design allows the incident light to be partially reflected and partially transmitted at the beam splitter film, thereby achieving a beam splitting effect. The beam splitting ratio can be designed according to application requirements, and common ratios include 50:50 (equal beam splitting), 70:30, 80:20, etc. The beam splitter film's coating process and material selection directly affect the beam splitter's performance parameters, such as the beam splitting ratio accuracy, transmittance, reflectivity, and working wavelength range.

The main features of general beamsplitter include:

1. Polarization insensitive: no selectivity for the polarization state of the incident light, different polarization states of light are separated by the same ratio of spectral separation

2. Wavelength dependence: spectral performance is usually wavelength-dependent, and the working wavelength must be considered when designing.

3. Angle of incidence effect: The spectral ratio will change with the change in the angle of incidence

4. Phase retention: There is usually a fixed phase relationship between transmitted and reflected light.

The advantages of ordinary beamsplitters are simple structure, low cost, and easy use; the disadvantages are that the beam splitting ratio is greatly affected by the angle of incidence and wavelength, and it is not possible to realize the separation or control of the polarization state. These beamsplitters are widely used in basic optical systems that do not need to consider the polarization state, such as ordinary microscopes, simple interferometers, optical path beam splitting, and other applications.

Operating Principle and Characteristics of Polarizing Beamsplitters

Polarizing Beam Splitter (PBS) is an optical element that splits light according to its state of polarization, separating incident light into two beams of linearly polarized light whose polarization directions are perpendicular to each other. This type of beam-splitting prism has important applications in polarized optics systems and precision optical measurements.

The core operating principle of polarizing spectroscopic prisms is the use of the birefringence effect or the polarization properties of dielectric films. The most common type of polarizing spectroscopic prism is a double right-angle prism structure with a special polarizing spectroscopic film coated on the gluing surface. This film has different reflection and transmission properties for s-polarized light (electric field vibrating perpendicular to the incident plane) and p-polarized light (electric field vibrating parallel to the incident plane): it is usually designed to be highly reflective for s-polarized light and highly transmissive for p-polarized light.

Typical polarizing spectroscopic prism performance parameters include:

-Extinction ratio: a key measure of polarization purity, up to 1000:1 and beyond.

-Transmittance and reflectance: different responses to p- and s-light.

-Operating angle: typically designed for 0 or 45 degree incidence.

-Damage Threshold: Especially important in high power applications.

The main features of polarizing beam-splitting prisms include:

-Polarization selectivity: able to effectively separate beams of different polarization states

-High extinction ratio: High purity linearly polarized light can be obtained.

-Wide band design: some PBS can work in a wide range of wavelengths.

-Angular sensitivity: performance is significantly affected by the angle of incidence.

Design and Application of Non-polarized Beamsplitters

Non-Polarizing Beam Splitter (NPBS) is a specially designed beam splitting element whose goal is to maintain the same beam splitting ratio for light in different polarization states over a range of wavelengths. Such beam-splitting prisms are important in applications where the polarization of the incident light needs to be maintained or where polarization is not sensitive.

The key to achieving unpolarized beam splitting is the precise design of the beam splitting film. By employing a complex multilayer dielectric membrane system, the difference in reflectance and transmittance between s-polarized and p-polarized light can be minimized for a specific wavelength range and angle of incidence. Common unpolarized beamsplitter prism structures include two forms of cube beamsplitter prisms and flat plate beamsplitters.

The main technical specifications of non-polarized beamsplitters include:

-Polarization dependent loss (PDL): a parameter that measures the variation of the spectral ratio with polarization state

-Wavelength range: the spectral width over which unpolarized properties are maintained

-Spectral ratio consistency: the difference in spectral ratio for different polarization states.

-Surface flatness: key factor affecting wavefront aberrations

The characteristics of unpolarized spectroscopic prisms can be summarized as follows:

1. Polarization-independent: the spectral ratios for s- and p-light are essentially the same

2. Design complexity: the design of the membrane system is much more complex than that of a normal spectroscopic prism.

3. Angular constraints: non-polarizing properties are usually maintained only near the designed angle of incidence.

4. Higher cost: due to the difficulty of design and manufacture, the price is usually higher than that of ordinary beamsplitting prisms.

Non-polarized beamsplitting prisms (NPBS) are widely used in fiber-optic communication systems, interferometric measurements, biomedical imaging, etc., especially indispensable in optical systems 

that need to maintain the polarization of light or are sensitive to polarization. For example, in optical communication, the use of NPBS can avoid signal fluctuations caused by polarization; in 

polarization-sensitive optical inspection systems, NPBS can ensure that the measurement results are not affected by the polarization effect introduced by the spectroscopic elements.