What is a polarizing beam splitter (PBS)? ——Definition and Technical Analysis

In modern optical systems, precise control of optical signals is the cornerstone of technological progress. As a core passive component, the function of a polarization beam splitter is to accurately separate and combine the optical path based on the polarization state of light. It can efficiently separate a beam of light containing any polarization state into two linearly polarized light beams with mutually perpendicular vibration directions for output, and is an essential component for constructing complex optical paths.

1、 Core working principle

Polarization beam splitter is an optical device that can decompose incident light into two linearly polarized light beams with orthogonal polarization directions. Its core structure is usually composed of prisms, multilayer dielectric films, or photonic crystals, which achieve polarization separation through physical interfaces or periodic micro nano structures. According to the working principle, it can be divided into two categories:

1. Prism type PBS: It is composed of two high refractive index prisms (such as N-BK7 glass, refractive index ≥ 1.6) glued together, and the glued surface is coated with a birefringent multilayer film. When light is incident at a specific angle, S-polarized light (polarized light perpendicular to the incident surface) is reflected, and P-polarized light (polarized light parallel to the incident surface) is transmitted, achieving beam splitting.

2. Grating type PBS: using a two-dimensional grating structure (such as square periodic arrangement of nano columns), utilizing diffraction effects to diffract TE polarized light (electric field direction parallel to the grating groove) and TM polarized light (electric field direction perpendicular to the grating groove) into different spatial directions. In the latest technology announced in 2024, the grating period can be as low as 200nm, the extinction ratio can reach 40dB, and the beam splitting efficiency can exceed 99%.

2、 The splitting principle of polarizing beam splitter

The core principle of a polarizing beam splitter can be traced back to Brewster's law in the 19th century: when light is incident at a specific angle (Brewster's angle), the reflected light is pure S-polarized light and the transmitted light is partially P-polarized light. Modern PBS achieves efficient beam splitting through the following technologies:

1. Multilayer dielectric film design: alternate deposition of high refractive index (such as TiO ₂, n ≈ 2.4) and low refractive index (such as SiO ₂, n ≈ 1.46) materials on the prism bonding surface to form a λ/4 optical thickness layer. By optimizing the number and thickness of film layers, the S light reflectance is close to 100% and the P light transmittance is greater than 99%. For example, a certain model of PBS has an extinction ratio (S light to P light power ratio) of 22dB at a wavelength of 1550nm.

2. Photonic crystal bandgap effect: In grating type PBS, the periodic arrangement of nanowires forms a photonic bandgap, allowing only specific polarized light to pass through. For example, the period d of a square grating and the incident light wavelength λ satisfy the TE light diffraction condition: d · sin θ=m λ (m is an integer), while TM light cannot be diffracted due to bandgap obstruction, thus achieving beam splitting.

3、 The difference between polarizing beam splitter and ordinary beam splitter

Ordinary beam splitters (such as cube beam splitters) only divide the beam according to the energy ratio, and the polarization state of the emitted light is the same as that of the incident light; PBS forces the emitted light to be in orthogonal polarization state while splitting the beam. For example:

1. Input light: Randomly polarized light (including P/S component)

2. Output of ordinary beam splitter: The power ratio of the two beams is 50:50, and the polarization state is the same as that of the input light

3. PBS output: One beam is pure P light, and the other beam is pure S light. The power ratio depends on the proportion of P/S components in the input light

4、 Relativity, high crosstalk design, and power controllability of P-light/S-light in polarizing beam splitter (PBS)

The core function of a polarizing beam splitter is based on the geometric definition of relativity between P-light and S-light. The properties of P-light (electric field parallel to the incident plane) and S-light (electric field perpendicular to the incident plane) are completely dependent on the incident plane and have different meanings as the incident angle changes. For example, under the Brewster angle, the reflected light is pure S light and the transmitted light is pure P light, which is widely used in lasers and polarization separation.

If it is necessary to artificially create high crosstalk PBS (such as for system limit testing), it can be achieved through "reverse optimization": firstly, deliberately introducing thickness errors or material inhomogeneity in multi-layer dielectric films to disrupt ideal interference conditions; The second is to intentionally reduce the purity of polarization separation by causing a mismatch in the grating period in the grating type PBS.

The answer to whether the power of the two beams of light after splitting is highly controllable. For an ideal linearly polarized input (pure P or pure S), one path has an output while the other theoretically has no output, resulting in completely unequal power; For natural light (random polarization) input, the two output powers are basically equal; More commonly, by pre adjusting the polarization state (P: S ratio) of the input light through a polarization controller, the power distribution ratio of the two output lights can be accurately and continuously controlled, achieving any ratio between 0:1 and 1:0. Therefore, PBS is not only a beam splitter, but also a flexible tool for polarization state management and power allocation.

5、 The function of a polarizing beam splitter

The core value of a polarizing beam splitter lies in its ability to accurately manipulate the polarization state of light, thereby achieving key functions in multiple fields:

1. Fiber optic communication expansion: By using polarization multiplexing (PDM) technology, two orthogonal polarized light beams are coupled to the same fiber optic transmission, doubling the channel capacity. For example, in a 400G optical module, PBS combines two 200G P/S polarized light beams to achieve single fiber 400G transmission.

2. Laser optimization: In fiber lasers, PBS serves as a pump multiplexer to couple the orthogonal polarized light output from two polarization maintaining fibers to the gain medium, reducing polarization sensitivity and improving output power stability. A certain model of PBS can compress power fluctuations from ± 5% to ± 0.5% at a wavelength of 1064nm.

3. Quantum information processing: In quantum key distribution (QKD), PBS is used to separate the polarization states of entangled photon pairs and achieve accurate measurement of quantum states. For example, in the BB84 protocol, PBS divides photons into H/V (horizontal/vertical) polarization bases for key encoding.

6、 The core indicators of PBS include:

Extinction ratio: The most critical parameter for measuring the purity of device separation, referring to the power ratio of target polarized light to non target polarized light (crosstalk) at the output port, expressed in decibels. The higher the extinction ratio, the better the polarization separation effect.

Transmittance: The percentage of polarized light transmission required.

Reflectance: The percentage of unwanted polarized light reflected.

Wavelength range: The effective operating wavelength range of PBS.

Incident angle: The angle at which a beam of light is directed onto a polarizing beam splitter (PBS). This will affect the performance of the device.

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