A polarization beam splitter (PBS) is an optical element that divides a beam of incident light into two beams of light whose propagation directions are perpendicular to each other. But unlike general optical beam splitting elements, there is a special relationship between the two beams of light separated by it, that is: they are all linearly polarized lights, and the polarization directions are perpendicular to each other. As shown below:
Polarization
Polarization is an inherent property of light, and polarization state is an independent parameter of light. If you want to completely describe the properties of a beam of light, in addition to frequency/wavelength, amplitude/intensity, and propagation direction, you also need to describe its polarization state. The so-called “polarized light” means that the vibration direction of the electric vector (E) of this light has a certain law. The polarization state can be divided into linear polarization, elliptical polarization (circular polarization in special cases). For example, for linearly polarized light, it’s electric vector-only reciprocates in one direction. While [unpolarized light], such as natural light, the vibration of their electric vector is disorderly, neither moving in some same direction nor having a fixed time correspondence (no fixed phase) when vibrating. Therefore, their Vibration is random and there is no fixed pattern. In the concept of [polarized light], in order to describe the relationship between vibration directions, for the most basic linearly polarized light (which can be combined into ellipsoidal light, of course, the reverse is also possible), we usually distinguish between P light and S light. Among them, P light represents linearly polarized light whose vibration direction is parallel to the incident surface, and S light represents linearly polarized light whose vibration direction is perpendicular to the incident surface.
Birefringence
Birefringence is an optical phenomenon. This phenomenon is: when a beam of light enters some material (usually a “transparent” crystal), one beam of incident light will become two beams! It’s amazing. This kind of birefringent crystal is called [birefringent crystal]. Then, the question is: what mechanism causes a beam of light to be divided into two beams? Moreover, is there any special relationship between the two separated beams of light? In fact, this phenomenon is the result of the interaction between light waves and crystals. Since the propagation of light can be described in accordance with the law of propagation of electromagnetic waves, the birefringence generation mechanism can be combined with the Maxwell equation and the dielectric of the crystal. Constant to explain. Analyze from the simplest case: a plane wave incident on an anisotropic crystal (these concepts will be explained in the next summary) can be described in the form of a Fresnel formula/equation, which is similar to the mathematics of an ellipsoid The equation (of course, the physical meaning is different), usually called [index ellipsoid] or [wave normal ellipsoid] and so on. If we solve this equation mathematically, we can get two solutions at the same time (this is normal for quadratic equations). These two solutions physically correspond to the two propagation directions of light (and there are countless solutions in special cases, which correspond to physical conic refraction, not shown here), which means: a beam of light and crystal After the interaction, two beams of light propagating in different directions will indeed be produced! ! !
There is a specific relationship between the two beams: they are both linearly polarized lights, and their polarization directions are perpendicular to each other. Their propagation direction is related to the following conditions: the direction of the incident light, the direction of the optical axis of the crystal, and so on. In order to describe more clearly, these two beams of light are usually called O light (ordinary light) and E light (extraordinary light) respectively. Among them, O light obeys the general law of refraction, and its wave surface is a circle, that is, along with various directions. The propagation speed of E light is the same; while the E-ray does not obey the law of refraction, its wavefront is an ellipse (for uniaxial crystals (a type of birefringent crystal, explained in the next section)), that is, it has different propagation speeds in different directions . Their propagation direction can be specifically determined according to the situation of the propagation wave surface. Now the question of concern is: What are the vibration directions of O light and E light? According to the Fresnel wave normal equation, for uniaxial crystals, the vibration direction of O light is always perpendicular to the main plane (the plane composed of the wave normal (propagation direction) of light and the optical axis), and the vibration direction of E light is always in In the main plane.
Crystal
Crystal is a relatively broad concept, here is only classified according to optical properties. According to the influence of the crystal on the polarization state of light, the crystal can be divided into two major categories and 3 sub-categories: isotropic and anisotropic (uniaxial crystal, biaxial crystal). Among them, the isotropic crystal does not produce the birefringence effect, and the anisotropic crystal produces the birefringence effect. In anisotropic crystals, there are certain special directions, and when light propagates in these directions, the birefringence effect will not occur either. If a crystal has only one such direction, it is called [uniaxial crystal]; if there are two such directions, it is called [biaxial crystal]. Using these properties of the crystal, a polarization beam splitting prism can be manufactured. However, there are many ways to form PBS, including reflection type, birefringence type, etc.