1 Polarization of light
Light has three basic properties, namely wavelength, intensity and polarization. The wavelength of light is easy to understand, taking the common visible light as an example, the wavelength range is 380~780nm. The intensity of light is also easy to understand, and whether a beam of light is strong or weak can be characterized by the size of the power. In contrast, the polarization characteristic of light is the description of the vibration direction of the electric field vector of light, which can not be seen and touched, so it is usually not easy to understand, however, in reality, the polarization characteristic of light is also very important, and has a wide range of applications in life, such as the liquid crystal display we see every day, the polarization technology is used to achieve color display and contrast adjustment. When watching 3D movies in the cinema, the 3D glasses are also applied to the polarization of light. For those engaged in optical work, a full understanding of polarization and its application in practical optical systems will be very helpful in promoting the success of products and projects. Therefore, from the beginning of this article, we will use a simple description to introduce the polarization of light, so that everyone has a deep understanding of polarization, and better use in the work.
2 Basic knowledge of polarization
Because there are many concepts involved, we will divide them into several summaries to introduce them step by step.
2.1 Concept of polarization
We know that light is a kind of electromagnetic wave, as shown in the following figure, electromagnetic wave consists of electric field E and magnetic field B, which are perpendicular to each other. The two waves oscillate in their respective directions and propagate horizontally along the propagation direction Z.
Because the electric field and magnetic field are perpendicular to each other, the phase is the same, and the direction of propagation is the same, so the polarization of light is described by analyzing the vibration of the electric field in practice.
As shown in the figure below, the electric field vector E can be decomposed into Ex vector and Ey vector, and the so-called polarization is the distribution of the oscillation direction of the electric field components Ex and Ey over time and space.
2.2 Several basic polarization states
A. Elliptic polarization
Elliptical polarization is the most basic polarization state, in which two electric field components have a constant phase difference (one propagating faster, one propagating slower), and the phase difference is not equal to an integer multiple of π/2, and the amplitude can be the same or different. If you look along the direction of propagation, the contour line of the endpoint trajectory of the electric field vector will draw an ellipse, as shown below:
B, linear polarization
Linear polarization is a special form of elliptic polarization, when the two electric field components are not phase difference, the electric field vector oscillates in the same plane, if viewed along the direction of propagation, the electric field vector endpoint trajectory contour is a straight line. If the two components have the same amplitude, this is the 45 degree linear polarization shown in the figure below.
C, circular polarization
Circular polarization is also a special form of elliptical polarization, when the two electric field components have a 90 degree phase difference and the same amplitude, along the direction of propagation, the endpoint trajectory of the electric field vector is a circle, as shown in the following figure:
2.3 Polarization classification of light source
The light emitted directly from the ordinary light source is an irregular set of countless polarized light, so it cannot be found in which direction the light intensity is biased when directly observed. This kind of light wave intensity that vibrates in all directions is called natural light, it has a random change of polarization state and phase difference, including all possible vibration directions perpendicular to the direction of light wave propagation, does not show polarization, belongs to the non-polarized light. Common natural light includes sunlight, light from household bulbs, and so on.
Fully polarized light has a stable electromagnetic wave oscillation direction, and the two components of the electric field have a constant phase difference, which includes the above mentioned linear polarized light, elliptically polarized light and circular polarized light.
Partially polarized light has two components of natural light and polarized light, such as the laser beam we often use, which is neither fully polarized light nor non-polarized light, then it belongs to partially polarized light. In order to quantify the proportion of polarized light in the total light intensity, the concept of Degree of Polarization (DOP) is introduced, which is the ratio of polarized light intensity to the total light intensity, ranging from 0 to 1,0 for unpolarized light, 1 for fully polarized light. In addition, linear polarization (DOLP) is the ratio of linearly polarized light intensity to total light intensity, while circular polarization (DOCP) is the ratio of circularly polarized light intensity to total light intensity. In life, common LED lights emit partially polarized light.
2.4 Conversion between polarization states
Many optical elements have an effect on the polarization of the beam, which is sometimes expected by the user and sometimes is not expected. For example, if a beam of light is reflected, its polarization will usually change, in the case of natural light, reflected through the water surface, it will become partially polarized light.
As long as the beam is not reflected or passes through any polarizing medium, its polarization state remains stable. If you want to quantitatively change the polarization state of the beam, you can use the polarization optical element to do so. For example, a quarter-wave plate is a common polarization element, which is made of birefringent crystal material, divided into fast axis and slow axis directions, and can delay the phase of π/2 (90°) of the electric field vector parallel to the slow axis, while the electric field vector parallel to the fast axis has no delay, so that when linearly polarized light is incident on the quarter-wave plate at a polarization Angle of 45 degrees, The beam of light through the wave plate becomes circularly polarized light, as shown in the diagram below. First, the natural light is changed into linearly polarized light with the linear polarizer, and then the linearly polarized light passes through 1/4 wavelength and becomes circularly polarized light, and the light intensity is unchanged.
Similarly, when the beam travels in the opposite direction and the circularly polarized light hits the 1/4 plate at a 45 degree polarization Angle, the passing beam becomes linearly polarized light.
Linearly polarized light can be changed into unpolarized light by using the integrating sphere mentioned in the previous article. After the linearly polarized light enters the integrating sphere, it is reflected several times in the sphere, and the vibration of the electric field is disrupted, so that the output end of the integrating sphere can get non-polarized light.
2.5 P light, S light and Brewster Angle
Both P-light and S-light are linearly polarized, polarized in perpendicular directions to each other, and they are useful when considering the reflection and refraction of the beam. As shown in the figure below, a beam of light shines on the incident plane, forming reflection and refraction, and the plane formed by the incident beam and the normal is defined as the incident plane. P light (first letter of Parallel, meaning parallel) is light whose polarization direction is parallel to the plane of incidence, and S light (first letter of Senkrecht, meaning vertical) is light whose polarization direction is perpendicular to the plane of incidence.
Under normal circumstances, when natural light is reflected and refracted on the dielectric interface, the reflected light and refracted light are partially polarized light, only when the incidence Angle is a specific Angle, the polarization state of the reflected light is completely perpendicular to the incident plane S polarization, the polarization state of the refracted light is almost parallel to the incident plane P polarization, at this time the specific incidence Angle is called Brewster Angle. When light is incident at Brewster Angle, the reflected light and the refracted light are perpendicular to each other. Using this property, linearly polarized light can be produced.
3 Conclusion
In this paper, we introduce the basic knowledge of optical polarization, light is an electromagnetic wave, with wave effect, polarization is the vibration of the electric field vector in the light wave. We have introduced three basic polarization states, elliptic polarization, linear polarization and circular polarization, which are often used in daily work. According to the different degree of polarization, the light source can be divided into non-polarized light, partially polarized light and fully polarized light, which needs to be distinguished and discriminated in practice. In response to the above several.
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Post time: May-27-2024
