Light not only has two basic properties, wavelength and intensity, but also polarization properties. Wavelengths describe the color range of light, and in the case of visible light, its wavelength is about 380–780 nm. The light intensity indicates the brightness or power of the light, which is easy to measure by instruments. However, polarization properties describe the direction of vibration of the photoelectric field vector, which is neither visible nor palpable, but plays a key role in optical systems. In everyday life, such as color display and contrast adjustment in liquid crystal displays, and 3D glasses in cinemas, the principle of light polarization is applied. This article will systematically introduce the polarization basis of light and the application of wave plates in polarization control in an easy-to-understand manner.
The Polarization Basis of Light
Light is an electromagnetic wave caused by electric fields that are perpendicular to each other and in the same phase (E) and magnetic field (B) composition, propagating in the direction of propagation (Z) and laterally. In practical applications, the polarization of light is often described by analyzing the vibration of the electric field vector. The electric field vector can be broken down into horizontal components (E_x) and vertical components (E_y); the polarization state of light reflects the distribution of these components in time and space.
Fundamental Polarization States
- Elliptic polarization: The most basic polarization state, where two electric field components have a constant phase difference and can be the same or different amplitudes. Along the propagation direction, the electric field vector endpoint trajectory is elliptical.
- Linear polarization: A special form of elliptic polarization, in which the two electric field components have no phase difference, and the electric field vector oscillates in the same plane, and the observation trajectory is straight along the propagation direction. A common 45° linear polarization is when two components are equal in amplitude.
- Circular polarization: Another special form of elliptic polarization, in which two electric field components have a 90° phase difference and equal amplitude, and the endpoint trajectory is circular.
Polarization Classification of Light Sources
Natural light is a wave of light emitted from an ordinary light source, a random combination of various polarized lights whose polarization states change randomly over time and do not exhibit specific polarization, such as sunlight or household light bulbs. Fully polarized light has a stable electromagnetic wave oscillation direction, including line-polarized, circularly polarized, and elliptically polarized light. Partially polarized light contains a mixture of natural and polarized light, such as laser beams. To quantify the proportion of polarized light in the total light intensity, the Degree of Polarization (DOP) is introduced, with a range of 0–1, where 0 is unpolarized light and 1 is fully polarized. Linear polarization (DOLP) and circular polarization (DOCP) represent the intensity ratio of linearly and circularly polarized light, respectively.
Transition Between Polarization States
Optical components can change the polarization state of the beam, sometimes expected and sometimes unexpected. For example, when a beam of light is reflected off the surface of the water, natural light becomes partially polarized. Precise polarization control is possible through polarizing elements. The quarter wave (λ/4) is commonly used as a polarization converter, which is made of birefringent crystals and is divided into fast and slow axes. When the linearly polarized light is incident on the λ/4 waveplate at an angle of 45°, it can be converted into circularly polarized light; when propagating backwards, circularly polarized light can also be converted back to linearly polarized light. In addition, integrating spheres can convert linearly polarized light into non-polarized light, disrupting the direction of electric field vibration through multiple reflections.
P Light, S Light, and Brewster Angle
The direction of the polarized light in the P light and parallel incident planes is the same, and the S light is perpendicular to the incident plane. When reflected or refracted at the dielectric interface, natural light is usually converted to partially polarized light. Only when the angle of incidence is Brewster, the reflected light is completely S-polarized, and the refracted light is almost P-polarized. Brewster angles can be used to efficiently generate linearly polarized light.
Basic Principle and Classification of Wave Plates
Waveplates, also known as delayers, are essential components that change the polarization state of light without attenuating, deflecting, or displacing the beam. The waveplate achieves the conversion of linear, circular, or elliptically polarized light by delaying the phase of one component of polarized light relative to its orthogonal component. Key terms include:
- Birefringence: The waveplate is made of birefringent material (such as quartz crystal), and the refractive index of light in different polarization directions is slightly different, breaking down the incident light into parallel and vertical components.
- Fast vs. slow axis: Light travels faster along the fast axis and slower on the slow axis, and the delay is the phase difference between the fast and slow axes, expressed in terms of retardance, wavelength, or nanometers.
- Multi-order and zero-order waveplates: The total delay of multi-order waveplates includes the target delay plus an integer multiple delay, and the zero-order waveplate delay is the target value. The zero-order waveplate has higher delay stability and low sensitivity to temperature and wavelength changes.
- Achromatic and superachromatic waveplates: Multi-material design for delay uniformity over a wider wavelength range for wide-spectrum optical systems.
It is difficult to manufacture waveplates, and it is necessary to control the crystal cutting accuracy within a few arc minutes, polish to achieve a laser-level finish, and maintain thickness tolerance within microns. Quartz waveplates are suitable for applications with high damage thresholds and temperature stability requirements; polymer waveplates are thin and light, with a good field of view, but are not suitable for high-power or high-temperature environments. The waveplate mounting method includes an air gap and optical glue bonding to ensure delay accuracy and beam deflection remains below the specified range.
Applications of Wave Plates in Optical Systems
- Rotating line-polarized light: The λ/2 waveplate rotates the line-polarized light at a specific angle to adjust the polarization direction of the system without the need for recalibration.
- Linear polarization and circular polarization conversion: A λ/4 waveplate combined with a linear polarizer can realize linear polarization light to circularly polarized light or circularly polarized light to linearly polarized light.
- Optical isolation: The combination of a linear polarizer and a λ/4 waveplate can build an optical isolator to prevent feedback light from affecting the light source or laser; the polarization beamsplitter and waveplate combination can efficiently route the return light.
- Crystalline quartz polarization rotator: Uses the optical rotation of quartz to rotate the polarization direction of incident light, which is an alternative to half-wave plates and can control the polarization direction of the entire beam.
Summary
Through this article, readers can have a comprehensive understanding of the polarization characteristics of light and the application of waveplates. The polarization of light reflects the direction of vibration of the electric field vector and is an integral feature in optical systems. Understanding the differences between elliptical, linear, and circular polarization, as well as the classification of unpolarized, partially polarized, and fully polarized light, is crucial for analyzing and designing optical systems. Polarization elements such as waveplates can achieve precise control of polarization states, including rotary line polarization, conversion between linear and circular polarization, and the construction of optical isolators. At the same time, the concepts of P-light, S-light, and Brewster angle provide a theoretical basis for understanding the polarization changes of light during reflection and refraction. Mastering the principles of polarization control and the use of waveplates not only helps to optimize optical systems but also plays a key role in lasers, displays, communications, and scientific research.
