Fiber Laser Series: What is Polarization of Light

Fiber Laser Series: What is Polarization of Light
Light is a transverse wave, and its electric field vector vibration direction is perpendicular to the propagation direction. Polarization refers to the regular distribution of vibrations along the propagation direction. Whether a laser can output polarized light depends on whether there is a mechanism within the resonant cavity that prioritizes the selection of specific vibration directions (such as gain medium characteristics, internal components, or external magnetic fields). Polarization characteristics are essential in fields such as laser cutting and interferometry, but have no effect in ordinary lighting.
1. Common polarization state classification
1.1 Random polarization (natural light): The vibration direction is randomly distributed in a plane perpendicular to the propagation, with no fixed preference, such as sunlight and incandescent light.
1.2 Linear polarization (planar polarization): The vibration direction is always fixed in the same plane. Two orthogonal polarized wave packets can be formed by completely synchronously superimposing their phases.
1.3 Circular Polarization: Composed of two orthogonal linearly polarized waves with equal amplitude and a phase difference of exactly 1/4 wavelength (90 °), the vibration direction rotates in a spiral along the propagation direction, divided into left and right rotation. By allowing linearly polarized light to enter a quarter wave plate at a 45 ° angle, it can be obtained.
1.4 Elliptical polarization: Between a line and a circle, two orthogonal waves have unequal amplitudes or non 90 ° phase differences, and their vibration trajectories are elliptical.
Note: Some lasers with nominal “random polarization” actually output “slowly changing polarized light”, and the polarization state will drift with temperature and mechanical deformation.
1.5 Physical Model: Any complex polarization state can be decomposed into two fundamental linearly polarized waves with mutually perpendicular vibration directions, which are superimposed by adjusting the amplitude and phase.
2. Simple polarization optical experiment
2.1 Polarization phenomenon can be observed at home using polarizing film, flat glass, and transparent tape:
2.2 Basic extinction: When the transmission axis of two polarizing plates is perpendicular, the light is completely blocked and the field of view becomes darker.
2.3 Brightening of intermediate components: Insert a third polarizer between two orthogonal polarizers and rotate it, and the light will light up again, in accordance with Marius’ law.
2.4 Birefringent Color Interference: Transparent tapes with different stretching directions are attached to glass and sandwiched between two orthogonal polarizing plates for observation, resulting in color fringes – this is the result of the birefringence of the tape causing different phase delays, interference enhancement or attenuation of light of different wavelengths.

Polarized light is a key optical state in optical experiments, and most lasers can output linearly polarized light, but there are also a few lasers that usually do not have natural polarization characteristics. Edge emitting diode lasers and lasers containing Brewster windows or external cavity components will inevitably produce polarized light. The ability of a laser to output polarized light depends on whether there is a mechanism within the resonant cavity that prioritizes the selection of a specific vibration direction, which can be attributed to factors such as the optical properties of the gain medium, the optical components within the cavity, or an external magnetic field. Polarization characteristics have varying impacts on applications: they are essential in laser cutting, interferometry, and holographic imaging, but have no effect on illumination or ordinary laser pointing.