What Is Polarization?

Polarization is a property of transverse waves, particularly electromagnetic waves, describing the geometric orientation of the oscillation in the plane perpendicular to the direction of propagation. For an electromagnetic wave, polarization specifies how the electric field vector is oriented and how that orientation evolves in time and space. In unpolarized light, such as the output of an incandescent lamp, the electric field orientation varies randomly; in polarized light, it follows a regular, predictable pattern. The study and control of polarization span optics, photonics, antenna engineering, and optical communications, making it one of the foundational concepts in modern electromagnetic science.

Polarization was first explained rigorously by Augustin-Jean Fresnel in the 1820s using a transverse wave model of light, resolving debates that had persisted since the 17th century. The mathematical formalism for describing polarization states advanced through the work of Stokes (1852), Poincare (1892, with the Poincare sphere), and Jones (1941, with the Jones matrix calculus). These representations remain the standard tools in optics, photonics design, and telecommunications today.

Polarization States

The three canonical polarization states are linear, circular, and elliptical. In linearly polarized light, the electric field oscillates along a fixed axis; in circularly polarized light, the field vector rotates at the carrier frequency while maintaining constant amplitude, tracing a helix along the propagation direction; in elliptically polarized light, the field traces an ellipse in the transverse plane. Circular and linear polarization are limiting cases of the general elliptic state. The four-element Stokes vector provides a complete, intensity-normalized description of any polarization state, including partially polarized light, while the Poincare sphere offers a geometric visualization in which each point on the sphere's surface represents a distinct fully polarized state. The NIST Physical Measurement Laboratory maintains reference resources and calibration standards for polarization measurement.

Birefringence and Polarization in Optical Media

When an electromagnetic wave enters a material whose optical properties differ along two crystal axes, the two orthogonal polarization components propagate at different phase velocities, a phenomenon called birefringence. Calcite, quartz, and certain liquid crystals are strongly birefringent. If the two components accumulate a phase difference of 90 degrees (a quarter-wave plate) or 180 degrees (a half-wave plate), the polarization state of transmitted light is transformed in a controlled and reproducible way. In optical fibers, unwanted birefringence arises from manufacturing imperfections and mechanical stress, causing the two polarization modes to travel at slightly different group velocities; this is the root cause of polarization mode dispersion, a limiting impairment in high-speed long-haul fiber communications. Polarization-maintaining fibers eliminate this by introducing strong, intentional birefringence that locks each polarization to a fixed fast or slow axis.

Polarization Control Devices

Engineering systems often require precise control or filtering of polarization states. Linear polarizers, based on dichroic materials (such as oriented iodine-doped polyvinyl alcohol), reflection at Brewster's angle, or wire-grid arrays, transmit one polarization while absorbing or deflecting the orthogonal one. Wave plates, fabricated from birefringent crystals or liquid crystals, convert between polarization states with predictable phase shifts. Electro-optic and acousto-optic modulators allow dynamic control of polarization for sensing and communications. In antenna engineering, polarization diversity, the use of antennas with orthogonal polarizations, mitigates fading in wireless channels and doubles the spectral efficiency of multiple-input multiple-output systems by transmitting independent data streams on each polarization.

Applications

Polarization has applications in a wide range of disciplines, including:

• Liquid crystal displays, where electrically controlled polarization rotation governs pixel transmission
• Fiber-optic telecommunications, for polarization multiplexing and polarization-division multiplexing
• Remote sensing and radar, where polarization signatures discriminate surface types and target geometries
• 3-D cinema and augmented-reality optics using polarized glasses or beam splitters
• Quantum key distribution, where photon polarization encodes and detects quantum information bits