What Is Polarization Shift Keying?

Polarization shift keying (PolSK) is a modulation scheme for optical communication in which information is encoded by switching between distinct states of polarization of a laser beam rather than by varying the intensity, frequency, or phase of the optical carrier. In a binary PolSK system, two orthogonal polarization states, such as horizontal and vertical linear polarization, represent the logical symbols 0 and 1; multilevel variants use four or more distinct polarization states drawn from a constellation on the Poincare sphere. Because the states of polarization of a propagating laser beam are relatively stable compared to its intensity profile, PolSK offers inherent robustness against impairments that distort amplitude-based signals, particularly in free-space and underwater optical links subject to turbulence.

The concept was introduced for optical fiber communications in the late 1980s, where it was explored as an alternative to on-off keying (OOK) in coherent detection systems. Interest expanded when researchers showed that polarization, unlike intensity, is largely insensitive to multiplicative atmospheric and oceanic turbulence, making PolSK attractive for free-space optical (FSO) links and underwater optical wireless communication (UOWC) systems.

Modulation and Detection Principles

In a PolSK transmitter, an electro-optic polarization modulator or a combination of a polarizing beam splitter and phase modulators impresses the desired polarization state onto a continuous-wave laser output in synchrony with the symbol clock. The receiver separates the incoming field into two orthogonal polarization components using a polarizing beam splitter, detects each component with a photodetector, and compares the photocurrent pair to determine which symbol was sent. Binary PolSK requires no intensity decision threshold, which is a practical advantage because optimal threshold setting in turbulent channels is difficult. Multilevel PolSK, with four states at the vertices of a tetrahedron on the Poincare sphere, maximizes the minimum distance between constellation points for a given number of states. An IEEE-published study on polarization shift keying for wireless communication analyzed PolSK performance under additive white Gaussian noise and turbulence channel models.

Robustness in Turbulent Channels

The primary motivation for PolSK in FSO and UOWC systems is its stability against turbulence-induced fading. Atmospheric turbulence causes intensity fluctuations through beam wander and scintillation, degrading the performance of OOK and pulse-position modulation systems that rely on an intensity decision threshold. Because turbulence affects the Stokes parameters that describe polarization far less severely than it affects intensity, PolSK receivers maintain stable decision boundaries even at moderate turbulence strengths. Experimental work in a 15-meter tank with thermally induced underwater turbulence showed that multilevel PolSK modulation significantly outperformed intensity-based schemes in terms of bit error rate versus signal-to-noise ratio. Relay-assisted FSO systems using PolSK also demonstrate better outage probability than equivalent OOK systems over strong-turbulence links.

System Design Considerations

PolSK receivers require polarization-tracking mechanisms to compensate for slow drift in the polarization state of the received beam caused by thermal variation, vibration, or fiber birefringence in the transmission path. In FSO links, pointing errors and beam wandering introduce additional coupling losses that must be addressed through beam tracking. Combining PolSK with forward error correction, spatial diversity, or cooperative relaying extends the reliable operating range and is an active area of research. The RP Photonics Encyclopedia entry on polarization provides background on the Stokes parameter framework that underpins PolSK signal characterization and receiver design.

Applications

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

• Free-space optical communication links in urban and satellite channels subject to atmospheric turbulence
• Underwater optical wireless communication in ocean environments where intensity fading is severe
• Quantum key distribution, where polarization states encode quantum bits
• High-capacity fiber-optic systems using polarization-division multiplexing alongside PolSK
• Short-range indoor visible-light communication as a turbulence-resistant modulation option