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Circular Polarisation

Key idea: H3 Photonics: Circular Polarisation — key ideas and exam-focused notes on reflection, refraction, polarisation, fibres, and attenuation.

  • Advanced Physics
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Learning objectives

  • Use advanced reference material to reinforce mathematical, optical, material, and modern-physics models.

Circular polarisation

  • At a fixed position, the electric-field vector rotates while its magnitude remains constant.
  • Circular polarisation is the superposition of two perpendicular components with the same frequency, equal amplitudes and a phase difference of ± 90° (±π/2).
  • The sign of the phase difference sets the sense of rotation. Because handedness names vary with viewing/sign convention, state the convention before calling a state left- or right-circularly polarised.
At a fixed position, four equal-length electric-field vectors point right, up, left and down at times separated by one quarter period. Their tips lie on a circle, showing constant field magnitude and a rotating direction.
Equal perpendicular components in quadrature make the electric-field tip trace a circle.

Circular polarisation using a quarter-wave plate

Linearly polarised light at 45 degrees to a quarter-wave plate's fast and slow axes splits into equal perpendicular components. The plate adds a relative pi-over-two phase delay, producing circularly polarised output; unequal components would produce an ellipse.
A 45° input supplies equal components; the quarter-wave plate puts them one quarter cycle out of phase.

  • A quarter-wave plate is birefringent: the two perpendicular field components acquire different phases as they travel along its fast and slow axes.

  • At the plate’s design wavelength (or specified order), the relative phase delay is π/2.

  • Incident linear polarisation at 45° to the axes gives equal component amplitudes. Equal perpendicular components in quadrature produce circular polarisation; unequal components produce elliptical polarisation.

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Course and syllabus information
Course
Advanced Physics
Edition
Advanced Physics