Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 186

The wave retarder also called the waveplate alters the relative phase between two orthogonal field components passing through it. In this respect, it is acting as a phase shifter. The waveplates are designed using the birefringent, i.e. anisotropic material with orthogonal fast‐axis and slow‐axis. The relative permittivity, also the refractive index, of the anisotropic material, has lower value along the fast‐axis and higher value along the slow‐axis, causing relatively slower phase velocity of the EM‐wave propagation along the slow‐axis. The half‐wavelength thick slab, called the half‐wave plate, changes the direction of the linear polarization at its output. Whereas, the quarter‐waveplate, i.e. a quarter‐wavelength thick slab, converts the linearly polarized incident waves into the circularly polarized waves at its output. The waveplates, i.e. the wave retarders, are characterized by the Jones matrices as discussed below.

For the EM‐wave propagating in the x‐direction, the field components at the output of a wave retarder could be written from equation (4.6.11), by normalizing the magnitude of field components to the unity, as follows:

(4.6.25)

The J_R(Δφ) is the Jones matrix of a wave retarder (waveplate) given by equation (4.6.25b). At the input, the incident wave is linearly polarized and at the output of the retarder slab of thickness d, the differential phase Δφ = φ_z − φ_y is the relative phase between the field components and .