Birefringent wavelength filters

Figure 8 Schematic of Lyot birefringence interference filter (A) transmittance of an individual Lyot stage as a function of wavelength (B) schematic of multiple Lyot stages cascaded end to end (C) and transmittance of multiple Lyot stages in series as a function of a wavelength (D).

The operation of an LCTF may be understood by considering a simplified Lyot filter stack, in which (N +1) polarizers are separated by N layers of liquid crystals sandwiched between birefringent crystals. The optical retardation, Rnm, introduced by birefringent crystals is dependent on the thickness of the crystal, tfnm, and the difference between the refractive index of the ordinary ray, and the extraordinary ray, tie, at the wavelength of interest ... 

Coarse wavelength tuning can be accomplished with a birefringent filter (Lyot filter, see Sect. 4.2.9) that consists of three birefringent plates with thicknesses d, q d, Q2d (where q, q2 are integers), placed under the Brewster angle inside the dye laser resonator (Fig. 5.93). Contrary to the Lyot filter discussed in Sect. 4.2.9, no polarizers are necessary here because the many Brewster faces inside the resonator already define the direction of the polarization vector, which lies in the plane of Fig. 5.93. 

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