Lyot filter

The basic principle of the Lyot filter [4.35] is founded on the interference of polarized light after having passed a birefringent crystal. Assume that a linearly polarized plane wave E = Aq cos(wt - kx) with 

The elementary Lyot filter consists of a birefringent crystal placed between two linear polarizers (Fig.4.55). Assume that the two polarizers are both parallel to the electric vector E(0) of the incoming wave. Let the crystal with length L be placed between x = 0 and x = L. Because of the different refractive indices nQ and n for the ordinary and the extraordinary beams, the two partial waves at x = L, 

The superposition of these two waves results in general in elliptically polarized light, except for phase differences 6 = Zmir, where linearly polarized light with E(L)1 E(0) is obtained. For 6 = (2m + l)ir and a = 45 , the transmitted wave is also linearly polarized but now E(L)iE(0). 

The second polarizer parallel to E(0) transmits only the projections 2 

Lyot filter, (a) Schematic arrangement, (b) Linearly polarized light passing through a birefringent crystal 

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Abstract This tutorial shows how fundamental is the role plaid by interferences in many of the physical processes involved in astrophysical signal formating and consequently instmmentation. It is obvious in interferometry. Grating spectroscopy is explained within the same framework as Young experiment, and Fabry-Perot filters are explained as Michelson interferometers.Polarization interferences, used in Lyot filters, are discussed, emphasizing the analogy with echelle gratings. 

Fabry-Perot interferometers, polarization interference, birefringence, Lyot filters... 

Figure 6. The transmission of a Lyot filter with five components (bottom plot) is given by the product of those of the five component filters (upper plots).

FIGURE 4 Lyot filter LCTF element. (Adapted from N. Gat, Proceedings SPIE, 4056,... 

Fig. 2. Set-up of the ILP laser system. Intracavity frequency-doubling is realized with a KTP crystal which, together with a Brewster plate, serves as a Lyot filter. This allows to frequency time the laser by more than 500 GHz by changing the temperature of the KTP crystal. The 532 nm laser radiation, after passing an acousto-optical modulator (AOM), is directed into an external I2 fluorescence cell. A photomultiplier (PM) detects the fluorescence signal over a solid angle of almost 0.2 n. The photodiode D is used to detect a fraction of the 532 nm laser beam to power stabilize the 532 nm light via the AOM...

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 ... 

In a typical Lyot filter, crystals are often selected so that transmission has its maximum value at the wavelength determined by the thickest crystal retarder, with... 

Some interferometers utilize the optical birefringence of specific crystals to produce two partial waves with mutually orthogonal polarization. The phase difference between the two waves is generated by the different refractive index for the two polarizations. An example of such a polarization interferometer is the Lyot filter [4.23] used in dye lasers to narrow the spectral linewidth (Sect. 4.2.9). 

Fig. 4.59a,b. Lyot filter (a) schematic arrangement (b) index ellipsoid of the birefringent crystal... 

The transmission of the Lyot filter is therefore a function of the phase retardation, i.e.. 

If N elementary Lyot filters with different lengths Lm are placed in series, the total transmission T is the product of the different transmissions i.e.,... 

Fig. 4.60. (a) Transmitted intensity Ij X) of a Lyot filter composed of three birefringent crystals with lengths L, 2L, and 4L between polarizers, (b) Arrangement of the crystals and the state of polarization of the transmitted wave... 

While this electro-optic tuning of the Lyot filter allows rapid switching of the peak transmission, for many applications, where a high tuning speed is not demanded, mechanical tuning is more convenient and easier to realize. 

For fast wavelength tuning of dye lasers, Lyot filters with electro-optic tuning are employed within the laser resonator. A tuning range of a few nanometers can be repetitively scanned with rates up to 10 per second [4.59]. 

Commercial cw dye laser systems (Sect. 5.5) generally use a different realization of single-mode operation (Fig. 5.41). The prisms are replaced by a birefringent filter, which is based on the combination of three Lyot filters (Sect. 4.2.9), and the thick etalon is substituted by a Fabry-Perot interferome-... 

Lasers as Spectroscopic Light Sources Lyot filter... 

Similar to the laser-pumped dye lasers, reduction of the linewidth and wavelength tuning can be accomplished by prisms, gratings, interference filters [5.160], Lyot filters [5.161], and interferometers [5.162,5.163]. 

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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