定义：基于双折射的光学滤波器，透射率与波长有关。

Lyot滤波器是一种光学滤波器，其功率透射率与波长有关。它包含了一系列双折射晶体片和偏振片。每一个晶体的双折射轴相对于偏振片的轴向旋转45°。在晶体中传播的光可认为包含两种正交的线偏振成分，产生不同的相位延迟。两偏振成分的相对相移与波长有关。因此，后面偏振器的光功率损耗与波长有关。 

图1：Lyot滤波器，包含一系列双折射晶体和偏振片。 
对于只采用一个双折射晶体的器件，功率透射率随频率的变化可近似用正弦振荡来描述。（色散会使变化稍微偏离正弦振荡。）通过采用不同厚度的多个晶体，可以得到很陡的滤波函数。根据Lyot设计，每个晶体的厚度都是前一个的一半（图1）。这样可以得到很小的透射带宽，很大的透射峰周期。 

图2：包含三个石英片的Lyot滤波器的透射函数，石英片厚度分别为5,2.5和1.25mm。 
具有电子可调谐透射峰的Lyot型滤波器可以采用普克尔盒得到，而不是无源双折射晶体。 
为了对激光器进行波长调谐，常采用双折射调谐器，但是不包含偏振片，因此s偏振光的菲涅尔反射损耗较大。

Definition: optical filter devices based on birefringence, exhibiting a wavelength-dependent transmission

More general terms: birefringent tuners, optical filters, wavelength tuning elements

A Lyot filter is a kind of optical filter, i.e. an optical device with a wavelength-dependent power transmission. It consists of a sequence of birefringent crystalline plates (e.g. of quartz) and polarizers.

In the simplest configuration, the birefringent axis of each crystal is oriented at 45° to the axis direction of the polarizers, and the beam is perpendicular to the plates. The light propagating in a crystal can be considered as containing two different linear polarization components, which experience a different phase delay. The relative phase delay for the two polarization components depends on the wavelength. Therefore, the loss of optical power at the subsequent polarizer is wavelength-dependent.

Figure 1: A Lyot filter, consisting of a sequence of birefringent crystals (BC) and polarizers (P).

For a device with a single birefringent crystal, the power transmission versus optical frequency can be described with an approximately sinusoidal oscillation. (Chromatic dispersion causes some deviation from an exact sinusoidal oscillation.) By combining multiple crystals with different thickness, a sharper filter function can be realized. According to the Lyot design (invented by Bernard Lyot), the thickness of each crystal is half the thickness of the previous one (Figure 1). In that way, a small transmission bandwidth combined with a large period of the transmission peaks (i.e., a large free spectral range) is possible.

Figure 2: Transmission function of a Lyot filter containing three quartz plates, with thickness values of 5, 2.5, and 1.25 mm.

A Lyot-type filter with electrically tunable transmission peaks can be realized by using Pockels cells or electrically controlled liquid crystal modulators instead of the passive birefringent crystals. It is a kind of tunable optical filter.

For wavelength tuning of lasers, one mostly uses birefringent tuners based on a similar principle, but not containing polarizers, since the losses via the Fresnel reflection for s-polarized light are sufficiently strong.

Fiber-based Lyot Filters

Lyot filters are generally built from bulk-optical elements as described above. However, one can realize optical filters based on the same operation principle with fiber optics. Here, the birefringent crystals are replaced with birefringent (polarization-maintaining) fibers, and combined with some fiber-based polarizers [3]. Tuning can be accomplished with a fiber polarization controller.

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