定义：包含两个平行反射表面的单片干涉装置。
 
光学标准具（也称为法布里-珀罗标准具）是具有平行反射表面的透明片状（通常由熔融石英制作得到）的法布里-珀罗干涉仪。然而，有时也通常采用法布里-珀罗腔两反射镜之间具有空气间隙的结构（空气间隔标准具）。 

图1：激光光束中放置倾斜的固态标准具。
若一束激光光束进入，那么标准具就具有光学谐振腔的功能，透射随激光频率周期性的变化。（严格来说，由于存在色散，透射随频率的变化并不具有完全的周期性。）在共振时，两表面的反射发生相消干涉。在反共振情况下反射损耗最大。透射率随频率的方程可有Airy方程描述，在表面反射率不太高时，该方程近似为简单的正弦方程。 
即使标准具有一定的倾斜角度也存在共振效应（如图1），前提是倾斜角度非常小，从而相反方向的波之间的交叠程度没有显著减小。因此可以利用一定的倾斜角度来控制共振频率。也就是说，标准具可以用作可调光学滤波器，调谐激光器的波长。 

图2：在正入射（实线）、倾斜角度为2°（短划线）和4°（虚线）情况下未镀膜、厚度为100μm熔融石英标准具的透射光谱。 
标准具表面的反射率可由标准具材料和空气之间折射率不连续性计算（菲涅尔反射）得到或者采用电介质涂层进行调节。通过提高反射率可以提高精细度，即在不减小自由光谱范围的情况下使共振峰变陡。 
标准具的有效精细度可能不能达到采用表面反射率计算得到的数值：可能偏小，例如，如果反射表面并不严格平行和平坦。如果表面反射率很高，需要表面质量非常高才能达到理论上的精细度值。精细度偏小还可能由于准直光束太小，倾斜角度太大或者光束质量降低。 

图3：标准具的透射光谱，与图2标准具相同，但是在两端都涂覆40%反射涂层。 
当工作在原理共振或者反共振处时，标准具存在色散。这在光纤通信的色散补偿模块中有时需要用到。

Definition: monolithic interferometric devices containing two parallel reflecting surfaces

An optical etalon (also called Fabry–Pérot etalon) was originally a Fabry–Pérot interferometer in the form of a transparent plate (often made of fused silica) with parallel reflecting surfaces (solid etalon). However, the term is often also used for Fabry–Pérots consisting of two mirrors with some air gap in between (air-spaced etalon).

Figure 1: Tilted solid etalon in a laser beam.

When inserted into a laser beam, an etalon acts as an optical resonator (cavity), where the transmissivity varies approximately periodically with the optical frequency. (Some deviations from perfect periodicity result from chromatic dispersion.) In resonance, the reflections from the two surfaces cancel each other via destructive interference. The highest reflection losses and thus the lowest transmissivity occur in anti-resonance. The transmissivity versus frequency can be described with an Airy function, which approximately fits a simple sinusoidal function for not too high surface reflectivities.

The resonance effects occur even with some tilt (Figure 1), provided that the tilt angle is so small that the overlap of counterpropagating waves is not significantly reduced. (That works well for a small etalon thickness and a large beam radius.) The tilt angle can then be used to control the resonance frequencies. An etalon can therefore be used as an adjustable optical filter, e.g. for tuning the wavelength of a laser.

Figure 2: Transmission spectra of an uncoated 100-μm thick fused-silica etalon for normal incidence (solid curve), and for tilt angles of 2° (dashed curve) and 4° (dotted curve). Note that the frequency shift is approximately proportional to the square of the tilt angle.

The reflectivities of the etalon's surfaces may simply result from the refractive index discontinuity between the etalon material and air (Fresnel reflection) or may be modified with dielectric coatings. By increasing the reflectivities, it is possible to increase the finesse, i.e., to sharpen the resonances without reducing the free spectral range.

The effective finesse of an etalon may not reach the value which could be expected based on the surface reflectivities: it can be reduced, for example, if the reflecting surfaces are not perfectly parallel and flat. For high surface reflectivities, one may require an extremely high surface quality (low roughness) to realize the theoretically possible finesse. A reduced finesse can also result from using a too small or not properly collimated beam, a too large tilt angle, or a reduced beam quality.

Figure 3: Transmission spectra of an etalon as before, but with a 40% reflecting coating on both sides.

When operated away from resonance or anti-resonance, an etalon provides chromatic dispersion. This is exploited in some dispersion compensation modules for optical fiber communications.

Bibliography