定义：具有与波长有关的透射率或反射率的器件。

光学滤波器通常是指透射率或反射率与波长有关的器件，当然也有的滤波器透射率或反射率是与偏振或空间分布相关的，或者施加的衰减是统一的。透射率与波长关系很弱的滤波器称为中性密度滤波器。 

光学滤波器的类型 
存在很多不同类型的采用不同物理原理的光学滤波器。例如： 

• 吸收玻璃滤波器，染料滤波器和颜色滤波器是利用了某些材料（例如玻璃掺杂，染料，颜料或者半导体）吸收与波长有关。由于吸收的光直接转化为热，因此这种滤波器不适合用于高功率光学辐射中。 
• 很多光学滤波器是利用干涉效应，与传播过程中相移随波长变化结合一起。这种滤波器反射率和透射率都与波长有关，被滤除的光还可以发送到束流收集器中，该器件可以承担很高的功率。一种很重要的干涉型滤波器包含电解质涂层。这些涂层可以用在电介质反射镜中（包括二色性反射镜），还可以用到薄膜偏振器中，偏振和非偏振分束器中。采用薄层设计可以实现边缘滤波器，低通、高通和带通滤波器，陷波滤波器等。光纤布拉格光栅和其他光学布拉格光栅（例如体布拉格光栅）采用的是相同的原理。除了阶跃折射率结构外，还有渐变折射率滤波器，称为梳状滤波器。该方案可以制作非常高质量的陷波滤波器。 
• 法布里-珀罗干涉仪，标准具和阵列波导光栅也是利用干涉效应，但是需要更大的光程差。因此，它们具有更加尖细的光谱特性。 
• Lyot滤波器涉及到与波长相关的偏振态变化。还有类似的器件，例如可调谐激光器中的双折射调谐器。 
• 还有一些滤波器是利用了棱镜（或棱镜对）与波长有关的折射或者光栅中与波长有关的衍射。 
• 还有声光可调谐滤波器，它利用了只在很窄频率范围内工作的声波的布拉格反射。 

图1：电介质边缘滤波器的反射率曲线，在小于980nm具有很高的透射率，而在大于1030nm具有很高的反射率。这种滤波器可以用来将泵浦光注入到激光器的掺镱晶体中。 
考虑到透射曲线的形状，还有下列滤波器： 

• 带通滤波器，只透射特定波长范围的光 
• 梳状滤波器，消除特定波长范围的光 
• 边缘滤波器，透射某一特定波长以上或者以下的光（高通和低通滤波器） 

应用 
光学滤波器的应用包括： 

• 滤波器可以消除不需要的光。例如，消除激光辐射实现人眼防护通常采用滤波器，可以消除红外激光只透射可见光（参阅激光安全）。类似的，太阳镜衰减可见光并且可以滤除紫外光。绿色激光笔也通常采用滤波器来消除杂散红外光。热控制滤波器用来透射可见光，且消除强的红外辐射，后者是由热表面辐射的。锋利的边缘滤波器或者带通滤波器可以用在荧光显微镜中消除荧光信号光中的泵浦光。 
• 与波长相关的损耗可以用在光纤放大器中进行增益平坦，与光纤通信中类似。类似的，滤波器还可以用来平衡光探测器响应或者光源中的非均匀光谱。 
• 以光纤分插复用器形式的滤波器可以用在波分复用光学数据传输系统中分离或者插入单个信道。 
• 激光器中的腔内滤波器可以进行波长调谐和激光器的单模工作，或者抑制在某些不需要的波长处的激光效应。 
• 滤波器可以抑制放大器链中放大的自发辐射效应。 
• 可调谐滤波器和宽带光探测器结合起来可以对光信号进行光谱分析。 
• 中性密度滤波器可以在不改变光谱形状的情况下衰减光信号。

Definition: devices with a wavelength-dependent transmission or reflectance

More specific terms: interference filters, dichroic mirrors, rugate filters, etalons, Fabry–Pérot interferometers, diffraction gratings, birefringent tuners, acousto-optic tunable filters, cold mirrors, hot mirrors

An optical filter is usually meant to be a component with a wavelength-dependent (actually frequency-dependent) transmittance or reflectance, although there are also filters where the dependence is on polarization or spatial distribution, or some uniform level of attenuation is provided. Filters with particularly weak wavelength dependence of the transmittance are called neutral density filters.

Types of Optical Filters

There are many different types of optical filters, based on different physical principles:

Absorption Filters

Absorbing glass filters, dye filters, and color filters are based on intrinsic or extrinsic wavelength-dependent absorption in some material such as e.g. a glass, a polymer material or a semiconductor. For example, one may exploit the intrinsic short-wavelength absorption of a semiconductor, or extrinsic absorption caused by certain ionic impurities or by semiconductor nanoparticles in a glass. As the absorbed light is converted into heat, such filters are usually not suitable for high-power optical radiation.

Interference Filters

Various kinds of optical filters are based on interference effects, combined with wavelength-dependent phase shifts during propagation. Such filters – called interference filters – exhibit wavelength-dependent reflection and transmission, and the light which is filtered out can be sent to some beam dump, which can tolerate high optical powers.

An important class of interference-based filters contains dielectric coatings. Such coatings are used in dielectric mirrors (including dichroic mirrors), but also in thin-film polarizers, and in polarizing and non-polarizing beam splitters. Via thin-film design it is possible to realize edge filters, low-pass, high-pass and band-pass filters, notch filters, etc.

The same physical principle is used in fiber Bragg gratings and other optical Bragg gratings such as volume Bragg gratings.

Apart from step-index structures, there are also gradient-index filters, called rugate filters. That approach allows one to make high-quality notch filters, for example.

Fabry–Pérot interferometers, etalons and arrayed waveguide gratings are also based on interference effects, but sometimes exploiting substantially larger path length differences than monolithic devices. Therefore, they can have sharper spectral features.

Lyot Filters

Lyot filters are based on wavelength-dependent polarization changes. Similar devices are used as birefringent tuners in tunable lasers.

Refractive and Diffractive Filters

Other filters are based on wavelength-dependent refraction in prisms (or prism pairs) or on wavelength-dependent diffraction at gratings, combined with an aperture.

Acousto-optic Filters

There are acousto-optic tunable filters, where it is exploited that Bragg reflection at an acoustic wave works only within a narrow frequency range.

Tunable Optical Filters

While most types of optical filters exhibit fixed optical characteristics, some types are tunable, i.e., their optical characteristics can be actively modified. Some examples:

• The resonances of an optical resonator can be tuned by modifying the resonator length with a piezo-controlled mirror. That way, one can tune the optical transmission peaks.
• Etalons can simply be tilted to shift their transmission peaks.
• Acousto-optic filters can be tuned through their electrical input, which can affect the amplitude or frequency of the generated acoustic wave. See the article on acousto-optic tunable filters.
• The principle of liquid crystal modulators can be used.

See the article on tunable optical filters for more details.

Different Filter Shapes

Concerning the shape of the transmission curve, there are

• bandpass filters, transmitting only a certain wavelength range
• notch filters, eliminating light of a certain wavelength range, e.g. by reflecting it
• edge filters, transmitting only wavelengths above or below a certain value (high-pass and low-pass filters)

Of course, a wide range of filter shapes can also be realized, particularly with interference filters.

Figure 1: Wavelength-dependent reflectance of a dielectric edge filter with high transmittance below 980 nm and high reflectance above 1030 nm. Starting from an analytically formulated design, the performance has been further optimized numerically (using the software RP Coating). Such a filter can be used for injecting pump light into the ytterbium-doped crystal of a laser.

Applications

Some examples for the many applications of optical filters are:

• Filters can eliminate some unwanted light. Some examples:
• Eye protection against laser radiation with laser safety glasses is often done with filters which can eliminate e.g. infrared laser light while transmitting visible light (→ laser safety).
• Similarly, sun glasses attenuate visible light and filter out ultraviolet light.
• Green laser pointers are often equipped with filters for removing residual infrared light.
• Heat control filters in the form of cold mirrors are used to transmit visible light while removing intense infrared radiation, as it is emitted e.g. by hot surfaces.
• Similarly, hot mirrors can remove infrared light from a beam path by reflecting it.
• Sharp edge filters or bandpass filters can be used in fluorescence microscopes for removing pump light from the fluorescence signal light.
• Wavelength-dependent losses are useful for gain equalization of fiber amplifiers, as used in optical fiber communications. Similarly, filters can be used for balancing a photodetector response or the non-uniform optical spectrum of a light source.
• In the image sensors of photo cameras, for example, RGB filters allow for separate detection of the intensity in different colors, so that color images are obtained.
• Filters in the form of fiber-optic add–drop multiplexers can extract or inject single channels in wavelength division multiplexing optical data transmission systems.
• Intracavity filters in lasers can be used for wavelength tuning and for single-frequency operation of lasers, or for suppressing lasing at unwanted wavelengths.
• Filters can suppress effects of amplified spontaneous emission in amplifier chains.
• The combination of a tunable filter and a broadband photodetector can be used for the spectral analysis of optical signals.
• Neutral density filters are used for attenuating optical signals without modifying their spectral shape.