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Optical Filters

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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

Category: article belongs to category photonic devices photonic devices

DOI: 10.61835/4wk Cite the article: BibTex plain text HTML Link to this page LinkedIn

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 of 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.

More to Learn

Encyclopedia articles:

Suppliers

The RP Photonics Buyer's Guide contains 261 suppliers for optical filters. Among them:

DayOptics

DayOptics offers various types of of optical filters for industrial measurement, environmental protection and other fields. We provide long-wave pass filters, narrow-band filters, band-pass filters, colored glass filters, etc. Size and coating requirements can be customized.

Sinoptix

We offer custom optical filters: band-pass, long-pass, short-pass, dichroic, neutral density, dichroic.

Universe Kogaku

Universe Kogaku offers various lens filters and accessories, specifically UV and IR band pass filters for use with our UV quartz lenses.

Exail

Exail (formerly iXblue)’s gain flattening filter based on fiber Bragg grating technology represents an easy and effective solution to flatten the gain in a WDM systems. The high accuracy (very low ripple) allows the use of cascaded filters, and the return loss is high (low reflections).

The IXC-FBG-PS bandpass filter has a sharp resonance peak in the transmission spectrum with less than 1 dB of insertion loss. It can be used in telecom as well as in sensing applications, also for distributed feedback (DFB) fiber lasers.

Exail offers a broad range of Fiber Bragg Gratings (FBG) to address a wide array of applications in telecommunications, sensing, harsh environments, research and development, etc.

The excellent know-how in the FBG manufacturing process gives Exail the capability to produce various filters on a variety of in house fibers. We stand ready to adapt our services to your needs from quick prototyping to volume production.

Avantier

Optical filters selectively allow certain wavelengths of light to pass freely while blocking other wavelengths. Avantier manufactures a wide range of optical filters for engineering, scientific, and photographic applications.

UM Optics

UM Optics currently has six different coating machines, including one for diamond-like carbon (DLC) and two for broadband anti-reflection coatings. We mainly produce infrared anti-reflection coatings, infrared filters, and diamond-like carbon coatings, providing customers with various coating services and products.

Technica Optical Components

World leader in Fiber Bragg grating and Fabry Perot filters. Product portfolio includes athermal Fabry–Pérot etalons (TWR30) and athermal FBGs (TWR50), as well as tunable FBGs (T10-T980) and tunable Fabry–Pérot filters (TFP10-TFP50).

UltraFast Innovations

UltraFast Innovations (UFI®) provides spectral filters for ultrashort pulse applications – for example, for preserving the pulse duration of reflected or transmitted pulses.

By using such spectral filters, researchers from Prof. Krausz’s group were able to build a so-called wave synthesizer. We were able to split or combine radiation from 250 to 1100 nm in four channels: 250–290, 290–350, 350–500, and 500–1100 nm. The phases of reflection and transmission are controlled. This allows one to precisely combine radiation from four channels, both spatially and temporally, resulting in the generation of sub-optical-cycle pulses. Based on that technology, we have developed the first commercial light field synthesizer.

Artifex Engineering

Artifex Engineering offers custom absorption filters and dielectric filters in almost any design. Bandpass, long pass, short pass or ND filters can be tailored to your wavelength range. The filters can be cut to any shape. Black anodized aluminium rings may be provided for ease of mounting. Visit our product page for more information. We look forward to your inquiry.

OPTOMAN

OPTOMAN optical filter coating designs are available in short pass, long pass or bandpass configurations. Our IBS-coated optics are optimized for high power applications and feature spectral drift-free performance, which is why very sharp edge configurations are feasible.

Standard in-stock IBS coated optics can be found in OPTOSHOP.

Edmund Optics

Edmund Optics offers a variety of optical filters for many applications, including bandpass interference, notch, edge, dichroic, color substrate, or ND. Edmund Optics also offers highly durable hard coatings for applications that require high optical densities with maximum performance.

EKSMA OPTICS

EKSMA Optics has introduced OD 6.0 notch filters for specific laser wavelengths in the range from 488 nm to 561 nm. Other filter choices are: neutral density reflective type filters designed to operate in the 400–2000 nm range, neutral density absorption type filters designed to operate at VIS wavelengths (450–650 nm) and Schott color glass filters.

Leukos

LEUKOS offers the widely tunable filter BEBOP. Combine this with our supercontinuum source Rock to obtain a broadband tunable light source.

The BEBOP can be tuned from 350 nm to 850 nm with a bandwidth between 7 nm and 100 nm. It reaches a transmittance of up to 90% and a blocking OD of >4. You can have a free-space output beam or a fiber-coupled output, and there is an additional infrared output.

See our data sheet.

Laserton

Laserton has various types of optical filters, including color class filters, birefringent filters, neutral density filters (absorptive or reflective, also versions with variable transmission) and bandpass interference filters.

Vortex Optical Coatings

We offer a complete design and manufacture service for optical filters from 300–6000 nm, all deposited using sputter deposition to ensure excellent environmental properties, adherence to very demanding optical requirements and a cost effective solution. Prototype volumes to many tens of thousands all covered.

Shalom EO

Shalom EO offers a vast variety of optical filters of both reflective and absorbing type: bandpass filters, neutral density filters, dichroic filters, notch filters, colored glass absorptive filters, IR filters and laser line filters.

IRD Glass

IRD Glass custom manufactures optical filters that expand the applications and improve the capabilities of almost any optical component. IRD works with a variety of filter glass materials from all the leading filter glass manufacturers: Schott, Kopp, Ohara, Hoya and others. IRD Glass also has the capability to apply thin-film optical coatings, from UV through IR spectrum.

Ecoptik

Ecoptik produces glass filters of different types (absorbing or with thin film coating) for selecting certain radiation bands. We can make many different types and custom optical filters, such as optical comb filters, high/low pass filters and linear variable filters.

Shanghai Optics

Shanghai Optics manufactures a wide range of custom optical filters for engineering, scientific, and photographic applications.

Knight Optical

Knight Optical's offers a large variety of stock and custom optical filters. Our bandpass, shortpass, and longpass filters are available as different filter types each with their own advantage including colour glass filters, dichroic filters, and interference bandpass filters. We also offer heat control filters. IR cut filters, neutral density (ND) filters, and Wratten filters. Knight Optical also have a fluorescence filter sets compatible with certain fluorophores, these consist of an excitation and emission filter, as well as a dichroic mirror.

Questions and Comments from Users

2021-05-20

We have a laser beam with several wavelength components. For example, we want to separate 780.00 nm from 780.04 nm, and can afford to discard one of the two beams; the other beam we want to use. I know the wavelengths are very close to each other, but is there a way to achieve this in practice? It would be really great to have a bandpass filter with 0.04 nm bandwidth.

The author's answer:

Such a narrow bandwidth is not feasible with a filter based on dielectric coatings; you will need some kind of larger resonator – for example, a Fabry–Perot interferometer made of two dielectric mirrors.

2022-01-19

Is an optical filter with center frequency 193 THz and a bandwidth of 50 GHz practically possible?

The author's answer:

Yes, e.g. with a combination of a resonator and some other (more broadband) filter for suppressing the unwanted resonance peaks.

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