What are Bandpass Filters Used for?

Light Notes

Bandpass filters are optical filters that only let a selected range of frequencies through. Bandpass filters are used for many optical applications in telecommunications, satellite communications and data transfer for light modulation. This article will explore some common use cases of bandpass filters and how they allow users’ applications to obtain “more signal, with less background”.

Raman and Fluorescence Spectroscopy

Bandpass filters are used for so many applications because they allow for shaping and controlling the wavelengths that pass through. The filtering process within the bandpass filter provides for the selective blocking of unwanted wavelengths. This filtering is beneficial in many spectroscopic applications where scattered light may contaminate a spectrum or cause saturation of a detector, or there may be residual fundamental wavelengths from some frequency conversion process.

If you would like to learn more, refer to our blog post on how bandpass filters work

Raman spectroscopy is one example of a technique where a bandpass filter can help ensure the laser illumination light is monochromatic (since the resultant Raman scattered light is observed as a shift relative to the illumination light) thus reducing unwanted background. There are also specific polymerase chain reaction (PCR) band pass filters designed to help discriminate the fluorescence signal produced by one probe from another, which is essential for identifying specific genetic material.

Telecom and Satcom

Telecom and satcom bandpass filters are used to filter the bandwidth of signals to select the signal of interest from the background. In modern fiber optic networks, it has become common to use designs that involve multiplexing of channels. Wavelength-division multiplexers help improve the efficiency of the transfer of information but the proximity of multiple channels risks cross-talk and potential degradation of the integrity of the data being transferred.

A high-quality bandpass filter can block all but a narrow range of frequencies and therefore eliminate contributions from nearby channels. Wavelength division multiplexing has become one of the technologies of choice for transferring large amounts of data and providing network infrastructures for large corporations. The technology allows for more efficient networks, but wavelength division multiplexing relies on bandpass filters to ensure that only the desired light colors pass through.

The high selectivity for a given central wavelength is crucial for satcom where high transmittance in the signal band, but good blocking of all other wavelengths is essential. Many satcom instruments also require very large diameter windows compared to other applications and need blocking with steep edges to maximize the isolation of particular channels and prevent cross-communication. Often, solar rejection bandpass filters are required at the input of a satcom optical communication port, transmitting the SWIR telecom bands while blocking the background solar radiation, improving signal quality, and additionally preventing solar induced heating within the satellite.

Spectral Shaping

A bandpass filter modulates the incident spectrum of the light by only allowing a fixed amount of bandwidth at a given frequency to pass through. The exact ‘sharpness’ of the filtering, amount of bandwidth transmitted and the central frequency are all dependent on the design of the particular bandpass filter.

Iridian Bandpass Filters

There are many reasons why your optical integrator may recommend bandpass filters; they are a mainstay in countless optoelectronic devices, are at the forefront of datacom filter solutions, and present many opportunities to reduce the footprint and design complexity of your systems.

At Iridian Spectral, we consider ourselves experts in bandpass filter design. We are uniquely positioned to help advise you on the best optical solution for your specifications. If we believe bandpass filters are suitable for your application, we can then craft custom optical filters based on your individual requirements.

Contact Iridian today to find out how we can help you.

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