How do bandpass filters work?

Light Notes

Bandpass filters are a crucial component in many optical devices, providing a means of filtering out undesired wavelengths of light. Myriad applications use bandpass filters, including telecom, spectroscopy, and remote sensing.

How do bandpass filters work? A bandpass filter is designed to transmit a specific region of the electromagnetic spectrum. The filter should block other wavelengths outside of this region of interest.

The main specifications of a bandpass filter describe the central wavelength and bandwidth of the transmission window, or preferably the wavelengths of the functional band needed to be transmitted, along with the percentage of transmission in this region and the wavelength range and blocking required outside of the signal band. The contrast between the transmission window and other regions is particularly important for many applications.

What does an optical bandpass filter do? 

It is crucial for optoelectronics and optical applications to have components that can modulate optical signals. This is analogous to the elements in an electronic circuit that provide amplification, filtering, and blocking for certain logic operations. How do bandpass filters work to achieve this?

An optical bandpass filter can help achieve light modulation by blocking contamination from unwanted wavelengths and shaping the usable light spectrum. Depending on the bandpass filter’s construction and the materials used, it is possible to achieve a high control over spectral modulation. For example, the onset of the filter region can be very sharp, or it may be more gradual depending on the filter stack deposited.

Bandpass filters are necessary for many applications as specific light sources do not always produce a single wavelength or a spectrum that is as narrow as desired. With harmonic generation from lasers, there is typically a residual of the fundamental frequency propagated along the same axis as the harmonics. Many spectroscopies rely on monochromatic light, and, for applications such as Raman spectroscopy, the presence of multiple excitation wavelengths complicates the final spectrum. A bandpass filter can help remove any unwanted spectral contamination.

Telecommunications often represent one of the most demanding applications for bandpass filters as the spectrum is often strongly modulated to maximize transmission and transfer range and encode any additional information. This means there needs to be a range of bandpass filters with different properties that operate across all wavelength regions used for the optical transfer of information, including the O, S, C, and L bands.

Iridian Bandpass Filters

Iridian are specialists in optical technologies, including the fabrication of custom bandpass filters. We have some standard, off-the-shelf filters for commonly used spectral filtering profiles, particularly in spectroscopy, but specialize in creating custom optical solutions for your specific application.

With over 20 years of experience in creating custom optics, the Iridian team can design and manufacture the flawless bandpass filter for you. Our bandpass filters are designed for excellent transmission (> 95 %) in the bandpass region and very accurate central wavelength control. This is key to ensuring the filter only lets through the desired wavelength range.

Our bandpass filters are available with extensive coverage of the electromagnetic spectrum, from ~340 nm to 15 µm. Filters are available in narrow pass or broader bandwidth variations depending on the specific needs.

Contact us today to find out how they can help support your application with a high-quality custom bandpass filter designed specifically for your needs.

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