Optical Bandpass Filters
Bandpass filters pass signals within a specific range of wavelengths while blocking others, this range being known as its bandwidth or “passband”.
Band pass filters have many applications in spectral radiometry, flame photometry and chromatography. Furthermore, they play an essential part in wavelength division multiplexing systems.
Optical Bandpass Filters
Optical bandpass filters enable transmission of specific wavelengths while blocking others. Their most important specifications are center wavelength (CWL), full width at half maximum (FWHM), and peak transmittance (T).
Additionally, it can be extremely helpful to understand a product’s blocking range – which refers to the upper and lower limits of frequency spectrum in which its peak transmission reaches at least 90% – as this provides an indication of its ability to reduce out-of-band interference and signal noise.
The microwave filter with narrow bandwidths is available for UV, visible and infrared applications like fluorescence microscopy, machine vision, laser line separation and spectral radiometry. They come in various aperture sizes and may either be hard or traditionally coated; hard-coated ones tend to feature more even transmission across their full wavelength width (FWHM), making them more moisture tolerant and often used in high speed applications.
Coating Technologies
Optical bandpass filters transmit only specific wavelengths while blocking others, creating narrow passbands depending on how many layers are employed in their construction. Each layer’s thickness plays a pivotal role in its performance while adding layers increases ripple and loss; Alluxa’s low-noise deposition control system ensures high optical transmission consistency across the filter by eliminating unnecessary ripple.
Hard oxide coating technologies produce highly durable, long-life filters designed to withstand extreme environments and handling. Hard-coated filters also boast tight tolerances on both their center and edge surfaces – unlike soft-coated filters which must be sealed before being shaped; hard-coated ones don’t require this step and can even be patterned by using optical epoxy for bonding, dicing or masking metal masks to achieve patterning effects.
Layer Design
Optical bandpass filters are composed of multiple layers of nonconductive materials which produce interference patterns to select certain wavelengths. Maintaining precise layer thickness and uniformity across the filter is of utmost importance as variations can greatly impact its center frequency and bandwidth; advanced techniques like ion beam sputtering and vacuum deposition are utilized to ensure these crucial criteria are met.
Center frequencies are determined by the quality factor (Q) of resonators, which inversely corresponds to their bandwidth. PCB-based bandpass filters must also be resistant to environmental influences like humidity changes in order to maintain stable performance; moisture absorption alters the dielectric constant of circuit materials which could result in unexpected variations of center frequency and bandwidth of these filters.
Fabrication
Bandpass filters allow signals to pass within a particular frequency band without attenuation or unwanted noise being introduced into it. Their bandwidth is defined as the range between two specific frequency cut-off points – typically three decibels above and below their peak position in terms of frequency – to provide this filter function.
Filters can be constructed using either a single mask or multi-mask system, with multi-mask systems being especially effective when used to fabricate multiple identical filters with different center wavelengths that will then be assembled together into an array.
Omega is an expert at designing and fabricating patterned filters using multiple-mask deposition and photolithography techniques, specifically targeting applications requiring high gradient spectral characteristics (nm/mm).
