Bandpass Filter Structure
Bandpass filters are constructed by laying down layers of material onto the surfaces of the substrate. Typically, there are several dielectric stacks, separated with spacers. The dielectric stack consists of several alternate layers of low-index as well as high-index dielectric materials. In each of the layers, the thickness of the stack of dielectrics is 4 which is the central frequency of the bandpass filter (i.e., the wavelength that has the greatest transmission across the filter). Spacer layers can be positioned between dielectric stacks, and have the thickness of (NL)/2 which is an integer. The spacer layers may be made of colored glass, epoxy, dyes, or metallic dielectric layers, or. A Fabry-Perot cavity is created through the sandwiching of each spacer layer in dielectric layers. The filter is encased inside a ring of metal with an engraved design to ensure safety and ease of handling.
Overview of Filter Operation:
The conditions for constructive interference in Fabry-Perot cavities allow light from the central wavelength and smaller wavelengths that are on either side of it which is transmitted with efficiency while destructive interference stops any light that is outside of passing through the band from getting transmitted. But, the range of wavelengths blocked to either end of that central wavelength range is very small. To extend the blockage range of the filter, materials that have large blocking ranges are utilized to coat the layer of spacers and the substrate. Even though these materials effectively block out-of-band transmission of incident radiation, they also limit the transmission of radiation through the filter’s passband.
An engraved arrow at the outside of the Optical bandpass filter is utilized to indicate the preferred direction to transmit light by the filters. While the filter can operate with any side facing towards the source, it is recommended to put the coated side in the direction of the source. This can minimize the thermal effects and thermal damage that blocking out-of-band radiation could cause because of the absorption of out-of-band radiation through the substrate or glass filters. The figure on the right was constructed by lighting the filter with an extremely low-intensity light and observing the light’s transmission depending on the wavelength. The plot shows that the direction of transmission through the filter does not influence the intensity and the spectrum of light that is transmitted via the filter. The small difference between back and forward traces is probably caused by a slight change within the incident angle of light that passes through the filter, which was created when the filter was taken off from the jig, turned over, and then replaced within the Jig.
The filter is designed to be used in conjunction with collimated light which is typically incident to the face of the lens. In the case of light that isn’t collimated, or that hits the surface with an angle not usually at a distance from the surface, that wavelength (wavelength which corresponds to the peak of transmittance) will shift to lower wavelengths, and the structure of the transmitted area (passband) will shift. Variation in the angle of the incident angle by a small amount could be utilized to alter the shape of the passband in the smallest area. Significant variations of the incidence angle can cause more significant changes in the center wavelength, but will also drastically alter the form of the passband, and most importantly, result in an important decrease in the transmission in the passband.
The temperature of the Filter:
The bandpass’s central wavelength filter may be altered to a degree (~1 nm across the range of operation that the filter operates in) by altering its temperature. This is because of the small temperature change or expansion of the layer.