An optical notch filter, also call as band-stop filter, band-rejection filter or negative pass filter.It is an optical filter component that allows the broad majority of wavelengths to pass through while precisely blocking light within a specific broad or narrow band — the exact opposite of a bandpass filter. In particularly, the spectral transmission curve of a notch filter shows a sharp dip or notch, which is where the name comes from.
How does a notch filter work?
In principle, high-performance notch filters are mostly based on thin-film interference. By alternately depositing dozens of layers of dielectric coatings with different refractive indices onto a glass substrate, the layer structure is precisely designed so that light of the target wavelength undergoes destructive interference after multiple reflections between the layers and is thus strongly reflected, while all other wavelengths are transmitted.
What are the main spectral characteristics for notch filters?
Normally,Key parameters that characterize notch filters include:
- Center wavelength: the center of the blocking wavelength band, e.g. 532 nm, 1064 nm.
- Notch depth: the degree of blocking, expressed as optical density (OD), or as a percentage of transmission at the specified wavelength.
- Notch width: generally refers to the full width at half maximum (FWHM) but in blocking band which is different with bandpass filters.
- Passband transmission: the average transmission over the wavelength ranges on either side of the notch where high throughput is required.
What are typical application for notch filters?
In practical applications, Notch filters are most typically found in:
- Firstly, in laser safety to make protective goggles that block a specific laser wavelength while maintaining the visibility of ambient light.
- Raman spectroscopy and fluorescence microscopy for filtering out the excitation light so that they extract only the signal light.
- Astronomical observations to remove specific spectral lines from sodium and mercury lamps in urban light pollution.
- Other application like optical communications for separating different wavelength channels.
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