What Is a Notch Filter?
A notch filter is an optical filter that attenuates (blocks) light in a specific wavelength band to a very low level, while transmitting light in other wavelength bands at a high transmission rate.
Notch Filters are also called bandstops or bandpass filters. Generally, bandpass filters are used to transmit only light in a specific wavelength band, whereas notch filters do the opposite (blocking the transmission of light in a specific band).
Uses of Notch Filters
Notch Filters are used to remove excitation light from a single wavelength laser beam. A laser is generally a device that oscillates with an excitation light to produce high intensity and high power.
It emits light at a single wavelength, but the excitation light can be mixed in. Notch Filters can be used for the purpose of extracting the light emitted by the laser and blocking the excitation light.
Notch Filters are also used in Raman spectroscopy and fluorescence spectroscopy, which are analytical scientific instruments that use lasers. By cutting off the light from the excitation light source, etc., and detecting only the Raman spectrum or fluorescence spectrum to be measured, measurements with little background can be made.
Principle of Notch Filter
Notch Filters are made of a dielectric multilayer film, which are composed of multiple layers of dielectrics with different refractive indices, on an optically polished glass substrate.
The dielectric multilayer film does not absorb light, and the difference in refractive index between the layers causes reflection and interference, thus blocking light in a specific band. The transmittance varies depending on the angle of incidence and polarization (S or P polarization). The center wavelength of the blocked light shifts to shorter wavelengths as the angle of incidence increases.
Dielectric multilayer films are composed of materials with high refractive index (refractive index 2~2.5) such as titanium oxide and tantalum, and materials with low refractive index (1~1.5) such as silicon oxide and magnesium fluoride. These films can be deposited by vapor deposition in a vacuum or by electron beam deposition on a dielectric material.
The surface of the filter is very strong and scratch-resistant due to the dielectric multilayer film and anti-reflective AR coating, etc. It is possible to suppress performance degradation caused by time, temperature, and humidity, and to increase transmittance outside of a specific wavelength region.
In addition, the direction of incidence is determined and is generally indicated by an arrow-like mark on the edge of the filter. Whether the incidence is from the end of the arrow or along the arrow differs depending on the manufacturer, so it is necessary to confirm the direction of incidence in advance.
Other Information on Notch Filters
Terms Used to Describe the Performance of Notch Filters
The following terms are necessary to define the performance of Notch Filters in the selection:
1. Optical Density (OD) indicates how well a Notch Filter blocks laser light of a particular wavelength. 6 indicates transmission to the power of 10 minus 6, or 0.0001%, and the higher the OD value, the higher the blocking ratio.
However, the higher the OD value, the more optical it is and the wider the blocking area tends to be. Therefore, you can select a Notch Filter with an appropriate OD value by checking in advance how much you need to cut from the intensity of the laser to be used and knowing the OD value sufficient for blocking.
2. Center wavelength
The center wavelength is the wavelength at the center of the wavelength range where light does not penetrate the Notch Filter and where the OD value is the largest. Since the main purpose of notch filters is to block lasers, most commercially available notch filters are generally designed so that the center wavelength matches the wavelength of the most frequently used lasers. Some manufacturers produce Notch Filters with a specially selected center wavelength by special order, but they are more expensive than the commercially available ones.
3. Blocking region
The blocking region is the wavelength region where light is blocked by the Notch Filter. It is defined by the full width at half maximum of the region where no light is transmitted. Notch Filters generally have a high transmittance in the wavelength region outside the blocking region, and some filters have a transmittance that decreases as the wavelength region moves away from the blocking region. Therefore, when measuring a broad spectrum, it is necessary to check the transmittance spectrum in the measurement wavelength range in advance, because there is a problem that the target light cannot pass through the Notch Filter.