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Notch filters, also commonly referred to as band-stop or band-rejection filters, are designed to transmit most wavelengths with little intensity loss while attenuating light within a specific wavelength range (the stop band) to a very low level. They are essentially the inverse of bandpass filters, which offer high in-band transmission and high out-of-band rejection so as to only transmit light within a small wavelength range.
Notch filters are useful in applications where one needs to block light from a laser. For instance, to obtain good signal-to-noise ratios in Raman spectroscopy experiments, it is critical that light from the pump laser be blocked. This is achieved by placing a notch filter in the detection channel of the setup. In addition to spectroscopy, notch filters are commonly used in laser-based fluorescence instrumentation and biomedical laser systems.
As with all dielectric stack filters, the transmission is dependent on the angle of incidence. The central wavelength of the blocking region will shift to shorter wavelengths as the angle of incidence is increased. Please see the Transmission Graphs Tab for measured transmission data for each individual filter. Additionally, plots of the meausured Optical Density (OD) of these filters in the blocking region may be found on the OD Graphs tab.
Thorlabs' notch filters feature a dielectric coating on a polished glass substrate, which has excellent environmental durability. The dielectric stack provides high rejection through destructive interference and reflection in the stop band: the optical density is greater than 6.0 (corresponding to a transmission less than 0.0001%) within the stop band. We currently offer filters with central stop-band wavelengths of 405, 488, 514, 533, 561, 594, 633, 658, 785, 808, 980, or 1064 nm. Regardless of the filter chosen, the transmitted beam's wavefront error for light at normal incidence will be less than λ/2 at 633 nm. These filters also have an AR coating on the back surface to ensure >90% average transmission within the passing bands.
Each filter is housed in a black anodized aluminum ring that is labeled with an arrow indicating the design propagation direction. The ring makes handling easier and enhances the blocking OD by limiting scattering. These filters can be mounted in our extensive line of filter mounts and wheels. We do not recommend removing the filter from its mount as the risk of damaging the filter is very high. However, custom unmounted filters are available; please contact Tech Support for more details.
Below are transmission plots for our notch filters, obtained at normal incidence. Although designed for use at normal incidence, the performance of these filters will not vary significantly if used within an AOI of ±3°, but the performance may differ slightly from that shown below. Please note that this measured data presented is typical, and performace may vary from lot to lot, especially outside of the specified wavelength range of each filter.
Our 633 nm filter (NF633-25) has a plot of the transmission as a function of the angle of incidence; this plot can be used as an example of how the center wavelength varies with AOI.
Plots of the Optical Density in the blocking region may be found on the OD Graphs tab above.
Click to Enlarge
Click for Angle of Incidence (AOI) Dependence Plot
Click Here for Raw Data for the 200 - 2600 nm Wavelength Range
The plots below detail the optical density in the blocking region of our notch filters. Please note that this measured data presented is typical, and performace may vary from lot to lot, especially outside of the specified wavelength range of each filter.
For plots of % transmission vs. wavelength, please see the Transmission Graphs tab above.
Optical Density (OD) is related to transmission by the following relationship: