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Transmission Direction Indicator
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. Above, the Transmission Graphs tab presents transmission vs. wavelength data, the OD Graphs tab presents optical density in the blocking region data, and the Spectra Graphs tab presents spectra vs. AOI data for both S- and P-polarization states of each notch filter.
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 of 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. As the mounts are not threaded, Ø1" retaining rings will be required to mount the filters in one of our internally-threaded SM1 lens tubes. We do not recommend removing the filter from its mount as the risk of damaging the filter is very high. However, select filters are available unmounted as well as in custom sizes; 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 the 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 and spectra vs. AOI may be found on the OD Graphs and AOI Graphs tabs, respectively.
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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 the measured data presented is typical, and performace may vary from lot to lot, especially outside of the specified wavelength range of each filter.
Plots of % transmission vs. wavelength and spectra vs. AOI may be found on the Transmission Graphs and AOI Graphs tabs, respectively.
Optical Density (OD) is related to transmission by the following relationship:
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Figure 1: Setup used to test notch filters. Light from the sources listed below were incident on a notch filter mounted on a rotation stage, and the spectra were recorded using Thorlabs' OSA201C Optical Spectrum Analyzer.
Modeling Off-Axis Performance: Optical Spectrum Analyzer Measurements
As the angle of incidence upon a thin-film interference filter increases, a phase shift will occur between each of the transmitted rays created from successive reflections within the filter. This will shift the filter's center wavelength downward, which can be modeled as:
where λ is the center wavelength at an incident angle of θ0, λ0 is the center wavelength at normal incidence, and n* is the effective index of refraction. The center wavelength shift of a multilayer interference filter can be characterized as equivalent to a monolayer with this effective index, which is between the high-index and low-index values for the thin films of the interference filter stack. The effective index can be experimentally determined from measurements of filter spectral transmission at various incident angles; the model is generally valid at small angles, but can work up to ~40° depending on the filter's structure. [1-2]
Measurements of filter transmission were made using the setup shown in Figure 1. The setup consisted of several Thorlabs' light sources (listed in the table under the Figure 1) in order to measure filters with center wavelengths ranging from 488 nm to 1064 nm. The spectral output from each filter was detected with our OSA201C optical spectrum analyzer. The filter was mounted in an LH1 adjustable lens mount mounted on an RP01 manual rotation stage in order to vary the AOI on the filter. Irises were used to control the beam size through the filter, and a half-wave plate was only used to maximize throughput.
Measurements were made of both S- and P-polarization states at AOI steps of 5° between 0° and 45°. All filters were tested except the NF405-13 filter, since a light source with a great enough power output at this wavelength was not available. The OSA software calculates the center wavelength from recorded spectra; these values were used to calculate the effective index n* using Equation 1.
The measured center wavelengths at each AOI are shown by clicking on the icons in the "OSA Measurements" column of the table below. Figures 2 and 3 show a summary of the results. In the graphs in the table, we also plot the theoretical curve based on the effective index for each filter; the value of n* used is the average of the values calculated at each AOI. The good agreement between the theoretical curve and the measured data points shows that the model can be used to determine the angle needed to achieve a particular center wavelength.
 Macleod, H.A, Thin-Film Optical Filters, 4th ed. Boca Raton: CRC Press, 2010.
 Pidgeon, C.R. and S.D. Smith, J. Opt. Soc. Am. 54, 1459 (1964).
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Figure 2: Plot showing the change in center wavelength from the on-axis value for various angles of incidence for S-Polarized light. These measurements were made using the setup shown in Figure 1.
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Figure 3: Plot showing the change in center wavelength from the on-axis value for various angles of incidence for P-Polarized light. These measurements were made using the setup shown in Figure 1.
Visualizing Spectral Performance: Spectrophotometer Measurements
Measurements were also taken using the spectrophotometer used to make the on-axis measurements shown on the Transmission Graphs tab. Transmission vs. wavelength plots for our notch filers, obtained at an AOI of 0°, 15°, 30°, and 45° for both S- and P-polarization states, can be found by clicking on the icons in the "Spectrophotometer Measurements" column of the table below. These measurements also show that the OD in the blocking region is decreased and the bandwidth is increased at higher AOI.
Plots of % transmission vs. wavelength and optical density in the blocking region may be found on the Transmission Graphs and OD Graphs tabs, respectively.
Please note: All measured data presented is an example of the performance of our notch filters. Performance will vary from filter to filter, especially at these off-axis angles of incidence that are not controlled during manufacturing.