1. What is the primary application of SureBlock™ Notch Filters?

 

These filters are specially designed patented integrated monolithic optical filters specifically designed for Low-Frequency THz-Raman Spectroscopy for operating in broadest operating conditions possible. Nevertheless, they remain highly alignment and environment sensitive. While standard thin-film filters typically block everything within 100–200 cm⁻¹ of the laser line, SureBlock™ filters have an ultra-narrow transition width, allowing researchers to capture both Stokes and anti-Stokes signals very close to the Rayleigh line, of 10 cm⁻¹. They are essential for observing "low-frequency" features that are usually hidden by the filter's cut-off.

 

2. How do I align the filter for maximum performance?

 

Alignment is critical to achieving high optical density (OD) rejection. 

 

• Mounting: Mount the filter in a 1" mirror mount (e.g., Thorlabs KM100) to allow for precise pitch and yaw adjustments.

• Initial Step: Center the collimated laser beam on the clear aperture (~diameter < 8 mm).

• Visual Check: From the input beam, identify the weak surface reflection and align the pitch and yaw so the reflection goes back toward the input beam.

• Fine Tuning: Adjust the yaw angle for maximum attenuation (OD3 or better).

• Optimization: Alternately adjust pitch and yaw until the signal is minimized.

 

3. What are the alignment sensitivities I should be aware of?

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• Angular Sensitivity: The filter is highly sensitive to the Angle of Incidence (AOI). For example, a shift of just a few degrees can move the transmission from 0% to nearly 80%.

• Pitch Limit: The optimum pitch angle should be no larger than out of the y-z plane.  

• Spatial Filtering: Because the filter produces a fan of weak diffracted beams behind it, high OD’s may only be achievable with a spatial filter in a compact setup or place your detector at a distance of ~120 cm to ensure these beams do not interfere with your signal.

 

4. How does temperature affect the filter's performance?

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• Center Wavelength Shift: The center wavelength of these filters is temperature-dependent at roughly 0.01 nm/degC.

• Tuning: If the target wavelength changes (due to temperature or laser drift), you can compensate by adjusting the yaw angle.    However, for overall system performance, it is best to adjust laser wavelength.

 

5.  What else do I need?

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General use case for low frequency Raman (LFR) requires laser, ASE filtering (depending on laser), beamsplitter and multiple notches to obtain >OD8 Rayleigh suppression.    See our integrated spectroscopy solutions page for complete solutions.

 

6. Why am I not seeing the promised >40 dB rejection?

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• ASE Noise: If your laser has significant Amplified Spontaneous Emission (ASE), the filter will block the laser line but let the "background" light through.   NoiseBlock ASE filters should be used for ASE filtering of semiconductor diodes.  

• Sensitivity to Beam Divergence: These gratings are highly angular-sensitive. If the input beam is not well-collimated (i.e., if it is diverging or converging), different parts of the beam will hit the grating at different angles, significantly reducing the effective Optical Density (OD).

• Wavelength Match: Ensure your laser wavelength falls within the "range of optimum performance".   Contact our engineers to verify your laser wavelength matches grating.

• Ghost Beams and Secondary Reflections: The filter produces a "fan" of weak diffracted beams and surface reflections. If these are not properly blocked using spatial filters or apertures, they can reflect off other optics and create additional Rayleigh to enter spectrograph

 

7.  Are these polarization sensitive?

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At typical use angles, these gratings are not polarization sensitive.