A notch filter is an ultra-narrowband suppression optical component that deeply blocks light within a few nanometers of the center wavelength (CWL) (OD6–OD8) while maintaining high transmission on both sides. It is commonly used in applications such as Raman spectroscopy, laser protection, astronomical observation, and biological imaging.

Key Parameters

• Center Wavelength: Common wavelengths include 405 nm, 532 nm, 632.8 nm, 785 nm, 1064 nm, etc.

• Bandwidth (FWHM): 10–25 nm (customizable to narrower widths)

• Peak Blocking Depth: OD6–OD8 (corresponding to transmittance of 10??–10??)

• Out-of-Band Transmittance: Tavg ≥ 90% (within a 200 nm range)

• Slope: < 1% relative bandwidth; edge steepness can achieve 99%→1% within 2 nm

• Damage Threshold: ≥ 5 J/cm2 @532 nm, 10 ns, 20 Hz; ≥ 1 MW/cm2 CW

Coatings and Substrates

• Substrate: UV fused silica, B270, Corning 7980, etc., with thicknesses of 0.5–5 mm

• Coating: Ion beam sputtering (IBS) or magnetron sputtering Ta?O?/SiO? multilayer films, with a total of 50–150 layers

• Surface Quality: 60-40 or 40-20 (MIL-PRF-13830B), flatness λ/10 @632.8 nm, roughness <0.3 nm RMS

Typical Applications

1. Raman Spectroscopy: Provides suppression of OD6 or higher at the excitation laser wavelength while allowing high transmission of Raman signals at ±50–4000 cm?1, improving signal-to-noise ratio

2. Laser Safety/Fluorescence Imaging: Blocks pump lasers while retaining Stokes or anti-Stokes fluorescence

3. Astronomical Observation: Suppresses urban light pollution from sodium lamps at 589 nm or mercury lamps at 546 nm while maintaining high transmission in other bands

4. Laser Beam Combining: Reflects a single wavelength while transmitting multiplexed combined beams, enabling compact optomechanical designs