Notch Filter

A notch filter is a type of optical filter that selectively rejects a narrow band of wavelengths while transmitting all others. It can be used to block unwanted light sources, such as laser lines, ambient light, or fluorescence emission, from reaching a detector or a sensor. A notch filter can also be used to isolate specific spectral features, such as absorption lines, emission lines, or Raman peaks, from a complex spectrum.

Features of a notch filter

  • High rejection: A notch filter can achieve very high optical density (OD) values, typically ranging from OD 3 to OD 6, within the rejection band. This means that the intensity of the rejected light is reduced by several orders of magnitude.
  • Low insertion loss: A notch filter has minimal impact on the transmission of the desired wavelengths outside the rejection band. The insertion loss, which is the ratio of the transmitted power to the incident power, is usually less than 5%.
  • Steep edges: A notch filter has very sharp transitions between the rejection band and the transmission bands. The slope, which is the change in OD per nanometer, can be as high as 1.5 OD/nm.
  • Customizable bandwidth: A notch filter can be designed to have a narrow or a wide bandwidth depending on the application requirements. The bandwidth, which is the full width at half maximum (FWHM) of the rejection band, can vary from less than 1 nm to more than 100 nm.

Major specifications of Notch Filter

  • Center wavelength: The wavelength at which the transmission is maximum.
  • Bandwidth: The width of the transmission band measured at half-maximum points on both sides of the center wavelength. It is also called full width at half maximum (FWHM).
  • Optical density: The measure of how much light is blocked by the filter outside the transmission band. It is expressed as a logarithmic ratio of incident and transmitted power. A higher optical density means a deeper blocking.
  • Cut-on and cut-off wavelengths: The wavelengths at which the transmission drops to a specified value, usually 50% or 10%, on both sides of the transmission band. They determine the steepness of the transition between high and low transmission regions.
  • Blocking range: The blocking range is the range of wavelengths that the notch filter blocks at least a certain level of OD, usually OD 3 or OD 4. It indicates how wide and deep the notch filter can block unwanted light sources. The wider and deeper the blocking range, the more versatile and robust the notch filter is. For example, a notch filter with a blocking range of 400 nm to 700 nm at OD 4 can block most of the visible light spectrum.

What is OD and FWHM

  • OD stands for optical density, which is a measure of how much light is blocked by a filter outside its transmission band. For example, an OD of 2 means that only 1% of the incident light is transmitted, while an OD of 4 means that only 0.01% of the incident light is transmitted.
  • FWHM stands for full width at half maximum, which is a measure of how wide the transmission band of a filter is. For example, a FWHM of 10 nm means that the filter transmits wavelengths within 5 nm on both sides of its center wavelength at half of its peak transmission value.

Message us