Introduction to Laser Filters

What is a laser filter?

Laser filters are designed for use with a specific laser wavelength or wavelengths. Design considerations can reduce the likelihood of filter surface damage due to the high-power coherent light produced by laser systems. Laser filters can be used for a variety of purposes and include a number of filter types:

A laser line clean-up filter is a narrow bandpass filter with high transmission at the laser wavelength and high blocking of other wavelengths. The blocking range for laser line clean-up filters can be over a smaller range than a typical bandpass filter. Center wavelength, edge steepness and FWHM are important specifications for this filter type.

Laser edge filters are typically used in laser spectroscopy applications, such as Raman, where the signal is very close in wavelength to the laser. Usually, they are specified by the laser wavelength, transition width, edge steepness and blocking at the laser wavelength. Both short-pass and long-pass laser filters can be used in these applications.

Laser dichroic beamsplitters are used in laser combining and laser steering applications in such diverse fields as fluorescence microscopy and multi-laser illumination and projection systems. Dichroic laser filters can be used in multi-band or single-band configurations.

Filters specifically designed to block laser wavelengths are called laser notch filters. They are specified by the amount of blocking at the laser wavelength and the bandwidth of the blocking. These types of laser filters are typically used in the detection path of the optical system.

GMBW: Guaranteed Minimum Bandwidth describes the wavelength range within which the T_avg (or T_abs ) specification is met. GMBW is always smaller than FWHM.

FWHM: Full-width half-max describes the spectral width of a bandpass filter. Laser line clean-up filters have a small FWHM centered around the laser line.

Transition width and edge steepness: These values describe the spectral width from high transmission (50%) to deep blocking (OD 6). Transition width is the maximum allowed spectral width between the laser line (where OD > 6) and the 50% transmission wavelength, while edge steepness is the actual steepness of a filter, measured from the highest wavelength with OD 6 to the 50% transmission wavelength. These can be specified in wavelength (e.g. 6 nm), percent of the laser wavelength (e.g. 1% of 633 nm), or in wavenumbers (cm^-1).

Wavelength to wavenumber conversion note: Wavenumbers are used in Raman spectroscopy to describe the shift of the non-elastically scattered signal from the laser wavelength. Learn more. The lower the wavenumber, the closer to the laser line. Wavenumber (△w) can be calculated by

Here's an online calculator: saviot.cnrs.fr

Learn more about measuring light with wavelengths and wavenumbers in our tech note.

Blocking: Measured as Optical Density (OD), defined as:

OD = – log ₁₀ (T)

For laser line clean-up filters, blocking is usually ~ OD 5 near the bandpass region. For Raman applications it is often OD 6 or higher at the laser wavelength.

Angle of incidence (AOI): Refers to the angle (with respect to the normal of the filter surface) at which the laser filter will be used in the end application. Most laser filters are designed to be used at an AOI of 0°, but dichroic beamsplitters are usually designed to be used at 45°. Filters are designed to be used at specific angles, so this should be specified from the beginning.

Flatness and Reflected Wavefront Error: Typically only specified for dichroic beamsplitters or when the reflected beam is used in the optical design. This value describes how much distortion of the beam is acceptable upon reflection off the beamsplitter. It is specified in waves / inch at 632.8 nm.

How are Laser filters used?

Laser filters are used in a number of applications that involve delivery of laser light to a sample and blocking of laser light from the detector.

• Laser launches are optical systems that combine the beams from several lasers into a common optical axis. The combined beams are used in systems such as flow cytometers and confocal fluorescence microscopes to enable simultaneous multi-wavelength excitation of a number of fluorophores. Laser filters used include:
• Laser line clean-up filters at the output of each laser
• Multi-line laser dichroic filters to combine the beams into a single axis

Figure 2. (Left) Typical laser launch configuration combining laser clean-up filters with dichroics. (Right) Spectra of dichroic beamsplitters used to combine beams.

• Laser Scanning Confocal Microscope (LCSM) systems are widely used to map out distributions of cellular components (including proteins, lipids, DNA, ions, etc.) in fixed and living tissues and cells. These imaging systems include a laser launch with up to 7 different laser lines for fluorescence excitation. Emission signals are detected between the laser lines, so a combination of multiband and bandpass dichroic mirrors and filters are used in the detection path. These all require high levels of laser blocking to eliminate noise from the system. Figure 3 illustrates the many laser filters used in such a system.

• Raman spectrometers and microscopes use inelastic laser scattering of molecules intrinsic to the sample to provide contrast and information about molecular composition. In microscopy, Raman is used to map out lipid, DNA, water and protein distributions in tissues. Raman has an advantage over fluorescence methods because no dyes or labels are required. The disadvantage is that the signal is very weak so high laser blocking is required (usually OD 6 or more at the laser line). Raman systems use laser notch filters (or edge filters) to remove elastic (Raleigh) scattering from the detection path. Raleigh scattering happens at the same wavelength as the laser, while the Raman signal is shifted from that wavelength. Shifts to longer wavelengths are termed “Stokes” signals while shifts to shorter wavelengths are termed “anti-Stokes” signals. The difference in wavelength between the laser and the Raman signal is measured in wavenumbers that match vibrational modes in molecules in the sample. Researchers doing Raman will use the following laser filters:
• Laser line clean-up filters in the illumination path
• Laser blocking filters- either edge filters or notch filters in the detection path




Figure 4. A simple Raman spectroscopy setup that uses a laser line clean-up filter in front of the laser and a laser blocking filter in front of the detector.

• Dichroic laser filters can also be used to direct the illumination or detection light through the optical system. This is common in microscopes as illustrated in Figure 5.

Figure 5. Raman microscopes use a laser dichroic beamsplitter to align the illumination and detection axes. Download our Raman catalog

Learn More About Laser Filter Product Families