Understanding Spatial Filters

Spatial Filters are designed to be used with lasers to "clean up" the beam. Often times a laser system does not produce a beam with a smooth intensity profile. In order to produce a clean Gaussian beam, a spatial filter is used to remove the unwanted multiple-order energy peaks and pass only the central maximum of the diffraction pattern (see illustration). Also, when a laser beam passes through a system, dust in the air or on optical components can disrupt the beam and create scattered light. This scattered light can leave unwanted ring patterns in the beam profile. The spatial filter removes this additional spatial noise from the system. The spatial filter assembly consists of a microscope objective, a pinhole aperture, and a positioning mechanism. The positioning mechanism has precision X-Y movements that center the pinhole at the focal point of the objective lens. Our TECHSPEC® Laser Objectives are designed for HeNe lasers (632.8nm) and provide the smallest spot sizes possible. Choosing the correct pinhole and objective combination will yield optimal results. The following equations were used to determine the values for the Aperture Selection Chart.

Where:
λ = wavelength of laser (microns)
f = focal length of objective lens (mm)
D = input beam diameter (mm)

Pinhole size is then determined for the table (see note):

Note: The factor of 1.5 in Equation 2 is determined as the optimal factor in order to pass the maximum amount of energy, while eliminating as much spatial noise as possible.

The following table shows the optimal pinhole diameter calculated using Equation 1 and Equation 2. The actual pinhole used should be as close to this value as possible. Smaller diameter pinholes will reduce energy throughput, while larger diameter pinholes will pass more spatial noise.