Spatial filters allow the user to remove random fluctuations from the laser beam’s intensity profile, significantly improving an optical system’s resolution. This laser beam cleanup is especially important for applications such as holography and optical data processing. To produce this clean Gaussian beam, a spatial filter removes the unwanted multiple-order energy peaks, while passing only the central maximum of the diffraction pattern. A spatial filter will remove any additional spatial noise from an optical system, including scattered light in the beam profile caused by dust particles on or around the objective. The spatial filter assembly is made up of the microscope objective, a pinhole aperture and a positioning mechanism.

Spatial filter alignment is straightforward. Start by positioning the spatial filter assembly in the laser beam while observing the back reflections from the objective lens to bring both into alignment. To align the pinhole, first defocus the lens, then while observing the pinhole substrate, adjust the pinhole position until a faint light appears at the output. Alternate the lens Z-axis adjustment with X-Y pinhole position corrections until the best throughput and focus are achieved. This is indicated when a deliberate pinhole movement causes the symmetric output pattern to quickly disappear. A slow smear means a poor focus.

Three-axis spatial filters come standard with a 25 µm high-energy ultra-thin molybdenum pinhole (900PH-25) pinhole and M-10X microscope objective. This pinhole and objective combination may not be correct for your particular application. Different objective lenses and mounted pinholes can be ordered separately. These spatial filters are designed to accommodate all M- and MV-Series Objective Lenses, the L-4X and L-50X Infinity-Corrected Objective Lenses, and 900PH-Series Mounted Pinholes.