The principle of a spatial filter
Many applications, such as holography, require the use of a beam of uniform intensity. Our KT311(M) spatial filter is ideal for generating clean Gaussian beams.
The input Gaussian beam has a spatially varying intensity"Noise"。When the beam is focused through an aspheric lens, the incident beam transforms into a central Gaussian spot (in the direction of the optical axis) and edge fringes, indicating that there is something we do not want to see"Noise"present (see Figure 2 below). The radial position of the stripes and"Noise"The spatial frequency is proportional.
By adding a pinhole in the center of the Gaussian flare,"Noise"The stripes will be blocked, and only the beams"Clean"Partially permeable (see Figure 3 below).
The diffraction-limited spot size at 99% of the profile is given by:
where denotes the wavelength, f denotes the focal length, and r denotes the radius at 1 e(2) of the incident beam.
Choose the right optics and pinholes for your spatial filter system
For your application, the right optical element and pinhole selection depends on the incident wavelength, the diameter of the light source, and the expected output spot diameter.
For example, let's say you use a 650 nm diode laser light source with a diameter of 12 mm (1 e(2)), after passing through the spatial filtering system, you want to get a diameter of about 44 mm light spot. Based on these parameters, the C560TME-B mounted aspheric lens is a suitable choice for the incident portion of a spatial filtering system, as it is designed for a wavelength of 650 nm with a clear aperture of 51 mm, which is sufficient to accommodate the entire diameter range of this laser light source.
The formula for calculating the diffraction-limited spot size at 99% of the profile has been given above, for this example, for C560TM-B, = (650 x 10(-9)M), F = 1386 mm ，r = 0.6 mm, substituting the above formula, we get.
Diffraction-limited spot size (650 nm light source, 1.)2 mm beam).
A pinhole that is about 30% larger than d should be chosen. If the pinhole is too small, part of the beam will be blocked;If the pinhole is too large, other parts except the TEM(00) die will pass through the pinhole. Therefore, for this case, the size of the pinhole is ideally 195 microns. For this reason, we recommend using the installed pinhole P20K, which has a pinhole size of 20 m. Changing parameters, such as the diameter of the input spot and the focal length of the focusing lens, can change the beam waist diameter and thus the desired pinhole size. Decreasing the incident beam diameter increases the beam waist diameter. Using a longer focal length focusing lens will also increase the beam waist diameter.
Finally, we need to select the optics at the output of the spatial filter to ensure that the diameter of the output beam is what we want to get44 mm。In order to determine the appropriate focal length of the lens, it is not drawn to scale, taking into account the chart in Figure 4. From the triangle on the left, we can deduce that the angle is about 248(o)。Using this angle in the triangle on the right, the focal length of the plano-convex lens can be calculated, which is approximately 50 mm.
For this focal length, we recommend the LA1131-B plano-convex lens [focal length f = 50 mm at the design wavelength [( = 633 nm) , which is an approximation of f at the wavelength of the light source ( = 650 nm)].
Note:The beam expansion ratio is equal to the focal length of the output portion divided by the focal length of the incident portion.
To optimize performance, large diameter aspheric lenses can be used for plano-convex lens positions if the output focal length is 20 mm (see AL2520-A, AL2520-B, AL2520-C). These lenses have a diameter of 25 mm and can be fixed with the included SM1RR snap ring.