SPATIAL FILTERING
LABORATORY TECHNIQUE
SPATIAL FILTERING
I.
OBJECTIVE
To filter out unwanted light intensity variations across a light beam, using a spatial filter.
II. BACKGROUND
A spatial filter is essentially a beam converging device coupled with a filter. The filter, or
pinhole, is used to remove interference patterns in a laser beam caused by diffraction from dust,
lint, lens imperfections, etc. that are part of any laser optical system. Diffraction interference
degrades the laser beam by producing phase and amplitude variations, or modulation, on the
otherwise uniphase laser output, leading to fresnel zone patterns in the beam. The interference is
removed from the beam in the following manner the laser output appears as a point source at
infinity; however, the interference producing sources appear as Huygens generators a finite
distance from the filter, due to the difference in the point of origin, focusing the beam will
produce an image of the "source" with all the "noise", or interference, defocused in an annulus
around the focused beam at the pinhole; therefore, the focused beam will pass through the
pinhole and the interference will be severely attenuated. Attenuations of 40dB or greater are
readily produced by this filtering method.
III. PINHOLE / OBJECTIVE SELECTION
The optimum pinhole diameter is a function of the laser wavelength, laser beam diameter,
and focal length of the microscope objective used. They are related by
8
Wavelength x Focal length
Pinhole diameter = — x ————————————
π
Beam diameter
Applying the above formula, we can match commercially available pinhole sizes and
objectives for spatial filtering purposes. Common helium-neon (HeNe) lasers have a wavelength
of 0.6328µm and a beam diameter of 1mm, and using these parameters gives the following
selection table:
Pinhole diameter
Objective
50 µm
25 µm
15 µm
10 µm
5 µm
5x
l0 x
20 x
40 x
60 x
1
Focal length
25.5mm
14.8mm
8.3mm
4.3mm
2.9mm
In practice, a slightly larger pinhole size is preferable to one smaller than the calculated optimum size; this
is reflected in the pinhole sizes above. In addition, the actual working distance between the objective and the
pinhole is quite a bit smaller than the focal lengths listed above.
IV. PROCEDURE
1. Before attaching the magnetic pinhole mount (PM) to the micrometer spindles, mount the
appropriate microscope objective (MO) onto the spatial filter unit; then align the MO so
that it is as close to the laser beam axis as possible. This will reduce aberrations, provide
optimum light economy, and ease alignment of the pinhole.
2. Use the z-axis adjustment micrometer to move the MO as far away as possible from the x
and y micrometer spindles.
3. Carefully remove the PM from its storage box, holding it by its integral handle. Do not
touch the flat pinhole substrate under any circumstances. Attach the PM first to the vertical
y-axis spindle, making sure the machined lip on the PM is against the side of the spindle
opposite the MO; then slide the PM towards the horizontal x-axis spindle. Before releasing
the PM, make sure it is attached squarely onto both spindles.
4. While observing the output side of the PM, adjust the x and y-axes until a faint light spot is
seen; be careful not to look straight into the output: always look from an angle. Place a
white card near the output and adjust the x and y-axes for maximum output
5. Slowly bring the MO closer in towards the PM with the z-axis control; the output will
probably drift a bit, so use the x and y controls to keep the output centered and symmetric.
As the focal point of the MO is brought closer-to-the pinhole location, the output will
become brighter and more sensitive to adjustments.
6. Continue alternate z and x, y adjustments until the output is a smooth speckle pattern; it
should look symmetric (round) with very faint or no ring patterns around it.
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