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A spatial filter is a device to 'clean up' a laser beam with an irregular intensity profile, and create a smooth Gaussian profile at the output.

It is usually said (e.g. here) that you need a microscope objective and a pinhole for this. The microscope objective creates the Fourier transform of the laser beam at its output. The pinhole acts as a low-pass filter in the Fourier plane of the lens, to remove unwanted high spatial frequency components of the beam.

Why is the microscope objective necessary, instead of any other lens with diffraction-limited performance (e.g. aspheric lenses)? What makes microscope objectives more suitable?

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    $\begingroup$ That's probably more of a practical question of "where do I buy a small diameter diffraction limited lens with near perfect optical performance CHEAPLY". The answer to the latter is that you could buy a medium quality microscope objective for like \$50, or so. You could, of course, also buy a \$500 "aspherical laser collimator lens" from folks who have the good business sense of relabeling a \$150 microscope objective and selling it for three times the price... for many applications a cheap (pennies) laser collimator from China will probably do, though. $\endgroup$ – CuriousOne Apr 29 '15 at 19:11
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Yes, that is a good answer. Microscope objectives are relatively cheap and well corrected on axis to provide a nice Airy pattern at the image. The pinhole usually sized so its diameter is the same as the first dark ring of the Airy pattern will provide a very clean beam when aligned properly. Also microscope objectives are easily held and small and you can handle them easily. Spatial filter hardware comes with mounts for microscope objectives. Can you imagine having to hold a tiny aspheric lens to do the same thing? Microscope objectives come in handy little packages and stuff.

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