I am wondering why pinholes are placed in the focal planes of two focusing lenses for the purpose of spatial filtering.Please look at the first image.
I do understand the reason in confocal microscopy.
However I cannot relate it to a case where the pinhole is used to clean up the Raman beam for example for removing the cladding Raman when the signal is coming from the core only. Why a pinhole setup like the one shown in the 2nd image is not usually done?
If the collimated beam size is very small (comparable to the pinhole size), then the pinhole can be placed in the collimated beam path. Or if the beam size is relatively large, a larger size pinhole (or really an aperture) can be used to clean up the beam. Is this reasoning right?
Thanks!
Dear Maryam,
conceptually there is no significant difference between confocal microscopy and spatial filtering. You must realise that in Fig 2 the light beams from all sources fill the whole aperture, so when you start to close the aperture, you affect all the sources in the similar way so it does not give the selectivity (the setup in Fig 2. is actually used to limit the outer parts of the beams, which suffer from aberrations). But in spatial filtering the first lens serves to transform directions to positions: at the lens, all beams still fill the whole aperture, but at the pinhole the focused images are spatially separated and allow for selection by the pinhole. The second lens serves merely to restore the parallel beam. In result, by selecting the size and shape of the pinhole you decide which light (from which sources) goes through. In confocal microscopy the pinhole is usually at the diffraction limit size, in the case you mention it can be more general.
Dear Maryam mam,
Greetings of the day !!
The practical imperfect laser Gaussian beam has spatially varying intensity as a central Gaussian spot with some side fringes, which is unwanted "noise". When a beam is focused by an aspheric lens, the input beam is transformed into a central Gaussian spot on the optical axis and side fringes are blocked. By centering a pinhole on a central Gaussian spot, the "clean" portion of the beam can pass while the "noise" fringes are blocked. Also a part of beam, which is not a perfect plane wave will not focus to a single spot, but rather will produce a pattern of light and dark regions in the focal plane due to diffraction. This undesired laser signal gets blocked and only the desired part will pass. That is why this is called Spatial Filtering. This filtering can be applied to transmit a pure transverse mode from a multimode laser by blocking other modes. Now the aperture is kept at the focal plane of the lens. Because, the plane beams converged to a point at the focal plane only and after that it diverges. So, if the aperture is kept at the focal plane, then all the desired beams will pass through the aperture. There will be no loss of desired signal. If the aperture is placed any other plane than the focal plane, then at that position, the spherical beam will not converged properly to a point and due to diffraction there will be loss of desired signal.
In figure 2 also, if we pass the plane beam directly through the aperture, there will be diffraction and the desired signal loss. However, the filtering operation is not performed here.
If the pinhole is too small, the beam will be clipped. If it is too large, more than the TEM00 mode will get through the pinhole. Therefore, no filtering will be done. The beam waist diameter as well as diameter of pinhole required also depends on the input beam diameter and focal length of focusing lens. If we decrease the input beam diameter, the beam waist diameter will increase. When the focal length o the focusing lens is large, then also a large beam waist diameter is required.
Reference :
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=10768
May this help you
Thank you.
Best Regards
Anik Ghosh
Dear Miss
The pinhole is very important filter to reduce nearly 80% of the stray back of Reighley scattering that comes from the the object back to the detector( if you are asking for a Raman scattering as the figure shown). so this pinhole will reduces the diffraction rings and should allow only for the Airy disk that comes from the first ordered of the diffraction pattern limited. that's why it should be in the right position where will be the focal length of the object lens. I hope I reached the answer of what you asking for. Good luck.
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