0
$\begingroup$

I have a simple setup with a diode laser and an object placed in front of it. I want to observe the edge's shadow clearly.

However, I get this very annoying effect of diffraction and interference around the edges of the object(see pic below)

Is there a way to reduce this effect? Spatial coherence is mandatory for my setup, but spectral/temporal isn't.

The two solutions I've thought of are:
(a) using a temporally incoherent source, like an arc lamp (which unfortunately won't be as spatially coherent as a laser) and
(b) somehow randomly scrambling the phase through the beam's section

I'm not good with optics though so I don't know if these will work or how to implement them.
Different approaches with regards to the light source are welcome.

edge diffraction razor

Improve this question
$\endgroup$
1
  • 1
    $\begingroup$ It is a pity that the diffraction pattern annoys you but it is due to the fact that light is behaving like a wave and therefore is not in the realm of geometrical optics which in theory should give you a sharp edge to the shadow. If you use a white light source then you may also be frustrated because there will be a visible penumbra unless the source is made very "small" or the incident light is parallel. $\endgroup$
    – Farcher
    Commented Nov 16, 2018 at 10:23

3 Answers 3

Reset to default
0
$\begingroup$

The diffraction can be greatly reduced by any of several methods. You haven't mentioned the objective of your experiment, so some of these might not be useful.

First method: You already suggested the first: reduce the temporal coherence of the source. For this, you can use an LED rather than a laser, but an LED typically has rather poor spatial coherence. Alternatively, you can put a rotating diffuser in the system which will reduce both spatial and temporal coherence. To get high spatial coherence from a very bright spatially incoherent source, focus it onto a pinhole. You're then trading brightness for spatial coherence.

Second method: Place a lens between the object and the screen to image the object on the screen. This will almost entirely eliminate the diffraction "ringing" around the edges of the image. A zoom lens will allow you to adjust the size of the image. I'm guessing this might work for you because you've said you need a spatially coherent source, which suggests you just want to obtain a "shadow" that amounts to an enlarged silhouette of the object.

Third method: Select your object to have optically "soft" edges rather than abrupt opaque-transparent transitions. This amounts to apodization. I'm guessing this won't serve your purpose.

Improve this answer
$\endgroup$
8
  • $\begingroup$ I disagree with 2nd method, or you have omitted something. I did try using a microscope with laser illumination and the interference noise is all over the image. The image pictured was already taken with a lens between the object and the (ccd) screen $\endgroup$
    – Manu de Hanoi
    Commented Nov 16, 2018 at 19:54
  • $\begingroup$ If so, the lens was not positioned correctly to focus the image. Please see the Thin Lens equation: [hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html] Of course, if the lens is dirty there will be a lot of diffraction from the dirt. If the object is a razor blade and you want an image that is a few inches across, you probably need a lens a couple of inches wide. $\endgroup$
    – S. McGrew
    Commented Nov 16, 2018 at 20:01
  • $\begingroup$ It seems like I'll just have to use classical optics (ie, a lens to project the object). However I'm disappointed that we don't have light sources with high spatial coherence and no temporal coherence. I suppose they would be very useful. $\endgroup$
    – Vasilis Papadimitriou
    Commented Nov 19, 2018 at 12:26
  • $\begingroup$ Actually there are such sources. For example, a "white light" laser or a supercontinuum laser which emits a nearly continuous broadband spectrum. However, diffraction fringes form in the shadow even when the light source is temporally incoherent. If you let sunlight project through a tiny hole into a dark room, and put a straight edge in the beam, the shadow will have diffraction fringes. See [en.wikipedia.org/wiki/White_light_interferometry]. $\endgroup$
    – S. McGrew
    Commented Nov 19, 2018 at 12:54
  • $\begingroup$ @S.McGrew You're right, I was confused, forgetting that a changing phase is equivalent to a broad spectrum emission (with Fourier analysis etc). However I still don't get why you'd get fringes if you have an arrangement with ie a point source that emits monochromatic light with a different, random phase per unit of angle (ie phase is 35° at dead center, 44° at 1° right, 87° at 2° right etc). Shouldn't the secondary (diffraction) sources be in sync with the incoming wave, hence cancelling out? Or am I missing something? $\endgroup$
    – Vasilis Papadimitriou
    Commented Nov 20, 2018 at 23:08
0
$\begingroup$

You can reduce wavelength (blue laser) so that the diffraction pattern will shrink

Improve this answer
$\endgroup$
0
$\begingroup$

Vibration of the object may help, you would need to get a time averaged measurement. Or can you do a mathematical correction based on observing the maxima and using a formula similar to single slit diffraction.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.