Hi,
I'm using a regular cheap low-power laser diode followed by a collimating lens and see several interference patterns on the laser spot (see attached pic). I know these patterns do not originate from the lens but from the diode itself, since they were there even after I removed the lens. Any thoughts about their origin and whether there's any way to eliminate them?
Thanks,
Assaf
Assaf,
Interference pattern is generated by a plane parallel glass plate rather than by a lens, when it is illuminated by the expanding or converging coherent light such as from laser diodes. Suspect firstly a thin glass plate such as the cover glass on CCD imager or the window glass on the cap of the diode. Attached find an example of the cover plate interference pattern seen as fixed pattern noise on CCD from James Janesick's book, Photon Transfer.
Shigeo
Shape of these patterns indicates that it can be a diffraction on small pieces of dirt, generally objects smaller or comparable size with the wavelength. Then the structure of diffraction pattern - Bessel like - is nearly the same as a diffraction on a disk. Simply try to clean it carefully.
As said, this looks like diffraction from a round object.
If careful cleaning does not help, you could make the beam better by using a spatial filter, whereby you focus the light through a small pinhole.
Assaf,
Interference pattern is generated by a plane parallel glass plate rather than by a lens, when it is illuminated by the expanding or converging coherent light such as from laser diodes. Suspect firstly a thin glass plate such as the cover glass on CCD imager or the window glass on the cap of the diode. Attached find an example of the cover plate interference pattern seen as fixed pattern noise on CCD from James Janesick's book, Photon Transfer.
Shigeo
You must add a pinhole in the output of the laser. With this you only let pass the zero order of diffraction and will see a clean beam of light.
Round patterns are formed on dirt and dust, as Radoslaw said. If you notice a thinner plain structure of fringes, those might form on the protection glass, if the diode has any.
There is some dust contribution in those patterns, but it is well known that speckle produced by the laser can cause this type of patterns. Moreover, you are using a laser diode, ¿what quality gives this light source? Usually laser types that are used for interference are stabilized, and with high-quality polarized etc. Then it is necessary to use auxiliary enabled devices to improve the quality of light, when you have not a quality laser. Now, What application will give you all this? . It is for interferometry, spectroscopy? or you are just generating a flat wavefront to ? or simply will you illuminate some object?
You have a combination of different noise sources. The decentred ones with a significant contrast are most probably coming from dusts. Flush your laser with dry air, or gently use an optical tissue slightly wet with IPA.
For the central one it is most probably coming from the diode cap window and it is not so easy to get rid of.
If you need a nice beam, ss mentioned by Javier you will need to filter the noise off through pinhole filtering, or find the same laser pigtailed to an optical fiber.
if you can not do this, you could also modulate the current of the laser at a large amplitude (at the expense of the mean power) and large frequency (significantly larger than your detection time equipment). However the window being quite thin, this might not completely extinct the fringe contrast.
You have two patterns from different noise sources. One is diffraction pattern around the dust, the other fringed pattern caused by interference between multiple reflection of the laser beam on the both sides of a thin glass plate located somewhere in the beam path.
Hi my freind
to eleminate this fringe pattern, I thik that depolarising the laser will be sollution to this problem, why you didn't try to add a diffuesed plate in the path of your beam may it resolve your problem
good luck
Since the itereference pattern is caused by mutiple beam reflection on the both surfaces of a thin glass plate, I believe the best way to eliminate it is to reduce surface reflectivity in a way such as applying V-coat at the laser wavelength.
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