by Din » Fri Oct 20, 2017 8:55 am
John Klayer wrote:If one gets a narrow peak with a Fabry Perot does it necessarily mean that it is spatially coherent?
No.
John Klayer wrote:Can't a beam be very monochromatic but still have terrible spatial coherence?
Yes.
John Klayer wrote:Is the Michelson technique the sure fire way?
For holography? Yes, based on the situation. In holography, the fringes, or planes, are formed by two point sources, even though one point source may be at infinity, and a specific frequency. However, if there were more than two point sources, additional fringes, or planes, would be formed. Similarly, if there were more than one frequency, additional fringes, or planes, would be formed. So, in order to get exactly one set of fringes, or planes, you need a source that is highly spatially and temporally coherent. Since no source is infinitely temporally coherent (all real sources have bandwidths), the laser is the best source. But, statements like "You need a laser to make a hologram" are not true. I can make a hologram with a led, albeit very thin hologram!
There are situations where you can relax one or other of these conditions. For example, when you make a shadowgram, the object has a very low spatial coherence deliberately.
If you record one set of fringes (pretty much), then, based on the parameters, every point source of the reconstruction source will "play off" the set of fringes. If the reconstruction source is broad, ie has a low spatial coherence, then multiple images result. It's said that the sun is the "best" source for reconstruction of a hologram. The sun subtends about half a degree at the earth, while some modern leds subtend less.
[quote="John Klayer"]If one gets a narrow peak with a Fabry Perot does it necessarily mean that it is spatially coherent? [/quote]
No.
[quote="John Klayer"]Can't a beam be very monochromatic but still have terrible spatial coherence?[/quote]
Yes.
[quote="John Klayer"]Is the Michelson technique the sure fire way?[/quote]
For holography? Yes, based on the situation. In holography, the fringes, or planes, are formed by two point sources, even though one point source may be at infinity, and a specific frequency. However, if there were more than two point sources, additional fringes, or planes, would be formed. Similarly, if there were more than one frequency, additional fringes, or planes, would be formed. So, in order to get exactly one set of fringes, or planes, you need a source that is highly spatially and temporally coherent. Since no source is infinitely temporally coherent (all real sources have bandwidths), the laser is the best source. But, statements like "You need a laser to make a hologram" are not true. I can make a hologram with a led, albeit very thin hologram!
There are situations where you can relax one or other of these conditions. For example, when you make a shadowgram, the object has a very low spatial coherence deliberately.
If you record one set of fringes (pretty much), then, based on the parameters, every point source of the reconstruction source will "play off" the set of fringes. If the reconstruction source is broad, ie has a low spatial coherence, then multiple images result. It's said that the sun is the "best" source for reconstruction of a hologram. The sun subtends about half a degree at the earth, while some modern leds subtend less.