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11012008, 05:34 PM  #1  
 
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I've been playing around with the little goodies to be found in the PHR803T sleds today, and have a question to ask about one of diffraction gratings. So the black and white ones in the lower right make the usual linear diffraction pattern: The ones on the top right however make a similar pattern, but only on one side of the beam axis. I know the theory behind the first one, and how it diffracts and interferes to produce the pattern, but can't figure out how the second one can do it on only one side :? Can anyone explain? However they do it, they make good patterns if you cross two at right angles:  
11242008, 06:03 PM  #2  
 
Class 2M Laser 
I don't understand the diffraction gratings but I love playing with those super magnets. What is that there on the lower left hand corner of the top pic?
 
11242008, 08:53 PM  #3  
 
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They're just some mini pots. Possibly for setting the laser currents (they're labeled HD, DVD and CD) thought they may be useful for something one day...  
11252008, 05:00 AM  #4  
 
Class 1M Laser Join Date: Oct 2008 Location: Toledo, OH
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I believe you are talking about Fresnel diffraction by a straight edge. Check out figure 3 in this pdf: (http://www.physics.udel.edu/~mfdecam...iffraction.pdf) Any optic book should have the equations that describe it. They are not fun, by the way!  
12052008, 04:33 AM  #5  
 
Class 2M Laser 
I pulled out one of those onesided diffraction gratings today and its pretty odd how it does that.
 
12192008, 03:48 AM  #6  
 
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It might be a blazed grating. Have you checked to see whether it's truly diffracting only to one side, or whether it is simply diffracting most of its energy in that direction? Are you using it as a transmissive grating, or as a reflective grating?
 
10272011, 08:11 PM  #7  
 
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Bringing this back from the dead. I actually study crossed diffraction, or diffraction from multiple gratings. What I can say as of now is that each grating separates the beam into different orders. For my case, I have multiple gratings arranged side by side, and the output is a collection of beams that is made up of the individual components of each beam. The diffraction occurs at the direction perpendicular to the grating, so each of the grating orientations diffracts the beam in a perpendicular direction. What you see there is the first beam going through one grating, and then the 0th order and diffracted orders from the first grating hit the second and diffract in a direction perpendicular to grating 2, so you get cross like patterns.
 
10282011, 08:35 AM  #8  
 
Class 3R Laser Join Date: Dec 2009 Location: 11.69,143.70
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Since this topic is already back from the dead.. Anyone happen to calculate the lines per mm/in on PHR803T or any sled diffraction grating? Or perhaps someone would kindly give an example for calculating lines per mm given a wavelength of 532nm & distance of 5 or 10 ft? I'm afraid I've been out of school too long to use the formula without help.
__________________ «Hĺlř» Openwrt Piranha! Debian on DockStar. 405nm=Crack 445=CrystalMeth RGB=Ecstasy Custom PCB GB . .Raspberry Pi $25 PC 'If it doesn't burn a hole in the wall then it's okay for my eyes.' *cute little smile* ッ . .Buy Goggles! OEM, Survival . Last edited by HaloBlu; 10282011 at 08:55 AM.  
10282011, 12:45 PM  #9  
 
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(1) m*lambda/d=sin(thetai)+sin(thetad) For simplicity, do it at normal incidence, so sin(thetai) disappears, and you can solve for d, if you know the wavelength lambda of the laser and the order and the diffraction angle: (2) d=m*lambda/sin(thetad) You can get theta d by knowing the distance from the grating to your "screen" (call this "a"), and the distance from the 0th order or center beam to the diffracted order (call this "b"). To get the angle, which is the diffraction angle for that particular order (remember to take order into account), use the trig relationship tan(thetad)=b/a, or: (3) thetad=arctan(b/a) Substitute (3) into (2) to get the grating spacing d. Keep everything in meters for now (wavelength and spacing, a and b). To get lines per mm, do: (4) lines per mm = 0.001/d  
02202012, 03:28 PM  #10  
 
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02212012, 03:36 PM  #11  
 
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These are pulled from optical disk drives, it will be the optic either right in front of the diode or the one after that.
 
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