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Message ID: 1069
Entry time: Wed Oct 22 17:48:58 2008
In reply to: 1062

Author: 
Yoichi 
Type: 
Update 
Category: 
General 
Subject: 
Lenses for focusing the optical lever laser (Re:divergence of He Ne 1035P) 


Steve had difficulty in finding lenses for focusing the HeNe laser for the ITM oplev.
Following his measurement of the beam divergence, I did some calculation to find a suitable set of lenses and positions.
First, I fitted Steve's data to get the waist size and location of the new HeNe.
The first plot shows the fitting result.
The size of the waist is 0.3mm at 367mm from the laser output (i.e. inside the laser).
(I only used horizontal beam size data.)
Then using the obtained beam parameter, I calculated the propagation of the beam through two lenses.
After playing with the focal length and location of the lenses, I found that with parameters {f1=0.125m, f2=0.2m, d1=0.2m, d2=0.1m} we get about 1mm beam at the QPD (about 4m away from the laser). f1 and f2 are the focal lengths of the lenses, d1 is the distance from the laser to the first lens and d2 is the distance between the two lenses.
The second plot shows the beam size as a function of the distance from the laser.
The Mathematica notebook used to plot the beam propagation is attached.
By running it on Mathematica 6, you can dynamically change the parameters (focal lengths and locations) by sliders, and the plot (like the one shown in the second attachment) updates in real time. It is cool. Please try it.
Quote:  The ITM oplevs laser diodes are noisy.
They will be replaced by JDS 1035P
SN T8093307 was measured with the beamscanner.
This will able us to calculate the right lenses to get a small beam on the qpd.
** The first column is distance from the front face of the laser in cm.
The second column is beam diameter in the horizontal direction in microns.
The third column is the beam diameter in the vertical direction in microns. (edit by Rana) 





