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Entry  Wed Jul 2 19:15:52 2008, Alberto, Update, General, Status of the alignment of the NPRO beam for the Absolute Length Measurement DSC_0156.JPGDSC_0158.JPG
    Reply  Thu Jul 3 11:53:30 2008, Koji, Update, General, Status of the alignment of the NPRO beam for the Absolute Length Measurement 
Message ID: 627     Entry time: Wed Jul 2 19:15:52 2008     Reply to this: 628
Author: Alberto 
Type: Update 
Category: General 
Subject: Status of the alignment of the NPRO beam for the Absolute Length Measurement 
Today I've tried to bring the frequency of the NPRO laser close enough to that of the IFO beam so that the beat between the two beams can be at a detectable frequency for the photodiode. The way I've been changing the frequency is by the NPRO's temperature control on its driver.

Looking at the signal from the AS OSA should enable us to monitor the direction in which the frequency is changing. Every time the resonances of the IFO beam and of the NPRO beam overlap, we know that the frequencies of the two beams are some FSR of the OSA away from each other. At the overlapping of the resonances, if the difference of frequency is within the detectable range of the photodiode, we should see a peak in the network/spectrum analyzer.

This way turned out not very easy in practice because from the AS OSA one can hardly distinguish the resonances of the primary beam from those of the secondary beam. The cause is mainly the flashing of the IFO beam at the AS port which produces a pattern of resonances of different amplitude. Also for some reason, triggering the output signal from the OSA at the oscilloscope doesn't work very well.

However, even if we didn't have these problems, I think that the two beams are not very well aligned, at least not anymore. I'm attaching some pictures from the AS port. The bright spot on the left is the NPRO beam and the one in the center which flashes is the IFO beam. We probably need some more work in the alignment of the NRPO beam.
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