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Entry  Tue Jul 26 11:21:53 2016, awade and Antonio, DailyProgress, , Initial search for beat note refcavs' beatnote 
    Reply  Tue Jul 26 12:56:13 2016, Koji, DailyProgress, , Initial search for beat note refcavs' beatnote 
       Reply  Tue Jul 26 15:12:53 2016, awade, DailyProgress, BEAT, Initial search for beat note refcavs' beatnote 
          Reply  Wed Jul 27 23:05:45 2016, awade and Antonio, DailyProgress, BEAT, Initial search for beat note refcavs' beatnote 
             Reply  Fri Jul 29 16:29:53 2016, awade, DailyProgress, BEAT, Initial search for beat note refcavs' beatnote 20160727_DirectLaserBeatNoteDetector.pdf
Message ID: 1691     Entry time: Fri Jul 29 16:29:53 2016     In reply to: 1689
Author: awade 
Type: DailyProgress 
Category: BEAT 
Subject: Initial search for beat note refcavs' beatnote 

I forgot the schematic.  Attached to this post.

Quote:

We built a temporary beat note detector before the cavities. The configuration is illustrated in the attached schematic.

We used some of the unused light from PBSs to provide a direct beat note measurement on a 1 GHz NewFocus 1611. This was before any EOMs or any other components so there is only a beat between the fundamental frequency of both lasers. Coated window optics were used to pick off a small amount of power and a 50/50 beam splitter was used to combine the light from the two lasers.  Some f = ~200 mm lenses were used to keep the beams reasonably collimated, but no big effort was put into mode matching.  There was plenty of light from both lasers to work with, so even poor spacial overlap was sufficient to give a beat note spectrum.  We achieved approximately -53 dBm beat note signal with a clearance of 10 dB above the dark noise level: this enough for our diagnostic purposes.

Using this diagnostic detector, it was found that the closest pair of refinances between north and south cavities was 300 MHz apart. We began the process of gradually tuning the north cavity temperature but believe it is taking a while to actually settle.  There is no feedback PID control on the cavity shield and then we only have control over one cavity with the other affected by variations in the lab environment.  This is something we will need to work on.  For now it looks like we can maybe dial some constants into a Newport 3040 to get a temperature readout from the existing sensor while electronics are organized to interface with the EPICS/acromag system.

For now we think we will keep the 125 MHz detector. This is convenent and from the data sheets it seems like the 1811's have a slightly better NEP. I couldn't see an actual noise spectrum on the manufacture's website so we might want to actually make a measurement of the dark noise of the two RF detectors for a true comparison (with the same power supply rigged up of course).

We left the north cavity to settle with the voltage slightly increased overnight.

 

 

Quote:

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

 

 

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