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40m Log |
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Tue Feb 2 18:35:47 2016, gautam, Update, Green Locking, Lightwave NPRO moved from PSL table to SP table
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Tue Feb 2 18:54:02 2016, Koji, Update, Green Locking, Lightwave NPRO moved from PSL table to SP table
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Wed Feb 3 23:23:47 2016, gautam, Update, Green Locking, Lightwave NPRO moved from PSL table to SP table 
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Thu Feb 4 09:16:46 2016, Koji, Update, Green Locking, Lightwave NPRO moved from PSL table to SP table
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Fri Feb 5 00:23:01 2016, gautam, Update, Green Locking, Lightwave NPRO moved from PSL table to SP table   
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Fri Feb 5 16:50:24 2016, gautam, Update, Green Locking, First pass at mode-matching   
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Mon Feb 8 15:36:37 2016, gautam, Update, Green Locking, Alternative mode-matching scheme
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Tue Feb 9 04:37:10 2016, ericq, Update, Green Locking, Laser swap initiated
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Tue Feb 9 11:49:47 2016, gautam, Update, Green Locking, Laser swap initiated
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Tue Feb 9 19:15:36 2016, gautam, Update, Green Locking, Laser swap - updates 
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Wed Feb 10 17:57:15 2016, gautam, Update, Green Locking, Laser swap - updates
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Fri Feb 12 17:05:40 2016, gautam, Update, Green Locking, Laser swap - green light recovered but no flashes in the arm
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Fri Feb 12 19:07:52 2016, Koji, Update, Green Locking, Laser swap - green light recovered but no flashes in the arm
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Fri Feb 19 00:42:19 2016, gautam, Update, Green Locking, Laser swap - beam ellipticity from laser?  
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Mon Feb 22 13:56:52 2016, gautam, Update, Green Locking, Laser swap -reflected beam from ETM aligned
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Tue Feb 23 23:01:16 2016, gautam, Update, Green Locking, Laser swap - Green PDH locking 
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Wed Feb 24 19:29:13 2016, gautam, Update, Green Locking, Laser swap - Green PDH locked
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Fri Feb 26 01:52:44 2016, gautam, Update, Green Locking, Laser swap - Green PDH OLTF
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Mon Feb 29 17:17:26 2016, gautam, Update, Green Locking, Laser swap - still no green beatnote
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Wed Mar 2 17:42:19 2016, gautam, Update, Green Locking, Laser swap - some progress
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Thu Mar 3 01:25:50 2016, gautam, Update, Green Locking, Laser swap - 2 IR + 1 green beatnotes found
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Fri Mar 4 01:11:41 2016, gautam, Update, Green Locking, Laser swap - both green beatnotes found
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Sat Mar 5 23:31:01 2016, gautam, Update, Green Locking, Laser swap - some updates    
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Mon Mar 7 23:51:36 2016, gautam, Update, Green Locking, Laser swap - some improvement   
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Tue Mar 8 18:22:20 2016, rana, Update, Green Locking, Laser swap - some improvement
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Message ID: 12026
Entry time: Mon Mar 7 23:51:36 2016
In reply to: 12023
Reply to this: 12027
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Author: |
gautam |
Type: |
Update |
Category: |
Green Locking |
Subject: |
Laser swap - some improvement |
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Quote: |
Next steps in recovering ALS and trying to lock again
- Having set the PDH modulation frequency to 256.62kHz, I took the spectrum of ALS noise using the IR beat (i.e. by piping the IR beat signal through the electronics the green beats usually go through - 6dB and 10dB attenuators were placed immediately after the beat PDs for the X and Y arms respectively, to make the signal levels compatible with the electronics), Attachment #5 unfortunately suggests that the noise performance is still poor, and I suspect the situation will be similar using the green beat (though I have not measured this yet).
- The modulation depth could be sub-optimal for the X-end PDH, I have to measure this and check that it is at an acceptable level. This will also tell me if I need to change the sum+HPF pomona box used to send the PDH control signal + piezo dither signal to the laser PZT. In order to do this, I need to know what the input impedance to the FAST control BNC is - the manual isn't very helpful, it just says the piezo has a capacitance less than 10,000pF. I suppose I will have to actually measure this.
- PDH loop OLTFs have to be re-measured for both ends to check that the servo gain's are appropriately placed.
- We know that the mode-matching into the arm for the X end is poor (I have yet to quantify this) - I suspect that the beam ellipticity is the main culprit. However, the DC transmitted power levels at the PSL table are comparable to (even slightly better than) the Y arm numbers, and so this cannot be the sole reason why the X-arm ALS noise is so much worse... I will continue my investigations next week...
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Attachment #1
Since I could not determine how many volts at the LO input of the pomona box input corresponds to how many volts at the laser PZT, I measured the transfer function between these points using the Agilent network analyzer. The measured TF suggests that for a function generator output of 2Vpp, we get approximately 75mrad of phase modulation, which compares reasonably well with the value of 120mrad reported here. I did not attempt to further increase the LO output signal to push this number closer to 120mrad, as with 2Vpp from the function generator we get +7dBm at the mixer, which is what it wants - so I wanted to avoid any attenuators etc...
Attachments #2 and #3
After ensuring that we have appreciable phase modulation, I set out to measure the PDH OLTFs and adjust the gain on the uPDH boxes accordingly. The X end gain is at 6.0, and the Y end gain is at 4.0. Before measuring the Y-end OLTF, I adjusted the steering mirrors to increase GTRY to ~0.45. GTRX remains a paltry 0.05... But the UGFs seem satisfactory..
Attachment #4
Finally, I took the ALS noise spectrum for the green beats. The beat note amplitudes on the network analyzer in the control room are still puny compared to what we had, -40dBm for Y and -45dBm for X. But the phase tracker Q values are ~1000 and ~3000 for X and Y respectively, which are pretty close to what these were if memory serves me right. There may still be some room for optimization of the PDH loop gains etc, and we could perhaps look at lowering the gain of the REFL PD at the X end? I also have yet to do the sweep for the 3 temperatures at which we can find a beatnote and park at the middle one...
These spectra suggest we could even possibly try locking? We are approximately a factor of 3 above the reference for X and on par with the reference for Y....
Unrelated to this work: I also realinged the PMC, PMC transmission is now 0.730V up from ~0.65V. |
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