27.0924 27.0934 [m]
Alberto and Koji,
Last Friday evening, Koji found that the power adj setting (indicated by ADJ) of the NPRO was somehow set to be
ADJ=-45 and yielded the output power of about 200mW instead of 700mW. This is not good because too small pump power
varies thermal conditions of the crystal such as thermal lensing, thermal gradient, and os on. The ADJ setting and the
crystal temperature had been restored to ADJ=0 and LT=~48deg (nominal of the controller), respectively.
Today we tried the quest of the beating again and the above power setting helped a lot! The beating was immediately
found at LT=48.55deg that is very close to the laser's nominal temp. Also the beating got significantly bigger.
After the alignment adjustment 50%-intenisity modulated signal was obtained. From the power calculation it was
estimated that the power coupling of the injected beam is to be 12%~13%. This not so good yet, but something which we
This time the modulation structure of the PSL beam was clearly observed. I could obtain the beating of the injection
beam with the carrier, the upper/lower sidebands of the 33MHz and 166MHz modulations, and the 2nd order of the
33MHz. They were beautiful as if working with an OSA. Very nice.
In reality, those additional intenisty modulations as well as the residual 33MHz signal from the main IFO are
disturbing for the PLL to be locked at the proper frequency. So, now Alberto is working on a passive LPF with
notch at 33MHz. The design was already done. This allows us to work up to 20MHz and at the same time, provides
60dB attenuation at 33MHz (in principle). Very cool.
Koji, on the other hand, continued to work with the PLL servo with some ready-made passive filters. Owing to the
fillters, the error signal was cleaner and the PLL was locked at the proper frequency. The PLL setup is as attatched.
Sideband rejection filter will be replaced to Alberto's one. The photo is the display of the RF spectrum analyzer with
beat locked at 8MHz.
So the next step, we try to find the resonances of the arm cavity with the injection beam once the IFO comes back.
At the last of the experiment "Last autoalignment" was restored, the flipper for the
inj beam was down, and the shutter for the NPRO was closed.
1) Continue mode matching into the PMC. Its transmission now is around the same as the
2) Put a UHV foil covered lead brick onto the PMC to quiet it down.
3) Characterize the PMC loop and retune the body notch for the new body.
4) Tweak the MZ alignment to minimize the RFAM. We can use StochMon to do this as
long as we have the MC WFS turned off or we can put in a flipper to take the
beam before the MC and send it to the StochMon RFPD.
5) Re-align onto the ISS.
6) Install irises around the periscope for the beam. The old iris there is way off.
7) Fix PSL ANG and center both POS and ANG.
Alberto and Koji
o We worked for the abs length measurement setup on Thursday night.
o At the last of the work Koji left the 40m lab at 2AM. "Last autoalignment" was restored. The flipper for the
inj beam was down. The shutter for the NPRO was closed.
o The alignment of the injection beam (NPRO) was re-adjusted.
o The laser crystal temp (LT) of the NPRO was scanned.
o After a long struggle the beat was found at about LT=61deg(!). I think this is almost at the maximum temp
for the NPRO. Note that this is not the diode temp, and therefore it will not damage the laser as far as the
TEC for the crystal works.
o Only the X arm was aligned.
o The alignment of the injection beam was adjusted such that the beating amplitude got maximum.
o At the faraday of the NPRO, we had 2.4V_DC and 1.8V_DC with and without the inj beam, respectively. The
beating amplitude was 200mVpp (at around 2.4V).
o With a simple calculation, the mode overlapping of tghe injection beam is only 0.0023. Ahhh. It is too weak.
In the modematching or something must be wrong.
o The position of the mode matching lens was tweaked a little. It did not help to increase the beat ampitude.
Even worse. (The lens was restored and the values above was obatined with the latest setting.)
o Then tried to build a PLL. It locks easily.
- Put the beat signal into the mixer RF input.
- Connect 10dBm @1MHz-10MHz from the marconi oscillator to the LO input. The supposed nominal LO level was
not checked so far. Just used 10dBm.
- The IF output was connected to an SR560 with 10Hz LPF (6dB/oct) with G=500 or so.We don't need to care
about the sign.
- The output of the SR560 was connected to FAST PZT input of the NPRO.
o The problem was that there was strong intermodulations because of 33MHz. No LPFwas used before the mixer.
Because of this spourious modulations, the PLL servo locks at the local zero crossings. These will be solved
o Eventually left the 40m lab at 2AM. "Last autoalignment" was restored. The flipper for the inj beam was
down. The shutter for the NPRO was closed.
1) Check cable between RFPD and FSS box for quality. Replace with a good short cable.
2) Using a directional coupler, look at the RFPD output in lock on a scope with 50 Ohm term.
I suspect its a lot of harmonics because we're overmodulating to compensate for the bad
3) Purchase translation stages for the FSS mode matching lenses. Same model as the PMC lenses.
Fix the mode matching.
4) Get the shop to build us up some more bases for the RFPDs on the PSL such as we have for the LSC.
Right now they're on some cheesy Delrin pedestals. Too soft...
5) Dump the beam reflected off the FSS RFPD with a little piece of black glass or a razor dump.
Anodized aluminum is no good and wiggles too much.