Your PZT is broken.

R

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 
next time. 

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.

Todo:
-----
1) Continue mode matching into the PMC. Its transmission now is around the same as the
   old one.

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.

• Recenter the beam on the 2 lenses that are just after the PBS and the waveplate
  
• Adjust pitch and yaw of the two steering mirrors until the beam reflected off the input mirror of the reference cavity is parallel to the incident beam
  
  • Use a sensor card to check the alignment of the incident and reflected beams, and adjust the steering mirrors to get the alignment close
    
    • Note the amplitude of the DC output of the Refl. PD with the iris completely open. Close the iris until the signal decreases by ~50%, then adjust the steering mirrors until the original amplitude is regained. Repeat until the iris can be almost completely closed but the Refl. PD signal doesn't change
      
  • Watch the DC output of the Refl. PD, and maximize the signal on a 'scope
    
  • Sweep the PZT of the laser using a function generator into the RAMP input on the FSS board (~10Vpp at ~1Hz), OR sweep the temperature of the laser using the trianglewave function on the SLOW FSS channel (amplitude~0.5, period~50)
    
  • Watch the modes that resonate in the cavity, and adjust pitch and yaw of the steering mirrors to get closer to the TEM00 mode
    
  • When the TEM00 mode appears in the sweep, stop the sweep, and lock the cavity
    
  • Watch the DC output of the Transmitted PD, and maximize the signal on a 'scope
    
• Celebrate!
  

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 
can work.

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.

Last night, I tried to find the resonance of Yarm by sweeping the frequency of the injection beam.

A strong beat was present at LT_NPRO=48.7856[C_deg], the power coupling of the injection beam was estimated to be 35%. 
(Vmax_beat = 1.060[V], Vmin_beat = 0.460[V], Vno_inject = 0.664[V])

The Yarm was locked and the alignment script was executed. The PLL between the PSL beam and the injection beam was 
somehow locked.

I tried to scan the freq offset (f_PLL) at around 3.88MHz first, then at around 15.52MHz. They are supporsed to be the 
first and fourth FSR of the Yarm cavity. The Yarm transmitted power (DC) was observed to find the resonance of the 
injection beam. It would have been better to use the RF power, but so far I didnot have the RF PD prepared at the end 
transmission. I just used the DC power.

I think I saw the increase of the transmitted power by 10%, at f_PLL = 15.517 +/- 0.003 [MHz]. This corresponds to the 
arm cavity length of 38.640 +/- 0.007 [m]. The previous measurement was not so bad!

Y-arm length
e-log             length [m]
-----------------------
556(2008-Jun-24)  38.70    +/- 0.08    Cavity swinging measurement
556(2008-Jun-24)  38.67    +/- 0.03    tape & photo
This              38.640   +/- 0.007

However, I had difficulties to have more precise measurement mainly because of two reasons:
o The PLL servo is too naive, and the freqency stability of the inj beam is not enough.
  The injected beam should have the linewidth (=freq stability) narrower than the cavity linewidth.

o The PLL servo may experience change of the transfer function at around the resonance. The PLL works the other 
frequencies. However, close to the resonance, it starts to be unstable.

So the next stuffs we should do is 
o Build the PLL just using the incident beams to the ifo, not by the reflected beams.
o Build sophisticated servo to have better frequency stability.
o RF PD at the transmission.

Left the lab with Yarm locked, flipper down, shutter for the NPRO closed.

Y-arm length
e-log    length [m]
-----------------------------
556      38.70   +/- 0.08   Cavity swinging measurement
556      38.67   +/- 0.03   tape & photo
776      38.640  +/- 0.007  Beam injection, poor PLL, Transmitted DC
this     38.6455 +/- 0.0012 Beam injection, independent PLL, Transmitted DC
-----------------------------

Y-arm length
e-log              length [m]           Measurement Conditions
----------------------------------------------------------------------------------------
 556(2008-Jun-24)  38.67    +/- 0.03    Cavity swinging measurement
 776(2008-Jul-31)  38.640   +/- 0.007   Beam injection, poor PLL, Transmitted DC
 782(2008-Aug-02)  38.6455  +/- 0.0012  Beam injection, independent PLL, Transmitted DC
this(2008-Aug-04)  38.64575 +/- 0.00037 Beam injection, independent PLL, Transmitted RF
----------------------------------------------------------------------------------------