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. |