[Akutsu, Hasegawa]

 

We worked about below two topic for TMS at Y end.

 

1. Background of two QPDs for green laser

Before working, background was about -200[cnt] for each segment of two QPDs because of room light.

To decrease this background, we put black cylinder just in front of QPD light receiving surface and black wall(?)

by the QPD modules (see Fig.1 and 2).

After this working, background was decreased to about -10[cnt] for each segment.

 

2. Transmitted power of green laser

As you can find in previous report(4609) , measured transmitted power of green laser just after periscope was 1.6 uW.

Today, we measured transmitted power just before and after the periscope.

Same equipments and setting were used(PDA100A with 40dB gain) and, using oscilloscope, we measured the mean value of the signals.

The measured value and corresponding laser power was…
     #Before the periscope : 1.09 [V]       18 [uW]

     #After the periscope : 312 [mV]       2.5 [uW]

       (W/O laser injection : 184 [mV])

So it means roughly (312-184)/(1090-184)~85% of laser was lose by periscope.

We found this is because the coating applying on the periscope mirrors is not suited for green laser.

After that, to increase the power somewhat, we put HWP before periscope and adjust its angle, finally it became to about 408 [mV]=4.4[uW].
These periscope mirrors will be replaced.