Yesterday we moved back the master laser (ML) in the atrium. In the first photo you can see the installation of the ML in the atrium's rack. The beam fibered path is: ML box --> pig-tailed EOM --> 300mW fiber AMP (S/N 609) --> laser bench
Here below are the details of the activities related to this installation.
We first blocked the Salve counter propagating beam with a beam dump between the two SL injection mirrors (LB_M1 and LB_M2). The ML beam power on the Laser bench was distributed as follows:
- Power ML fiber output on LB = 263 mW
- Power at Fiber AMP pick-off = 81 mW
- Power injected into SL = 182 mW
We switched-off the ML, and then moved it and its driver in the atrium. We put the same settings: ML Current = 2.000 A, ML Temperature = 22.79°. We optimized the fiber alignment and we could increase the coupling up to 60%. To calculate the coupling we measured the output power at the end of a 2m-fiber connected to the ML box and the input power before the diaphragm inside the box.
The long fiber with the thin metallic jacket that was used to bring the ML beam from the atrium into the laser lab had a damaged end (the one in the laser lab), on saturday the fiber was polished to remove the burned part (55150). Yesterday we cheked that there aren't any important losses by replacing the 2m-fiber used for the ML fiber coupling measurement by this long fiber. The output power was the same as the one measured with the 2m-fiber within 1%, so the fiber is validated.
5 BNC cables were put back to the atrium through the cable entry: EOM_CORR_IN, EOM_CORR_MONIT, ML_TH_CORR, ML_PZT_CORR, ML_PD. The snake with the lemo cables for the monitoring signals of the ML pump diodes is still in the laser lab.
Since the pig-tailed EOM can accept only up to 150 mW of input power, most of the ML power must be dumped, for that an absorbing blade provided by the EGO optics group was installed on the reflection of the OI's input polarizer (see second photo). By turning the half-wave plate after the ML head, the power measured at the end of the 2m-fiber is: 80 mW at 265°, 100 mW at 263°, 120 mW at 261°.
We plugged the 2m ML fiber with 120 mW at the EOM input of the EOM box, at the output we measured 45 mW (using a 1m fiber). We plugged the 300mW fiber AMP input fiber at the EOM output box. As the seeded AMP was off the power measured at the its fiber output was 25 mW. Then we switched-on the fiber AMP, the output power was 314 mW. We connected the long fiber with the metallic jacket. Each time we connected a fiber we checked it with the fiberscope and we often had to clean the fiber end. The bleu fiber connected on the laser bench was damaged and we measured 12% of power losses so we decided to plug directely the long fiber on the laser bench. In order to be sure that we do not misalign the SL input beam by unplugging and replugging the fiber on the collimator, we monitored the output power at the pick-off for the Fiber AMP. After the ML fiber replacement, the pick-off fiber was still well aligned, we optimized by ajusting the mirror LB_M0B. We turned the half-waved plate before the pick-off to send more power to the SL. The power distribution is now:
- Power ML fiber output on LB = 265 mW
- Power at Fiber AMP pick-off = 5 mW
- Power injected into SL = 231 mW
We removed the beam dump from the slave input, it locked without any problems.
