[Tahara, Sakakibara, Nakano]

We found that a back reflected light from the lens after the output collimator lens and WPs went back into the collimator lens. We installed an iris between the output collimator lens and those optics and tilted those optics. Now back reflection is caught by the iris.
After improving the back reflection problem, we re-measured the RIN.  Figure 1 shows the RIN measured with the current configuration with each power.

The RIN seemed to be more stable than last week. Opposite from the consideration in the report, the fluctuation of the spectrum seemed to be because of the back reflection. But once in several minutes, we saw the non-stationary noise as shown in the curve of 15 A in Fig.1. Possibly the back reflection from the collimator lens attached to the output of the fiber causes that instability.

Fig. 1 RIN plot. The legend represents the current to the fiber amplifier and the vertical axis represents the RIN.