[Tahara, Sakakibara, nakano]

Since the Nufern 40-W fiber amplifier which we had tested (klog5138 klog5124, and so on) was broken, we tested another amplifier unit which was used in the coherent addition experiment in Mio lab. The measurement we have done is as follows:

1. Output power measurement

The output power was measured. The relationship between the input current to the fiber amplifier and the output power is shown in the figure.

2. Beam profile measurement

We measured the beam profile with the same configuration with klog5084. The result is shown in the figure. The result is different from the previous measurement, and it might be because of the lens position. In any case, the collimation lens attached to the output port of the fiber amplifier is different from the one we will use in Kamioka, so we need to measure again.

3. RIN measurement.

We measured the relative intensity noise with the same setup reported in klog5124. As reported in klog5124, the RIN had non-stationary noise. However, the one we tested today, we did not see any non-stational noise in a couple of hours. If the current is smaller than 15 A, a large peak at 35 Hz and the harmonics of it appeared, and that is gone in the spectra with the larger current than 20 A. This is the stational and reproducible noise. At frequencies below 1 kHz, the RIN is getting the larger when the current to the fiber amplifier is the larger. And at the frequencies higher than 1 kHz, it is difficult to understand the relation between the RIN and the current. however, there is not so large difference.