[Tahara, Sakakibara, Nakano]

We continue to investigate the laser for the bKAGRA phase-2.

1. Adjustment of the current to the NPRO laser diode.

The fiber amplifier should be injected with the proper power, which is 1mW~15mW according to the datasheet. We checked the input power to the fiber amplifier and adjusted it to 10 mW. The proper current was 1.69 A.

2. Adjustment of the NPRO crystal temperature

The NPRO crystal temperature should be set at a temperature where the mode hop does not happen. The chosen temperature is 27.0 degree.

3. Beam profile measurement.

=== Setup ===

We measured the beam profile. The setup is shown in Fig.1. An HWP and a QWP to linearize the polarization and a lens to collimate the laser was installed.

Fig1. setup for the beam profile measurement.

=== Result ===

The result is shown in Fig.2. The beam waist position and the waist size were 1.1 m from the lens and 150 um in a radius, respectively. The measurement has been done with the power of 2 W and 40 W. Both results were almost same. Also, I attached the beam shape at each power. They don't have large differences.
From this result, the position and the size of the output beam waist from the fiber amplifier can be predicted as w0 = 43 um, z0 = -0.323, respectively. The waist position is about 15 mm inside from the fiber output.

   Fig.2 Measured beam profiles with each laser power. z = 0 is the position where the lens located.

4. Tomorrows plan

The FI we will install has the aperture size of 9 mm. Therefore, the beam with the size of 3 mm in diameter is the proper size to inject the FI. We will install the FI at the proper position according to the today's measurement. Then, we will re-measure the RIN.