NAOJ GW Elog Logbook 3.2
Shalika, Mitsuhashi-san.
This elog report covers the following aspects:
1. We observed that the beam after EOM was experiencing astigmatism and in order to make corrections we tried increasing the diffraction efficiency of the AOM.
2. The desired operable voltage for the EOM was calculated.
(Details below)
1. a. Initially we hadn't placed any HWP after the Faraday Isolator(FI). Since mirrors reflect one particular polarisation more efficiently than others. As a result in our case, the mirror was not reflecting efficiently. Yesterday, we placed a HWP after the FI and observed the reflected power with respect to the degree of rotation HWP (see Graph 1). The angle was set to 100° and the maximum power found was 11.50 mW.
b. We then observed the diffraction power (We had chopped the zero order beam) of AOM with respect to the degree of HWP(placed just before AOM). See 2nd graph. This HWP was set at 100° too. The power of beam before AOM was 11.5 mW. The AOM was aligned efficiently and the power of 1st order beam obtained was 4.38mW. The diffraction efficiency is now 38%. (The connector being used for the RF driver is correct but loose. The loose connection alters the diffraction power from 0.1 to 0.9 mW. We feel that the proper connector can remove this issue)
c. Since we changed the alignment of AOM we had to do beam fitting after lens LB1901C (see Graph 3 and 4). This would help the beam to enter EOM efficiently, as we will place the EOM at the minimum waist position i.e 128mm from the lens. The energy filters used were, 3.0(attached to beam profier) and 1.0(placed after lens to avoid saturation). We also made sure that there was no zero order beam (see images 5 and 6). Before the adjustment the beam waist was at 91.4 mm and 142.9 mm for major radius and minor radius respectively. After doing the adjustments the beam waist is at 128.6 mm and 137.1 mm for major radius and minor radius respectively.
(See Image 7 for experimental setup)
| position(mm) | Radius Major(mm) | Radius Minor(mm) |
| 87 | 0.259±0.002 | 0.187±0.002 |
| 109 | 0.267±0.002 | 0.191±0.001 |
| 134 | 0.268±0.003 | 0.208±0.002 |
| 163 | 0.270±0.002 | 0.228±0.002 |
| 180 | 0.267±0.002 | 0.245±0.001 |
| 222 | 0.305±0.001 | 0.282±0.001 |
| 249 | 0.352±0.001 | 0.288±0.001 |
2. We calculated the input power for the EOM. Since 1W is the maximum RF power, the maximum voltage(peak-peak) that can be applied was found to be 20 V. The impedance was considered to be 50 ohms at the termination. For 1 rad phase shift, the required RF power is 19.8dBm. Therefore, Vp-p decided to be applied is 6.177 V. (Since we didn't have the datasheet for this EOM in the lab, we took this EOM (being the closest one) into consideration for calculation).
Next Step:
We will place the EOM at the optimum position obtained after the results from the above beam fitting, and place the last lens before the cavity. We will then try to lock the cavity.
