NAOJ GW Elog Logbook 3.2
Yohei,
All the files are available here.
Polarization Beam Splitter is a key element of polarization circulation speed meter. The PBS we obtain is made by Layertec.
I measured the transmissivity and reflectivity of the PBS. Schematics are here.
Glan-Thompson polarizer (GTP) is use to transmit one-polarization. The extinction ratio in a spec sheet of 10GT04AR.18 is 100000:1. This value is high enough to negrect a leakage of the orthogonal polarization, because as mentioned later the S and P polarization power were set to 2.2 and 4.9 mW with the same input polarization. The power fluctuation of the laser it self is an order of 1/1000, which is ~100 times larger than the orthogonal beam power estimated from the extinction ratio and input beam power. For those reasons and for the sake of simplicity, the output of GTP is assumed to be perfectly-linearly polarized.
The table below shows how the output power of GTP are changed by changing the angle of the HWP and GTP.
| Mode | ø_H [degree] | ø_G [degree] | Power (\pm 1 uW) |
| P mode | 141 | 295 | 4.880 |
| S mode | 141 | 205 | 2.176 |
| - | 96 | 295 | 2.368 |
| - | 96 | 205 | 4.427 |
The first two configurations are denoted as P and S mode in the folllowing discussion.
Photo detector calibration
The GTP output is picked off by a beam sampler, and its reflection goes into a photo detector.
To cancel out power fluctuation of the laser, we need calibration factors between photo-detector output voltage V [V] and power-meter's beam power P [W]. Measurement results are shonw in the table below:
| V (sigma) [mV] | P (sigma) | A (sigma)[W/V] | |
| P mode | 26.61 (0.13) | 4.90 mW (513.42 nW) | 0.167 (1e-3) |
| S mode | 211.95 (0.16) | 2.30 mW (223.06 nW) | 1.070e-2 (1e-5) |
| Background | -2.67 (0.14) | 2.98 uW (480.84 pW) | - |
Here the measurement time is 10 seconds and acquisition rates are 100 Hz and 10 Hz, for the PD and power meter, respectively. A is the calibration factor with uncetainty sigma, which take into account sigmas of V and P propagation. To derive A, background noise is subtracted from P and S mode data.
Transmission measurement
Using these calibration factors, we were able to perform simultaneous measurement of input and transmission beam power. The table below shows the measurement results:
| V (sigma) [mV] | P (sigma) | Transmissivity (sigma) [%] | In spec [%] | |
| T_P | 26.24 (0.15) | 4.61 mW (569.35 nW) | 95.3 (0.9) | - |
| T_S | 200.11 (0.16) | 5.17 uW (4.47 nW) | 0.2382 (0.0004) | 0.2 < |
Here uncertainties are all propagated into sigma of transmissivity.
Reflection measurement
| V (sigma) [mV] | P (sigma) | Transmissivity (sigma) [%] | In spec [%] | |
| R_P | 26.54 (0.14) | 8.53 uW (560.61 pW) | 0.175 (0.002) | < 3 |
| R_S | 200.47 (0.15) | 2.18 mW (174.02 nW) | 100.3 (0.1) | - |
Here uncertainties are all propagated into sigma of transmissivity.
Discussion
We found that T_P and R_S behaves a bit weird: T_P is smaller than that inferered from R_P, i.e. T_P=1-R_P. Loss should not be so large in an order of few percent, and R_S went beyond 100 % within an error of one sigma. There seems to be some systematic error.
On the other hand, T_S and R_P are close to the designed values. With those amount of losses, cutoff frequency in transfer function will be low enough to see its speed behaviour.