We aligned the single mode fiber again and We obtained the linear polarization without QWP as shown in Fig. 1.
P-pol. and S-pol. are able to be obtined at 34.8 deg and 81.0 deg in angle of HWP #3, respectively.
Then, We checked the PD read-out circuit and the circuit diagram is shown in Fig. 2.
Important resistanve values are as follows:
R4 = 20.2 Ohm, R5 = open, R11 = 1.000 kOhm, R16 = 9.82 kOhm, R7 = 50.2 Ohm.
In this circuit, the current generated by PD (I_PD) can be calibrated from the DC output voltage (V_out) using the following equation:
(In other words, the trans-impedance of the readout circuit is 219 Ohm.)
After that, we measured the DC output voltage at every 10 deg in the incident angle with the window glass as shown in Fig. 3.
The PD position is 5 cm after the beam waist and, at this position, the beam sizes in x-direction and y-direction are 215 um and 210 um, respectively.
The incident angle is determined with +/- 0.5 deg error by following way:
step1 (in this step, PD is alomost perpendicular to the beam) seeing reflected light from the PD, we centerd the beam to the PD using the steering mirror.
step2 We miscenterd the beam to the border of the PD, and checked the incident angle seeing the reflected light. The incident angle was 0.5 deg (-0.5 deg in the opposite side.) Then the beam was re-centerd to the PD by the steering mirror.
step3 The PD is tilted by the rotational stage (the angle of the stage is called "alpha"), and, if the beam is miscenterd, we centerd the beam to the PD using steering mirror (the angle caused by the steering mirror is called "beta"). Thus the incident angle is "alpha + beta"
step4 Without touching the steering mirror, the PD is rotated to be perpendicular to the beam (i.e. "alpha" became 0 deg).
step5 We checked whether the beam is still on the PD or not (in today's measurement, the beam is on the PD in this step). If the beam is on the PD, the incident angle at step3 is estimated as follows,
alpha - 0.5 < alpha + beta (incident angle) < alpha + 0.5 (degree)
Therefore, the incident angle is determined with +/- 0.5 deg error.
We are going to measure again the reflectivity tomorrow with this method.