We added and aligned the fast photodetectors (PDA50B) to the HWS table, per T1500579. The outputs from these photodetectors go into a whitening board (D1500430). The input channels in the ADC are adc0_22 and adc0_23 in the TCS CO2 model.
Alignment:
We used an IR card with mounted in a C-Ring and the Green Beam from the ALS to align to the center of the aperture on the photodiodes. See the before and after alignment photos:
Also attached is a photo of the whitening board.
Calibration:
The input channels in the real-time code are: H1:TCS-IFO_POLZ_S_HWS and H1:TCS-IFO_POLZ_P_HWS.
The filter banks in these channels:
- Undo the whitening board (2x poles at 0.5Hz and 2x zeros at 10.9Hz)
- Convert from counts to volts
- Convert from volts to amps (+50dB transimpedance setting of PDA50B)
- Convert from amps to Watts (divides by response = 0.56 A/W at 1064nm)
Also, the following DC offsets are added at the input based on measurements of the background value when the photodiodes were not illuminated:
- S_HWS_OFFSET = -28.76 counts
- P_HWS_OFFSET = 20.92 counts
From here, the outputs go into a 2x4 matrix for combination to produce S and P estimates for the power incident on the BS_AR surface. A rough estimate for these values are as follows:
- S_BSAR_INC = (1/R_BSAR_S) * (1/T_DCBS) * S_HWS= 1/0.027 * 1/300E-6 = 1.235E5 * S_HWS
- P_BSAR_INC = (1/R_BSAR_P) * (1/T_DCBS) *P_HWS = 1/34E-6 * 1/300E-6 = 9.8E7 * P_HWS
Based on the complex reflectivites of the BS for S and P, the ratio of S to P at the AS PORT increases by a factor of 39 from the BS_AR surface. We use the following parameters to calculate the polarization angle (in radians) at the AS PORT.
- S_ASPORT = 38.8*S_BSAR_INC = 4.79E6 * S_HWS
- P_ASPORT = P_BSAR_INC
- AS_THETA = ATAN( SQRT( S_ASPORT/P_ASPORT ) ) ~ SQRT( S_ASPORT/P_ASPORT )
