As an additional data analysis for the TMS-X polarization monitor calibration, we have also determined the calibration factor that converts the s-pol PD spectrum (V/rtHz measured by DL950) into the polarization rotation spectrum (rad/rtHz).
The analysis procedure follows the same steps as in (klog #35743), but since the HWP dependence of the s-pol PD signal is opposite in phase to that of the p-pol PD signal, the sign of the model function has been modified accordingly.
Also, because the output of the X_IR PD was not plotted in Figures 1 and 2 of (klog #35743), we have added those plots and attached the modified figures in this post.

Calibration: s-pol PD to Polarization rotation (V/rtHz → rad/rtHz)

Model: $V_\mathrm{s} = A(1+\cos (4(\theta_\mathrm{HWP}-B)))+C$

Fitting parameters: A[V], B[deg], C[V]

Result:
A = 1.311(9) V
B = 8.1(2) deg
C = 0.22(1) V

Calibration factor:
$\frac{d\phi}{dV_{s}} = -\frac{1}{2A\sin(4(\theta_\mathrm{HWP}-B))}\times\frac{3.79(3)\,\mathrm{V} + 5(1)\times10^{-3} \mathrm{V}}{V_\mathrm{IR}+ 5(1)\times10^{-3} \mathrm{V}}$
$=-0.52(2) \,\mathrm{rad/V}\quad (\mathrm{at}\, \theta_\mathrm{HWP} = 20.0(5)\,\mathrm{deg}, V_\mathrm{IR}=3.79(3)\,\mathrm{V})$

Calibration: s-pol PD to Polarization rotation (cnt/rtHz → rad/rtHz)

Model: $C_\mathrm{s} = A(1+\cos (4(\theta_\mathrm{HWP}-B)))+C$

Fitting parameters: A[cnt], B[deg], C[cnt]

Result:
A = 2.16(7)e3 cnt
B = 8.4(5) deg
C = 3(2)e2 cnt

Calibration factor:
$\frac{d\phi}{dC_{s}} = -\frac{1}{2A\sin(4(\theta_\mathrm{HWP}-B))}\times\frac{3.17(2)\times10^3 \mathrm{cnt} + 0.3(2)\,\mathrm{cnt}}{C_\mathrm{IR}+ 0.3(2)\,\mathrm{cnt}}$
$=-3.2(2) \times 10^{-4}\,\mathrm{rad/cnt}\quad (\mathrm{at}\, \theta_\mathrm{HWP} = 20.0(5)\,\mathrm{deg}, C_\mathrm{IR}=3.17(2)\times10^3 \mathrm{cnt})$​​​​​​​