Temperature change causes the change of cavity length. However, PPKTP has a different refractive index for p and s polarization. So the FSR change for s and p polarization are different. If we want to make both s and p polarization resonant inside the crystal, we need to choose a good temperature or choose a good frequency difference between s and p polarization. As pointed out as the entry about the temperature we should measure, we did the measurement. Besides, we measured the calibration factor before so we can know the s-p frequency difference change according to temperature change.

The measurement was done like this, we firstly check the rotation of HWP so that the transmission of HWP keeps s-pol. At this time(almost all the light is reflected by PBS cube), the angle is 331 degrees. Then put it just in front of OPO housing, rotate HWP to have some p-pol inside OPO. In the end, we make s-pol is higher than p-pol. So we can differentiate them by hight. Then we changed temperature and made measurement.

The measurement result is listed in the last attached figure as a sheet.(detail is in the attached figure 1-17)

Most of the case, the find of frequency different is based on finding the highest and second highest peak. And then use the calibration factor and the slope of ramp signal. However, the find of frequency difference in temperature of 307.4K is performed by fitting the scanning by the addation of two airy functions.

By using these data I plot the birefringence effect in figure 18, the birefringence constant is 397.76 +/- 0.99 MHz/K.