In the last calibration calculation, I didn't consider the loop gain. Therefore, the calibration factor must have some error.

Nevertheless, we can use another way to do this calibration without considering the loop gain. 

0. Lock filter cavity.

1. Change slightly the temperature of main laser.

2. Read how much main laser frequency is changed.

3. Check how much length correction is sent to end mirror.

I did these procedures. The frequency change is read from the attached two figures. The correction signal change is in the attached figure three.

And get calibration factor (frequency difference)/(correction signal) = (248.6-235.2) [MHz]/ (5200) [counts] = 2.56 [MHz] / 1000 [counts]

Since 1pm = 1Hz, we can calibrate the factor above as 2.56 [um]/[kcounts].