until the phase difference is 90 degrees at step 5 5 once the phase difference is 90 degrees then p is at the quad point note that alternatively instead of scanning 416 q in steps 5 1 and 5 2 416 i could be scanned next control circuit 450 tunes the i phase according to steps 5 6 through 5 10 in step 5 6 a dither tone is applied by heater 416 i heater 416 q is scanned and the modulator output at photodiode 440 is recorded applying eq 7 this should result in curve 701 in fig7 in step 5 7 the value of q is determined where curve 701 reaches its maximum value this value of q is the u201c course null value u201d for q in step 5 8 the course null value determined in step 5 7 is set on heater 416 q in step 5 9 a dither tone is applied by heater 416 i heater 416 i is scanned and the modulator output at photodiode 440 is recorded applying eq 7 this should result in curve like curve 801 in fig8 in step 5 10 the value of i is determined where curve 801 crosses zero this value is the null point for i finally control circuit 450 tunes the q phase according to steps 5 11 through 5 13 first in step 5 11 i is set to the i phase.