Linear range df of the delay line technique is about df ~ c/(2D). So, the linear range for the fine signal(delay line length D=30m) is about 5 MHz.
Arm cavity FSR = c/(2L) = 3.7 MHz.
So, I think we need phase shifting to do mode scan for more than 2 FSRs by holding the arm length finely with fine servo.
For the coarse (D=1.5m), the linear range is about 100 MHz, so if we can do mode scan using coarse servo, it is OK.
In any case, I think it is nice to have linear signal with fixed slope even if we don't adjust the phase every time.
That sounds goofy.
With the delay line technique, you can get a linear signal over 50 MHz with no phase shifting. What is with all this I/Q stuff?