Representation of the tuning technique based on the measurement of ϑ. A sweep (solid line) is carried out to look for the resonances where ϑ = 0. The tone is fixed at the corresponding frequency (diamond) and the slope evaluated (dashed line). If the optical load increases, ϑ changes allowing the appropriate correction Δfⁱ (black arrow) to be evaluated. The new tone (circle) will be nearer to the actual position of the resonant frequency, , and will be used to update the estimated slope of the ϑ(f) curve, thus making the iterative tunings increasingly more accurate.