Phase forward model Clause Samples

Phase forward model. The main feature of the forward model is the expression of the total demodulated phase Φ(t) in to three separate components φω(t), φL(t) and φn(t). The term φω(t) is driven primarily by the 200 MHz frequency difference between the zeroth and first order beams. The term φL(t) is driven by the length and composition of the free space propagation paths of the two beams. The term φn(t) is sensitive to the refractive index of the sample. All three terms contain a contribution from the path length imbalance ΔL of the two beams, which has the effect of introducing both an offset and a slope to the spectrum baseline. A further feature is the derivation of a time-dependent chirp rate to replace of the assumption of constant value. Frequency calibration is performed by determining the index positions, n, of peaks and troughs in the etalon trace and associating a relative wavenumber ν rel to each successive index given by the FSR of the etalon. Fitting the optimum polynomial P of order up to 8 and converting index to time gives a parameterization ν rel(t) = P(t), from which the chirp rate S(t) is extracted by differentiation : S(t) = P'(t). Typical results are shown in Fig. 36.