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The VIPA is not quite a G-T etalon. It's close, but the tilt and the
AR coated input window in the 100% reflective side make it a very
different beast. The VIPA produces angular dispersion. The G-T etalon
is a phase filter and produces no angular dispersion.

You could try a Fourier approximation, but I think you would run into
trouble because the harmonics have progressively smaller FSRs and
correspondingly longer etalons. This composite structure adjusts the
center frequency setting of the etalons and the reflectivity of the
non-100% mirror. These correspond to phase and amplitude from a Fourier
point of view. Frequency is not changed, so it's not quite a Fourier

The period of the composite compensator would be something like 50 GHz
or 100 GHz depending on your trade off decisions. So, around each ITU
point, there will be a "dispersion passband". In this "dispersion
passband" (+/- 10 GHz around all ITU channels, for example), the
composite dispersion compensates your fiber dispersion. Outside of this
dispersion passband, all bets are off. You can create a slope
compensator by slightly modifiying the free spectral range of the
etalons (varying them from 95 GHz through 105 GHz for a 100 GHz
version, for example). This would correct slope over the band of

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