Wavelength-order-dependent phase-noise degradation from short-wavelength loss in cascaded WDM interferometric FBG arrays.

Point-by-point femtosecond-written fiber Bragg gratings (FBGs) exhibit Mie-scattering-induced short-wavelength loss. Its array-level noise consequences in cascaded wavelength-division-multiplexed (WDM) interferometric arrays remain unclear. We show that this device-level loss causes wavelength-order-dependent phase-noise degradation. Reversing the long- and short-wavelength FBG order gives an 18.5 dB short-wavelength-channel noise-floor difference at 1 kHz. The penalty increases as return power decreases and as more long-wavelength FBGs are placed upstream, identifying wavelength order as a key design parameter.