Figure 1: General schematics of a MWP multi-tap filter.
From: Integrable microwave filter based on a photonic crystal delay line
(a) Traditional approach based on a single optical source in combination with multiple non-dispersive short fibre coils. The basic delay T is implemented by means of a dispersionless fibre coil of length cT/n, where c stands for the speed of light in vacuum and n is the refractive index of the core. The RF-modulated optical signal is divided by a 1×N coupler into N taps or samples. The weight of each sample can be changed by means of an individual attenuator or amplifier. For instance, the k-th tap is weighted in power by the attenuator/amplifier providing a value of ak and it is delayed by (k−1)T by means of a fibre coil of length (k−1)cT/n. The signal samples are combined by means of a N×1 coupler and fed to the photodetector. The boxes connected with green arrows show the time and frequency domain structure of the signals at selected points of the layout. (b) A more compact approach based on a multi-wavelength optical source together with a single dispersive element. The optical sources emit at different wavelengths so the spectral separation of a pair of consecutive modes implementing consecutive taps is Δλ, and their basic delay is given by T = DLΔλ where L represents the length of the dispersive element and D its dispersion parameter. Here, the intensity or weight ak of the k-th tap can be adjusted by changing the optical power emitted by the optical sources. The boxes connected with black arrows show the time and frequency domain structure of the signals at selected points of the layout.
