A more complex design, this time in Silicon, has been also recently presented in [69] and [70] where 1–2 GHz-bandwidth filters with very high extinction ratios ( dB) have been demonstrated. The silicon waveguides employed to construct these filters have propagation losses of dB/cm and inser- tion losses (excluding fiber to waveguide coupling) where in the
Fig. 16. Tunable incoherent MWP filter based on multiple phase shifters imple- mented by periodic resonances of an integrated SOI RR. (Upper left) Filter con- figuration. (Upper right) Amplitude and phase response of the SOI RR. (Lower left) Spectral locations of the four optical carriers and subcarriers to achieve, re- spectively, 0 and 90 phase shifts. (Lower right) Filter transfer functions cor- responding to the two cases (0 and 90 phase shifts) showing tunability. After [72].
range of 2–3.5 dB. Each ring of a filter is thermally controlled by metal heaters situated on the top of the ring. With a power dissi- pation of mW, the ring resonance can be tuned by one FSR, resulting in wavelength-tunable optical filters. Both the second- order and fifth-order RRs have been demonstrated, which can find ready application in RF/microwave signal processing.
Work on integrated MWP incoherent filters has been reported by various groups [25], [26], [72]–[78] as well. Recent efforts have focused towards the implementation of complex-valued sample filters by means of exploiting several techniques to integrate MWP phase shifters. For instance, two-tap tunable notch filter configurations where phase shifting was achieved by means of coherent population oscillations in SOA devices followed by optical filtering were reported in [25] and [26]. In another approach [72], the periodic spectrum of an integrated SOI RR is employed as a multicarrier tunable and independent phase shifter. The configuration for a four-tap filter is shown in Fig. 16.
Here, the basic differential delay between samples is imple- mented in the first stage, while an independent phase coefficient for each tap is selected in the RR by finely tuning the wave- length of each carrier. A similar configuration based on a hybrid InP-SOI tunable phase shifter has also been recently reported [73]. In this case, tuning is achieved not by changing the source wavelength but by carrier injection into the III–V microdisk. A more versatile configuration which can provide both phase and optical delay line (ODL) tuning has been recently reported based on the Si N TripleX technology [74]–[76]. It consists of a reconfigurable ODL with an SCT [75] unit and an optical sideband filter on a single CMOS compatible photonic chip. The processing functionalities are carried out with optical RRs as building blocks demonstrating reconfigurable MWP filter oper- ation in a bandwidth over 1 GHz.
Most of the incoherent MWP filters reported so far require a dispersive delay line which is usually implemented by either a dispersive fiber link or an LCFBG which being bulky devices,