# Fully Reconfigurable Photonic Filter for Flexible Payloads
## 4 Discussion  
By tuning either one RR or both, the bandwidth or the central frequency can be tailored, respectively. When applying an electrical power ranging from 0 to 4.60 \(\upmu\)W to the second RR, the bandwidth expands from about 500 MHz to 520 MHz, while maintaining the stopband rejection value (=45 dB). This broadening of the bandwidth is correlated to a red shift of the resonance peak of the second RR. As the two resonators cascade, this shift results in an overall enlargement of the filter envelope. Another consequence is the slight shift of the central frequency, which varies by approximately 37 MHz (as shown in Figure 3). Providing a higher current to the heater enables a broader bandwidth but also leads to an increase in in-band ripple, which compromises the desired filter flatness (as illustrated in the inset of Figure 3). Figure 3: Photonic filter spectra vs. power (P) fed to second RR. The inset represents a zoom on the stopband region. If a shift in the central frequency is desired, both RRs need to be tuned. This configuration allows for adjustment of the central frequency of the notch filter without influencing the bandwidth. The Free Spectral Range (FSR) of both RRs is sufficiently wide to avoid resonance peak overlap when the feeding electrical power ranges from 0 to 11.50 mW. Therefore, this approach guarantees a central frequency shift spanning up to 180 MHz (as depicted in Figure 4). The maximum achievable shift is limited by the chosen tuning mechanism as higher power values can lead to more significant shifts but also higher losses. These losses lead to an increase in BW and a decrease in the SBR. During the device operation, power values may deviate from their nominal ones. However, this does not strongly affect the overall performance. Indeed, as regards the bandwidth reconfigurability, the electrical power rounding up and down to the nearest integer yields a 5-10 MHz oscillation of the nominal bandwidth value. In the case of central wavelength tuning, the rounding up or down of the electric power poses no issue as the central frequency remains within the tuning range. Considering manufacturing tolerances of \(\pm\)10 nm, the coupling coefficient could change within the range of \(\pm\)1.4 \(\times\) 10\({}^{-3}\), consequently impacting both stopband rejection (34-72 dB) and the bandwidth (472-534 MHz) of the individual resonator. The proposed filter successfully meets most of the requirements outlined in Table 1, including in-band ripple (<0.5 dB), stopband rejection (45 dB), and power consumption (ranging from 0 to 11.50 mW). It accomplishes the target performance while maintaining a simple and compact architecture that aligns with the constraints of the _NewSpace_ Economy. A further improvement of the FWHM could be achieved through the use of alternative technological platforms, such as silicon nitride (SiN) [39], which shows losses 2 orders of magnitude lower than the SOI one. However, the main bottlenecks regard the thermo-optic efficiency of SiN, which results in one order of magnitude lower than the Si one, with a resulting smaller tuning range and/or larger power consumption. The limited reconfigurability is associated with the chosen tuning mechanism, the thermo-optic effect, since higher power values can lead to more significant shifts but also higher losses, which strongly worsens the bandwidth and the stopband rejection. Concurrently, efforts are underway to explore techniques that can enhance the tuning range and reduce reconfiguration time, like the electro-optic effect [34]. However, a fast reconfiguration time and low power consumption is achievable at the expense of a limited tunability range and a complex fabrication. One potential improvement lies in the use of nonvolatile materials. This innovative tuning mechanism seems to be promising in the development of switches, filters, lenses, absorbers, sensors, and more [40; 41].