# Fully Reconfigurable Photonic Filter for Flexible Payloads
## 2 Reconfigurable Photonic Filters  
Reconfigurable photonic filters emerge as key components for next-generation transceivers as they possess the ability to adapt to the dynamic space environment by adjusting both the central frequency and bandwidth to meet new constraints. They excel in harsh conditions, including extreme temperatures, radiation exposure, and mechanical stress, while maintaining consistent and reliable performance levels. These devices can swiftly change between various filtering configurations or dynamically track alterations in received signals. This capability facilitates the efficient utilization of available bandwidth and enhances communication reliability, particularly in situations where signal requirements continuously fluctuate. It is important to note that in such scenarios, reconfigurable photonic filters may sacrifice some performance compared to traditional ASPIC filters [9]. Through the adaptive suppression of unwanted interference, reconfigurable photonic filters enhance both signal reception and transmission, enabling precise and targeted communication with Earth stations or other space assets. Reconfigurable photonic filters can be categorized into two groups: coherent filters and incoherent filters [10; 11]. In coherent filters, an optical source in the form of a narrow-linewidth laser is used. Its output is appropriately modulated to produce either a single sideband or two sidebands. The light beam at the modulator's output then undergoes filtration through a photonic filter, which can take the form of a fiber Bragg grating, a planar ring resonator, or an integrated ring-assisted Mach-Zehnder interferometer [12]. Conversely, incoherent filters rely on either an array of photonic delay lines or a dispersive element that introduces a wavelength-dependent delay (multi-tap) to generate the desired spectral response [13]. A novel filtering approach is founded upon programmable photonic filters, offering compactness and flexibility at the cost of somewhat reduced performance. Specifically, a waveguide mesh comprising seven hexagonal cells fabricated in SOI has been proposed in [14] for the implementation of over 100 distinct circuits. Typically, reconfigurable filters are designed by defining a grid resolution. However, when the grid resolution is too coarse, as in liquid-crystal-on-silicon filters, the resolution of the central frequency tuning is about tens of GHz. Conversely, an ultra-high-resolution \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance** & **Value** \\ \hline Tunable central frequency (f\({}_{0}\)) & 4–40 GHz \\ Tunable bandwidth (BW) & 30–300 MHz \\ Insertion loss (IL) & \textless{}5 dB \\ Bandpass ripple & \textless{}0.5 dB \\ Stopband attenuation & \textgreater{}30 dB \\ Power consumption & \textless{}1 W \\ Reconfiguration time & \textless{}1 us \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photonic BPF requirements for next-generation payloads [8]. grid (tens of MHz), e.g., by using Stimulated Brillouin Scattering-based filters, requires a large number of elements with several design challenges [15]. The presented solution has been designed to overcome the main limitations of the competing technologies, such as fiber Bragg gratings, Mach-Zehnder interferometers (MZI), Ring Resonators (RRs) with MZI architecture [16], liquid crystal-on-silicon filters, and Stimulated Brillouin Scattering-based filters [15; 17], mainly in terms of reconfigurability range, bandwidth (BW), and stopband rejection (SBR). During the last few years, great research effort has been spent on the design of fully reconfigurable filters with limited bandwidth and a large stopband rejection. Solutions similar to the one reported here have been proposed in the literature. In 2022, [PERSON] et al. [18] proposed a photonic bandpass filter based on four cascaded silicon microring resonators, characterized by a coupling region with a complex engineered shape.