# Optical Technology for NFV Converged Networks
## 4 Tunable Filters in Silicon Photonics for Full Tunable Transceivers
In this design, the input from the modulated external tunable laser is sent to the SM duplex fiber port via a ring filter, through the drop port ('TX Ring') that is tuned to the optical wavelength of the laser. The TX Ring has a FSR of 6 \(\times\) 0.8 nm; this low FSR design is possible thanks to a wavelength management strategy that is implemented according to the wavelength plan described in Figure 8. The wavelength plan comprises two groups  
Figure 7: Scheme of a design based on a 4 rings resonator for the RX filter and a simple ring resonator for the routing of wavelengths from the TX. The DPGC on the left is a duplex fiber port: UL wavelengths are fed to the fiber and DL wavelengths are fed to the PIC. The DPGC on the right couples the output of the filter to the APD. of channels for the Uplink (UL) and the Downlink (DL): a first group of 20 T channels, from \(\lambda_{1}\) to \(\lambda_{20}\), are spaced by 0.8 nm on the lower C band spectrum, and a second group of twenty Rx channels, from \(\lambda_{21}\) to \(\lambda_{40}\), are spaced by 0.8 nm in the upper C band spectrum. A 0.8 nm guard band, consisting in one unused channel, separates the highest TX channel \(\lambda_{20}\) from the lowest Rx channel \(\lambda_{21}\). The wavelength plan assumes that Tx and Rx wavelengths from a given Optical Network Unit (ONU), e.g., ONUi, are paired as \(\lambda_{1}\)/\(\lambda_{1}\)+20, respectively, for the TX and RX channels (i.e., \(\lambda_{5}\) TX paired with \(\lambda_{25}\) RX, \(\lambda_{7}\) Tx paired with \(\lambda_{27}\) RX). As shown on Figure 8 for the case of ONU\({}_{5}\), when the TX filter is tuned to \(\lambda_{5}\) for inserting the external laser, there will not be any impact on the light received at the RX, having wavelength \(\lambda_{25}\). With such a small FSR, a large micro-ring radius of 20 \(\upmu\)m can be used. The coupling coefficients of the rings are set to minimize the laser add loss to 0.5 dB, while the through loss seen by the RX signal on its way to the tunable receiver is kept below 0.3 dB (see Figure 5a). When using a ring filter for adding the laser light, one should minimize the nonlinearities of the ring: this is achieved by using a low Q ring (2000) which minimizes the power within the ring. The RX filter is more complicated as it requires to have a 20 nm FSR. A single ring cannot be used due to the flatness requirements, BW and extinction specifications; the design is based on 4 coupled rings. For the realization of both TX and RX filters, one needs to consider the dispersions that characterize the fabrication process. The Silicon on Insulator (SOI) thickness variation (\(\pm\)10% of the SOI thickness) accounts the most in the ring performances variations. This adds on to the width variation of the waveguides. Figures 9 and 10 show the range of performance variation calculated via Monte Carlo (MC) simulations for each filter, assuming a dispersion in the coupling coefficients of \(\pm\)10%. These assumptions were verified in a first fabrication run with a set of rings with different coupling coefficients. Figure 8: Scheme of the spectrum management for the design solution of Figure 7 with a TX Ring filter having FSR of 4.8 nm and RX filter having an FSR of 20 nm. Figure 9: Spectral response of the TX drop and through filter ports according to MC simulations. The estimated footprint for the photonic chip comprising filters, couplers and additional monitor photodiodes is 2.5 \(\times\) 3.7 mm, where the RX (APD and amplifiers) and TX (modulators and tunable laser) are external.