# Optical Technology for NFV Converged Networks
## 4 Tunable Filters in Silicon Photonics for Full Tunable Transceivers
Figure 5 shows two options for the fiber-PIC coupling: the input fiber in Figure 5a is coupled to the chip via edge coupler, whereas the output fibers in Figure 5b are coupled via dual polarization grating couplers. The choice of the type of couplers in this architecture is determined by the bandwidth of the input and output spectrum; this is approximately 40 nm for the input coupler, therefore wide-band element, such as an edge coupler, may be preferred. On the other hand, the output couplers in this configuration operate just on one half of the BW (TX or RX) and have more relaxed constraints on the passband, that can be easily ensured by a DPGC. It must be noted that state of the art DPGC based on SiN-on-Si can presently reach a 1 dB bandwidth of at least 29 nm, with insertion losses of around 2.4 dB and 0.2 dB polarization dependent loss (PDL) [24]. The BW can be further extended over 40 nm by exploiting a design based on SiN-on-Si platform and back mirrors as shown in [25], where a value of 70 nm was achieved for a single polarization grating coupler. Therefore, the functional group comprising Edge Coupler (EC), Polarization Splitter (PS), and Polarization Rotator (PR) in Figure 5a can be substituted with a wide bandwidth DPGC (WB-DPGC) as shown in Figure 6.  
Another design option consists in replacing the band splitter elements with solutions that operate with two interleaved sub bands. This requires an appropriate spectrum management: it is possible, e.g., to associate all odd channels to the TX and the even  
Figure 6: Tunable transceiver configuration with interleavers, wideband DPGC and (**a**) external TX and RX or (**b**) integrated TX and RX. WB-DPGC: wide band dual polarization grating coupler. channels to the RX, using a filter with the appropriate FSR (equal to twice the channel spacing). This architecture solution is shown in Figure 6a. This kind of interleaved solutions requires both the input and the output couplers to be wideband. However, band splitters based on this scheme may consume very low power since they need minimal temperature stabilization to keep the interleaver comb filter in place. Finally, Figure 6b shows an architectural solution where TX and RX are integrated in the transceiver. In this architecture, laser source, modulators, and photodiode are integrated in the photonic chip and electronic divers and amplifiers can be connected to these elements through wire bonding or integrated on top of the PIC e.g., in a 3D integration scheme [55]. In the following, it is described a design implementation of an architecture of the type shown in Figure 6b, which does not include the integrated SOA. The SOA can be easily integrated according to the integration process described in [56; 57], enabling III-V integration at the backside of standard silicon photonics wafers. A detailed implementation is given in Figure 7. The filtering function at the receiver input is operated by a set of 4 coupled micro-ring resonators, whose parameters have been optimized to achieve a flat central BW of 37 GHz at \(-\)1 dB and a rejection BW of more than 25 dB. The resonator has FSR of 25 \(\times\) 0.8 nm = 20 nm, corresponding to 25 WDM channels with 0.8 nm (100 GHz) spacing. Coupling of the Photonic Integrated Circuit (PIC) with fibers is performed using surface grating couplers:  
1. The external modulated tunable laser is fed to the chip via a Polarization Maintaining (PM) fiber, coupled with a Single Polarization Grating Coupler (SPGC) to the integrated TX ring. 2. The Single Mode (SM) fiber duplex, comprising the TX output and the RX input to the tunable filter is coupled to the chip via a DPGC. 3. The output of the RX tunable filter is coupled out from the chip to a Single Mode fiber (SMF) pigtailed with a DPGC, to reach an external avalanche photo detector (APD). 4. The dual polarization grating couplers separate the two orthogonal polarization components (named TE and TM in Figure 7) of an input light into two TE polarization components propagated by two separate waveguides.