# Towards Large-Scale Fast Reprogramable SOA-Based Photonic Integrated Switch Circuits
## 5 On-Chip Distributed Power Monitoring  
The recent demonstrations of multi-path, multi-wavelength packet scheduling highlight the potential for highly reconfigurable networking with relatively simple control planes, but this places stringent demands on the channel monitoring. So far signal monitoring is being performed at the edge of the network, and is not well adapted for an optical packet paradigm. Distributed optical performance monitoring (OPM) is the key tool for future optical networking. In particular, the optical signal-to-noise ratio (OSNR) monitoring can provide information on the intra-chip signal transmission quality. Intra-chip OSNR measurement has been demonstrated through both discrete [37] and integrated [38; 39; 40] solutions, but has never been integrated within a switching circuit. However, the most advanced circuits do already contain the essential components. A combination of SOA gate and AWG technology can provide on-chip integration of active elements and filtering functionality for a more straightforward route to signal and noise detection. We have recently proposed and demonstrated an OSNR meter co-integrated with the 8 \(\times\) 8 cross-connect, presented in Section 4, using switch gates as photo-detectors (PDs), to enable the analysis of signal quality in an integrated packet routing engine [36]. An electronic pre-calibration scheme is proposed which can be performed before chips are selected for packaging and is done to measure the filtered integrated optical power after the AWGs, which is then used to calculate a noise power spectral density correction factor to be used for the intra-chip OSNR calculation. For the in-service operation, the pre-amplifier is used to compensate fiber-chip coupling of the in-coming signal, and the wavelength-select gates are used again in detector mode to separately measure signal and noise power. Figure 5a shows the integrated currents measured at all the PDs. The signal level increases exponentially with input power. At low input powers of \(-\)30 dBm power, the measured on-chip signal power approaches the integrated in-band noise. This is expected to be dominated by the space-select ASE. The OSNR is calculated as the ratio between the total integrated current received at the PD6, which is corrected for in-band noise and the appropriately scaled total out-of-band noise contribution. This is plotted as a function of the input power in Figure 5b. The red curve shows the OSNR versus in-fiber  
Figure 4: Time traces (a) rise and fall time (b) for dynamic multi-port WDM routing at output 0.  
optical input powers: An OSNR dynamic range extending from 6 to 40 dB/0.1 nm is demonstrated. The calculated noise figure of the last SOA gate is also plotted as the difference between the calculated on-chip OSNR and the measured off-chip OSNR. The possibility of providing intra-chip measurement of signal OSNR opens up the opportunity to provide real-time equalization per channel in WDM optical switches: on-chip OSNR and power monitoring per channel increase network awareness at the physical layer and allow for power channel equalization. Concepts of WDM space switches followed by co-integrated channel separation for per-channel monitoring and equalization can become feasible at the packet level. The schematic of the foreseen concept for an aware space switch is displayed in Figure 6a. Figure 5: (**a**) Integrated photocurrent measurements (**b**) On-chip OSNR calculation (red curve) and off-chip OSNR measurement. The noise figure of the SOA gate is also plotted (black curve). Figure 6: (**a**) Concept of a co-integrated per-channel monitoring and per channel-equalization block for broadband photonic switches and (**b**) tentative photonic integrated circuit design. A tentative design for an aware WDM 4 \(\times\) 4 space switch is shown in Figure 6b: the light blue box indicates the 4 \(\times\) 4 space switch and the four pairs of AWGs are for each of the multiplexed channels.