# Towards Large-Scale Fast Reprogramable SOA-Based Photonic Integrated Switch Circuits
## 3 InP SOA-Based Broadband Switch Matrices
However, due to the moderate scale of integration, the space switch scheme presents high path losses: The fiber-to-fiber signal losses increase as path length and complexity increase, because of the additional traversed components. The measured total passive losses along the thirty-two paths include an inherent 30 dB loss contribution resulting from the broadcast-select capability and an excess 37 dB loss resulting from component-level imperfections, with a maximum loss variation of 12.8 dB. The gain from the SOA gates can compensate much of the excess component losses, while the inherent losses of about 30 dB coming from the broadcast-enabled architecture are  
Figure 1: (a) A composite photograph of the active-passive indium phosphide (InP) 16 \(\times\) 16 switch matrix, (b) Projected performance of chip (a) for a data signal input power of \(-\)2.1 dBm and optical signal to noise ratio (OSNR) of 60 dB/0.1 nm for paths from input 1 to output 1-to-16. present in any given path. The circuit-level losses were analyzed by evaluating the losses in comparable circuits and through direct measurement. The noise performance was predicted by estimating the optical power map for the signal and noise within the switch matrix circuit and including the additional amplified spontaneous emission noise contributed at each SOA gate. Each of the three stages of the SOA gates in the circuit was expected to operate with a mean gain of 12.9 dB and a mean amplified spontaneous emission power density of \(-\)47.3 dBm/0.1 nm. The operating optical input power for the input SOA was \(-\)11.6 dBm and for the central and output SOAs was \(-\)21 dBm, so the 500-\(\upmu\)m-long gates were operated far from the optical saturation power. The electrical bias conditions were 75 mA and 1.5 V. The agreement for the implemented elements is excellent, giving confidence that the loss and noise performance of the circuits can be clearly attributed to the excess losses of the components. The excess losses can, however, be significantly reduced by using optimized components like mode-size adaption at the facets [30], low-parasitic-reflection splitters [31], low-loss waveguide crossing through a broadening at the intersecting waveguides [32], and low waveguide losses of the order of 0.5 dB/cm for minimized p-doping level in the passive waveguides [33]. The projected performance for the 16 \(\times\) 16 switch matrix with optimized component losses are shown in Figure 1b: lossless circuit level operation is enabled and a radical impact on the optical signal to noise ratio is found by being increased up to about 50 dB/0.1 nm. This would correspond to a circuit level noise figure of 13.7 dB, of which 5.3 dB is attributable to losses between the input side fiber and the first SOA gate. This analysis suggests how a further increase in connectivity is possible, but requires careful design of the chain of cascaded SOAs in a multistage optical switch: reduced losses at the input of the first SOA stage can provide improved switch noise figures. Also, the losses per stage must be designed to be balanced by the SOA-gain stage, keeping the SOA far from the saturation point, in a chain of multiple optical amplifiers. The operating current and voltage per path are currently 240 mA and 1.5 V for the three gates. If an on-chip preamplifier is included for on-chip gain, the operating current (power) is expected to increase to 340 mA (0.55 W/path). A few watts of power consumption is then expected for the fully loaded chip. The temperature is kept at 15 \({}^{\circ}\)C by fixing the chip on a temperature-stabilized water-cooled copper mount. When compared with an all-active approach, the active-passive integration scheme enables order-of-magnitude noise performance improvement as well as reduced energy requirement. This analysis opens a route to broadband switches with connectivity higher than sixteen IOs.