The operation principle can be explained as follows. By acting on the phase shifters (PSs) placed in the interferometer arms and on the SOA currents, the device is initially biased so that, in absence of any pump signal, the probe data would experience destructive interference at OUT 1. In normal operation, however, the data probe and data pump signals are always simultaneously applied to the SOA-MZI. In this way, when the gate pumps are switched off, a phase shift in the upper arm of the interferometer is produced by the marks in the data pump as effect of carrier depletion in SOA 1. The power level of the data pump signal can then be chosen such that the induced phase shift in SOA 1 cases constructive interference to occur for the data probe at OUT 1. Thus, in absence of gating signals, input data are normally presented at OUT 1, where they are retrieved at the device output by means of an optical filter centered at
λdata (pass-through data). When the gate signal turns high, gate probe, gate pump 1, and gate pump 2 signals enter the device from their respective inputs. In particular, the power levels of gate pump 1 and gate pump 2 are such to restore the original gain/phase balance between the two interferometer arms set by the initial bias condition. The weaker delayed gate pump 2 plays an important role for 40 Gb/s applications, since it cancels out the slow part of the phase transient induced by the onset/release of gate pump 1 in SOA 2, as explained in Ref. [
15]. In this way, sharp-edged selective cancellation of the data signal at OUT 1 is obtained, which is required for operation with high data rates. It should be noted that the gate pump 2 signal is thus typically not required for operation at 10 Gb/s [
31,
32]. The gate pump 2 signal could be also avoided for higher data rates if faster SOAs, optimized for high-speed operation were employed in the SOA-MZI [
18]. Because of the device symmetry, the initial bias settings of the device are such that the gate probe signal entering from IN 2 would also experience total destructive interference at OUT 2 if no pump signal were applied. This condition is broken by the gate pump 1 and the gate pump 2 signals that make gate probe light at
λgate to appear at OUT 2. However, with a proper choice of the data pump power level, the initial condition of destructive interference at OUT 2 for the gate probe light entering from IN 2 can be restored again by the marks in the data pump signal. This results in transferring the input data pattern onto the gate signal at OUT 2 with inverted logic. An optical band-pass filter, tuned at
λgate at OUT 2 can thus be used to select this inverted replica of the gated data (the wavelength-shifted signal). Clearly, the wavelength-shifted signal is presented at OUT 2 only when the original signal data are suppressed at OUT 1.