Feature: Communications


WG1 and WG2 (or vice versa). Here Mode 1 and Mode 2 refer to the fundamental Hermite-Gaussian mode HG00 and a higher order HG10-like mode, respectively. In the circuit’s configuration, the first


MZI row of the photonic processor is lined up to maximise the coupling from one of the two modes (for instance Mode 1) to the output port WG1, which leads to nulling of the other mode (Mode 2) at this port. Figure 3b shows the relative transmission of the two modes measured at both output ports aſter the configuration of the photonic processor. An intensity ratio of over 30dB between the extracted and rejected modes is observed at both ports. If mode sorting is swapped, that is if Mode 2 is coupled to WG1 and Mode 1 is coupled to WG2, the same level of isolation (over 30dB) is obtained. By reversing the propagation direction


of the light, the far-field profile of the beam generated by the 2D optical antenna array can be observed for the different configurations of the photonic processor. All the possible cases handled by the two- diagonal photonic processor are shown in Figure 3c. For instance, considering the situation where the first row of MZIs is configured in forward propagation to couple Mode 1 to output port WG1; in the reversed direction – when WG1 is used as an input port – the far-field radiated back by the 2D optical antenna array is well shaped as the fundamental HG00 mode (panel c1). If the second MZI row is configured to couple Mode 2 to WG2, the far-field radiated back when WG2 is used as input port is shaped like the HG10-like mode (panel c2). Panels c3 and c4 show the far-field


pattern for the opposite coupling scenario. Notably, in all these cases the photonic processor automatically self-configures by simply minimising the power of the relevant mode at each stage of the MZI rows, without any prior knowledge of the incoming beam shapes. Te performance of a programmable


photonic processor as a multibeam mode-diversity receiver was assessed by means of data-channel transmission. Two intensity-modulated 10Gbit/s OOK data streams were transmitted on the spatial and


The linear


transformation performed by the mode mixer can be inverted by the photonic processor


direction overlapped modes (HG00 and HG10-like) at the same carrier wavelength of 1550nm and same polarisation state. Te eye diagrams of the received signals, aſter the separation performed by the photonic processor, are shown in Figures 3c and 3d. As a quantitative assessment of the


effectiveness of the mode separation performed by the photonic processor, the bit error rate (BER) of the received channels was measured versus the optical signal to noise ratio (OSNR). Te noise power in the OSNR is evaluated across the same bandwidth as the signals’; see Figure 3e. As reference curves, we measured the BER of the two modes (HG00 blue squares, HG10 red squares) when they are individually transmitted through the photonic processor to output port WG1 in the absence of the other mode. Te other curves show the BER measured with both data channels switched on, and the modes are sorted out at the output ports WG1 and WG2 in all four possible configurations. Tanks to the high optical crosstalk rejection between the separated modes, no significant OSNR penalty is observed in all these cases. We also evaluated the wavelength range


across which the photonic processor can guarantee high isolation in the separation of the two modes. To this end, the carrier wavelength of the two modes was swept across a 35nm-wide range, from 153nm to 1570nm. Te width of this range is mainly limited by the wavelength-selective response of the grating couplers of the 2D optical antenna array. In the results shown in Figure 3f, for every wavelength considered, the photonic processor was configured to extract Mode 2 at output port WG1 and Mode 1 at output port


WG2; these are the cases considered in the eye diagrams of panels d3 and d4 for the central wavelength of 1550nm. Te red curve shows that the intensity


rejection of Mode 1 at port WG1 is above 30dB across the entire wavelength range. Te drop of mode rejection versus wavelength is due to the wavelength dependence of the 3dB directional couplers of the photonic processor’s MZIs, which can be reduced by replacing the directional couplers with broadband 3dB multimode interference couplers. Te rejection is somewhat lower for Mode 2 at port WG2, yet higher than 20dB across 35nm.


Mixed-mode receiver Here we apply the mode-diversity receiver to other spatially-overlapped beam pairs that can be disentangled by the programmable photonic processor. In particular we show that the photonic processor can separate two orthogonal free-space beams even aſter they have propagated spatially overlapped through a mode-mixing obstacle or a free-space path perturbation. Mathematically this means that the linear transformation performed by the mode mixer, which maps the original orthogonal modes to another pair of orthogonal modes, can be inverted by the photonic processor. First we consider the mapping of an


input pair of mutually orthogonal modes to an output pair of orthogonal modes belonging to the same mode set. In this case, the process can be simply considered as a mode conversion. As shown in the schematic of Figure 4a, a 0-π phase mask converts the fundamental mode HG00 into a 45°-rotated HG10-like mode (Mode 3), while the higher-order mode HG10-like mode (-45° rotated) is transformed to a 45°-rotated HG11-like mode (Mode 4). Figures 4b-d show the results achieved


when the photonic processor is tuned to extract Mode 3 at output port WG1 and Mode 4 at output port WG2. From the intensity ratio of the extracted modes at each port, we observe mutual isolation of over 30dB. Figures 4b1 and b2 show the NIR camera


acquisition of the far-field patterns radiated back by the photonic processor when the


www.electronicsworld.co.uk July/August 2023 23


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