Feature: Communications
crosstalk suppression (> 25dB) of the beam arriving from a different direction (1.25°) is achieved because the photonic processor can control both the amplitude and the phase of the field coupled by each grating coupler of the 2D optical antenna array. Such a low crosstalk level enables us to use the integrated photonic processor as a direction-diversity receiver in an FSO communication system, where the optical beams are used to transmit two independent data channels. To this end, the two beams TX1 and TX2 were used as carrier wavelengths for the transmission of independently-modulated 10Gbit/s on-off keying (OOK) signals. Te results of the transmission
Figure 4: Mode separation after controllable mode-mixing
a. Representation of two free-space modes (Mode 3 and Mode 4) that arrive at the receiver after a mode conversion performed by a 45°-rotated phase mask;
b. Backward far-field intensity pattern radiated by the 2D optical antenna array when the photonic processor is configured to couple Mode 3 to WG1 (b1) and Mode 4 to WG2 (b2). Circles indicate the position of the zero-order diffraction;
c. Measured eye diagrams of two intensity-modulated 10Gbit/s OOK signals transmitted simultaneously, using Mode 3 and Mode 4 for the configurations considered in (b);
d. BER measurements of 10Gbit/s OOK channels transmitted simultaneously in the free space on spatially-overlapped modes (Mode 3 and Mode 4) and separated by the photonic processor. Blue and red squares indicate the reference BER of individually transmitted channels
an angle of about 1.25°. Notably, optical crosstalk suppression (TX1 in WG2 and TX2 in WG1) of over 25dB is measured and reported; see Figure 2b. If we swap the mode sorting status, meaning coupling TX2 to WG1 and TX1 to WG2, the same level of optical isolation is observed. To better understand the behaviour of
the photonic processor in the tuned state, we reversed the direction of the light propagation by injecting the light into WG1 and WG2 ports and measuring the far-field pattern radiated by the 2D optical antenna array when the photonic processor is configured in the case of Figure 2b. For clarity, we restrict the view to one period
of the radiation pattern, which is zoomed in around the zero-order diffraction (main lobe). Panels 2c1 and 2c2 refer to the far-field
radiated by the 2D optical antenna array when the light is injected from WG1 (transmission back to TX1) and from WG2 (transmission back to TX2), respectively. It should be noted that in each case the position of the main lobe (highlighted by the dashed circle and indicating the direction of maximum radiation) coincides with the position of the null in the other case, which is consistent with the high rejection between the two beams shown in Figure 2b. Notably, the high optical
22 July/August 2023
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experiment are shown in Figure 2d. When the photonic processor is not set up in any particular way (panel d1), the MZIs’ thermal tuners are at arbitrary working points, the two data channels are randomly overlapped at output port WG1 and the measured eye diagram is completely closed. A similar result is observed at the WG2 port. In contrast, open eye diagrams are
recorded when the photonic processor is tuned to simultaneously extract the signal TX1 at output port WG1 (d2) and the signal TX2 at output port WG2 (d3). Both eye diagrams then show no degradation with respect to the reference eye diagram of the individual channels.
Mode-diversity receiver In principle, the coupling, separation and sorting of free-space beams can be achieved by the programmable photonic processor on any set of orthogonal beams. As a second example, we consider
two free-space beams sharing the same wavelength (1550nm) and state of polarisation (TE), and coming from the same direction but shaped according to different orthogonal spatial modes. In this case, the programmable photonic processor receiving the two beams operates as a multibeam mode-diversity receiver. Figure 3a shows this setup, where two free-space beams, Mode 1 and Mode 2, simultaneously shine on the 2D optical antenna array; the photonic processor is configured to separate them at output ports
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