Feature: Communications
(Figure 1f), with the ASIC for the read-out of the on-chip detectors and connections to an FPGA-based board for real-time data processing and actuating of the tuners.
Direction-diversity receiver Here we show that the integrated photonic processor can operate as a direction- diversity receiver, i.e., a multi-beam receiver capable of individually detecting beams arriving simultaneously from different directions. Tis concept is demonstrated by considering N = 2 beams, although this functionality can be generalised to N beams arriving from N directions, using a photonic processor with N rows of MZIs. As shown in Figure 2a, two free-space
beams with identical Gaussian shape, wavelength (1550nm) and polarisation status (transverse-electric [TE] polarisation to match the polarisation sensitivity of the grating couplers) are shone from two different directions onto the 2D optical antenna array of the photonic processor. Tis implies that orthogonality is given only by the arrival direction of the beams, which is obtained by different positions of the two transmitters, TX1 and TX2. Te experiment aims to demonstrate that the photonic processor can effectively separate the two beams TX1 and TX2 at the two output ports WG1 and WG2 (or vice versa), with negligible residual crosstalk. Te beam impinging on the 2D optical
antenna array from each source can be individually coupled to the desired output waveguide (WG1 or WG2) of the photonic processor through automatic tuning and stabilisation algorithms applied to each MZI row. Upon tuning the rows of the photonic processor to extract each transmitted beam, identical optical powers from two collimators are measured at ports WG1 and WG2, shown as 0dB normalised insertion loss in Figure 2b. Te end-to- end loss from each collimator to output waveguide WG1 or WG2 is about 28dB, which accounts for on-chip losses, the coupling loss of the grating couplers, the geometric loss of the 2D array, and the loss of the free-space optics. In this experiment, the two beams arrive
overlapped at the inputs of the photonic processor from different directions, with
Figure 3: Mode-diversity receiver
a. Schematic representation of two free-space modes (Mode 1 and Mode 2), sharing the same wavelength and state of polarisation, and arriving at the receiver from the same direction;
b. Bar chart showing the normalised received power of the Mode 1 and Mode 2 at output waveguides WG1 and WG2;
c. Backward far-field intensity pattern radiated by the 2D optical antenna array when the photonic processor is configured to couple Mode 1 to WG1 (c1), Mode 2 to WG2 (c2), Mode 2 to WG1 (c3) and Mode 1 to WG2 (c4) entering from WG1 and WG2. Circles indicate the position of the zero- order diffraction;
d. Measured eye diagrams of two intensity-modulated 10Gbit/s OOK signals transmitted with Mode 1 and Mode 2 for the configurations considered in (c);
e. BER measurements of 10Gbit/s OOK channels simultaneously transmitted in free space on spatially-overlapped modes (Mode 1 and Mode 2) and separated by the photonic processor. Blue and red squares indicate the reference BER measured when only one data channel is switched on. Negligible OSNR penalty is observed for any data channels sorted out by the photonic processor;
f. Wavelength dependence of the mutual mode rejection at the output ports of the photonic processor
www.electronicsworld.co.uk July/August 2023 21
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