Feature: Communications


used to coherently reconstruct the power of the second beam at the output port WG2. Generalising the concept to a photonic processor with M inputs and N rows of MZIs (with M > N), we can conclude that such a device can couple, reconstruct and separate N orthogonal free-space beams (or, N + 1, if N = M-1) that are spatially sampled by M optical antennas, and transmit them to N single-mode output waveguides, with arbitrary sorting order and no mutual optical crosstalk. Tis is the basic concept of the multibeam FSO receiver developed in this work.


Figure 2: Direction-diversity receiver


a. Representation of two free-space beams (TX1 and TX2) that share the same wavelength and state of polarisation, and arrive at the receiver from different directions;


b. Bar chart showing the normalised insertion loss of the beams TX1 and TX2 (relative angle 1.25°) at the 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 beam TX1,2 to WG1,2, launching the light from WG1 (c1) and WG2 (c2) ports;


d. Measured eye diagrams of two received intensity-modulated 10Gbit/s on-off keying (OOK) signals transmitted by using the two beams TX1 and TX2: (d1) when the photonic processor is not configured, the eye diagrams of TX1 and TX2 at port WG1 are severely overlapped; while after the configuration of the photonic processor both eyes, TX1 at port WG1 (d2) and TX2 at port WG2 (d3) are clearly open with neither evident distortion nor inter-symbol interference


to such orthogonal vectors, this processor can separate them essentially losslessly and automatically. We used this ability to implement a


multi-beam receiver for FSO beams. A light beam shone on the chip is sampled by the 2D array of M optical antennas and is coupled to the single-mode waveguides at the input of the programmable photonic processor. Te light field in these M waveguides can be coherently summed by configuring the first row of MZIs in such a way that the light in the first beam is entirely extracted out of WG1 with no residual power transmitted to the other output ports. By simultaneously


shining a second free-space optical beam orthogonal to the first one onto the 2D antenna array, the two can share the same wavelength and polarisation, a technique known as “mode orthogonality”. When the second beam is shone onto


the photonic chip, its field is spatially sampled by the 2D antenna array, and co-propagates with the first beam in the same M single-mode waveguides, making the two apparently indistinguishable inside each waveguide. However, because of the orthogonality, no portion of the second beam is transmitted to output port WG1. Te second row of MZI stages can be


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


Multi-beam transmitter If the propagation of the light is reversed, the photonic processor can operate as a multi-beam transmitter, allowing us to map the light intensity carried by N single- mode waveguides into N orthogonal free-space beams. By tuning the integrated MZIs, we can control both the amplitude and the phase of the light radiated by each element of the 2D optical antenna array, to modify the shape and direction of the far-field beam. Note that these approaches change amplitudes by re-routing the light, not by absorbing or attenuating the beam, so there is no fundamental loss when relative amplitudes are adjusted. Figure 1b shows the entire chip, 5.8mm × 1.3mm large. As shown in Figure 1c, the 2D optical antenna array is made of M = 9 identical grating couplers, all aligned in the same direction and arranged in a 3×3 square. Te centre-to-centre spacing between the grating couplers of the 2D optical antenna array is 49μm (corresponding to about 32λ), which leads to the presence of several diffraction orders (grating lobes) in the far-field radiation pattern, with a minimum angular spacing of about 1.7°. As an example, Figure 1d shows the


collimated far-field intensity profile measured with a near-IR camera for a uniformly-excited array (i.e., when all elements radiate a light beam with the same intensity and phase). All 15 MZIs in the mesh (eight in the first row, seven in the second) are identical and controlled with thermal tuners; see Figure 1e. Te photonic chip was mounted on a PCB


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60