Feature: Communications


As a final example we show that the


photonic processor can separate beams that have experienced a generalised mode-conversion process (mode mixing), in which the input modes are mapped to output modes belonging to a different mode set and not in any mode family. To prove this concept, let’s consider the case of Figure 5a, where the 0-π phase mask responsible for the mode mixing is rotated at an arbitrary angle. Presuming that the phase mask introduces no losses, it produces an arbitrary mixing of the incoming modes, resulting in two beams (A and B) that are still orthogonal but no longer resemble any of the modes of the HG family (although still describable as a linear combination of HG-like modes, with possibly more than two terms). The shape of these beams is not known if the axis of the phase mask is unknown. If we want to know the shape of the


two beams A and B, we can reverse the direction of propagation, injecting the light at ports WG1 and WG2 and looking at the far-field radiated by the 2D optical antenna array with NIR camera. Tese beams’ field profiles are shown in Figures 5b1 and 5b2 and, as expected, they exhibit an arbitrary shape that doesn’t match any of the tabulated optical free-space modes. Nevertheless, they are still orthogonal and can be separated with extremely low mutual crosstalk, as confirmed by the BER measurements shown in Figure 5c.


Chip design and fabrication Te processor was fabricated on a standard 220nm system-in-package platform by the AMF foundry. All the circuit’s waveguides are single-mode 500nm channels. Te grating couplers are designed to


operate on transverse-electric polarised light. Te emission angle with respect to the chip surface is 12°, whilst the radiation diagram has an angular beamwidth of 5° × 9°. Te nine waveguides connecting the grating couplers to the photonic processor share the same optical length, in order to minimise the wavelength dependence of the multipath interferometer implemented by the mesh so the circuit can have the widest possible wavelength range of operation.


The photonic processor can separate beams that have experienced a generalised mode- conversion process (mode mixing) where the input modes are mapped to output modes belonging to a different mode set


Each MZI has two 3dB directional


couplers, implemented by two 40μm- long waveguides, spaced by 300nm. Two thermal tuners made of TiN metal strips (2μm × 80μm) are integrated in each MZI stage, one in the lower input waveguide, the other in the waveguide of the upper internal interferometer arm. Tese enable the control of the relative phase shiſt between the optical fields at the input ports of the MZI and the amplitude split ratio of the MZI, respectively. A chip-mounted turning mirror steers


the beam emitted vertically by the 2D array of grating couplers to the horizontal direction to facilitate coupling with the free-space optical setup employed in the experiments presented here. Te photonic processor self-configures


and self-stabilises by exploiting dithering signals for the thermal tuners and thermal- crosstalk mitigation strategies. Te control circuitry is a custom-electronics board, implementing two parallel electrical control chains for calibrating the two MZI rows of the processor. Each MZI is independently controlled, with a local feedback loop that minimises the optical power at the integrated monitor detectors; see Figure 1e. Te integrated detectors are read using


a custom ASIC, wire-bonded directly to the PIC to reduce the measurements’ noise figure. Dithering signals are used to identify the magnitude of deviation from the


optimum bias point of the MZI tuneable couplers. To this end, different pairs of orthogonal frequencies, ranging from 6kHz to 21kHz, are employed for each MZI. Te optical power measured by the


detector is demodulated using the dithering frequencies to understand the tuning status of the individual MZIs, and the DC currents fed to the thermal actuators are modified to minimise the evaluated error. To distinguish the two optical beams


coupled from free space and co- propagating inside the waveguides of the photonic processor, each FSO beam is labelled with a suitable tone, superimposed as a shallow amplitude modulation at a specific frequency that can be identified by the integrated detectors. For the pairs of FSO beams in the experiments, we used 500Hz and 900Hz, respectively. Te reconfiguration time of each MZI from random initial status is about 5ms, with sub-millisecond tracking time.


Photonic processor applications Te optical bandwidth of the photonic processor, spanning the extended telecommunication C band (1530- 1570nm), can be used as a receiver in high- data-rate systems as well as in wavelength- division multiplexing communications links. Te photonic processor can self- configure through simple automated control actions, without requiring global multi-variable optimisation techniques, giving its architecture scaleability for a larger number of optical antennas or more rows, to handle more orthogonal beams simultaneously, without complex control. Besides the massive increase of data capacity


offered by multi-beam SDM transmission, the adaptive nature of the photonic processor also allows the possibility of compensating for dynamic changes in the FSO link, caused by, say, moving obstacles or atmospheric turbulence, establishing and maintaining an optimal communications link in real time. Finally, many applications can use


advanced processing of FSO beams, including wave-front sensing, phase-front mapping and reconstruction, multiple- beam transmission and imaging through scattering media, as welll as chip-to-chip optical wireless communications.


www.electronicsworld.co.uk July/August 2023 25


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