Unscrambling light-automatically undoing strong mixing between modes

Abstract

Propagation of light beams through scattering or multimode systems may lead to the randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here we show that we can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters. Transparent light detectors integrated in a photonic chip are used to directly monitor the evolution of each mode along the mesh, allowing sequential tuning and adaptive individual feedback control of each beam splitter. The entire mesh self-configures automatically through a progressive tuning algorithm and resets itself after significantly perturbing the mixing, without turning off the beams. We demonstrate information recovery by the simultaneous unscrambling, sorting and tracking of four mixed modes, with residual cross-talk of -20 dB between the beams. Circuit partitioning assisted by transparent detectors enables scalability to meshes with a higher port count and to a higher number of modes without a proportionate increase in the control complexity. The principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications.

Keywords: optical processing; photonic integrated circuits; silicon photonics; tuneable photonic devices.

Figure 1

Self-configuring mode unscrambler integrated in a silicon photonic chip. (a) Schematic concept of an N × N (N=4) triangular mesh of tuneable beam splitters implementing any arbitrary transformation on N-dimensional input vectors. Transparent detectors at the output port of each beam splitter monitor the evolution of the optical field E_m,k along the entire mesh enabling local control operation on each beam splitter individually. (b) Guided-wave implementation of the mesh through a lattice of two-port cascaded MZIs realizing the tuneable beam splitters controlled through a pair of integrated phase shifters. (c) Silicon photonic four-mode unscrambler consisting of six thermally actuated MZIs individually monitored by transparent CLIPP detectors. Mode scrambling is induced on chip through a multimode waveguide section (mode mixer). Self-configuration and stabilization of the circuit is performed through a CMOS ASIC (d) bridged to the silicon chip, which is connected to an FPGA controller.

Figure 2

On-chip unscrambling of optical modes. (a) Mixed modes are reconstructed at the output port of the 4 × 4 silicon photonic mesh by sequentially tuning the MZI beam splitters. To reconstruct the first mode (m=1, Mode D) at port Out₁, the first row of the mesh (M₁) is configured by progressively nulling the light intensity at the lower output arms of MZI S₁₁, S₁₂ and S₁₃, where a CLIPP detector is integrated (b). (c) Normalized power of Mode D measured by CLIPP1 integrated after S₁₁. Depending on the initial MZI biasing (Sⁱ₁₁), convergence to different equivalent solutions S^f₁₁ (local minima of the map) may be achieved.

Figure 3

Experimental setup used for the demonstration of all-optical unscrambling of mixed MDM channels.

Figure 4

(a) Mesh configuration makes the transmission of the mode reconstructed at port Out₁ progressively increase a₄, while the crosstalk due to the concurrent modes A a₁, B a₂ and C a₃ reduces. (b) Reconstruction of mode A b₁, mode B b₂, mode C b₃, and mode D b₄ at port Out₁ can be achieved with less than −20 dB residual crosstalk of the three concurrent modes over a bandwidth of ~ 10 nm.

Figure 5

On-chip mode sorting. The mesh transmission matrix H_mesh can be configured in order to sort the reconstructed modes {A,B,C,D} arbitrarily at the output ports {Out₁, Out₂, Out₃, Out₄} of the mesh according to any 4 × 4 permutation power transmission matrix |H_meshH|². Given the mode scrambling introduced by the mode mixer H, spreading the power of the input modes almost equally in the input waveguides of the mesh (a), panels (b–f) show the normalized light power at the output ports of the mesh, when it is configured to extract the modes in the follow order: (b) A, B, C, D; (c) D, C, B, A; (d) D, C, B, A; (e) C, A, D, B; (f) C, B, A, D.

Figure 6

On-chip unscrambling of MDM optical channels. (a) Information encoded in four scrambled 10 Gbit s⁻¹ intensity modulated MDM channels is recovered after mode reconstruction performed by the silicon photonic mesh. (b) As a consequence of mode mixing, the spectrum of the four mixed channels (black curves) exhibits deep time-varying oscillations, which disappear after mode reconstruction (mode A, blue curves). Displayed curves refer to 10 successive measurements taken at output port Out_1. The corresponding time domain signals are shown in the eye diagrams in the insets. Eye diagram (c) and BER (d) measurements (port Out₁) demonstrate that information encoded in each channel can be retrieved with a very small power penalty independent of the number of mixed modes.

Figure 7

Reconstruction of modes scrambled by time-varying mixing. (a) A light source (980 nm) is used to perturb the mode mixer integrated in the silicon chip in order to modify the relative amplitude and phase of the mixed modes. (b) After configuring the mesh to reconstruct channel A at port Out₁ (reference state, b₁), the 980-nm source is switched on to modify the mode mixing, thus impairing mode reconstruction at the mesh output (perturbed state, b₂). In the track mode b₃, the mesh adaptively self-configures by controlling each MZI through a local feedback loop, in order to automatically compensate against time-varying mixing of the modes.