Structured light
Can a chip untangle two overlapping light beams of the same colour?
Open access · cc by · source: Europe PMC
A self-tuning optical chip split two overlapping laser beams of identical wavelength and polarisation into separate outputs with almost no leakage, even after the beams had been scrambled.
Study at a glance
- Design
- Other — Optical bench experiment on a silicon photonic chip (3x3 grating-coupler array feeding two rows of Mach-Zehnder interferometers) receiving pairs of free-space beams at 1550 nm.
- N
- No sample size; one fabricated chip tested with several beam pairs (direction-diverse, mode-diverse, and mode-mixed).
- Population
- A single programmable silicon photonic processor with 9 optical antennas and 15 interferometers
- Outcome
- Crosstalk suppression between separated beams, eye diagrams and bit error rate versus optical signal-to-noise ratio for 10 Gbit/s data channels
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Key findings
Beams arriving 1.25 degrees apart were separated with more than 25 dB crosstalk suppression, and beams differing only in mode shape with more than 30 dB. Data channels showed open eye diagrams and no measurable signal-to-noise penalty in bit error rate compared with each beam sent alone. Mode rejection stayed above 20 dB over a 35 nm wavelength range, and even arbitrarily mixed beams with unfamiliar shapes were separated.
Methodology
The authors built a silicon chip in which 9 tiny antennas sample an incoming light beam and feed a mesh of 15 tunable interferometers arranged in two rows. Each interferometer is tuned by a local feedback loop that simply minimises light at a monitor detector, so the chip configures itself without knowing the beam shapes. They sent in pairs of beams that differed only in arrival direction, only in spatial mode shape (Hermite-Gaussian modes), or that had been mixed by a phase mask, and each beam carried a 10 Gbit/s data signal.
Limitations
Only two beams were separated at once, using one chip with just 9 antennas, so scaling to many channels is argued rather than demonstrated. The end-to-end loss was about 28 dB, largely from geometric coupling into the sparse antenna array, which would matter in a real link. The mixing tested was a static lab phase mask, not real time-varying atmospheric turbulence, and separation only works when the sampled beams remain orthogonal on the antenna array.
How this study connects
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