Structured light
Can a twisted fibre act as a one-way valve for twisted light?
Open access · cc by · source: Europe PMC
Inside a twisted glass fibre, a backward control beam can strip power only from a forward vortex beam with the matching twist, letting light carrying orbital angular momentum pass one way but not the other.
Study at a glance
- Design
- Other — Optical-fibre experiment: pump, seed and control beams with set spin and topological charge launched into two twisted photonic crystal fibres, with gain and isolation measured and compared to theory.
- N
- No sample size; two fibres (threefold- and sixfold-symmetric chiral PCF, each 200 m) and several vortex modes were tested.
- Population
- Chiral (twisted) silica photonic crystal fibres with threefold and sixfold rotational symmetry
- Outcome
- Brillouin frequency shift, Brillouin gain coefficient, isolation and amplification factors of circularly polarised vortex modes
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Key findings
Scattering was strong only when the backward wave had the opposite topological charge and opposite spin to the pump, as angular momentum conservation predicts; with matched charge and spin there was almost no gain. Measured peak gain coefficients were close to theoretical values. Isolation exceeded 22 dB for two different vortex orders and stayed roughly constant over a 35-dB range of signal power, and shifting the control frequency upward turned the device into a one-way amplifier. Isolation saturated at high control power, which modelling attributed to cascaded second-order Brillouin scattering.
Methodology
The authors drew two twisted photonic crystal fibres, one with threefold and one with sixfold rotational symmetry, each 200 m long. They launched circularly polarised vortex beams and measured the backward Brillouin (sound-wave) scattered light, its spin and topological charge, and the Brillouin gain as a function of frequency. They then built an isolator in which a counter-propagating control beam, shifted down or up by the Brillouin frequency, attenuates or amplifies a signal, and compared the results to coupled-wave equations.
Limitations
The device needs hundreds of metres of fibre and watt-level control beams, so it is far from a compact component; the authors suggest non-silica glasses might shorten it. It only works within the narrow Brillouin gain linewidth unless the control is tuned along with the signal. Imperfect launch optics excited unwanted modes, leaving a small residual gain for matched modes, and the multiplexing test used only two vortex modes as a proof of concept.
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