Reprogrammable photonics tune optical path on-the-fly
While a photonic chip functionality is usually hard-wired by design (geometry and layered refractive indexes route the incoming light beams), the researchers were able to alter and control the optical properties of a multimode interference power splitter (approximately 40um long) to freely steer a light beam at the 1550±1.7 nm wavelength. Re-routing of the light between the ports was performed dynamically, with more than 97% total efficiency and negligible losses, according to the researchers.
In what they call an “all-optical wavefront shaping” process, the researchers used a digital micromirror device (DMD) as a spatial light modulator to project a pattern of femtosecond ultraviolet laser pulses onto the MMI device surface. Presenting their results in the April issue of the journal Optica under the title “All-optical spatial light modulator for reconfigurable silicon photonic circuits”, the researchers detail how under each illuminated position, plasma dispersion locally decreases the refractive index of silicon (by approximately 0.25 refractive index units).
Modulating the refractive index simultaneously across a large number of positions (around 500) significantly affects the light flow, essentially implementing a dynamically reprogrammable refractive index profile that changes the optical route of the incoming beam (in a static silicon element).
Principal investigator Professor Otto Muskens, from Physics and Astronomy at the University of Southampton admits the current set up is quite bulky due to the external laser source and the use of a digital micromirror.
“It is a proof of concept, but in the future, we could design a more compact system, using a laser diode and an integrated LCD mask instead of a micromirror”, he told eeNews Europe, adding that he was quite open for others to look into new solutions to make these field-programmable photonic circuits a commercial reality.
1×2 multimode interference splitter with a
projected pattern of perturbations induced
by femtosecond laser.
Such integrated platform could someday find their way into all-optical reconfigurable routers, ultrafast optical modulators and switches for optical network, or even in silicon chips. Examples cited in the paper include new gradient designs based on transformation optics, or the use of phase change layers for writing nonvolatile patterns for reconfigurable optical memory devices.
Wavefront shaping could also be used to design optical systems with wavelength dependent responses, achieving spectral control, or specific time domain characteristics. For now, even a bulky setup could be useful for exploratory work, to prototype and test new photonic chips through quick iterative work.
Access the full paper at https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-4-396
Visit the University of Southampton at www.southampton.ac.uk
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