Reprogrammable photonics tune optical path on-the-fly

April 19, 2016 // By Julien Happich
By combining a spatial light modulator with a multimode interference (MMI) device, researchers from the University of Southampton (UK), and the Institut d’Optique in Bordeaux (France) have demonstrated reconfigurable photonic circuits able to change an optical wavefront in free-space optics, on demand.

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).

Concept of wavefront shaping by ultrafast photomodulation spectroscopy. The transmission spectra of TE-polarized 150 fs probe pulses with a central wavelength of 1550nm are monitored through a multimode interference (MMI) device. Simultaneously, a 2D pattern of 400nm pump light is projected onto the device (blue overlay), locally decreasing the refractive index of the silicon MMI material by plasma dispersion. The pump beam is spatially modulated by employing a digital micromirror device (DMD), and the pattern of the DMD is imaged onto the MMI surface by means of a lens and a microscope objective.