Complex light from a photonic chip

By Thomas Farrell on April 28, 2017

In a recent paper in JPhysB, published as part of our Emerging Leaders special issue, Andrea Crespi and Francesca Bragheri investigate a new method to generate complex light beams from integrated-optics circuits. We asked them to explain their work for us.

The use of light beams to perform tasks more complex than just illumination strongly relies on modulation capabilities. Time modulation, even in its simplest form of blinking a light source on and off, has allowed the transmitting of binary information, from the old times of manually actuated signal lamps, to modern electronically-modulated sources for optical communications. Indeed, a new and fascinating world is unveiled by also introducing spatial modulation. Space multiplexing is used to increase the bandwidth in optical communications, and light beams with non-trivial wavefronts can be employed to manipulate microparticles or living cells through optical forces.

The common choice to generate these elaborate beams are static phase and intensity masks, as well as dynamic spatial light modulators, based on arrays of liquid crystal cells or micromirrors. However, these devices are bulky and difficult to interface with miniaturized photonic circuits or fiber networks, thus posing problems in developing portable and compact systems.

Examples of the far-field distributions that can be obtained from the output of a three-dimensional waveguide array composed of ten waveguides, placed on the vertices of a regular decagon with radius 25 μm. Each waveguide is assumed to support a single Gaussian mode at at 633 nm wavelength. All waveguides here carry the same light intensity but phase delays are arranged cyclically in different ways, thus resulting in different values for the orbital angular momentum l of the overall field. The graphs represent normalized intensity (top pictures) and the phase of the electromagnetic field (lower pictures) numerically simulated at a distance z = 15 mm from the output of the array. Each picture represents an area of 1 mm × 1 mm. From Andrea Crespi and Francesca Bragheri 2017 J. Phys. B: At. Mol. Opt. Phys. 50 014002 © IOP Publishing, All Rights Reserved.

In our recent paper, we propose and theoretically explore a new method to produce complex free-space optical beams directly from the end facet of an integrated optical circuit. In particular, we investigate the potentials of engineered three-dimensional waveguide arrays to generate tailored intensity and phase patterns in the far-field, which may include phase singularity points or may carry orbital angular momentum. In such devices, input light would be spread across a two-dimensional pattern of optical modes, with controlled phase and intensity ratios, so that at the output they collectively emit an engineered light beam in the free-space.

Our proposed devices should in fact be at reach of current technology. Three-dimensional waveguide arrays can be realized in glass chips by direct inscription with femtosecond laser pulses: a powerful microfabrication technique that is employed in our laboratories with strong expertise (The labs, headed by Dr. Roberto Osellame, are research facilities shared between Politecnico di Milano and the Institute for Photonics and Nanotechnologies (IFN-CNR)).

Our approach may be applied in the future to free-space optical communications, including quantum-secured cryptography protocols, or to enhance the manipulation capabilities of living cells in integrated analysis devices.

About the authors:

Andrea Crespi received his PhD in Physics at Politecnico di Milano in 2012. From 2012 to 2015 he worked as research fellow at IFN-CNR (Institute for Photonics and Nanotechnologies of the National Research Council). Since 2016, he is Junior Researcher at the Physics Department of Politecnico di Milano. His research interests mainly regard femtosecond laser microfabrication of three-dimensional optical circuits in glass, for applications in quantum information processing, simulation of quantum phenomena, and biosensing in integrated lab-on-a-chips.

Francesca Bragheri received her degree and PhD in Electronic Engineering at University of Pavia in 2003 and 2007 respectively. From 2007 to 2011 she worked as a postdoc at the Quantum Electronics Lab, University of Pavia. Since October 2012 she is staff researcher at IFN-CNR in Milan. Her scientific activity is mainly related to the development of new biophotonic devices for optical manipulation and imaging of cells and to their fabrication by femtosecond laser micromachining.

Read More:

Projecting light beams with 3D waveguide arrays
Special issue: emerging leaders – The article that this blog post is about belongs to the special issue: emerging leaders, which features invited work from the best early-career researchers working within the scope of JPhysB. This project is part of the Journal of Physics series’ 50th anniversary celebrations in 2017. Andrea Crespi was selected by the Editorial Board of JPhysB as an Emerging Leader.

This work is licensed under a Creative Commons Attribution 3.0 Unported License