David A. B. Miller, a professor of electrical engineering and applied physics at Stanford University (Palo Alto, CA), has determined in a theoretical study that the optical-waveguide-based Mach-Zehnder interferometer (MZI) -- a device commonly used in integrated photonics to modulate light -- can, when properly configured into meshes, realistically perform a wide variety of information-processing functions.1 Such a mesh has two very important characteristics: 1) it can be made of many identical mass-fabricated MZIs, and 2) the MZIs that make up the mesh can be individually imperfect (deviating from the ideal 50:50 split) and thus fabricated using broad process tolerances.
Potential uses include optical processing and computing, optical quantum computing, spatial mode converters, and many linear optical components; essentially, MZI meshes could give integrated optics the kind of versatility normally seen in electronics.
"Recently, optical researchers have begun to understand that these interferometers can be thought of as universal 'building blocks' that could enable us to construct essentially any optical device we could imagine," says Miller.
Previously, this approach would have only been feasible if the Mach-Zehnder interferometers were able to achieve perfect 50:50 performance, and thus modulation from maximum down to zero -- a seemingly unattainable goal.
Very imperfect (85:15) MZIs operate perfectly
In the new approach, the mesh of interferometers can, when properly programmed, compensate for its less-than-perfect parts and deliver overall perfect performance. Such a mesh can, in principle, perform any linear optical operation, much like computers are able to perform any logical application by controlling on/off functions of semiconductors.
"It's this larger scheme that allows us to use reasonable but imperfect versions of these components," says Miller.
The scheme relies on algorithms that allow the mesh to be self-configuring: the light paths within the mesh are directed based on signals received by simple optical power sensors embedded in the system (or even, using a sequential setup process, by sensors external to the mesh). The algorithms then control the MZIs' internal phase shifters to imitate "perfect" performance.
Miller's study included simulations of meshes with double MZIs (DMZIs) having split ratios anywhere in the range 85:15 to 15:85 -- which are very large imperfections -- that acted as if all the mesh components had a perfect 50:50 split.
"By using small amounts of electronics and novel algorithms, we can greatly expand the kinds of optics and applications by making completely custom optical devices that will actually work," says Miller.
Source: OSA (http://www.osa.org)
1. David A. B. Miller, Optica (2015); doi: 10.1364/OPTICA.2.000747