progressively work along row 1, from MZ4
to MZ1, while nulling powers in detectors
3 to 1.
The second key concept to our design is
that a second orthogonal input beam will
be entirely transmitted through detectors
1 to 3, where it can be aligned similarly
with row 2 to the second output, and so
on, thus separating multiple overlapping
orthogonal beams without loss. If we use
a second device, running it backward, we
can convert these single modes to any output modes, and interspersed modulators
can set the coupling strengths to give a
completely arbitrary spatial linear optical
device. Such schemes are well suited to
silicon photonics, for example. Extensions
include spatial add-drop multiplexers
and fully arbitrary linear optical devices,
including polarizations and different
frequencies, at least in theory. 4, 5
Hence, any physically legal linear
optical device can be designed without
Though we have many useful linear optical
components, like lenses,
mirrors and gratings, as well
as opportunities with micro-and nano-fabrication for new
devices, we have not known
how to design “legal” optics.
Is it even possible to separate
arbitrary overlapping modes,
such as the multi-fiber modes
of current communications interest,
without loss? Classic approaches to
complex optical functions, like planar
holograms with imaging or Fourier
transform optics, cannot generate such
arbitrary linear optics. 1
We recently reported that a mode
converter capable of separating arbitrary
overlapping modes could produce any
physically legal linear optical component. 2 We then demonstrated how
to make an arbitrary mode converter,
proving any such optical component is
possible in principle. 3, 4 Our design method
is completely progressive, without
complex global optimization. Also, the
design doesn’t require calculations—it
self-configures with simple local
feedback. 3, 4
The core idea is a new device concept;
a self-aligning beam coupler that couples
an arbitrary input beam into a single
mode guide. 3 We can theoretically divide
the beam into enough patches, each
approximately uniform in intensity and
phase. 1 Then, we couple power from the
different patches to the single output with
a set of controllable phase shifters and
reflectors (as shown in the video), or more
practically, with a set of Mach-Zehnder
(MZ) waveguide interferometers. We
Schematic of a self-aligning mode coupler and splitter.
David A.B. Miller
Stanford, Calif., U.S.A.
1. D. A. B. Miller. J. Opt.
Soc. Am. A 30, 238
2. D.A.B. Miller. Opt. Express 20, 23985 (2012).
3. D.A.B. Miller. Opt. Express 21, 6360 (2013).
4. D. A. B. Miller. Photon.
Res. 1, 1 (2013).
5. D.A.B. Miller. Opt. Express 21, 20220 (2013).
Visit www.opnmagazine-digital.com to view
the video that accompanies this article.
Input light beam Row 1 Row 2 Row 3