Sandia National
Laboratories researcher Charles Reinke studies a tiny phononic/photonic
filter nestled
among test equipment on a green, stamp-sized substrate toward the
picture’s
bottom-right. (Photo by Randy Montoya)
(January 12, 2016) A
unique filtering technology that combines light and sound waves on a single
chip is expected to better detect radar and communications frequencies.
“We have developed a powerful signal filtering technology
that could revolutionize signal processing systems that rely solely on
conventional electronics,” said Patrick Chu, manager of applied photonic
microsystems for Sandia National Laboratories.
The radio frequency (RF) filters, which promise both high
bandwidth and wide functional flexibility, would form the basis for
spectrometers that would let users “see” energies placed in various frequency
bands across a wide spectral range.
The novel, very thin filter structures are in the laboratory
stage. A system demonstration — complete with lasers, modulators, detectors and
battery — should be a bit larger than a computer hard drive, weigh only a few
pounds and become available within three to five years.
Photon-to-phonon
conversion
The filter uses a relatively new concept called
photon/phonon coupling. This technique lets the hybrid device temporarily
change RF signals propagating as photons (light) into phonons (sound), enabling
efficient analog manipulation of those slower-moving signals.
In the upper
image, two green silicon optical waveguides are shown embedded
in a gray photonic
crystal membrane. In the bottom image, the violet and blue curves
represent optical
input and output signals; the yellow curves represent transduced
phonon waves.
(Image courtesy of Sandia National Laboratories)
With this hybrid approach, also known as nano-optomechanical
coupling, the researchers were able to combine the high bandwidth offered by
light — demonstrated at frequencies up to 20 gigahertz and easily extended to
100 gigahertz — with the linearity and sharp resonances provided by phononic
filters. The energy cost of this photon-to-phonon conversion is offset by the
high-resolution filter responses that exhibit very little signal distortion
over a wide frequency range, says Charles Reinke, who leads the Sandia effort.
Like a tin can
telephone
A simple analogy for the photon-phonon information transfer
is the tin can telephone: two cans connected by a string that transmits sound
between a speaker and listener. The speaker’s cup is like the emitter
waveguide; it converts audible sound to vibration in the string. The cup by the
ear is the receiver waveguide, which converts the vibration back into sound.
The string, representing an engineered material called a phononic crystal, not
only carries the message but changes its tone by filtering out high-pitch
sounds, a kind of signal processing.