Frequency Comb Pushes Limit of Fiber-Optic Strain Sensing New

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Frequency Comb Pushes Limit
of Fiber-Optic Strain Sensing
B     y integrating an optical frequency
      comb into a fiber-optic strain-sens-
ing system, a team of Italian researchers
                                                Italy) designed a sensor that can
                                                track slow deformations. They
                                                built a fiber-Bragg-grating
has made a sensor that is three orders of       resonator sensing system with a
magnitude more sensitive than exist-            1,560-nm diode laser stabilized
ing instruments (Science Express, doi:          against an optical frequency
10.1126/science.1195818).                       comb—a familiar high-accuracy
    Optical sensors using standard tele-        tool for today’s physicists.
communication fibers and fiber Bragg                The actual sensor consists of
gratings already measure mechanical             two identical single-mode fiber-
strain—relative displacements and defor-        Bragg-grating reflectors con-
mations—with sensitivities between 10 –6        nected to a 130-mm-long silica                     the researchers wrote. The sensitivity
and 10 –9 e Hz –1/2, where e is the frac-       fiber in an isolated cavity. A piezoelectric       improves markedly when the laser is
tional length change of the object being        actuator applied a periodic displacement           phase-locked to the frequency comb.
measured. To get strain sensors with            of 6.1 3 10 –11 e—in other words, a tiny              According to the researchers, the
pico-e sensitivity or better, engineers         stretching increase over the length of the         sensor system’s resolution is now limited
have had to build extremely complex             fiber inside the cavity. Results showed            to thermal noise in the fiber. They hope
gear stretching out over several kilome-        the lower limit of the system’s sensitivity        that their experiments yield new pos-
ters (for gravitational-wave experiments)       was roughly 20 3 10 –12 e Hz –1/2.                 sibilities for using compact, all-optical-
or limit their sensing to high-frequency            “The laser serves as a secondary               fiber sensors in gyroscopes, gravimeters
strain signals.                                 length standard that is directly com-              and the next generation of gravitational-
    Gianluca Gagliardi and colleagues at        pared to the fiber cavity optical path-            wave detectors.
the National Institute of Optics (Naples,       length during strain measurements,”                                     —Patricia Daukantas

New Technology Produces Real-Time 3-D Holograms
R     esearchers at the University
      of Arizona have combined a
photorefractive polymer that can be
                                             A refreshable holo-
                                             graphic medium
                                                                                                                between two indium-tin-oxide-
                                                                                                                coated glass plates, 100 µm
                                                                                                                apart. Transparent ITO layers
                                             made of a photore-
refreshed every two seconds with a           fractive polymer                                                   act as electrodes and apply an
system of cameras, computers and             displays an F-4                                                    external electric field required to
an Ethernet link to provide a holo-          Phantom Jet.                                                       make the photorefractive mate-
graphic version of Skype—transmit-                                                                              rial work. Samples of the materi-
ting and displaying 3-D images over                                                                             als have been used for months
a network in near real time. Lead                                                                               without any sign of degradation
author Pierre-Alexandre Blanche                                                                                 from crystallization and dielec-
and others in Nasser Peyghambar-                                                                                tric breakdown. Sizes of the
ian’s team developed the system                                                                                 samples vary from 4 to 17 in.
using a polymer from Nitto Denko                                                                                    A frequency-doubled Nd:YAG
Technical Corporation (Nature, doi:                                                                             laser emitting at 532 nm writes
                                                           University of Arizona
10.1038/nature09521).                                                                                           each 1-mm-cubic pixel in a
   The technology could be used for 3-D         and depth reproduction on large-scale                   6-ns 200-mJ pulse at a repetition rate
video conferencing as well as medical           3-D static images,” the authors write,                  of 50 Hz. By using a holographic optical
applications prototyping, advertising,          “but has been missing dynamic updating                  element to split the beam, one can write
updatable 3-D maps and entertainment.           capability until now.” The photorefrac-                 multiple pixels in parallel. (See movie at:
   “Holographic stereography has been           tive material that makes this possible is      In the
capable of providing excellent resolution       based on a copolymer that is sandwiched                 Nature paper, the researchers described

6 | OPN Optics & Photonics News                                                                                        

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