FIBER SENSING
Chiral-fiber gratings
sense the environment

VICTOR I. KOPP, VICTOR M. CHURIKOV,

AND AZRIEL Z. GENACK

FIGURE 1. Side and face images of the twisted ber show single- (top) and double-helix (bottom) gratings. (Courtesy of Chiral Photonics)

Transmission through optical bers is sensitive
to a wide range of factors including stretching,
compression, axial twist, and temperature of
the ber, as well as the refrac-
tive index of the surrounding
medium. is sensitivity can be exploited
to create ber-optic sensors that imprint in-
formation about these factors upon the light
passing through the ber. Precise periodic
modulation of ber structures produces
sharp spectral features in the re ection or
transmission spectrum of the ber. Changes
in the spectra can therefore describe varia-
tions in strain, axial twist, liquid level, pres-
sure, and temperature.

Chiral ber gratings, in which a periodic structure is created via a helical variation along the ber axis of birefringence perpendicular to the axis, are used in a variety of applications. Such bers di er from traditional ber Bragg gratings (FBGs), in which the periodic structure is created via a variation in the index of refraction along the ber axis.

Traditionally, periodic structures are fabricated by
exposing the ber to a periodic pattern of ultraviolet
light that produces a modulation of the index of refrac-
tion along the length
of a photosensitive -
ber. ¹ When the peri-
od of modulation of
the ber structure is
as short as half the op-
tical wavelength, the
light within the core is
evanescent within the
structure over a band of
wavelengths and is thus
strongly re ected. e
central vacuum wave-
length of the re ec-
tion band in such FBGs
is equal to the prod-

uct of the structure and the e ective refractive index of the core mode. If the period greatly exceeds the optical wavelength, core and cladding modes can still be coupled via the grating to produce several narrow dips in the transmission spectrum. In this case, in addition to sensitivity to temperature and elongation, the spectral position of the dip is sensitive to the refractive index of the surrounding medium, which a ects the propagation constant of the cladding modes.

Twisted fibers create
chiral gratings that can
facilitate the deployment of
fiber sensors for sensing
liquid level, temperature,
elongation, axial twist,
and pressure in harsh
environments.

VICTOR I. KOPP is the director of research and development, VICTOR M. CHURIKOV is a senior scientist, and AZRIEL Z. GENACK is the chief technology of cer at Chiral Photonics, 26 Chapin Road, Unit 1104, Pine Brook, NJ 07058; GENACK is also Distinguished Professor of Physics at Queens College of the City University of New York, Flushing, NY 11367; e-mail: vickopps@chiralphotonics.com; www.chiralphotonics.com.