We have fabricated a variety of chiral fiber sensors by twisting one or more standard or custom optical fibers with
noncircular or nonconcentric core as they pass though a miniature oven. The resulting structures are as stable as the
glass material and can be produced with helical pitch ranging from microns to hundreds of microns. The polarization
selectivity of the chiral gratings is determined by the geometry of the fiber cross section. Single helix structures are
polarization insensitive, while double helix gratings interact only with a single optical polarization component. Both
single and double helix gratings may function as a fiber long period grating, coupling core and cladding modes or as
a diffraction grating scattering light from the fiber core out of the fiber. The resulting dips in the transmission
spectrum are sensitive to fiber elongation, twist and temperature, and (in the case of the long period gratings) to the
refractive index of the surrounding medium. The suitability of chiral gratings for sensing temperature, elongation,
twist and liquid levels will be discussed. Gratings made of radiation sensitive glass can be used to measure the
cumulative radiation dose, while gratings made of radiation-hardened glass are suitable for stable sensing of the
environment in nuclear power plants. Excellent temperature stability up to 900°C is found in pure silica chiral
diffraction grating sensors.
Long period fiber gratings couple core and co-propagating cladding modes to produce dips in the transmission
spectrum and have been widely utilized as sensors and filters. We have recently developed a new approach to long
period fiber gratings utilizing optical fibers, which are uniformly twisted at elevated temperatures to produce
double or single helices. Because these fibers are not manufactured by exposing photosensitive glass to patterned
UV illumination, as is the case for traditional fiber Bragg gratings (FBGs) or long period gratings (LPGs), they are
more robust in harsh thermal and chemical environments. Double helix fibers are polarization sensitive and are
fabricated by twisting fiber preforms with high-index noncircular cores while single helix gratings are polarization
insensitive and are created by twisting standard optical fibers with cores that are not perfectly centered. Here, we
present a new approach to single-helix chiral long-period gratings (CLPGs). The CLPG is created in a glassforming
process in which two optical fibers are twisted together to form a helix in the signal fiber as the fibers pass
through a miniature oven. "Dual-twist" CLPGs may be fabricated from any conventional or specialty fiber and
provide reproducible spectra that may be tailored to specific applications.
We demonstrate two types of in-fiber linear polarizers based on PANDA PM fibers with < 0.7 dB insertion loss, > 30 dB
extinction ratio, and > 40 dB return loss. The first is a chiral fiber, realized by twisting an etched and tapered PANDA
fiber. A form birefringence of 10<sup>-2</sup> is achieved in the noncircular fiber produced by differentially etching the pure silica
cladding and the doped silica stress members. The chiral grating scatters one polarization component out of the fiber
over a broad band while the orthogonal polarization component propagates freely. The second polarizer is produced in
an untwisted tapered PANDA fiber. Polarization selectivity is the result of opposite modal interferometric oscillation of
two orthogonal polarization components. Both types of in-fiber polarizer are all-glass, scattering, and non-absorbing
devices without back-reflection and are therefore suitable for high-power applications.
Chiral fiber gratings are produced in a microforming process in which optical fibers with noncircular or nonconcentric
cores are twisted as they pass though a miniature oven. Periodic glass structures as stable as the glass material itself are
produced with helical pitch that ranges from under a micron to hundreds of microns. The geometry of the fiber cross
section determines the symmetry of the resulting structure which in turn determines its polarization selectivity. Single
helix structures are polarization insensitive while double helix gratings interact only with a single optical polarization.
Both single and double helix gratings may act as a fiber long period grating, coupling the core and cladding modes. The
coupling is manifested in a series of narrow dips in the transmission spectrum. The dip position is sensitive to fiber
elongation, twist and temperature, and to the refractive index of the surrounding medium. The suitability of chiral
gratings for sensing pressure, temperature and liquid levels is investigated. Polarization insensitive single helix silica
glass gratings display excellent stability up to temperatures of 600°C, while a pressure sensor with dynamic range of
nearly 40 dB is demonstrated in polarization selective double helix gratings.
We describe the development of fiber chiral gratings and discuss salient similarities and differences from planar chiral structures. Planar chiral structures include cholesteric liquid crystals and structured thin films produced by oblique deposition of dielectric materials on a rotating substrate. These are composed of uniform anisotropic planes with 180 degrees rotation symmetry which rotate uniformly with displacement perpendicular to the planes so that the pitch is equal to twice the period. The sinusoidal modulation of the structure which possesses double-helix symmetry results in a single band gap for co-handed light with the same sense of circular polarization as the handedness of the helical structure. Orthogonally polarized light is freely transmitted. Within the band gap the wavelength in the medium equals the structure pitch. Double-helix symmetry may also be implemented into a fiber geometry by twisting glass optical fiber with noncircular core cross section as it passes through a miniature oven. In addition to the polarization-selective resonant band observed in planar chiral gratings, we observe two additional modes of optical interaction when the pitch exceeds the wavelength in the fiber. In chiral long period gratings, dips in transmission are observed at wavelengths associated with coupling of the core mode and distinct cladding modes mediated by the chiral grating. In chiral intermediate period gratings, a broad scattering band is observed due to scattering out of the fiber into a continuum of states. Gratings with uniform pitch as well as with a specially designed pitch profile can be utilized to produce a variety of polarization selective devices. In addition to describing optical chiral gratings, we describe studies of microwave planar and fiber gratings, which played a key role in the development of optical fiber chiral gratings.
We find that the evolution of an optical polarization along a twisted optical fiber may be spatially synchronized with polarization-selective light scattering. We demonstrate experimentally that linearly polarized light initially oriented along the fast axis of an adiabatically twisted single-mode, polarization-maintaining optical fiber is converted into elliptically and then into circularly polarized light with the same handedness as the chiral structure. As the state of polarization is changing along the length of the fiber, the light is scattered out of the fiber core. By choosing an appropriate twist acceleration profile in custom-made, rectangular-core polarization maintaining fibers, scattering and conversion may be synchronized, allowing the orthogonal polarization to freely propagate through the fiber. When the portion of the fiber with accelerated twist is combined with another fiber segment with decelerated twist, the fiber becomes a broadband, low-insertion-loss, in-fiber, linear polarizer. In this polarizer, the passing component of the incident light oriented along the slow axis, is converted to circularly polarized light of the opposite handedness, and then converted back to linearly polarized light oriented along the same slow axis. While the polarization evolution may be calculated using 1D model of light propagation through a birefringent medium, the calculation of light scattering requires a full 3D calculation.
We have produced chiral fiber Bragg gratings with double-helix symmetry and measured the polarization and wavelength selective transmission properties of these structures. These gratings interact only with circularly polarized light with the same handedness as the grating twist and freely transmit light of the orthogonal polarization. The optical characteristics of chiral fibers are compared to those of planar cholesteric structures. The resonant standing wave at the band edge or at a defect state within the band gap, as well as the evanescent wave within the band gap is comprised of two counterpropagating components of equal amplitude. The electric field vector of such a circularly polarized standing wave does not rotate in time; rather it is linearly polarized in any given plane. The standing wave may be described in terms of the sense of circular polarization of the two counterpropagating components. The wavelength dependence of the angle q between the linearly polarized electromagnetic field and the extraordinary axis, which is constant throughout a long structure, is obtained in a simple calculation. The results are in good agreement with scattering matrix calculations. Resonant chiral gratings are demonstrated for microwave radiation whereas chiral gratings with pitch exceeding the wavelength are demonstrated at optical wavelengths in single-mode glass fibers. The different functionalities of these fibers are discussed.
Optically induced second harmonic generation in glasses with large PbO content was studied. Tensor and phase properties of encoded (chi) <SUP>(2</SUP>) susceptibility indicate the presence of electrostatic field, which is proportional to E(EE) where E is a sum of real recording waves E<SUB>(omega</SUB> ) and E<SUB>2(omega</SUB> ). The model describing charge separation as a result of (chi) <SUP>(3</SUP>) optical rectification is discussed.