In this paper, the two different mechanisms of supercontinuum generation in single crystal sapphire fibers according to
fiber lengths longer and shorter than dispersion length are theoretically and experimentally investigated. When the fiber
length is shorter than the dispersion length, self-phase modulation is the dominant factor for supercontinuum broadening.
A broad spectrum ranging from near-IR (1.2 μm) to the lower end of mid-IR (2.8 μm) is obtained. But, when the fiber
length is longer than dispersion length, soliton-related dynamics with self-phase modulation is the dominant factor for
supercontinuum. We further demonstrate that supercontinuum in a sapphire fiber can extend beyond the range of silica
fibers by showing the spectrum from 2 μm to 3.2 μm. Also, we successfully apply the supercontinuum source generated
from a sapphire fiber to IR spectroscopy. The spectra of pseudo-TNT chemical measured using our own supercontinuum
source is in good agreement with those obtained by FTIR. Supercontinuum generation using a sapphire fiber, which has
high damage threshold and broad transmission ranges can be used in many applications such as IR spectroscopy,
broadband LADAR, remote sensing, and multi-spectrum free space communications.
In this paper, we report laboratory test results of an LPG that can maintain a constant resonant peak depth over an
enhanced tuning range when it is coated with an ITO electrode that has optimized thickness and refractive index.
Without the ITO layer, LPG tuning ranges as large as 50 nm have been achieved when the ambient index is
increased from 1.00 (air) to ~1.444 (index of the silica cladding), but the peak depth cannot be maintained. When a
properly designed, high-index ITO overlay is coated onto the silica cladding, mode transition effects coincide with
the LPG's intrinsic sensitivity to changes in the ambient index, resulting in a stable peak depth over an enhanced
tuning range. The authors have experimentally demonstrated an LPG coated with ITO that can be tuned in excess of
150 nm with an ambient refractive index change of less than 0.01. To the best of the authors' knowledge, this is the
highest sensitivity reported for an LPG to date. In addition to the tuning performance, the resonant peak remains
within 1 dB of its maximum depth for at least 100 nm of the tuning range, which allows the tunable LPG to be used
in real applications.
Novel supercontinuum generation by launching ultra-short femtosecond laser pulses into single crystal sapphire fibers is
demonstrated. Supercontinuum generation using sapphire fiber exhibits many advantages that include high transparency
up to 5 micron, low material dispersion in the 0.8 micron to 5 micron spectral range, and an extremely high laser damage
threshold (500 times higher than that of silica). Thus, supercontinuum spectrum with high power, super broadband, and
spatial coherence can be realized by pumping single crystal sapphire fibers. By experimental comparison, we prove that
sapphire fiber can provide a broader supercontinuum spectrum than that of bulk sapphire counterpart under the same
exciting conditions. Since supercontinuum generation in single crystal sapphire fibers can radiate high power
supercontinuum in the middle-IR regime, it will have a great impact on many applications, including sensing and
broadband multi-spectrum free space communications.
In this paper, we report the fabrication of higher-order-mode rejected fiber Bragg gratings (FBGs) in sapphire crystal fiber using infrared (IR) femtosecond laser illumination. The grating is tested in high temperature furnace up to 1600 degree Celsius. As sapphire fiber is only available as highly multimode fiber, a scheme to filter out higher order modes in favor for the fundamental mode is theoretically evaluated and experimentally demonstrated. The approach is to use an ultra thin sapphire crystal fiber (60 micron in diameter) to decrease the number of modes. The small diameter fiber also enables bending the fiber to certain radius which is carefully chosen to provide low loss for the fundamental mode LP01 and high loss for the other high-order modes. After bending, less-than-2-nm resonant peak bandwidth is achieved. The grating spectrum is improved, and higher resolution sensing measurement can be achieved. This mode filtering method is very easy to implement. Furthermore, the sapphire fiber is sealed with hi-purity alumina ceramic cement inside a flexible high temperature titanium tube, and the highly flexible titanium tube offers a robust packaging to sapphire fiber.
Our high temperature sapphire grating sensor is very promising in extremely high temperature sensing application.
A unique all-fiber tunable filter is based on the combination of a single resonant band long period grating (LPG) and an electro-optic polymer second cladding layer. The single resonant band LPG is fabricated by etching the cladding of a 125 μm thick fiber and using ultraviolet (UV) illumination to write the grating. Once a single resonant band has been achieved, an ITO electrode is sputtered onto the thin silica cladding and then a polymer second cladding layer is applied. The refractive index of the polymer determines the resonant wavelength of the filter. After a second electrode is coated onto the second cladding, the polymer index is tuned by applying an external electric field. Recent modeling and experimentation has shown that a high index ITO inner electrode can increase the tuning range of the filter up to 10 times by inducing cladding mode transitions.
In this work, an investigation of the tuning characteristics of electrically tunable long-period gratings (LPGs) is
presented. A precise four-layer model is used to quantitatively analyze the tuning potential of the gratings and
experimental data is provided to support the analysis. The four-layer model includes a silica core layer with an inscribed
LPG, a thin silica cladding layer (~40 μm), an ultra-thin (~ 50 nm) high refractive index indium-tin dioxide (ITO) inner
electrode layer, and a tunable electro-optic polymer layer. It has been found that the inner electrode layer, made of high
refractive index ITO, can be modeled as a high index overlay and causes the forward propagating modes in the thin silica
cladding to reorganize as the ambient refractive index changes. This reorganization effect can lead to a significant
increase (10 plus fold) in the tuning range of LPG tunable filters. Moreover, the required specifications of the tunable
polymer layer are quantitatively analyzed. Finally, the required characteristics of the electro-optic polymer are realized
by using a nano-composite of zinc sulfide and ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer.
We demonstrate an widely electrically tunable long-period fiber grating with ultrathin first cladding and ferroelectric relaxor poly(vinylidene fluoride - trifluoroethylene - chlorofluoroethylene) terpolymer as the second cladding. Large Kerr effect is found in the terpolymer where a refractive index change of -2.6% can be induced under an electric field of 80MV/m. Simulations and experiments show that electrodes and the index matching between terpolymer and fiber have significant effect on the tuning range. An 18nm resonant wavelength shift is achieved by the terpolymer when electric field of 50MV/m is applied. On the other hand, over 100nm shift is observed by index matched terpolymer/PMMA blend as the temperature changes from 25oC to 100oC (temperature tuning). To realize this index matching condition, ZnS/terpolymer nanocomposite was developed which allowed the index of the composite to be varied over a large range while maintaining large electro-optical response. A simulation result predicts that large electrical tuning of the resonance band can be realized by the the index matched nanocomposite.
A unique all-fiber tunable filter is based on the combination of a single resonant band long period grating (LPG) and an electro-optic polymer second cladding layer. The single resonant band LPG is fabricated by etching the cladding of a standard 125 μm thick fiber and using either ultraviolet (UV) illumination or electric arc discharge to write the grating. Once a single resonant band has been achieved, a polymer second cladding layer is applied to the LPG. The refractive index of the polymer cladding determines the resonant wavelength of the filter and is tuned by applying an external electric field. The grating fabrication method and type of polymer used for the second cladding affect filter performance, and both must be considered when designing an application specific all-fiber filter.
In this paper, a brief review on ultrasensitive fiber optic sensors and their applications, done recently at Penn State, is presented. Our discussions will mainly focus on two types of highly sensitive fiber optic sensors. One type is based on the combination of single resonant band long period gratings (LPGs) with the second refractive index matched polymer cladding layer. The other one is based on the LPGs fabricated in photonic nanostructured fibers and waveguides. It is found that a significantly increased sensitivity (two order plus) can be achieved by harnessing these approaches, which will benefit a variety of applications, in particular, low concentration chemical/biological agents detection.
In this paper, we will provide a brief review on the progress of a unique all-fiber tunable filter based on the combination of single resonant band long period grating (LPG) and harsh environment electro-optic polymer second cladding layer recently developed at Penn State University. The single resonant band LPG is used to select the resonant wavelength and the tuning of resonant wavelength is realized by changing the refractive index of electro-optic polymer cladding layer via external electric field. Although the basic operational principle and implementation of this unique tunable filter have been previously reported by authors, this paper is focused on recent progress in this project.
In this paper, a unique all-fiber tunable filter based on the combination of single resonant band long period grating (LPG) and harsh environment electro-optic polymer second cladding layer is presented. The single resonant band LPG is used to select the resonant wavelength and the tuning of resonant wavelength is realized by changing the refractive index of electro-optic polymer cladding layer via external electric field. Although the basic operational principle and implementation of this unique tunable filter have been previously reported by authors, this paper is focused on athermal operation design and synthesis of harsh environment electro-optic polymer, which enhances the practicability of proposed tunable filter.
In this paper, a unique all-fiber tunable filter based on the combination of single resonant band long period grating (LPG) and harsh environment electro-optic polymer second cladding layer is presented. The single resonant band LPG is used to select the resonant wavelength and the tuning of resonant wavelength is realized by changing the refractive index of electro-optic polymer cladding layer via external electric field. Although the basic operational principle and implementation of this unique tunable filter have been previously reported by authors, this paper is focused on athermal operation design and synthesis of harsh environment electro-optic polymer, which enhances the practicability of proposed tunable filter.
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