Fiber & ElectroOptics Research Center (FEORC)

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Characterization And Modeling Of Drift Noise in Fourier Transform Spectroscopy: Implications For Signal Processing And Detection Limits
Hazel, G.; Bucholtz, F.; Aggarwal, I. D. (Optical Society of America, 1997-05-01)
A theoretical analysis of long-term drift noise in Fourier transform spectroscopy is presented. Theoretical predictions are confirmed by experiment. Fractional Brownian motion is employed as a stochastic process model for drift noise. A formulation of minimum detectable signal is given that properly accounts for drift noise. The spectral exponent of the low-frequency drift noise is calculated from experimental data. A frequency-dependent optimal spectrum averaging time is found to exist beyond which the minimum detectable signal increases indefinitely. It is also shown that the minimum detectable signal in an absorbance or transmission measurement degrades indefinitely with the time elapsed since background spectrum acquisition. (C) 1997 Optical Society of America.
Dynamics of the 5-EV optical-absorption in SIO2 glass
Tsai, T. E.; Jewell, J. M.; Sanghera, J. S. (AIP Publishing, 1993-06-01)
The optical absorption at 5 eV in SiO2 glass was observed, using laser calorimetry, to change reversibly depending on the intensity of UV light. The generation and bleaching of an absorption band at 5 eV by two- and one-photon absorption processes, respectively, can explain these reversible changes. This observation supports the structural model of unrelaxed oxygen deficiency center for the 5 eV absorption band in silica.
Effect Of External Index Of Refraction on Multimode Fiber Couplers
Wang, G. Z.; Murphy, Kent A.; Claus, Richard O. (Optical Society of America, 1995)
The dependence of the performance of fused-taper multimode fiber couplers on the refractive index of the material surrounding the taper region has been investigated both theoretically and experimentally. It has been identified that for a 2 x 2 multimode fiber coupler there is a range of output-power-coupling ratios for which the effect of the external refractive index is negligible. When the coupler is tapered beyond this region, the performance becomes dependent on the external index of refraction and lossy. To analyze the multimode coupler-loss mechanism, we develop a two-dimensional ray-optics model that incorporates trapped cladding-mode loss and core-mode loss through frustrated total internal reflection. Computer-simulation results support the experimental observations. Related issues such as coupler fabrication and packaging are also discussed. (C) 1995 Optical Society of America
Fabry-Perot Fiber-Optic Sensors in Full-Scale Fatigue Testing on an F-15 Aircraft
Kent A. M.; Gunther, Michael F.; Vengsarkar, Ashish M.; Claus, Richard O. (Optical Society of America, 1992-02-01)
We report results from fiber-optic-sensor field tests on an F-15 aircraft mounted within a full-scale test frame for the purpose of fatigue testing. Strain sensitivities of the order of 0.01 _m/m have been obtained.
Fiberoptic Temperature Sensors Based on Differential Spectral Transmittance Reflectivity And Multiplexed Sensing Systems
Wang, A.; Wang, G. Z.; Murphy, Kent A.; Claus, Richard O. (Optical Society of America, 1995-07-01)
A concept for optical temperature sensing based on the differential spectral reflectivity/transmittance from a multilayer dielectric edge filter is described and demonstrated. Two wavelengths, lambda(1) and lambda(2), from the spectrum of a broadband light source are selected so that they are located on the sloped and flat regions of the reflection or transmission spectrum of the filter, respectively. As temperature variations shift the reflection or transmission spectrum of the filter, they change the output power of the light at lambda(1), but the output power of the light at lambda(2) is insensitive to the shift and therefore to the temperature variation. The temperature information can be extracted from the ratio of the light powers at lambda(1) to the light at lambda(2). This ratio is immune to changes in the output power of the light source, fiber losses induced by microbending, and hence modal-power distribution fluctuations. The best resolution of 0.2 degrees C has been obtained over a range of 30-120 degrees C. Based on such a basic temperature-sensing concept, a wavelength-division-multiplexed, temperature-sensing system is constructed by cascading three sensing-edge filters that have different cutoff wavelengths along a multimode fiber. The signals from the three sensors are resolved by detecting the correspondent outputs at different wavelengths.
Investigation And Application Of The Frustrated-Total-Internal-Reflection Phenomenon in Optical Fibers
Rahnavardy, K.; Arya, Vivek; Wang, A.; Weiss, J. M. (Optical Society of America, 1997-12-01)
A detailed investigation of the frustrated-total-internal-reflection (FTIR) phenomenon in silica-glass-based optical fibers and its application to simple intensity-modulated strain and pressure sensors is presented. Such sensors may be readily fabricated with silica-based fibers and can be easily modified with sapphire fibers for high-temperature industrial applications where conventional silica-based fiber sensors are not feasible. We present the all-fiber FTIR sensor and show good correlation between theory and experiment. We also present results for the design and implementation of a prototype FTIR-based fiber pressure sensor. (C) 1997 Optical Society of America.
Layer-by-layer electrostatic self-assembly of nanoscale Fe3O4 particles and polyimide precursor on silicon and silica surfaces
Liu, Y. J.; Wang, Anbo; Claus, Richard O. (AIP Publishing, 1997-10-01)
Monolayer and multilayer ultrathin films comprised of nanosized iron oxide (Fe3O4) particles and polyimide molecules have been fabricated on single crystal silicon and quartz substrates by a novel layer-by-layer electrostatic self-assembly process. This process involves the alternate dipping of a substrate into an aqueous solution of anionic polyimide precursor (polyamic acid salt, PAATEA), followed by dipping into an aqueous solution of polycation polydiallyldimethylammonium chloride (PDDA) which coats on nanoscale Fe3O4 particles as a stabilizer. The growth process and the structure have been characterized using UV-vis spectroscopy, contact angle, and ellipsometry measurements. The results suggest that well-ordered uniform monolayer and multilayer magnetic films have been formed on silicon and silica surfaces. A recently developed highly sensitive fiber optic magnetic field sensor was used to probe the small magnetic field intensity produced by the multilayer films. (C) 1997 American Institute of Physics.
Microbend loss fiber optic direction and amplitude sensors for underwater applications
Vengsarkar, Ashish M.; Murphy, Kent A.; Tran, Tuan A.; Claus, Richard O. (Acoustical Society of America, 1990-07-01)
Dual purpose fiber optic microbend loss sensors have been developed for measurement of underwater acousticwave amplitudes and for detection of the direction of wave propagation. Three different construction schemes for cylindrical sensing elements are considered. The dual purpose hydrophones have been characterized for frequencies ranging from 15 to 75 kHz. They exhibit sensitivities in the range of -175 to -200 dB r e:1 V/uPa and directionality sensitivity limited by geometrical construction. 1990 Acoustical Society of America
Miniaturized Fiber-Optic Michelson-Type Interferometric Sensors
Murphy, Kent A.; Miller, W. V.; Tran, Tuan A.; Vengsarkar, Ashish M.; Claus, Richard O. (Optical Society of America, 1991-03-01)
We present a novel, miniaturized Michelson-type fiber-optic interferometric sensor that is relatively insensitive to temperature drifts. A fused-biconical tapered coupler is cleaved immediately after the coupled length and polished down to the region of the fused cladding, but short of the interaction region. The end of one core is selectively coated with a reflective surface and is used as the reference arm; the other core serves as the sensing arm. We report the detection of surface acoustic waves, microdisplacements, and magnetic fields. The sensor is shown to be highly stable in comparison to a classic homodyne, uncompensated Michelson interferometer, and signal-to-noise ratios of 65 dB have been obtained.
Minimum Detectable Phase-Shift in Spectrum-Analysis Techniques Of Optical Interferometric Vibration Detection
Sudarshanam, V. S. (Optical Society of America, 1992-11-01)
The minimum detectable phase shift indicated in recent experimental reports of new linear spectrum-analysis techniques of optical interferometric vibration detection is established as the direct consequence of the 1/f noise voltage in the system components. The dynamic range and in accuracy predicted by the simple theoretical model presented is in good agreement with experimental measurements. The conclusions of the analysis are compared with experimental reports of heterodyne shot-noise-limited optical systems. With this effective tool the generic class of spectrum-analysis techniques can be analyzed and relatively weighed to assess the effect of noise. This analysis is applicable to optical interferometry in general, although the experiments specifically involved fiber-optic modulators.
Modeling Of Fiber-Optic Sensors Based on Micromechanical Vibrations in Liquid
Prokhorov, A. M.; Claus, Richard O.; Popov, Andrey A.; Tulaikova, T. V. (Optical Society of America, 1997-08-01)
Fiber-optic chemical sensors based on optical power absorption or wavelength changes are well known. A new type of sensing element is considered. A micromechanical vibrated tiber-optic tip changes its resonance frequency during its operation. Sensors of this type are simple and convenient and do not require adjustment while in use. They are useful in industry and in medical applications. The action of this sensitive element in a liquid is considered. (C) 1997 Optical Society of America.
Optical Scanning Extrinsic Fabry-Perot Interferometer For Absolute Microdisplacement Measurement
Li, Tianchu Li; May, Russell G.; Wang, Anbo; Claus, Richard O. (Optical Society of America, 1997-02-01)
We report an optical-scanning, dual-fiber, extrinsic Fabry-Perot interferometer system for absolute measurement of microdisplacement. The system involves two air-gapped Fabry-Perot cavities, formed by fiber end faces, functioning as sensing and reference elements. Taking the scanning wavelength as an interconverter to compare the gap length of the sensing head with the reference-cavity length yields the absolute measurement of the sensing-cavity length. The measurement is independent of the wavelength-scanning accuracy, and the reference-cavity length can be self-calibrated simply by one's changing the sensing-head length by an accurate value. (C) 1997 Optical Society of America.
Sapphire Fibers: Optical Attenuation And Splicing Techniques
Barnes, Adam E.; May, Russell G.; Gollapudi, Sridhar; Claus, Richard O. (Optical Society of America, 1995-12-01)
The optical attenuation in sapphire fibers was examined. Attenuation was found to depend heavily on injection conditions. A number of techniques for making sapphire-silica fiber splices were attempted, with an effort toward optimizing injection conditions in the sapphire fiber. The most successful of these techniques, interior capillary-tube splicing, produced robust splices with an attenuation of less than 1 dB.
Separation of Intrinsic and extrinsic Optical-Absorption in a Fluoride Glass
Jewell, J. M.; Williams, Glen M.; Jaganathan, J.; Aggarwal, I. D.; Greason, P. (AIP Publishing, 1991-07-01)
The contribution of impurity ions to the total optical absorption of a heavy metal fluoride glass has been determined at 532 and 1064 nm. Four ZrF4-BaF2-LaF3-AlF3-NaF glasses were prepared from various purity raw materials. The absorption coefficients of these glasses range from 0.92 to 45.4 x 10(-4) cm-1 at 1064 nm and from 7.43 to 11.1 X 10(-4) cm-1 at 532 nm as determined by laser calorimetry. The concentrations of Fe, Ni, Cu, and Co ions in each glass were determined by graphite furnace atomic absorption spectroscopy. These two measurements enable the absorption, due to transition metal ions to be differentiated from the intrinsic absorption of the glass. At 1064 nm, the absorption coefficient of these glasses is controlled entirely by the transition metal ion content. However, at 532 nm, the absorption by the transition metal ions accounts for 4-42% of the total absorption depending on impurity concentration. The intrinsic absorption of this fluoride glass calculated from these data at 532 nm is (7.69 +/- 0.99) X 10(-4) cm-1.
Split-Spectrum Intensity-Based Optical Fiber Sensors For Measurement Of Microdisplacement, Strain, And Pressure
Wang, Anbo; Miller, Mark S.; Plante, Angela J.; Gunther, Michael F.; Murphy, Kent A.; Claus, Richard O. (Optical Society of America, 1996-08-01)
A self-referencing technique compensating for fiber losses and source fluctuations in reflective air-gap intensity-based optical fiber sensors is described. A dielectric multilayer short-wave-pass filter is fabricated onto or attached to the output end face of the lead-in-lead-out multimode fiber. The incoming broadband light from a white light or a light-emitting diode is partially reflected at the filter. The transmitted light through the filter projects onto a mirror The light returning from the reflecting mirror is recoupled into the lead-in-lead-out fiber. These two reflections from the filter and the reflecting mirror are spectrally separated at the detector end. The power ratio of these two reflections is insensitive to source fluctuations and fiber-bending loss. However, because the second optical signal depends on the air-gap separation between the end face of the lead-in-lead-out fiber and the reflecting mirror, the ratio provides the information on the air-gap length. A resolution of 0.13 mu m has been obtained over a microdisplacement measurement range of 0-254 mu m. The sensor is shown to be insensitive to both fiber-bending losses and variations in source power. Based on this approach, a fiber-strain sensor was fabricated with a multilayer interference filter directly fabricated on the end face of the fiber. A resolution of 13.4 microstrain was obtained over a measurement range of 0-20,000 microstrain with a gauge length of 10 mm. The split-spectrum method is also incorporated into a diaphragm displacement-based pressure sensor with a demonstrated resolution of 450 Pa over a measurement range of 0-0.8 MPa. (C) 1996 Optical-Society of America
Stress-dependent growth kinetics of ultraviolet-induced refractive index change and defect centers in highly Ge-doped SiO2 core fibers
Tsai, Tsung-Ein; Taunay, Thierry; Friebele, E. Joseph (AIP Publishing, 1999-10-01)
The evolution of the index change of type-IIa gratings observed in 28 mol % Ge-SiO2 core fibers with 1.8 mu m core diameter under various strains was measured from the optical spectra, and the induced defects at high and low strains were studied with electron spin resonance. Data will be presented to show that the index modulation (Delta n(mod)) of type-IIa gratings is likely associated with Ge E-' centers. (C) 1999 American Institute of Physics. [S0003-6951(99)04141-8].
Structural origin of the 5.16-EV optical-absorption band in silica and GE-doped silica
Tsai, Tsung-Ein; Friebele, E. Joseph; Rajaram, M.; Mukhapadhyay, S. (AIP Publishing, 1994-03-01)
The origin of the 5.16 eV absorption band observed in silica and Ge-doped silica was studied using optical and electron spin resonance (ESR) measurements. The band was observed only in samples containing Ge, suggesting that it is related to the Ge impurity in silica, while a lack of correlation between the ESR intensity of the induced hydrogen-associated doublet and the absorption coefficient of the 5.16 eV band indicates that it is not related to two-coordinated Si or Ge. The observation of the absorption coefficient increased as the square root of the Ge concentration demonstrates that the 5.16 eV band is not related to two-coordinated Ge defects but that it is an oxygen deficiency center of the divacancy type associated with Ge.
Uniform component of index structure induced in Ge-SiO2 fibers by spatially modulated ultraviolet light
Tsai, Tsung-Ein; Williams, Glen M.; Friebele, E. Joseph (AIP Publishing, 1998-06-01)
Experimental data are presented to show that Ge(1) and Ge(2) centers are induced by trapping photoinduced electrons from the conduction band, in agreement with our previous proposal that both are trapped electron centers. The spacing (Lambda) dependence of ultraviolet (UV) light bleaching of the pre-existing Ge E' centers illustrates that the electron diffusion length is greater than Lambda of the spatially modulated UV light used in the fabrication of fiber Bragg gratings (FBGs) with Bragg wavelengths less than or equal to 1.5 mu m (short period grating) for laser powers as low as 25 mJ/cm(2). The Ge(1) and Ge(2) centers are uniformly induced by the spatially modulated UV light and therefore contribute to the uniform component of the index structure of FBGs. [S0003-6951(98)01325-4].

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