Fiber Bragg gratings (FBGs) embedded in conventional fibers may serve as temperature sensors over a wide temperature range and withstand temperatures around 1200 K. A variety of linearly polarized (LP) modes for the wavelengths between 400 and 700 nm may be sustained in fibers with and without FBGs. The composition of the LP modes and their competition is instrumental for understanding physics of thermo-optics and thermal expansion effects in silica-based fibers. The first objective of this work was to model mathematically the competition between LP modes and modal distribution using the solutions of Bessel equations for the fibers with and without the gratings. Computer generated modes were constructed and the cut-off V-numbers (and Eigen values W and U) were determined. Theoretical results then were compared with experimental observations of LP modes for two separate ranges of temperatures: 77– 300 K and 300-1200 K. To study the formation of LP modes over the first temperature range, liquid nitrogen was used to cool down the fiber and a thermocouple was used to monitor the temperature of the fiber. Real time recording of the modal structure was performed using digital imaging and data acquisition instrumentation. To study LP modes between 300– 1200 K, the fibers were inserted into a tube furnace with temperature control. The wavelength of the infrared radiation was reflected by a FBG and detected by an optical spectrum analyzer. Radiation at the visible wavelength propagated through the fibers, and transmitted visible light was collected, analyzed and recorded with a CCD camera to monitor distribution of the LP modes in the samples with and without the FBGs.
Two sets of resonances in glass microspheres attached to a standard communication grade single mode optical fiber have
been observed. It has been found that the strength of the resonances depends strongly on the polarization of the coupled
light. Furthermore, the position of the resonances in the wavelength domain depends on the polarization of light in the
optical fiber with maximum magnitudes shifted by approximately 45°.
New light-driven actuators based on films of polymer polyvinylidene fluoride are described. The actuators
employ the photomechanical bending of the polymer film caused by low power (10 mW and less) laser radiation.
The photomechanical effect combines various physical mechanisms, such as anisotropic thermal expansion,
converse piezoelectric mechanism along with photovoltaic and pyroelectric ones, while the mechanism of
thermal expansion is dominant for slow motion. Mechanical vibrations of the strips of the photomechanical
polymer were observed with periodic pulsed laser excitation. The resonance frequency is inversely proportional
to the square of the length of the strip, in full agreement with the theory. Resonance frequency measurements
were used to determine the modulus of elasticity of the films, which was close to 3.0×109 Pa. Two possible
applications were discussed: optical fiber switch and adaptive mirror propelled by the proposed actuators. The
actuators have a potential of being used as the components of future light-driven micro/nano systems.
We describe new light-driven actuators based on films of polymer polyvinylidene fluoride, known as PVDF. The actuators employ the photomechanic bending of the polymer film caused by low-power (10 mW and less) laser radiation. The photomechanic effect combines various physical mechanisms, such as thermal expansion, the converse piezoelectric mechanism, and the photovoltaic and pyroelectric mechanisms, while the mechanism of thermal expansion is dominant. The force applied by the actuators to external objects is measured with a torsion balance. It is proportional to the power of laser beam and could be as high as 10–4 N for a 50-µm film illuminated with a 10-mW laser beam. We demonstrate mechanical vibrations of a 1×7-mm strip actuator at a frequency of 0.3 kHz. As examples of possible applications, a photonic switch and an actuator with a closed-loop motion that could drive the inner workings of a conventional mechanical clock were demonstrated. The proposed actuators have a potential of being used as propulsion components of future light-driven micro/nano systems.
New light-driven actuators based on films of polymer polyvinylidene fluoride are described. The actuators employ the photomechanical bending of the polymer film caused by low power (10 mW and less) laser radiation. The photomechanical effect combines various physical mechanisms, such as anisotropic thermal expansion, converse piezoelectric mechanism along with photovoltaic and pyrolelectric ones, while the mechanism of thermal expansion is dominant for slow motion. The static force applied by the actuators to external objects was measured with a torsion balance. It is proportional to the power of laser beam and could be as high as 10-4 N for a 50- micron film illuminated with a 10-mW laser beam. Mechanical vibrations of the strips of the photomechanical polymer were observed when the periodic laser pulses were used. The resonance frequency is inversely proportional to the square of the length of the strip. Resonance frequency measurements were used to determine the modulus of elasticity of the films, which was close to 3.0x109 Pa. Two possible applications are discussed: photonic switch and adaptive mirror. The proposed actuators have a potential of being used as the components of future light-driven micro/nano systems.
Fiber Bragg gratings (FBG) have become preferred sensory structures in fiber optic sensing system. High sensitivity, embedability, and multiplexing capabilities make FBGs superior to other sensor configurations. The main feature of FBGs is that they respond in the wavelength domain with the wavelength of the returned signal as the indicator of the measured parameter. The wavelength is then converted to optical intensity by a photodetector to detect corresponding changes in intensity. This wavelength-to-intensity conversion is a crucial part in any FBG-based sensing system. Among the various types of wavelength-to-intensity converters, unbalanced interferometers are especially attractive be-cause of their small weight and volume, lack of moving parts, easy integration, and good stability.
In this paper we investigate the applicability of unbalanced interferometers to analyze signals reflected from Bragg gratings. Analytical and experimental data are presented.
We describe new light-driven actuator based on films of the polymer polyvinylidene fluoride known as PVDF. The actuator employs the photomechanic effect of bending of the polymer film caused by low power (10 mW and less) laser radiation. The photomechanic effect combines various physical mechanisms, such as thermal expansion, converse piezoelectric along with photogalvanic and pyrolelectric, while the thermal mechanism is prevailing. The force applied by the actuator to external objects was measured with a torsion balance. It is proportional to the power of laser beam and could be as high as 10-4 N for a 50-micron film illuminated with a 10-mW beam. We demonstrated mechanical oscillations of a 1-mm by 10-mm actuator at a frequency of 0.3 kHz. The frequency could reach 1 MHz and higher for actuators of micron size. The actuators could be easily made of various shapes. Illumination could be in multiple regions of the actuator body with various time delays between laser pulses in different regions. All this can provide a lot of flexibility in terms of the trajectory of mechanical motion. As an example, we demonstrated an actuator with elliptical motion that could drive inner workings of a conventional mechanical alarm clock. The proposed actuator has a potential of being used as a core element of future optical micro- and nanomotors.
The development of integrated fiber optic sensors for smart propulsion systems demands that the sensors be able to perform in extreme environments. In order to use fiber optic sensors effectively in an extreme environment one must have a thorough understanding of the sensor’s limits and how it responds under various environmental conditions. The sensor evaluation currently involves examining the performance of fiber Bragg gratings at elevated temperatures.
Fiber Bragg gratings (FBG) are periodic variations of the refractive index of an optical fiber. These periodic variations allow the FBG to act as an embedded optical filter passing the majority of light propagating through a fiber while reflecting back a narrow band of the incident light. The peak reflected wavelength of the FBG is known as the Bragg wavelength. Since the period and width of the refractive index variation in the fiber determines the wavelengths that are transmitted and reflected by the grating, any force acting on the fiber that alters the physical structure of the grating will change what wavelengths are transmitted and what wavelengths are reflected by the grating. Both thermal and mechanical forces acting on the grating will alter its physical characteristics allowing the FBG sensor to detect both temperature variations and physical stresses, strain, placed upon it. This ability to sense multiple physical forces makes the FBG a versatile sensor.
This paper reports on test results of the performance of FBGs at elevated temperatures. The gratings looked at thus far have been either embedded in polymer matrix materials or freestanding with the primary focus of this paper being on the freestanding FBGs. Throughout the evaluation process, various parameters of the FBGs performance were monitored and recorded. These parameters include the peak Bragg wavelength, the power of the Bragg wavelength, and total power returned by the FBG. Several test samples were subjected to identical test conditions to allow for statistical analysis of the data. Test procedures, calibrations, and referencing techniques are presented in the paper along with directions for future research.
In this paper the sensitivity of embedded fiber optic sensors to changes in modal characteristics of plates is discussed. In order to determine the feasibility of embedded fiber Bragg gratings for the detection of modal shapes and modal frequencies, a comparison of holographically imaged modes and the detected dynamic strain from embedded fiber optic Bragg gratings is made. Time averaged optical holography is used for the detection of mechanical defects, or damage, in various aerospace components. The damage is detected by measuring an alteration in structural dynamics, which is visually apparent when using time-averaged holography. These shifts in the mode shapes, both in frequency of the resonance and spatial location of vibration nodes, are caused by changes in parameters that affect the structure's mechanical impedance, such as stiffness, mass and damping, resulting from cracks or holes. It is anticipated that embedded fiber optic sensor arrays may also be able to detect component damage by sensing these changes in modal characteristics. This work is designed to give an initial indication to the feasibility of damage detection through the monitoring of modal frequencies and mode shapes with fiber optic sensors.
All-optical actuators based on static or moving holographic gratings could have an advantage over current actuators because of their smaller size, less power and less RF interference. Instead of an ultrasonic wave produced by an electrically driven piezoelectric actuator as in ultrasonic motors, the wave resulted from mechanical deformation of the crystal caused by photo-generated electric charge distribution due to the converse piezoelectric effect. The charge distribution was periodical since it was produced by a holographic grating generated by two interfering coherent laser beams. Surface gratings associated with holographic volume gratings in photorefractive crystals of iron-doped lithium niobate have been studied using diffraction of a reflected probe beam and high-resolution phase-shifted interferometric profilometry. Both techniques show that the surface gratings do in fact exist in the form of periodical corrugations of the same period as that of the volume grating. The maximum amplitude of the surface grating measured by both techniques was close to 6.5 nm. We also demonstrated that the periodical electric forces on the surface were capable of assembling polystyrene microspheres along the fringes of the grating.
We describe an optical fiber splicing process based on the phenomenon of optical beam self-trapping in photocurable organic materials with light induced increase of the refractive index. The ends of two optical fiber cables to be spliced are immersed in a photocurable material with a gap maintained between them. The terminal ends of individual fibers in each cable are facing their counterparts. Two optical beams are sent from opposite fiber cores to meet each other. The beams increase the refractive index of the material. This results into formation of an optically induced bridge that traps and directs the like beams toward the opposite fiber core. This permanent optical connection can be simultaneously built for all the fibers within each of two cables. The splice connection is tolerant to some inevitable tilt or displacement. The advantages of such an approach are high productivity, maintainability, reliability, and cost efficiency. We report the results of theoretical simulations and experiments that demonstrate the feasibility of the approach.
We report a novel single-arm double-mode double-order waveguide interferometer being used as a chemical sensor for detection f toxic industrial materials such as ammonia in air. The sensor is based on thin films of polymers poly(methyl methacrylate) and polyimide doped with indicator dyes bromocresol purple and bromothymol blue. These dye- doped polymer materials exhibit a reversible optical absorption in a band near 600 nm being exposed to ammonia in wet air. The rise of absorption is accompanied by the change of the refractive index in near IR region out of the absorption band. The distinguished feature of the sensor is that is uses for reading the change of the refractive index of the dye-doped polymer film the interference of two propagation waveguide modes of different orders. The modes TM0 and TM1 are simultaneously excited in the light- guiding polymer film with a focusing optics and a prism coupler. The modes are decoupled from the film and recombined producing an interference pattern in the face of an output optical fiber. The sensitivity of the sensor to ammonia is 200 ppm per one full oscillation of the signal. We analyze effects of various factors such as polymer composition, light wavelength, ambient humidity and atmospheric pressure on the performance of the sensor. Various design and fabrication issues are also discussed. The problems of particular interest are reduction of losses and sensitivity improvement.
Fiber optic Bragg gratings have been used for years to measure quasi-static phenomena. In aircraft engine applications there is a need to measure dynamic signals such as variable pressures. In order to monitor these pressures a detection system with broad dynamic range is needed. This paper describes an interferometric demodulator that was developed and optimized for this particular application. The signal to noise ratio was maximized through temporal coherence analysis. The demodulator was incorporated in a laboratory system that simulates conditions to be measured. Several pressure sensor configurations incorporating a fiber optic Bragg grating were also explored. The results of the experiments are reported in this paper.
We report on a single-arm double-mode waveguide interferometer being used as a chemical sensor for detection of presence of ammonia and other pollutants in ambient air. The sensor is based on thin films of polymers such as poly(methyl methacrylate) and polymide doped with various indicator dyes such as bromocresol purple. These dye-doped polymer materials significantly increase their optical absorbance in the presence of the pollutants. The absorbance change is accompanied by the change of the refractive index of the materials in the region of optical transmittance. The change of the refractive index can be converted into a change of the light intensity in the single arm interferometer. The sensor employs interference between two propagating modes of the same polarization. Single-arm configuration makes it rigid but yet sensitive enough to detect concentrations of ammonia of the order of 10 ppm and less. The device also exhibits high sensitivity to ambient temperature change (of the order of 1 degree(s)C per 2(pi) -phase shift). We analyze effects of various factors such as polymer composition, light wavelength, ambient humidity and atmospheric pressure on the performance of the sensor. Various design and fabrication issues are also discussed. The problem of particular interest is coupling the sensor to the optical fiber transmission line, reduction of losses and sensitivity improvement.
Generating a moving holographic grating in a slab os photorefractive crystal can turn it into a smart optical material making up an optically driven actuator. Actuation occurs as a result of mechanical deformation of the crystal caused by photogenerated electric charge distribution due to piezoelectric effect. The charge distribution is periodical since it is produced by a holographic grating generated by two interfering coherent laser beams. In case of static grating with oscillating amplitude or grating vibrating near the static position the effect results in appearance of a surface acoustic wave or in bending bulk crystal as has been recently demonstrated for BSO. In this paper we focus ourselves on periodical surface deformation moving synchronously with running holographic grating. This running surface wave of deformation is similar to that being used to push a rotor pressed by its surface against a vibrating stator in so-called ultrasonic motors. It can be potentially used in optically driven actuators, which could be small in size (nanometers), less power consuming, and less affected by FR interference. We report on an appearance of a periodical surface deformation in photorefractive crystals of iron doped lithium niobate that is produced by a holographic grating. The amplitude of the deformation is of the order of 10 nm. We also report light-induced mechanical motion of micron-size particles associated with self- diffraction of weak laser beams in strongly absorbing solutions of phthalocyanines. These results might find some applications in new types of all-optical and electro-optical actuators.
Fiber optic sensors have the potential to be used in the very hostile environments necessary for advanced aerospace platforms. This paper reviews some of the key issues associated with the implementation of distributed fiber optic sensors in harsh environments and outlines baseline system designs.
We describe a simple yet efficient technique of delineation of various polymer MEMS structures using computer driven liquid jet dispenser. A syringe-type dispenser with a replaceable needle is filled with a polymer solution and mounted on a vertical translation stage. The substrate is mounted on two-coordinate horizontal translation stage. Additional vertical translation arm is connected to the plunger of the syringe. Vertical translation provides positioning of the needle at a certain height over the substrate. Delineation is accomplished when the syringe plunger, being pushed down by the vertical translation arm, dispenses the liquid on the substrate at a programmable flow rate simultaneously with horizontal motion of the substrate with respect to the needle. Horizontal motion is done along a programmable trajectory at a programmable sped. Comparing to conventional methods, the proposed device consumes three order of magnitude less amount of material for the fabrication of similar MEMS structures. In addition, we describe a supplement to the liquid delineation system in the form of a microscope objective head connected to a noncoherent UV source with a multimode optical fiber. The head can serve as a tool for UV curing of the polymer material deposited with the jet dispenser as well as a separate UV writing tool. In conclusion, we describe the calculation technique for the distribution of the electric field in electrically or optically controlled ferroelectric polymer MEMS fabricated by the proposed technique.
To compete globally in the next millennium, designers of new transportation vehicles will have to be innovative. Keen competition will reward innovative concepts that are developed and proven first. In order to improve reliability of aerospace platforms and reduce operating cots, new technologies must be exploited to produce autonomous systems, based on highly distributed, smart systems, which can be treated as line replaceable units. These technologies include photonics, which provide sensing and information transfer functions, and micro electro mechanical systems that will produce the actuation and, in some cases, may even provide a computing capability that resembles the hydro- mechanical control system used in older aircraft systems. The combination of these technologies will provide unique systems that will enable achieving the reliability and cost goals dictated by global market. In the article we review some of these issues and discuss a role of photonics in smart system for aerospace platforms.
Single arm dual mode optical waveguide interferometer utilizes interference between two modes of different order. Sensing effect results from the change in traveling conditions of the modes caused by the environment. The waveguide is made as an open asymmetric structure containing a dye-doped high temperature polyimide film onto a silica substrate. It is more sensitive to the change of environment than its conventional polarimetric analog using orthogonal modes (TE and TM) of the same order. The sensor still preserves the option of operating in polarimetric regime using a variety of mode combinations such as TE0/TM0 (conventional), TE0/TM1, TE1/TM0, or TE1/TM1 but can also work in nonpolarimetric regime using combinations TE0/TE1 or TM0/TM1. Experimental temperature sensor based on TE0/TE1 combination demonstrates 2(pi) -phase shift between interfering modes in response to 2 degree Celsius-change of the ambient temperature. Utilization of different mode combinations simultaneously makes the device more versatile. Application of the sensor to gas sensing is based on doping polymer film with an organic indicator dye targeting a particular gaseous reagent. Change of the optical absorption spectrum of the dye caused by the gas results in change of the reactive index of the dye-doped polymer film which is detected by the sensor. We propose to use indicator dyes based on temperature durable metal substituted phthalocyanines (such as Octadecyloxy copper phthalocynaine) which demonstrates a significant change of the absorption spectrum being exposed to acidulous or alkaline atmosphere. We discuss the design of the experimental gas chamber for testing the interferometer as a NOX sensor.
Wave propagation in inhomogeneous media has been studied for such diverse applications as propagation of radiowaves in atmosphere, light propagation through thin films and in inhomogeneous waveguides, flow visualization, and others. In recent years an increased interest has been developed in wave propagation through shocks in supersonic flows. Results of experiments conducted in the past few years has shown such interesting phenomena as a laser beam splitting and spreading. The paper describes a model constructed to propagate a laser beam through shock-like inhomogeneous media. Numerical techniques are presented to compute the beam through such media. The results of computation are presented, discussed, and compared with experimental data.
We present the results on fabrication of plastic integrated optical elements using molding, liquid jet delineation, and UV irradiation. Coupling gratings with a period of 620 mm-1 and a diffraction efficiency of 3.3 percent were molded in a preheated poly(methyl methacrylate) planar waveguide using master saw shaped grating. An experimental device is also described which uses computer controlled micro syringe to dispense polymer solution onto a moving substrate. This device consumes thousand times less material to produce integrated optical circuits than equivalent spin- casting equipment.We have made various structures such as Y- branches and Mach-Zehnder interferometers. All the structures demonstrated good performance. The same device was also equipped with a microscope-type UV illuminator for noncontact delineation of optical structures in polyimide films. The operation is based on the refractive index increase due to photo-oxidative decomposition of the material. The index difference 0.02 between exposed and unexposed regions is sufficient to define light guiding channels in polyimide without using liquid developing. UV radiation was also used to make graded-index polyimide waveguides. Their refractive index profile was successfully reconstructed by a specially developed calculation technique. All the techniques combined together give us the cost efficient tool for the fabrication of plastic integrated optics.
We present the results of study of thin film optical waveguides based on photocrosslinkable polyimide with glass transition temperature approaching 400 degrees C. The birefringence of the waveguides can possibly be reduced by eliminating the effect of gravity on the conditions of the film fabrication. We propose fabrication technique that allows us to change the orientation of the substrate and to analyze the gravity effect on the polyimide waveguide. This technique uses UV assisted film deposition from polyimide solution on a transparent substrate mounted as a removable wall of the container with the solution. The technique is also suitable for film fabrication at low gravity conditions. In order to investigate possible effects that can occur during UV assisted liquid deposition at low gravity, we studied UV exposed polyimide films made by spin coating at normal conditions. We describe the appearance of a gradient index waveguide on the top of polyimide coat exposed to UV light. The proposed mechanism of the waveguide formation includes photocrosslinking followed by UV assisted modification of the material which leads to the increase of the optical absorbance in UV region accompanied by the refractive index increase in visible region. Theoretical model based on this approach allowed us to reconstruct the refractive index profile n the waveguide. The profile is in good agreement with that obtained from the waveguide mode spectrum measured with prism coupling technique.
While the classical methods of shadowgraph and schlieren do yield a shadow in the neighborhood of a shock, they often suffer from low power densities and the need for relatively long distances. Scanning methods may help in solving these problems. The paper describes various scanning techniques, presents experimental data obtained by mechanical scanning, and identifies conditions at which the data were taken.
We present a simplified 1D theory for detecting locations of normal shocks in a converging- diverging nozzle. The theory assumes that the flow is quasi 2D and the flow is accelerated in the throat area. Optical aspects of the model consider propagation of electromagnetic fields transverse to the shock front. The theory consists of an inverse problem in which it reconstructs from the measured intensity an index of refraction profile for the shock. From this profile and the Dale - Gladstone relation, the density in the flow field is determined, thus determining the shock location. Experiments show agreement with the theory. In particular, the location is determined within 10 percent of accuracy. Both the theoretical as well as the experimental results are presented to validate the procedures in this work.
A fiber optic sensing system with an intensity sensor and an RF modulated source has been shown to have sensitivity and resolution much higher than a comparable system employing low modulating frequencies or DC mode of operation. Also the RF modulation with an appropriate configuration of the sensing system provides compensation for the unwanted intensity losses. This paper describes the basic principles and applications of a fiber optic sensing system employing an RF modulated source. In addition the paper discusses various configurations of the system itself, its components, and modulation and detection schemes. Experimental data are also presented.
Techniques for the quantitative determination of shock position in supersonic flows using direct and indirect methods is presented. A description of an experimental setup is also presented, different configurations of shock position sensing systems are explained, and some experimental results are given. All of the methods discussed are analyzed to determine the ease of technology transfer from the laboratory to in-flight operation.
A design for a birefringence transducer for high-temperature applications is described. The spring element and the readout instrumentation are addressed. A pressure transducer based on the concept has been built and successfully tested at temperatures up to 600 C.
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