A fiber optic refractive index sensor based on Fabry-Perot interferometer formed by two Chirped Fiber Bragg Gratings on a seven-core fiber is successfully demonstrated. A small part of the fiber cladding is etched to expose the outer 6 cores to the ambient environment. While optical modes supported by the outer 6-cores are affected by both temperature and refractive index changes of the surrounding liquid, the optical mode in the central core is affected by the temperature changes only. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.
In our work, transverse Anderson localization is introduced for the first time in a simple wedge-type optical waveguide, which is formed by a triangular air hole imbedded into a fused silica material via a conventional fiber drawing technique. The micro tube is filled with a polymeric medium consisting of fluorescent dye molecules and naturally formed air inclusions caused by the capillary effect to offer a scattering medium for photons to localize the interfered electromagnetic waves. Anderson localization is explored through various single modes at different emission wavelengths within the photoluminescence spectral bandwidth of dye molecules. The photonic design of the optical waveguide allows the guidance of a single Anderson localized mode and suppression of the other modes to enable investigation of the spontaneous emission rate of the emitters, which are principally coupled into a single Anderson localized mode. The physical mechanism behind the changes in the emission dynamics of the fluorescent emitters is investigated by the time-resolved spectroscopy, which is found to be on resonance dependent with a particular cavity mode. The fastest decay rate of the light emission from the excited dye molecules is attributed to be due to the photons that couple into the localized optical modes without any spectral detuning. The enhancement of the spontaneous emission rate by a factor of 2.2 is achieved as the majority of the photons are coupled into an Anderson localized mode. Thus, a simple wedge-type optical waveguide is demonstrated to provide an opportunity to enhance light-matter interaction and opens new avenues to understand the nature of the spontaneous emission dynamics of the fluorescent emitters that are trapped in quasi optical cavities.
A four-core optical fiber is introduced as a strain based temperature sensor to investigate the phase shift based on the temperature variations. An interferometric fringe pattern is obtained by the coherent waveguides from the four cores. A small piece of a four-core fiber is winded around a solid stainless steel cylinder to form a tight circular loop, which is exposed to a temperature change from 50 °C to 92 °C. Shear strain due to the expansion of the steel rod at this temperature interval causes an optical path length difference between the inner and outer core pairs, resulting a total phase shift of 20.4±0.29 rad, which is monitored with a CMOS camera. Using the phase changes, two dimensional shear strain is determined.
A four-core optical fiber is used to investigate one-dimensional heat transfer measurements. Heat pulses from a Nd:YAG laser of 600 ms duration with a repetition rate of the order of 10 s are delivered onto one of the fiber cores. This results in an optical path length difference between the guiding cores due to the change in the refractive index and physical length of the targeted fiber core. As a result of this process, a phase shift of 1.30 rad is measured with a digital camera for 140 mW pulses in reflection scheme. The heat diffusion length in the selected fiber core is determined to be 2.8 mm, which contains 33.2 kJ/m2s heat, causing a temperature rise of 4.30 K.
This study defines measurements of three-dimensional rigid-body shapes by using a fiber optic Lloyd’s mirror. A fiber optic Lloyd's mirror assembly is basically a technique to create an optical interference pattern using the real light point sources and their images. The generated fringe pattern thanks to this technique is deformed when it is projected on an object's surface. The introduced surface profilometry algorithm depends on a multi-step phase shifting process. The deformed fringe patterns containing information of the object's surface profile are captured by a digital CCD camera. While each frames are captured, required π∕2 phase shifts for interference fringe pattern are obtained by mechanically sliding the Lloyd assembly via an ordinary micrometer stage. Some preprocess algorithms are applied to the frames and are processed with an algorithm to accomplish 3D topographies. Finally, the continuous data determines the depth information and the surface topography of the object. The experimental setup is simple and low cost to construct, and is insensitive to the ambient temperature fluctuations and environmental vibrations that cause unwanted effects on the projected fringe pattern. Such a fiber optic Lloyd’s system which provides an accurate non-contact measurement without contaminating and harming the object surface has a wide range of applications from laser interference based lithography in nano-scale to macro-scale interferometers.
In this paper, an interferometric fiber optic vibration sensor based on a four-core optical fiber is described. When the light is coupled into the four cores, each core acts as a mutually coherent waveguide with the other ones, which allows obtaining an interference fringe pattern at the far field. Vibrating a section of the four-core optical fiber causes a path difference between the light beams guiding in the separate cores, which results in a shift in the fringe pattern. Such a mechanism allows one to relate the fringe shift to the vibration amplitude and frequency. In this study, a source, which is capable to generate 100 Hz frequency sound waves is attached to the optical fiber to maintain vibration of the section of the fiber. A single slit and a photodetector are used to detect the shifting of the fringe pattern that causes a change in the phase of the guiding light. When a He-Ne laser beam is coupled into the optical fiber, the structured fringe pattern is projected onto the slit behind the photodetector, then a small part of the fringe pattern is analysed. Thus, an interferometric fiber optic vibration sensor based on a four-core optical fiber, which has a simple structure and high sensitivity, is accomplished.
Confocal fluorescence lifetime imaging microscopy method is used to obtain individual fluorescence intensity and
lifetime values of aromatic Perylene dye molecules encapsulated into PMMA based nanofibers. Fluorescence spectrum
of aromatic hydrocarbon dye molecules, like perylene, depends on the concentration of dye molecules and these dye
molecules display an excimeric emission band besides monomeric emission bands. Due to the dimension of a nanofiber
is comparable to the monomer emission wavelength, the presence of nanofibers does not become effective on the decay
rates of a single perylene molecule and its lifetime remains unchanged. When the concentration of perylene increases,
molecular motion of the perylene molecule is restricted within nanofibers so that excimer emission arises from the
partially overlapped conformation. As compared to free excimer emission of perylene, time-resolved experiments show
that the fluorescence lifetime of excimer emission of perylene, which is encapsulated into NFs, gets shortened
dramatically. Such a decrease in the lifetime is measured to be almost 50 percent, which indicates that the excimer
emission of perylene molecules is more sensitive to change in the surrounding environment due to its longer wavelength.
Fluorescence lifetime measurements are typically used to confirm the presence of excimers and to construct an excimer
formation map of these dye molecules.
This study defines measurements of three-dimensional rigid-body shapes by using a fiber optic Lloyd’s mirror.
A fiber optic Lloyd's mirror assembly is basically a technique to create an optical interference pattern using real light
point sources and their images. The generated fringe pattern thanks to this technique is deformed when projected on an
object's surface. The deformed fringe pattern containing information of the object's surface profile is captured by a digital
CCD camera. The two-dimensional Fourier transformation is applied to the image, which is digitized with a frame
grabber card. After applying a band-pass filter to this transformed data in its spatial frequency domain, the twodimensional
inverse Fourier transform is applied. Using the complex data obtained by the inverse Fourier transform, the
phase information is determined. A phase unwrapping algorithm is applied to eliminate discontinuities in the phase
information and to make the phase data continuous. Finally, the continuous data determines the depth information and
the surface topography of the object. It is illustrated for the first time that the use of such a fiber optic Lloyd's system
increases the compactness and the stability of the fringe projection system. Such a fiber optic Lloyd’s system which
provides an accurate non-contact measurement without contaminating and harming the object surface has a wide range
of applications from laser interference lithography (LIL) in nano-scale to macro-scale interferometers.
Humidity induced changes in the refractive index and thickness of polyethylene glycol (PEG) thin films are in situ
determined by optical waveguide spectroscopy. PEG brushes are covalently attached to the surface of a thin gold film on
a borosilicate crown glass (BK7) using a grafting-from chemical synthesis technique. The measurements are carried out
in an attenuated total internal reflection setup. At low humidity levels, both the refractive index and the thickness change
gradually due to swelling of the PEG thin films upon water intake. At around 80% relative humidity, a steep decrease in
the refractive index and a steep increase in the thickness are observed as a result of a phase change from a
semicrystalline state to a physical gel state. The hydrogenation of PEG films causes a less pronounced phase change
from a semicrystalline state to a gel state. Due to fewer ether oxygen atoms available for the water molecules to make
hydrogen bonding, the polymer has a more stable structure than before and the phase change is observed to shift to
higher humidity levels. It is discussed that such a humidity induced change in the index of refraction can be utilized in
constructing of a PEG based humidity sensor.
The radiative decay rates of perylene dye molecules, attached to silicon nano-rods are investigated by means of timeresolved
fluorescence experiments. The decay rates of dye molecules in the vicinity of silicon nano-rods are inhibited
due to their various diameters and therefore the modification of the surrounding environment. Inhibition is caused by an
increased nonradiative rate due to resonant energy transfer described by the Gersten-Nitzan model.
Use of a structured light pattern from a four-core optical fibre in three dimensional shape measurements with the Fourier profilometry technique is described. It is demonstrated that the use of such a four-core optical fibre increases the compactness and the stability of the fringe projection
An experimental determination technique for simultaneous measurements of the thermal optical and the linear thermal expansion coefficients of tantalum pentoxide thin films at infrared wavelength region is described. A tantalum pentoxide thin film deposited directly onto the end face of a single mode optical fiber was illuminated with a SLD source and its spectrum on reflection was monitored at various temperatures using an optical spectrum analyzer. Temperature induced change in the index of refraction and the film thickness were determined from the spectrum to calculate the thermal optical and the linear thermal expansion coefficients simultaneously.
A two-dimensional (2-D) photonic band gap (PBG) structure was utilized for the temperature mapping of ultra-small structures, such as microelectromechanical systems (MEMS). Optical properties of GaAs were considered in the design of the device since GaAs is nearly transparent and lossless in the chosen infrared region, and also has a reasonably high dielectric constant of 11.4. The structure consist of a triangular lattice of air holes etched into GaAs, with a lattice constant, a, of 0.382-micrometers , including one linear waveguide and three isolated point defects with radii 0.51 a, 0.54 a, and 0.57 a, respectively. The operational principle of the device is based on guiding and selecting the specifically tuned wavelengths through the corresponding point defects. It has been sown that having processed the intensities, obtained from each defect, in accordance with the blackbody radiation characteristics and the transmission properties of the device, the temperature reading of the target in concern can be obtained. Despite many studies concerning guided modes in 2D PBG materials, few sensor applications exist in the literature. Future work on defects, taking advantage of strongly directional behavior, frequency selectivity and specific polarization, will highlight the much richer possibilities of the PBG technology for novel applications in the fields from optical MEMS to quantum computing.
A fiber optic wavelength modulation sensor based on the spectral shift of tantalum pentoxide thin films for absolute temperature sensing up to 650 degrees Celsius is described. A tantalum pentoxide single layer was deposited directly onto the cleaved end-face of a single mode optical fiber and was illuminated with a super luminescent diode (SLD) through an addressing fiber. Interference fringes of the film on reflection were obtained within the optical bandwidth of the SLD using an optical spectrum analyzer. The spectral shift versus temperature rise showed no turning points and the output was unambiguous, linear, monotonic and gave about 0.016 nm wavelength shift in the spectrum per 1 degree Celsius. A semi-empirical calibration procedure for reading absolute thermometric measurements is presented.
We describe the use of a four-core optical fibre as the basis of a sensor capable of measuring the angle through which the fibre is bent in two dimensions. The intended application of the sensor is in measuring the shape of flexible structures.