Fiber Bragg gratings (FBG) are one of several fiber optic sensor technologies currently being used in structural
health monitoring systems. When the effective refractive index of a fiber Bragg grating is changed by external
environmental variations (e.g. temperature, pH), the wavelength at which incident light experiences a maximum
reflection from the grating will correspondingly shift. To detect small environmental variations that occur during certain
chemical processes, one can enhance the sensitivity using either side-polished or tilted fiber Bragg gratings. Enhanced
sensitivity in each case is achieved by polishing the fiber on one side or writing the grating at some tilt angle. Side
polished FBG sensors having a 1542 nm Bragg wavelength and cladding thickness values from 1-3 &mgr;m provide a
maximum refractive index sensitivity of 7×10-4. Tilted FBG sensors having a 1566 nm Bragg wavelength and written
with a 4° degree tilt angle provide a maximum refractive index sensitivity of 5×10-5. Experiments on the tilted gratings
were done using 50, 80, 125 &mgr;m diameter fibers immersed in solutions in the index range 1.31-1.44. Since tilted FBGs
have enhanced sensitivity and the advantage of maintaining their full mechanical strength, they show greater promise as
reliable sensors for structural health monitoring applications.
Fiber Bragg grating sensing is a relatively mature fiber optic sensor technology currently being used in structural health monitoring systems. Therefore, there are significant benefits to using this technology as a platform for other sensing modalities. In this work, a side polished fiber Bragg sensor is described for sensing refractive index changes. The effective refractive index of a fiber Bragg grating is a function of the refractive index of the media surrounding it, and its sensitivity may be optimized with appropriate design. As the external refractive index changes, the wavelength at which incident light experiences a maximum reflection from the grating will shift. The sensitivity of a fiber Bragg grating to external refractive index changes increases when the grating is polished on one side. This side-polishing technique enables the Bragg grating to preserve a greater portion of its mechanical strength compared with other techniques such as chemical etching. This work utilizes side-polished fiber Bragg grating sensors centered at a 1542.9 nm wavelength with cladding thickness values of approximately 1-2 μm. The response of these sensors to small refractive index changes was studied. Previous work on fiber Bragg grating sensors has shown that the peak wavelengths can be measured with 3 pm repeatability. With this repeatability, this study demonstrated that a 0.001 refractive index change can be observed. By using materials that change index with moisture or pH, this technique can be used to construct both pH and moisture sensors.
The manufacturing process has a huge impact on the characteristics of the all optical fiber sensors array. By automating the manufacturing of fiber Bragg gratings, FBG arrays with much larger count of sensing points, stronger mechanical strength, tighter optical parameters tolerances and enhanced reliability are produced in a cost effective
manner. Such fiber Bragg grating arrays are now commercially available with both acrylate or polyimide coating widening the range of applications for FBG sensors to larger scale of services for strain and temperature in a distributed configuration.
We describe a simple low temperature method to produce integrated optics devices in a photosensitive, hybrid, organically modified sol-gel silica glass. In particular, we report on fabrication and characterization of slab and channel waveguides, waveguides with grating and a directional coupler. The fabrication process is appealing for its simplicity, entailing few steps and utilizing elementary photodefinition to give robust, mechanically rigid devices.
We propose a two step ion exchange process to minimize the losses in silver ion-exchanged waveguides using aluminum as mask material. In a first step the sample with the Al-layer is treated in sodium nitrate in order to oxidize the aluminum. The second step involves an ion exchange in an AgNO3/NaNO3 salt mixture. We applied this method to prepare strip waveguides in a special glass substrate, BGG31, used for telecommunication devices. The low losses of the strip waveguides in BGG31 are important for applications such as integrated optical laser amplifiers that we suggest in this paper.
We optimize experimentally the double ion-exchange process parameters to achieve a designed phase modulation for a wavefront passing through a computer-generated waveguide hologram. We also demonstrate a gradient-thickness waveguide hologram (kinoform) for 1/8 beam splitting.
Nonsymmetrical integrated optical Mach-Zehnder interferometers are produced in glass by potassium ion exchange and by silver ion exchange with ionic masking. Two- and three-port nonsymmetrical Mach-Zehnder interferometer structures with path length difference ranging from 10.75 micrometers to 259.7 micrometers are studied.
Fabrication of single-mode glass waveguide devices by ion exchange with ionic masking are studied. In channel waveguides, low losses (0.1 dB/cm) at 1.3 micrometers wavelength are measured. As a device example a nonsymmetric Mach-Zehnder interferometer at 1.55 micrometers wavelength region is demonstrated. A Y-branch wavelength multiplexer, in which 'tapered ionic masking' is utilized, is also described. Design of a 0.98 micrometers /1.55 micrometers wavelength multiplexer is presented.
A potassium ion-exchange process is employed to make integrated optical Mach-Zehnder interferometers in a glass substrate. An extensive study is carried out to determine the contribution of different components of the interferometer in its performance. Propagation properties of Mach-Zehnder interferometers, straight and S-shape waveguides, and Y-junctions are investigated.