Tyson L. Lowder

High-temperature sensing using surface relief fiber Bragg gratings

We present a new type of fiber Bragg grating (FBG) that can be used in high-temperature sensing applications. We use the flat side of a D-shaped optical fiber as a platform to etch the grating into the surface of the fiber. Because the grating becomes a physical feature of the fiber, it is not erased at high temperatures as are standard FBGs. These surface relief fiber Bragg gratings will operate up to high temperatures. We provide a brief explanation of the fabrication process and present our results for operation up to 1100/spl deg/C.

Volatile organic compound sensing using a surface-relief D-shaped fiber Bragg grating and a polydimethylsiloxane layer

The authors use a surface-relief fiber Bragg grating with a polydimethylsiloxane (PDMS) layer as a volatile organic compound chemical sensor. A PDMS layer is used because it is compatible with the optical properties of the grating and exhibits good chemical selectivity. As the analyte is absorbed the refractive index of the PDMS changes, causing the Bragg wavelength to shift, and this shift is correlated to chemical type and concentration. The direction and amount of the Bragg wavelength shift is dependent on the absorbed chemical. The authors demonstrate chemical differentiation between dichloromethane and acetone in gaseous states.

Surface Relief D-Fiber Bragg Gratings for Sensing Applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.

Optical D-fiber-based volatile organic compound sensor

A fiber-optic sensor used to detect volatile organic compounds is described. The sensor consists of a single-mode D-fiber with a 2.5 microm polydimethylsiloxane layer. The layer is applied to the fiber flat after removal of a section of the fibers cladding to increase evanescent interaction of the light with the layer. Absorption of volatile organic compounds into the polymer alters the refractive index of the layer, resulting in a birefringent change of the fiber. This change is observed as a shift in polarization of the light carried by the fiber. The sensor has a short length of 3 cm and a response time of around 1 s. The sensor is naturally reversible and gives an exponential response for gas and liquid concentrations of dichloromethane and acetone, respectively.

Temporal response of surface-relief fiber Bragg gratings to high temperature CO2 laser heating

The authors use a fiber sensor integrated monitor (FSIM) as a fully functioning system to characterize the temporal response of a surface-relief fiber Bragg grating (SR-FBG) to temperature heating above 1000 degrees C. The SR-FBG is shown to have a rise time of about 77 ms for heating and a fall time of about 143 ms for cooling. The FSIM also provides full spectral scans at high speed that can be used to gain further insights into the temperature dynamics of a given system.

Packaging of surface relief fiber Bragg gratings for use as strain sensors at high temperature

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments - where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using surface relief fiber Bragg gratings (SR-FBG). These gratings, instead of being written in the core of a photosensitive or hydrogen-loaded fiber, are formed by introducing a periodic surface relief - through photolithographic and etching processes - in the cladding above the core. Samples of SR-FBGs were successfully encapsulated and mounted onto metal shims. The packaged sensors displayed a linear response with temperature and a sensitivity factor of 11pm/°C.

Advanced FBG sensing through rapid spectral interrogation

A fiber Brag grating sensor interrogator has been developed which is capable of gathering vectors of information from individual fiber Bragg gratings by capturing the full optical spectrum 3 kHz. Using a field programmable gate array with high speed digital-to-analog converters and analog-to-digital components, plus a kilohertz rate MEMS optical filter, the optical spectrum can be scanned at rates in excess of 10 million nanometers per second, allowing sensor sampling rates of many kilohertz while maintaining the necessary resolution to understand sensor changes. The autonomous system design performs all necessary detection and processing of multiple sensors and allows spectral measurements to be exported as fast as Ethernet, USB, or RS232 devices can receive it through a memory mapped interface. The high speed - full spectrum - fiber Bragg grating sensor interrogator enables advanced interrogation of dynamic strain and temperature gradients along the length of a sensor, as well as the use of each sensor for multiple stimuli, such as in temperature compensation. Two examples are described, showing interrogation of rapid laser heating in an optical fiber, as well as complex strain effects in a beam that had an engineered defect.

Polarization analysis of surface-relief D-fiber Bragg gratings

Surface-relief fiber Bragg gratings exhibit substantially more polarization dependence than standard fiber Bragg gratings. Using D-fiber with different core orientations, surface-relief gratings are analyzed and fabricated to determine the polarization dependence. We show that the largest Bragg reflection occurs for the polarization state with a dominant TE field component parallel to the flat surface of the fiber. The polarization dependence is adjusted by changing the index of refraction of the surrounding media and by fabricating the surface relief grating using rotated core D-fiber.

Multi-use D-fiber sensors

This paper demonstrates the value of D-type optical fibers (D-fibers) in a variety of sensing applications. The principal advantage of the D-fiber is that it allows for interaction with light traveling in the core of an optical fiber with materials or structures placed in contact with the fiber. This permits stimulus sensitive materials to be placed on the D-fiber to interact with the light in the core of the fiber. The presentation shows that this feature of D-fibers can be used to create alternatives to sensors formed in standard optical fibers for measuring temperature, strain, and shape change. In addition, D-fiber sensors have been fabricated to measure chemical concentrations, and electric fields.

Etched in-fiber Bragg gratings for temperature sensing at high temperatures

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface relief fiber Bragg gratings (SR-FBGs) are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. In this work we describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors. We show that SR-FBGs resist much higher temperatures than standard FBGs by demonstrating their operation up to 1100 degrees Celsius.