Stephen J. Mihailov

Apodization technique for fiber grating fabrication with a halftone transmission amplitude mask

Experimental results of fiber Bragg gratings fabricated with halftone amplitude transmission masks and 10-cm-long phase masks are presented for the first time to our knowledge. The performance of the devices is evaluated in terms of their spectral characteristics and deviation from linear group delay. Good out-of-band sidelobe suppression of -27 dB and group-delay ripple of ?9.5 ps is achieved for fully apodized grating devices.

Long-term thermal stability tests at 1000 degree C of silica fiber Bragg grating made with ultrafast laser radiation

Using an ultrafast Ti:sapphire IR laser and a phase mask, fiber Bragg gratings (FBGs) with high thermal stability were fabricated in SMF-28 fiber for sensor applications and were subjected to long annealing tests at 1000 °C. FBGs that maintained more then 99.95 % reflectivity after several hundred hours at this temperature are demonstrated. The gratings perform well in cycling experiments up to 1000 °C and hysteresis in the wavelength response was not detected. The capability of this new type of FBG to be used for high temperature sensors is briefly discussed.

Self-packaged Type II femtosecond IR laser induced fiber Bragg grating for temperature applications up to 1000 °C

We propose a thermally stable Type II grating made with ultrafast infrared radiation in a silica-based 400 μm clad single mode fiber cane as a self packaged grating due to its ability to maintain good mechanical integrity after more 100 hours at 1000°C.

Single and low order mode interrogation of a multimode sapphire fiber Bragg grating sensor with tapered fibers

Multimode sapphire fiber Bragg gratings (SFBG) made with an IR femtosecond laser and a phase mask were probed using tapered single mode fibers of different taper diameters producing single and low order mode reflection/transmission responses. A configuration made of an input single mode tapered fiber and multimode silica fiber used for output coupling was also tested and has delivered a filtered multimode transmission spectrum. The tapered coupling improved the spectral resolution of the SFBG as compared to its multimode responses previously reported. Such improvements facilitate the utilization of the SFBG as a high temperature sensor. Wavelength shifts of the single mode response were monitored as a function of temperature up to 1500 °C and were consistent with the measurement obtained from the multimode response published previously.

Influence of fringeless UV post-exposure on index modulation amplitude in fiber Bragg gratings

Dynamics of the refractive index modulation amplitude during uniform UV post-exposure of fiber Bragg gratings at 248 nm are examined. The change in index modulation amplitude depends on the strength of the grating prior to the post-exposure and also on the degree of zero-order suppression in the phase mask used to write the grating. Furthermore, during the post-exposure, diffraction orders are observed emanating from the fiber in the absence of a phase mask. This observation is incorporated into the development of a model that successfully explains the post-exposure dynamics.

High Temperature Stable Fiber Bragg Gratings (FBGs) Inscribed Through Polyimide Coating of Optical Fibers Using a Phase Mask

High temperature stable FBGs are observed after 800°C-annealing of Type I gratings that were written through the polyimide coating of H2-loaded high Ge-doped silica fibers with a fs-IR laser and a phase mask.

Reflection Characteristics of Type II FBG Made With Femtosecond Radiation

We have designed an experiment to show that in spite of the broadband loss along the grating, type II gratings manifest low loss in a reflective configuration

Radiation resilient fiber Bragg grating sensors for sensing applications in nuclear reactor cores

This paper reports testing results of radiation resilient fiber Bragg grating (FBG) in radiation resistant fibers in the nuclear reactor core at MIT Research Reactor Lab. FBGs were fabricated by 140-fs ultrafast laser pulse using a phase mask approach. In-core test of fiber Bragg gratings was carried out in the core region of a 6-MW research reactor at temperature > 600°C and an average fast neutron (>1 MeV) flux >1×1014 n/s/cm2. First 100-day tests of FBG sensors shows less than 5 dB reduction in FBG peak strength after over 1×1020 n/cm2 of accumulated fast neutron dosage. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal optical coefficients and temporal responses of FBG sensors are determined at different accumulated dosages of neutron flux. Results presented in this paper reveals that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers could be used as sensors to perform in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

Laser Cavity Made with Ultrafast IR Type II Gratings In Heavily Doped Er-Yb fiber

We are reporting for the first time a laser cavity built in heavily doped Er-Yb fiber with type II Bragg gratings mirrors made with IR femtosecond radiation using the phase mask method.

Probing of sapphire fiber Bragg gratings using intrinsic black-body radiation

Thermal radiation that is present when sapphire fiber Bragg grating sensors are operated at high temperatures (> 1000 °C) is used to obtain a spectral response of the grating without the use of an external optical source to probe the sensor. Depending on the localization of the heat source, transmission or reflection type grating spectra can be obtained.