No abstract

A new technique for writing extremely short-length Bragg gratings in optical fibers is demonstrated. A physical model describes the diffraction effects on the spatial and wavelength spectra of the Bragg gratings. Selection of appropriate diffraction patterns and related parameters permits self-apodized Bragg gratings with a typical spatial length of several hundred micrometers and a bandwidth of several nanometers to be obtained. These gratings with well-defined spectra are suitable for use as miniature distributed strain sensors and other applications requiring small physical dimensions and broadband spectra.

A new technique has been developed for sensing both temperature and strain simultaneously by using dual-wavelength fiber-optic Bragg gratings. Two Bragg gratings with different wavelengths were inscribed at the same location in an optical fiber to form a sensor. By measuring the wavelength shifts that resulted from the fiber being subjected to different temperatures and strains, the wavelength-dependent thermo-optic coefficients and photoelastic coefficients of the fiber were determined. This enables the simultaneous measurement of temperature and strain. In this study, measurements were made over the temperature range from room temperature down to about 10 K, addressing much of the low temperature range of cryogenic tanks. A structure transition of the optical fiber during the temperature change was found. This transition caused splitting of the waveforms characterizing the Bragg gratings, and the determination of wavelength shifts was consequently complicated. The effectiveness and sensitivities of these measurements in different temperature ranges are also discussed.

Cryogenic temperature sensing was demonstrated using pressurized fiber Bragg gratings (PFBGs) with polymer coating of various thicknesses. The PFBG was obtained by applying a small diametric load to a regular fiber Bragg grating (FBG). The Bragg wavelengths of FBGs and PFBG were measured at temperatures from 295 K to 4.2 K. The temperature sensitivities of the FBGs were increased by the polymer coating. A physical model was developed to relate the Bragg wavelength shifts to the thermal expansion coefficients, Young's moduli, and thicknesses of the coating polymers. When a diametric load of no more than 15 N was applied to a FBG, a pressure-induced transition occurred at 200 K during the cooling cycle. The pressure induced transition yielded PFBG temperature sensitivities three times greater than conventional FBGs for temperatures ranging from 80 to 200 K, and ten times greater than conventional fibers for temperatures below 80 K. PFBGs were found to produce an increased Bragg wavelength shift of 2.2 nm compared to conventional FBGs over the temperature range of 4.2 to 300 K. This effect was independent of coating thickness and attributed to the change of the fiber thermo-optic coefficient.

The NMR line shape and spin-echo behavior of 13 C and 15 N NMR in powder samples of K(CN)o, 5 Bro,5 are reported. The line shape and spin-echo data are best explained by the molecules' developing increasingly preferred orientations as the temperature is decreased. Models in which the molecular reorientations simply slow down with decreasing temperature do not fit our data. The quadrupolar freezing is nearly complete at temperatures well above the dielectric freezing temperature. The NMR results from KCN-KBr are compared to results from the orientational glasses 0-H 2 -/?-H 2 and N 2 -Ar.