G. Meltz's research while affiliated with CT Associates and other places

The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described

Fiber Bragg grating sensors can be used to measure the refractive index of fluids by etching the surface of a D- shaped fiber to expose the evanescent field to the superstrate layer. The cladding is modified and the modal propagation constants are changed thereby shifting the Bragg wavelength of a grating in the etched region. Experiments are reported with etched e-core D-fiber that demonstrate the effect. The Bragg lines of both the fast and slow eigenmodes are blue-shifted when the silica cladding layer is removed and replaced with water or methanol films. Changes in the fiber birefringence are also observed because the perpendicular and parallel modes decay into the cladding at different rates. By using a tunable laser, such as an ECL, with a narrow band Bragg grating filter or three-grating Fabry-Perot interferometer, it may be possible to resolve refractive index variations of 5 by 10-6. Temperature compensation methods are also discussed including the use of an isolated reference grating and the simultaneous combination of birefringence and Bragg line wavelength shift measurements.

Experimental transmission spectra of ultraviolet (u.v.)-written Bragg gratings in depressed- and matched-cladding fibre are characterized and compared. In particular, we discuss how the location and strength of the spectral features vary with the degree of blazing, or angular tilt of the grating. The fine-structure detail on the short-wavelength side of the fundamental Bragg line is attributed to power coupling between the forwardpropagating fundamental (LP01) mode and discrete, backward-propagating cladding modes. Resonances corresponding to backward-propagating LP0n and LP1n modes are identified, and their relative strengths are compared with theoretical overlap calculations. Physical arguments are presented that explain the pronounced ghost-grating notch that appears in the transmission spectra of blazed, fibre Bragg gratings in depressed-cladding fibre.

Considerable interest has been shown in the use of fiber gratings as multiplexed sensor elements for civil and aerospace structures and for underwater acoustics. Fiber gratings can also act as transducer elements for many generic sensor types such as temperature, strain, pressure, and E-field for electric utility power plant applications. Properly annealed grating sensors can be used under load conditions to temperatures of 400 degree(s)C with no significant loss in reflectivity or wavelength changes over long periods of time. New fiber grating sensor demodulation schemes also look attractive for decoding the sensor's wavelength encoded output. There are also new developments using fiber gratings in short cavity fiber lasers for ultra high sensitivity sensors.

Experimental transmission spectra of UV-written Bragg gratings in depressed-cladding fibre are characterised. The fine-structure features on the short-wavelength side of the fundamental Bragg line are shown to be due to coupling between the LP₀₁ and higher-order LP_0n and LP_1n modes

Properly annealed fiber gratings can be used as sensor transducer elements at temperatures of 400° C over long periods of time without measurable changes. At 650° C, however, a problem was observed with creep or hysteresis of the grating response. At even higher temperatures diffusion of the core material will become a problem for long term operation. A couple new decoding schemes for fiber grating sensors use matched gratings and acousto-optic tunable filter in the decoding unit These systems can measure many grating transducer elements simultaneously with high sensitivity. Short cavity fiber lasers that utilized fiber gratings can also act as sensor transducer elements giving one ultra high sensitivities that are limited by the fundamental noise in the fiber. One fiber laser sensor uses hetrodyning to generate a signal that can be measured on an RF spectrum analyzer. New techniques for simultaneous measurement of temperature and strain will also be discussed.

Fiber Bragg grating (FBG) devices have received wide attention because they are useful in a variety of passive and active WDM network components and for sensing various measurands. These applications take advantage of either the broad- or narrow-band spectral characteristics of the Bragg resonance and its sensitivity to extrinsic perturbations. In this paper, we examine FBG tuning by controlling the effective index of the optical waveguide through modification of the cladding. This causes a change in the evanescent field which is reflected in a proportional shift in the Bragg wavelength. If the change is large enough then it can be detected by measuring the shift in the resonance line or equivalently by determing phase-path imbalances in an auxiliary interferometer. Very small wavelength changes are more easily measured by forming a compound FBG transmission filter, such as a three-grating Fabry-Perot interferometer, and using a tuneable laser to determine the transmission or reflection spectrum.

A series of blazed, or tilted gratings were fabricated in both depressed-cladding (AT&T Accutether) and matched-cladding (Corning SMF-28) single-mode fibre. In each case, a phase mask was orientated to set the nominal blaze angle (i.e., the complement of the angle between the fibre axis and the etched lines in the mask) from 0° to 3°. The fibres were soaked in hydrogen at 2120 psi and 97.6°C for 15 hours to enhance their photosensitivity. Gratings were formed by exposing the fibre core to a 248 nm KrF laser for 5 minutes with a fluence of 130 mJ/cm ² /pulse at a rate of 20 pulses/s.