A long period fiber grating (LPG) device is formed to exhibit a "turn-around-point" (TAP) in a phase matching curve when the group velocities of two propagating modes are matched. When the grating period of the LPG is selected to coincide with the TAP, a large tuning bandwidth is formed. This device has been found to be highly sensitive to changes in the refractive index of the ambient surrounding the LPG (recognizing a change in refractive index as low as 10^-4), allowing the device to be used as a sensor for trace elements in the atmosphere. The ability of the TAP LPG to modify the intensity of a propagating optical signal as a function of changes in the refractive dielectric of a surround material also allows for this device to be used as an all-fiber high speed optical signal modulator.