Patent ID: 8144315

Claim:
A method for characterizing a multilayered periodic structure in an optical fiber in-situ during its formation, comprising: illuminating the optical fiber with a first optical signal, the optical fiber forming part of an interferometer wherein the first optical signal is reflected in the optical fiber, the reflected first optical signal being received by a photodiode; illuminating the optical fiber with a second linearly translatable optical signal through a phase mask, the second optical signal causing a perturbation in the fiber which changes a refractive index in the fiber at a point of illumination by the second optical signal, a multilayered periodic structure being formed layer-by-layer by successively illuminating portions of the optical fiber with the second optical signal through the phase mask, wherein at each point in its formation the multilayered periodic structure has an associated complex reflectivity as a function r(δ m ;z′) of a perturbation position z′ along a length of the multilayered periodic structure and a measurement wavenumber detuning δ m such that the reflected first optical signal has an interference pattern on the photodiode caused by the complex reflectivity of the multilayered periodic structure, a value of the complex reflectivity being determinable from the interference pattern; determining ⅆ r ⁡ ( δ m ; z ′ ) ⅆ z ′ , a spatial rate of change of the complex reflectivity of the multilayered periodic structure, after each successive layer is formed; deriving a complex coupling coefficient q*(z) of the multilayered periodic structure from the value of ⅆ r ⁡ ( δ m ; z ′ ) ⅆ z ′ as each successive layer is formed, wherein q*(z) is approximately equal to ⅆ r ⁡ ( δ m ; z ′ ) ⅆ z ′ by the relation ⅆ r ⁡ ( δ m ; z ′ ) ⅆ z ′ ≃ - q * ⁡ ( z ) · exp ⁡ ( 2 ⁢ iδ m ⁢ z ) ; comparing the derived complex coupling coefficient q*(z) to a target complex coupling coefficient associated with a target multilayered periodic structure to determine a difference between the extracted coefficient and the target coefficient; and adjusting the formation of the next layer of the multilayered structure if the difference between the derived coefficient and the target coefficient is not zero.