Methods of measuring the refractive index profile of a transparent cylindrical object

A method of measuring a refractive index profile of a fiber preform having a central axis, a radial coordinate r, an outer edge at a radius r=R, and an actual refractive index profile ηi(r) at a wavelength λ, the method comprising:

• transmitting a light beam of wavelength λ through the preform in a direction transverse to the central axis for different heights relative to the central axis;
• measuring deflection angles of the transmitted light beam to define a measured deflection function ψm;
• and
• numerically fitting a target deflection function ψt to the measured deflection function ψm within a first preform region RFIT that does not include the outer edge, and to within a select tolerance δ by varying yet unknown radius R and yet unknown refractive index ηi to obtain an estimated refractive index profile η*i(r) over a second preform region equal to or greater in size than the first preform region.

A method of measuring a refractive index profile of a fiber preform having a central axis, a radial coordinate r, an outer edge at a radius r=R, and an actual refractive index profile ηi(r), the method comprising:

• a) generating a measured deflection function comprising discrete data points by transmitting light rays transversely through the preform and measuring corresponding light ray deflection angles;
• b) identifying respective opposite approximate edges and a center of the measured deflection function, thereby defining first and second halves of the measured deflection function corresponding to first and second halves of the preform;
• c) defining a first preform region RFIT in the first half of the deflection function that excludes the corresponding preform approximate edge;
• and
• d) fitting a first target deflection function to the measured deflection function data points within the first preform region by varying the yet unknown radius R and yet unknown refractive index ηi to obtain a first estimated refractive index profile and estimated radius for the first half of the preform.