Measurement of Refractive Index Profile of Optical Fiber Preform

Optical fiber preform is the preliminary product fabricated according to the fiber design, it basically determines the characteristics and quality of the finished optical fiber. Consequently if relative measurements are carried out at this stage on the preform whether or not the preform meets the design specifications can be seen according the testing results, thereby judging if or not proceeding the next step, fiber drawing. Of which refractive index profile measurement is the most important item, from this measurement a series of geometric parameter, such as core/cladding ratio, concentricity, non-circularity,  can be measured

Compared with optical fiber, optical fiber preform is characterized by its bigger size, so the measurement method for its refractive index profile has uniqueness. Generally side light-beam scanning on the preform is employed, and the P104 PREFORM ANALYZER from NETTEST Company, US, is just the  measurement system based on this method. The basic principle of this method is described briefly as follows:

A thin laser beam is used to scan a optical fiber preform that is placed transversely(laterally), the beam passing through the preform is angle deflected due to preform’s geometry and refractive index profile, which is so called refraction (similar to the effect of lens). According to specific testing system the refraction angle can be indicated as: 

                               

Where D and F are the system parameters; θ is the factor related to the position of scanning beam (i.e. the  radial position of the preform, θ=ωnΔt（ω is the angular velocity of the chopper that periodically occludes the exit beam; n is the times that the chopper occludes the exit beam; Δt the time space of acquiring optical power. Obviously the refractive angle, Φ, is the function of preform’s radial position, r. That refractive angle function is the radial refractive index profile function of the preform after Abel transformation (a kind of integral transformation of special function), the mathematical expression of that transformation is: