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Measurement of optical fiber’s Refractive Index Profile

2010/5/7

Some main transmission  properties  of  optical fiber, such as bandwidth, dispersion and so on, are  dependant  on the design of  fiber’s  refractive  index  profile,  which is the base of optical waveguide constitution. From  the design  of optical  fibers to  their fabrication, refractive index profile is a very important basic parameter of optical fibers, and often the practical optical fiber’s refractive  index  profile is the key  that determines  if or not the above-mentioned transmission performance  can achieve  the expected goal. Therefore, the exact  measurement of a  finished fiber’s refractive index profile is absolutely necessary.

The  earliest  way (about in the end of 1970s) for the measurement of  refractive  index  profile adopted so-called “Reflection method” which is based on the reflection principle of the light. This principle is reflected in the following formula of normal incidence (i.e. 0 incident angle) reflectivity：
                            R=[（n1－n0）/（n1＋n0）]2

                       R=[（n1－n0）/（n1＋n0）]2

Where n0  is  the  refractive  index  of  air; n1  is  the  refractive  index of  the medium material.
Afterward refracted near-field technique was proposed, which is ranked as the standard method both at home  and abroad. Just as its name implies,  the refracted  near-field technique utilizes the refraction  principle of  light. That is  if the numerical aperture of  the  launching light  at  the incident end is larger  than that (NA) of the optical  fiber, the launching light will refract from the core into  the  cladding of the optical  fiber. The intensity of  the refracted light is related  to the “local” NA at the incident point, and the NA of optical fiber is related to the refractive index. That is: 
                                   NA＝（n12－n02）1/2

                             NA＝（n12－n02）1/2

Where n0 is the refractive index of the cladding; n1 is the refractive index of the fiber core, it can be the function of the radial position of the fiber core.

According to the refraction law of light, we can derive out the following formula at last:
                            n(r)－ncl =k[Pcl－P(r)]/ Pcl

                            n(r)－ncl =k[Pcl－P(r)]/ Pcl

Where k is the correction  factor depending  on system  parameters; ncl  and Pcl are constants，being the refractive index of fiber cladding and the refracted optical power, respectively.

Therefore the refractive optical power, P (r), is related to n (r), and we can get n (r) through measuring P (r).
 
Our test system is NR9200 optical fiber analyzer from EXFO company, Canada, and it is specially
 for measuring optical fiber’s refractive index profile using the standard method, “ refracted near
-field technique”. When it scans along the radial direction of the fiber, very high  spatial measuring
  resolution can be  achieve due to the high precision  mechanical scanning mechanism of 0.1μm
 precision: the spatial measurement resolution is better than 0.5μm, and  the  absolute  refractive
index resolution is 0.0001.