Error analysis of optical fiber refractive-index profiling using the focusing method

Area of Science:

• Optics and Photonics
• Materials Science
• Metrology

Background:

• Refractive-index profiling is crucial for characterizing optical fibers.
• The focusing method is a common technique for this measurement.
• Understanding sources of error is essential for accurate fiber characterization.

Purpose of the Study:

• To investigate the causes of measurement error in refractive-index profiling using the focusing method.
• To compare experimental results with theoretical models (ray tracing and diffraction integral).
• To identify the impact of diffraction on measurement accuracy.

Main Methods:

• Experimental measurement of intensity distributions and refractive-index profiles.
• Ray tracing simulations.
• Diffraction integral calculations.
• Comparative analysis of obtained data.

Main Results:

• Experimental results for intensity distribution and refractive-index profiles fell between ray tracing and diffraction integral predictions.
• The focusing method, based on ray optics, introduces significant errors for small fiber samples due to diffraction.
• Diffraction from index depressions in MCVD fibers severely degrades measurement accuracy for both the depression and the overall profile.

Conclusions:

• Diffraction is a major source of error in refractive-index profiling of small optical fibers using the focusing method.
• Ray optics approximations are insufficient for accurate profiling when diffraction effects are significant.
• Accurate characterization of MCVD fibers requires accounting for diffraction, especially around index depressions.