What is "interferometer" inspection?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
1

How can the principle of light interference be used to check if an optical fiber end face is polished round?

2

In the grinding process of fiber optic connectors, the use of optical interference principles (typically based on Fizeau, Michelson, or Mirau interferometric optical paths) to detect the microscopic 3D topography and ‘roundness’ of the end face is the standard industry testing method.

This method visually translates nanometer-scale surface micro-topography into optical contour lines through the interference fringes generated by optical path differences. The specific detection and judgment mechanisms are as follows:

I. Core Physical Principle: Interferometry of Equal Thickness and Optical Contour Lines

  1. Formation of Interference Light Path:
    Light emitted from a monochromatic or quasi-monochromatic source is split into two beams by a beam-splitting system: one beam is directed towards an internal standard optical reference surface, and the other is directed towards the polished end face of the fiber optic connector under test. The two reflected beams coherently superimpose on an imaging sensor (CCD/CMOS).
  2. Optical Path Difference and Bright/Dark Fringes:
    When the spatial gap d between the reference surface and the end face under test satisfies the optical path difference formula:
    • Constructive Interference (Bright Fringes): 2d = m\lambda (where m is the interference order, and \lambda is the wavelength of light)
    • Destructive Interference (Dark Fringes): 2d = (m + \frac{1}{2})\lambda
      The height difference between any two adjacent interference fringes is precisely equal to half a wavelength (\frac{\lambda}{2}, typically in the hundreds of nanometers range). Therefore, the interference fringes are equivalent to 3D contour lines of the end face’s microscopic geometric topography.

II. How to Judge ‘Roundness’ from Interference Fringes

Fiber optic connectors (such as PC, UPC, APC ferrules) are typically ground into a microscopic spherical bulge. The interferometer assesses the geometric quality of the grinding by analyzing the shape, spacing, and symmetry of the interference fringes:

1. Fringe Roundness and Symmetry (Judging Grinding Symmetry)

  • Standard Spherical End Face : When the microscopic topography of the end face is uniform and symmetrical, the interference pattern appears as standard concentric and coaxial circular rings (Newton’s rings).
  • Elliptical or Asymmetrical Deformation : If there is uneven grinding pressure or wear on the polishing pad, the end face may exhibit astigmatism or ellipticity, causing the interference fringes to appear as elliptical rings, saddle shapes, or distorted fringes.

2. Radius of Curvature (ROC)

  • The radial spacing \Delta r between the interference rings determines the steepness of the surface. Denser fringes indicate a smaller radius of curvature (more convex surface); sparser fringes indicate a larger radius of curvature (flatter surface).
  • The algorithm fits the radius of curvature of the sphere by extracting the radius r_m of each interference ring using a geometric formula:
    R \approx \frac{r_m^2}{m \lambda}
  • Industry standards (e.g., IEC/Telcordia standards) typically require R to be within the range of 10 \sim 25\text{ mm} for PC/UPC and within 5 \sim 12\text{ mm} for APC.

3. Apex Offset

  • Definition : The horizontal displacement between the highest point (apex) of the ground microsphere and the physical center of the fiber core.
  • Judgment Method : In the interferogram, if the center of the innermost ring (i.e., the apex of the curvature) coincides with the fiber core center, the apex offset is 0; if the ring center deviates from the core center, the interferometer uses image algorithms to accurately calculate the planar distance between them (typically required to be < 50\ \mu\text{m}). If the deviation is too large, a proper physical contact (PC) will not be formed in the core contact area when two fibers are mated.

4. Fiber Height (Undercut / Protrusion)

  • At the junction edge between the fiber and the ceramic ferrule, if the fiber is protruding (Protrusion) or recessed (Undercut), the interference fringes will exhibit fringe breaks or phase jumps at the interface. By calculating the fringe displacement, the relative height difference of the fiber end face with respect to the ferrule end face can be precisely measured (typically within \pm 50\text{ nm}).

III. Modern Phase Shifting Interferometry (PSI) Technology

Modern fiber end face interferometers commonly employ Phase Shifting Interferometry (PSI): a piezoelectric ceramic (PZT) drives the reference mirror to shift with sub-nanometer steps, capturing multiple interference images (e.g., 4 or 5 steps) with known phase delays. This allows for precise demodulation of the absolute phase \phi(x,y) at each point across the entire end face, reconstructing a 3D color topographic map of the fiber end face, and automatically outputting quantitative indicators such as radius of curvature, apex offset, and fiber height.

Note: Fiber end face interferometers are optical metrology instruments specifically for the processing and testing of fiber optic passive components and are not part of Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) core product line.