Calibration of a portable interferometer for fiber optic connector endface measurements E. Lindmark Ph.D Light Source Reference Mirror Beamsplitter Camera Calibrated parameters Interferometer Interferometer configuration for measuring the endface of a PC connector Here are the parameters that PROMET allows the user to calibrate on its line of interferometers: X Y scale camera pixel coordinates to actual measured distances Phase shifter reference mirror needs to move specific distances so that fringes move in prescribed steps Z height related to the wavelength of the interfering light Tilt of the interferometer s optical axis X Y coordinates In order to have measured data with true distances, the camera pixels need to be scaled. An interferometer system typically uses an artifact with fiducials that are known distances apart (measured by some other microscope system) and then scales the image so that the distances in the image match the known distances. Each interferometer has slightly different optics and magnification, so a fixed magnification number cannot be used. PROMET uses a custom target made by e beam lithography with + shaped fiducials spaced at 125 microns in both X and Y:
Microscope image of 2D calibration target with 125 micron spaced fiducials Some other systems use the diameter of a fiber as a known 125 microns. In addition, the optics of the interferometer have to be well corrected so that the image is not distorted and causes measurement errors. Phase Shifter For phase shifting interferometry, several images with different amounts of phase shift are combined to extract the 3D information of the surface. A phase shift is created by slightly changing the length of the reference arm of the interferometer by moving the reference mirror. There are many different phaseshifting algorithms, but typically fringes are shifted by a quarter of a fringe for each step using a reference mirror mounted on a piezo actuator. Here are images from a 5 image algorithm spaced a quarter of a fringe apart with the resulting 3D data map:
0 Degree Frame 90 Degree Frame 180 Degree Frame 270 Degree Frame 360 Degree Frame Resulting 3D Map Images for a 5 frame phase-shifting algorithm (note the changing fringes) and resulting 3D data A piezo actuator changes its length when a voltage is applied to its electrical leads. The voltages applied to the piezo actuator need to be calibrated so that the piezo actuator steps the correct distance. There are different methods to accomplish this task, but they usually involve the software looking at how much the fringes are moving in the image with the different applied voltages and adjusting these voltages so that they move in the correct steps. There is no one correct way to calibrate the phase shifter, and in addition there are some phase shifting algorithms that can minimize the effects of less than perfect phase shifting as well as other systematic errors (non linear camera response, vibration, etc.). The most important thing is that the piezo is calibrated well enough so that it can make repeatable and reproducible measurements of an acceptable quality. Z or Height After a phase shift measurement is performed, the images are combined using the correct algorithm chosen for the number and type of image frames which results in a phase map of the surface. This modulo phase map is unwrapped and the result is a 3D height map of the surface with the heights in terms of the wavelength of the light used to create the interference.
Modulo Data Unwrapped Data Example of a hemisphere as modulo data and unwrapped data Large frame commercial interferometers typically use frequency stabilized Helium Neon lasers whose wavelength changes less than 1/1000 th of a nanometer over time and temperature. The heights in terms of wavelength from these interferometers can then be scaled correctly using the wavelength of the HeNe laser or λ = 632.8nm. For portable interferometers, stabilized HeNe lasers are too large and too expensive to use, so diode lasers or light emitting diodes (LEDs) are used instead. Diode lasers can have wavelength stability issues unless dealt with correctly (and expensively) so LEDs are the much more common choice. LEDs emit over a broad wavelength band compared to lasers and their peak wavelength changes with temperature and drive current, so it is important to calibrate the wavelength of the LED to calculate the correct scale factor. PROMET uses an artifact with a known radius of curvature to calibrate its FiBO interferometers. The artifact is a convex lens with a radius of curvature within the range of typical optical fiber connector radii of curvature (from 7 to 25mm). The radius of curvature of the lens is characterized by means of a wellknown technique using a large frame commercial interferometer equipped with a precision translation stage and measuring the distance between the confocal and cat s eye positions.
Reference radius of curvature measurement showing confocal (ghost position) and cat s-eye (right) positions The radius of curvature of the lens is then measured on the FiBO interferometer using the same methods described for measuring the radius of curvature of fiber optic connector endfaces in Telecommunications Industry Association document TIA 455 218 Measurement of Endface Geometry of Single Fiber Optical Connectors (fitting a sphere to a donut shaped portion of the 3D data centered on the fiber center). The measured radius using a nominal wavelength value is compared to the known radius and a scale factor is calculated and used in subsequent measurements. Fringe image of a lens target (left) and resulting 3D data (right) Angle of the Interferometer Axis or Apex Offset Calibration If the optical axis of the interferometer (including the reference mirror) is tilted relative to the axis of the ferrule of the connector endface being measured, excess Linear Apex Offset will be measured. To combat this problem, PROMET uses a kinematic mechanical mounting system to allow the interchange of different connector adapters while maintaining very precise connector alignment. In order to remove any residual tilt of the interferometer system from the apex offset measurements, an angular apex offset calibration is performed. A ferrule endface is mounted in a kinematic holder and three apex offset measurements are performed while rotating the kinematic holder 120 in between measurements. The X and Y angular components of the three apex offset measurements are averaged
and are subsequently subtracted from future measurements. A system that is perfectly aligned will have zero average angular errors. 3 rotations of a connector endface with fiber centers (+), sphere peak (square) and linear apex offset (dashed-line) shown Other connector endface interferometers have similar calibration systems but involve rotating the ferrule by hand in a sleeve. Some systems may require hand adjustment of the tilt of the reference mirror. Surface quality of the reference mirror The interferograms produced by the interferometer are a combination of light from the connector endface and light from the reference mirror. If the surface quality of the reference mirror is not good, the interferometer cannot tell if the problem is from the reference mirror or the connector endface. The reference mirror has to be of sufficient flatness and smoothness so that the surface defects and shape measured by the interferometer is due to the surface of the connector endface. PROMET uses reference mirrors that have a flatness of less than 1/10 th of a wave of power and irregularity of less than 1/20 th of a wave over an aperture of 3mm. Only a small portion of this mirror is typically used, so the flatness and irregularity are even better than the above specifications. In addition, the scratch dig specification is very stringent so that there are basically no defects on the reference surface.