What are White Light Interferometers?

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- GoPhotonics

Jan 17, 2023

White Light Interferometers are devices that use interference effects between light beams. The interference occurs when the light reflected from a sample is superimposed with the light reflected by a high-precision reference mirror. The basic principle behind this method is Michelson interferometry. A broadband white light source is used to illuminate the sample in white light interferometry. The collimated light beam from a coherent light source is split into two beams, a measurement beam and a reference beam using a beam splitter. The measurement beam will hit the sample and the reference beam is the beam that hits the reference mirror. Light reflected from the sample and reference mirror is recombined at the beam splitter and is focused onto a CCD camera. Figure 1 shows the optical setup for white light interferometry.

Figure 1: Setup for White Light Interferometry

If there is any difference in the optical path length (distance travelled by light) between the light reflected from the sample and the reference beam to the camera, interference occurs at the camera. The formed interference pattern is then studied. If the optical path length is same, constructive interference takes place and the camera pixel shows high intensity at those points. For different path lengths, destructive interference occurs and low intensity is shown on camera pixels. The white light interferometers are used for high-precision distance and thickness measurements. Surface roughness can be measured using this method. Color cameras that can split light into red, blue, and green are preferred for this technique. Complex optics are involved in this method.

Advantages

  • Measuring a wide field of view in the sub-nanometer range is possible
  • Noise that occurs due to spurious interference fringes is avoided
  • Different wavelengths can be filtered from white light to measure the sample
  • Capable to do quick measurements
  • Precise thickness measurement is possible

Disadvantages

  • Limited or zero angular characteristic
  • Limited use to certain objects: Measurement using white light interferometer is possible only when there is a good reflection. Hence, it cannot do measurements on all objects. Measurements cannot be done if there is a significant difference between the light reflected from the measurement area and the light reflected from the reference mirror. White light interferometers cannot measure spiky or bumpy samples or non-reflective parts even though they can handle mirrored surfaces well.
  • Tilt correction is required: Tilted samples may cause closely spaced interference patterns affecting measurement accuracy. So before performing the measurement, tilt correction of the sample must be done using a goniometric stage. There are some white light interferometry systems with a tilt mechanism that can automatically correct the sample tilt.
  • Resolution is low for XY stage measurements: Due to the low number of sampling data sets (approximately 300,000), the resolution for XY stage measurements is low. 
  • Sensitive to vibrations: Due to the equipment’s high sensitivity to vibrations, the place of installation is very limited. Shock-absorbing tables are needed for installation purposes.

Applications

  • Interferometer for industry and automation: The white light interferometer used for industrial applications consists of a robust sensor, a highly flexible sensor cable, and a controller in an aluminum housing that can be mounted on a DIN (Deutsche Institut fur Normung) rail. The controller compensates for the ambient temperature changes and helps to attain temperature stability. The interferometer can be integrated as it has a robust design. A spatial separation of sensor and controller is possible with cable lengths up to 10 m. 
  • Reflection Measurements: White light interferometry can find reflections inside a photonic integrated circuit. 
  • Medical Imaging: White light interferometry is used in medical imaging for optical coherence tomography. 
  • Finding surface roughness: Inspection of scratches on metal surfaces. 
  • Distance and dispersive measurements