Measuring Total Thickness Variation (TTV) with an Interferometer

Measuring Total Thickness Variation (TTV) with an Interferometer

Measuring Total Thicness Variation

Front to back measurement of TTV (left); Direct measurement of TTV (right).

 

Total Thickness variation (TTV), and often wedge, is specified in optical windows and substrates. There are two methods for measuring TTV: Front-to-Back (left image above) and Direct (right image above). Both methods utilize Fizeau interferometers, a product line of Äpre Instruments.

Front-to-Back Measurement

TTV can only be calculated if the front and back measurements are performed without translating or moving the part. While this restriction poses challenges for traditional laser Fizeau systems, this is accomplished easily with Spectrally Controlled Interferometry (SCI) as the fringes are moved rather than the part.

The process is as follows:

  1. The front surface is measured, named, and stored in REVEAL’s clipboard.
  2. The back surface is measured, and a correction is applied for the index of refraction.
  3. The two measurements are added to report the TTV.

An advantage of Front-to-Back TTV is that parts as thin as 150 µm optical path length (OPL) can be characterized.

Direct Measurement

The Direct measurement is accomplished by locating the SCI interference fringes inside the substrate itself. These fringes are called “internal Fizeau” fringes because the fringes are between the front and back surfaces, or internal to the substrate. Once the SCI has localized fringes inside the substrate, the interference phase is directly measured and hence the TTV acquired. Phase measurement of internal Fizeau fringes is only possible by wavelength modulation (wavelength phase shifting) and SCI phase shifting.

The OPL limitations for Direct TTV measurement are:

  • For wavelength modulation: OPL must be >2 mm, and phase measurements can take more than 60 seconds.
  • For SCI: OPL of 500 µm are possible with measurement times measured in milliseconds.

Summary

Measurement of a glass plate has many complications, mostly due to interfering back reflections as found in a laser interferometer. SCI solves this problem and expands what can be measured on a substrate in one setup and with no preparation.

Blog, Interferometers, Systems