Chapter 2 Description - THORLABS Chromatis User Manual

Chromatis™ Dispersion Measurement System Chapter 2: Description
Chapter 2 Description
The Chromatis is a white light interferometer (WLI) designed to measure the dispersion characteristics of reflective
and transmissive optics. The WLI itself can be implemented as a simple interferometer as shown in Figure 1.
Figure 1 White Light Interferometer: A broadband, temporally incoherent light source (such as a
halogen bulb) is collimated and sent through an interferometer consisting of a broadband, low
dispersion beamsplitter, and two delay arms. One arm (reference arm) uses a low dispersion mirror,
while the other arm (test arm) contains the test mirror. The time delay (Δt) between the two arms is
scanned across Δt = 0. In the regions around Δt = 0, the white light signals from each arm interfere
with each other, giving an interference pattern that is detected by a photodiode and recorded by a
computer. To provide a distance reference for the time delay scan, a coherent source (HeNe laser) is
co-propagated with a vertical offset to the white light. The HeNe fringes are recorded on a separate
photodiode.
The white light interference pattern contains information about the relative phases of the light from the test arm
and the reference arm. A computer is used to process the interference pattern and extract the phase and
dispersion properties of the mirror under test. This device also can measure the dispersion of a transmissive
sample by replacing the test mirror with a low dispersion mirror and placing the transmissive sample in the beam
path.
The Chromatis dispersion measurement system directly measures the phase of the optic under test and converts
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the phase to dispersion through numerical derivatives :
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More in-depth information can be found in: S. Diddams and J. Diels, "Dispersion measurements with white-light
interferometry," J. Opt. Soc. Am. B 13, 1120-1129 (1996).
Rev. A, September 27, 2022 Page 3