Vibration on
the factory floor. Jitter induced by air filtration systems in industrial cleanrooms, or pumps in environmental test chambers. Air turbulence over long measurement distances when measuring large-aperture mirrors where it is difficult to mechanically isolate
them and the structures needed to support the optical test; in addition, long
optical paths produce air turbulence problems.
For many years these types of conditions precluded the use of laser interferometers in real-world environments. These extremely-accurate systems were instead relegated to air isolation tables in metrology labs with pristine environments.
A traditional phase shifting interferometer (PSI) makes a measurement by introducing known
phase shifts between the signal and reference beams and recording multiple
sequential interferograms over time. A typical measurement requires hundreds of microseconds to complete—an unfortunately slow rate compared
with typical mechanical vibrations and air turbulence. Vibration will blur the fringe
signal during each 30-ms exposure. The fringe pattern will also change
significantly between exposures.
Instantaneous, "dynamic" interferometers overcome vibration and turbulence by acquiring all phase data simultaneously, in a single frame. A polarization-based pixelated phase sensor consists of a high definition camera with a "phase mask" that enables the camera to sense 0°, 90°, 180° and 270°
phase shift information simultaneously.
Dynamic interferometry enables exposure times in the range of tens of microseconds, which can be used to completely freeze a rapidly-changing interference pattern, as in the presence of extreme vibration.
In long-path measurements averaging can be used to greatly reduce the effects of air turbulence as well. Over path lengths of tens of meters, air
turbulence can introduce anywhere from 0.2 to 2.0 fringes of wavefront
distortion that varies in a random way over the test pupil. By averaging over a
period longer than the coherence time of the air turbulence, it is possible to
greatly reduce the effects of turbulence. The amount of reduction is simply a
matter of how much data is averaged. Using Dynamic measurement and averaging it is possible to
accurately map surface features of a few hundredths of a wave in height even with
a round trip air path of 20 meters.
Dynamic
interferometers provide
sufficient speed to measure moving and vibrating parts accurately. By
synchronizing data acquisition with a part being excited with a periodic signal,
it is possible to map resonant modes up to 5
to 10 kHz. With higher
power lasers and more rapid A-O shutters, modes can be measured in the 10 to 50
kHz range for measuring vibrating parts and scanning optics.
