OVERCOMING VIBRATION ISSUES WHEN USING LASER INTERFEROMETRY
Erin McDonnell, Associate Product Manager (Optical Test), Zygo Corporation
Before looking at the problems that vibration causes when using a laser interferometer, and the ways that its effects can be overcome, it is worth briefly describing the way that laser interferometry works.
HOW AN INTERFEROMETER WORKS
Interferometers can measure objects with extreme accuracy. They work by taking a beam of light and splitting it into two equal halves using a beam splitter, effectively a piece of glass thinly coated with silver. When light is shone on the beam splitter, half the light passes through and half is reflected back. One of the beams (known as the reference beam) shines onto a mirror and from there to a detector. The other beam shines at or through the object being measured, onto a second mirror, back through the beam splitter, and onto the same detector. So saying, the second beam travels an extra distance (or in a somewhat different way) to the first beam, and is therefore slightly out of phase.
When the two beams meet up on the detector, they overlap and interfere, and the phase difference between them creates the interference fringes. The light areas are where the two beams have added constructively, the dark areas are places where the beams have subtracted from one another destructively. The exact pattern of interference depends on the different way or the extra distance that one of the beams has travelled. By inspecting and measuring the fringes, it is possible to calculate this with great accuracy — and that gives a precise measurement of whatever it is that the user is trying to measure.
COMPROMISING INTEGRITY
Today, state-of-the-art interferometers can measure with nanometer-levels of precision, and so can be susceptible to errors if used in loud and highly dynamic manufacturing environments where significant vibration is present. When you’re measuring on the order of nanometers, even seemingly small amounts of motion in the measurement path can be significant. Data acquisition takes place over time, so any environmental change during the measurement shows up in the data. The presence of vibration during a laser interferometer measurement can cause fringe print through, drop out, and in some cases, the inability to collect data at all.
QPSI & FT-QPSI
As mentioned, vibration causes fringes to wash-out, resulting in bad data or a signal that is impossible to acquire. High powered laser sources can be a part of an anti-vibration solution as they provide enough power to shutter the camera faster as there is more light, which means faster data acquisition and less sensitivity to vibration because the measurement takes less time,
As a company, ZYGO has had a “laser-like” focus on the elimination of the negative effects of vibration, which has resulted in the development of a number of solutions that can be used in relatively hostile manufacturing environments.
In addition to providing proprietary high power HeNe lasers in its interferometers, the company has innovated the QPSI and FT-QPSI solutions, technologies that are a nod to the fact that laser interferometry is transitioning from carefully controlled lab environments as a growing number of applications demand easy-to-use solutions in environments where it was previously impossible to achieve quality metrology. While appropriate in a range of current industrial scenarios, QPSI and FT-QPSI solutions also address the fact that — moving forward — an increasing number of applications require vibration robustness, including large aperture lightweight mirrors used in space telescope applications which will require testing in vacuum chambers and at cryogenic temperatures where the control of vibration is at best extremely challenging, and at worst impossible.
Introduced in 2014, ZYGO’s QPSI acquisition is an on-axis vibration robust solution. QPSI acquisition is a model-based approach that addresses rigid body vibrations, and increases vibration tolerance without sacrificing lateral resolution. It uses an identical set up to traditional Phase Shifting Interferometry (PSI) acquisition.
QPSI technology has been recognized throughout industry as a breakthrough in precision optical testing, eliminating noise and ripple print-through in phase data due to small vibrations, and providing reliable data that would otherwise be “noisy” with traditional PSI acquisition. QPSI measurements require no special setup or calibration, and cycle times are typically equivalent to standard PSI measurements. QPSI is today included as standard on the company’s range of interferometers, enabling reliable high-precision measurements in the presence of vibration from manufacturing equipment through the use of unique software algorithms.
QPSI acquisition, however, only supports two surface cavities, meaning a single interference pattern. To address this, ZYGO has recently introduced FT-QPSI acquisition, a measurement mode with vibration robust performance that can be used anywhere FT-PSI was used. This includes simultaneous front and back surface measurements, optical thickness variation measurements, and homogeneity measurements. FT-QPSI uses the same model-based technique as QPSI with an expanded physical model to account for the additional surfaces and works with the company’s Multi-Surface Test (MST) interferometers.
MST applications are particularly susceptible to vibration, so FT-QPSI measurement mode is an important development. Thin glass (parts < 1mm thick) is an increasingly in demand application for an MST interferometer, FT-QPSI measurement mode being invaluable as samples deform more easily than thicker samples, so the data is more likely to be affected by vibration. Homogeneity measurements are also a core capability for MST interferometers, but homogeneity data tends to have a low PV (peak-valley) so even small amounts of print-through from vibration are visible in the data and greatly effects the measurement quality. Again, the need for FT-QPSI measurement capabilities in such scenarios is obvious.
DYNAPHASE®
Within the context of overcoming issues with vibration when undertaking ultra-precise metrology measurements, in addition to QPSI and FT-QPSI solutions, ZYGO also offers DynaPhase® Dynamic Acquisition technology.
DynaPhase® offers the versatility and performance to address a wide range of challenging optical testing environments and applications like cryogenic and vacuum chamber testing or telescope components and tower applications with long path lengths. DynaPhase® enables the highest vibration tolerance in a Fizeau interferometer, using carrier fringe acquistion facilitated once again by the ZYGO-manufactured high-powered laser and fast acquisition speeds. Patented in-situ calibration enables the highest precision, lowest uncertainty measurements, and excellent correlation to temporal PSI.
SUMMARY
It is the very precision of interferometry that can be its Achilles heel, as even the minutest of environmental vibration can distort measurements. Because of this, and recognizing that interferometry along with other highly-accurate metrology solutions are increasingly being used in-process and on the production floor rather than in controllable lab environments, ZYGO is pioneering a raft of developments that overcome vibration issues.
