Displacement Sensor

A Guide to Laser Interferometry - Why Choose Fabry-Pérot?

unlock the highest level of precision in cutting-edge applications

Imagine possessing a ruler so precise it could measure the width of an atom, or a microscope so powerful it could detect the ripples in spacetime. This isn't science fiction - it's the reality of laser interferometry, a technology revolutionizing our ability to explore the minutest corners of the universe.

A laser interferometer is an instrument used to measure microscopic displacement, changes in refractive index, or to examine surface irregularities. It offers extremely precise non-contact measurements and is among the most accurate methods in metrology. Industries like semiconductor manufacturing, aerospace, and material research depend on this precision, making it a pivotal enabler of technological progress.

How does interferometry work?

Picture yourself at a pond, skipping stones. When two ripples from different stones meet, they create a pattern - sometimes the waves add up, making bigger waves, and sometimes they cancel out, creating calm spots. 

Optical interferometry works on a similar principle. Overlapping light waves interfere and thereby create one combined wave. The interference can lead to an increase (constructive interference) or decrease (destructive interference) of the resulting amplitude, depending on how well the crests and troughs of the waves match each other.

The interference is influenced by, amongst others, the  phase and wavelength of the individual waves. By keeping the wavelength constant, detecting the intensity and analysing the interference pattern, it is possible to draw conclusions to the phase shift between individual waves. As the phase shift can be linked to external changes, like the displacement of a mirror, it´s possible to translate the phase shift to a displacement with ultra-high precision. In contrary to white light interferometry (mostly surface analysis) in the following the focus will be on laser interferometry, which uses monochromatic, directional and coherent light ideally suited for punctual and precise displacement measurement.

Optical Laser Interferometer Systems:

For optical laser interferometry at least two light waves must interfere. One of them is reflected by your object of interest and the resulting interference pattern is monitored with a detector. There are two major types of laser interferometers:

1. Heterodyne: Interference of light with slightly different wavelengths
2. Homodyne: Interference of light with identical wavelength

Due to lower complexity and therefore significant higher cost-effectiveness, homodyne systems enable the expansion of cutting-edge laser interferometry into a broader field of applications and overcome the precision limitations of e.g. optical encoders. There are multiple designs for homodyne laser interferometers, but in the following the focus will be on the most common one - the Michelson Interferometer - and a special design optimized for highest robustness, thermal stability and compactness – the Fabry-Pérot Interferometer.

attocube’s IDS3010

The IDS3010 operates by directing light from a semiconductor laser through an optical fiber to a sensor head which is a purely optical component based on the described Fabry-Pérot principle. At the end of the optical fiber 4% of the light is reflected to form the reference beam, while the rest is collimated or focused by the sensor head optics into a beam aimed at the target. The measurement beam reflects off the target and when re-entering the fiber, it interferes with the reference beam. This interference signal is sent back through the fiber to a detector, which displays a sinusoidal interference intensity based on the target's position. To detect positional changes within a fraction of the light's wavelength and determine direction, the system generates a second, 90° phase-shifted cosine signal through high-frequency wavelength modulation.

The IDS3010 features a compact base module housing, the semiconductor laser diode, and electronic controls. The semiconductor laser's wavelength is kept extremely stable by a gas cell filled with acetylene gas, which features naturally constant absorption peaks at certain wavelengths. A control loop adjusts the laser wavelength with an expanded uncertainty (k=2) of just ±0.3 pm, certified by NIST. This precise wavelength reference ensures the accuracy and traceability necessary for reliable high-precision measurements.

Compared to conventional laser interferometers, the unique fiber-based concept with ultra-compact optical Fabry-Pérot sensor heads and a durable semiconductor laser enables nanometer precise displacement measurements combined with straightforward integration and low cost of ownership. The sensor heads can even be implemented in systems operating in extreme environments like ultra-high vacuum or cryogenic to high temperatures, and thereby allowing completely new approaches to solve critical challenges in cutting edge applications.