How do interferometric systems work?

How does interferometry work?

Introduction

The most common tool in interferometry, the Michelson Interferometer was invented by Albert Abraham Michelson in 1887, the first American to win a Nobel Prize for science. He came up with a system of mirrors and semi-transparent mirrors (beam splitters) for merging separated beams of light, which are coming from the same source. Laser interferometry is a well-established method for measuring distances with great accuracy.

Basic principles

Typically a single incoming beam of coherent light source will be split into two identical beams by a Michelson interferometer. Each of these beams travels a different route, called a path, and they are recombined before arriving at a detector. The difference in the distance travelled by each beam creates a phase difference between them. It is this introduced phase difference that creates the interference pattern between the initially identical waves, which is identified on the detector. If a single beam has been split along two paths (measurement and reference), then the phase difference is diagnostic of anything that changes the phase along these paths. This could be a physical change in the path length itself or a change in the refractive index through which the beam travels.

Michelson interferometry

The laser beam (1) emerges from the laser source and gets split into two beams (reference (2) and measurement (3)) at the interferometer. These beams get reflected back from the two retroreflectors, recombine at the interferometer before reaching the detector.

The use of retroreflectors ensures that the beams coming from the reference and measurement arms are parallel when they recombine with each other at the interferometer. The recombined beam (4) reaches the detector where they interfere with each other either constructively or destructively. During the constructive interference the two beams are in phase and the peaks of both beams reinforce each other resulting in a bright fringe, whereas during the destructive interference the beams are out of phase and the peaks of one beam are cancelled by the troughs of the second beam resulting in a dark fringe.

Signal processing

The optical signal processing in the detector allows the interference of these two beams to be observed. The displacement of the measurement beam causes change in the relative phase of the two beams. This cycle of destructive and constructive interference causes the intensity of the recombined light to undergo cyclic variation. One cycle of variation in intensity from light to dark to light occurs every time the measurement beam/retroreflector (3) is moved by half the laser wavelength.

Accuracy of the system

The accuracy of the linear positional measurements depends on the accuracy to which the wavelength of the laser beam is known. The operational wavelength of the laser beam depends on the frequency of the laser and the refractive index of the air through which it passes. This refractive index changes with any variations in air temperature, air pressure, and relative humidity.

What is in an RLE laser encoder system?

Laser source	Fibre coupling	Interferometer optics	Measurement optics	Detection scheme	Encoder feedback signals
Frequency stabilised Class 2 HeNe laser	One or two fibre optic laser outputs that deliver laser light directly to the detector heads	Interference of laser light taking different optical paths	Highly reflective hard oxide coated dielectric mirrors	Converting the interference fringes into an electronic signal	Standard digital or analogue quadrature position feedback

What does the RLU laser unit do?

Outputs laser light to detector head

Generates analogue quadrature signals from measurement beam and reference beam