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Laser-Based Interferometry

Laser-based interferometry is an incredibly precise metrology technique. It employs light to accurately measure distances such as length, outside diameter, pitch diameter, lead, major/minor diameters and taper angles – as well as linear displacement in precision slip gauges.

An ideal method of measuring target illumination involves splitting one coherent light source into two optical paths using a beam-splitter and lighting each path with the target to be measured, before recording their interference patterns on a photodetector.

Optical Interference

Interferometers utilize light interference to precisely measure lengths with nanometer precision. A laser is split into two beams that travel different optical paths before coming together again to form a pattern of interference fringes on the detector. A displacement sensor or photodetector measures this pattern and converts it into digital signal that provides information about any differences in length between their paths.

Optical interference works on the principle that two identical waves can interact constructively or destructively depending on how their peaks and troughs intersect, creating either constructive or destructive interference effects. When their peaks meet perfectly in one direction, their interference produces an additional wave larger than any individual height of either of its individual troughs – this principle forms the cornerstone of all optical interferometry techniques.

An interferometer typically consists of a reference arm with a fixed path length and a moving sample arm that can change position relative to it, creating phase variations in light output by the reference arm as its position changes; these fluctuations cause errors in measured displacement which reduce accuracy of measurements; in order to compensate for these errors terms, measurement systems typically feature hardware control and correction methods which produce more precise measurement results.

One method involves employing a synchronized modulated laser source to perform wavelength shifting and phase shifting on both reference and test arms of an interferometer, using Fourier transform analysis on captured interferograms of phase shifting data captured from these arms to reveal wavelength-dependent geometric dispersion (GDD) and chromatic dispersion – making possible measurements with multiple reflective surfaces that would otherwise produce complex interference patterns difficult or impossible to interpret with traditional interferometers.

An additional way to reduce error caused by nonlinearity of an interferometer is the use of a micro-displacement stage that provides more precise movement control of measurement mirror than traditional mechanical systems can, and can eliminate movement control errors due to thermal distortion and hysteresis.

Speckle Interference

Speckle interference occurs when a laser beam is scattered off of a rough surface, creating a complex pattern of interference fringes which can be detected using a speckle interferometer. These fringes reveal phase differences between two wavefronts comprising the beam; such differences result from differences in optical path length caused by variations in surface shape and texture.

An image recording device (CCD) is typically employed in laser interferometers to detect intensity variations that constitute fringe patterns. An expander splits object and reference beams; object beam is used to illuminate target, while reference beam is directed toward fixed point on CCD device and recorded. Shifted and correlated speckle fields created from these recorded speckle fields then form fringe pattern images shown in Figure 1.

These fringe patterns reveal an abundance of information about the surface that is being compromised, such as dimensions and inclination of rough areas that help identify defects; as well as being used to calculate distances from known points on the surface – invaluable data in many measurement applications.

For optimal performance, superimposed shifted and unshifted speckle fields should not exceed an average speckle size shift. To assess its effect on performance, this restriction was examined through superimposing simulated unshifted and shifted fields onto each other systematically, with results reflecting experimental findings quite accurately.

Speckle patterns depend on the magnification and numerical aperture of objective lenses in a lens system, for instance by selecting four objective lenses (10x/0.25NA, 20x/0.45NA, 60x/0.7NA and 120x/1.2NA) sequentially inserted in an object arm while keeping one reference arm objective lens 10x/0.25NA fixed as reference arm objective lens (10x/0.25NA) constant as reference objective lens (10x/0.25NA). Figure 2 displays normalized correlation maps showing speckle fields from both reference and object arms simultaneously in two dimensions normalized normalized correlation maps which compare both speckle fields within two dimensions (referral and object) through two dimensions normalized correlation maps showing both reference (referral arm 10x/0.25NA) while keeping one reference arm objective lens 10 x/0.25NA fixed as reference arm objective lens 10 x/0.25NA) while keeping another fixed 10x/0.25NA lens constant as reference arm objective lens 10x/0.25NA to generate two-dimensional normalized correlation maps depicting two-dimensional normalized correlation maps that compare reference (left arm), making up 120×1.2NA in terms of both reference (left arm), with respect to (referred arm). See Figure 2, normalized correlation maps). Figure 2, normalized correlation maps representing reference field as well as object field 10 x/0.25NA objective lens). Figure 2, shows 2D normalized correlation maps illustrating 2D correlation maps depicted both.

Doppler Velocimetry

Doppler velocimetry is a noninvasive optical technique for measuring velocity of moving objects. It relies on Doppler effect caused by scattered light from microscopic particles moving with fluid flow to measure their velocity. Doppler velocimetry is widely utilized for medical diagnostics and research applications to measure speed, displacement and velocity profiles as well as track objects in space. A typical Doppler interferometer setup based on Michelson’s principle can also be found here. In this set up, a single wavelength laser is reflected off two retroreflectors to form two separate retroreflective arm mirrors; one attached directly to the beam splitter serves as the reference arm while its counterpart further away acts as measurement arm by having its distance from it vary with time – thereby measuring time differences between fringes proportional to tracer particle velocity.

To enhance this technique’s performance, a single mode CW Argon laser is employed. This laser features narrow spectral width and high coherence length characteristics ideal for interferometry; its low noise floor enables frequency resolutions below 1Hz while offering precise beam geometry control with high-resolution measurement signals.

Doppler interferometers use two light beams that are reflected off a fixed point into a laser cavity, where they are then combined and mixed with the reference beam through a polarizer to form an interference image of target movement. As its position changes, its Doppler shift causes changes to occur in frequency of reflected light which are detected by detectors for measurement. This data provides accurate displacement measurement.

This technique has many clinical applications, such as measuring blood flow. Ophthalmologists use this method to measure velocity of blood flow through the choroid and optic nerve in order to diagnose and treat eye diseases like glaucoma; other applications include studying uterine blood flow during fetal development as well as arterial blockages that block it off.


As its name implies, holography uses laser interferometry to create three-dimensional holograms of objects or surfaces using laser interferometry. By simultaneously recording the amplitude and phase of both reference light beam (split by a beam splitter) and returned scattered light from an object’s surface (which is recorded on a holographic plate), an exact replica can be reconstructed using this technique; also known as computer-aided speckle pattern interferometry or television/video holography.

Interferometers rely on interference. A laser beam is split into multiple beams that combine again, producing interference fringes that show how different parts of a system differ in optical path lengths. Common types include Michelson, Mach-Zehnder and Fizeau interferometers as well as Fresnel bi-prisms (Rayleigh mirrors) and Fresnel lenses (Fabry-Perot, Michelson-type).

Un essential step to accurately measuring optical path differences lies in ensuring the beams recombine in perfect sync, which requires using high-quality single-mode laser sources with long coherence length. HUBNER Photonics offers a broad portfolio of single-mode laser sources suitable for optical interferometry applications at wavelengths from 405nm to 2.1 microns.

LIGO’s laser interferometers are engineered to be perfectly in sync when merging at the end of each arm, so that when gravitational waves pass through they result in alternate lengthening and shortening as they travel through it, producing an intermittent flicker of light that ranges in intensity from no light at all where waves destructively interfere to almost as bright a beam where waves constructively interfere.

LIGO uses sophisticated techniques that enable it to measure the linear displacement of objects over very small ranges, from fractions of nanometers to meters, while meeting stringent accuracy and precision requirements. Interferometers make these measurements possible due to being capable of measuring not only linear motion but also angular displacement and changes in air refractive index; as a result, these systems represent one of the highest precision length measurement instruments currently in existence.
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