National Physical Laboratory

Frequency Scanning Interferometry research

Technology

Frequency Scanning Interferometry (FSI) is a technique which allows absolute distances to be measured to interferometric accuracy. An external cavity laser diode can be used which allows its output wavelength (frequency) to be altered over a relatively wide range (tens of nm for a 1550 nm nominal wavelength). When this scanned laser light is used in an interferometer, the output signal of the interferometer can be processed to calculate the absolute length of the measurement beam of the interferometer, provided the reference beam is of fixed, known length.

By using more complex arrangements involving a second frequency scanning laser, it is possible to compensate for variations in the measurement beam arising, e.g. from vibrations. More detailed processing of the acquired interferometer data can give the ability to determine measurement beam lengths at high repetition rate, essentially allowing vibrational motion to be time-resolved. Early FSI systems used conventional bulk, air-spaced optics, but recent research has evolved into the use of fibre-fed interferometers, where the FSI laser light is launched into single mode optical fibres, and the output end of the fibre forms a miniature interferometer, with the reference beam formed from the internal reflection at the fibre-air interface.

Frequency Scanning Interferometry (FSI) research - Figure 1

Miniaturisation

Recent collaborative research between NPL, the University of Oxford and Etalon AG has resulted in a significant reduction in the size of hardware required for FSI with the additional bonus of building in a traceable length reference, linked to the SI metre. The use of fibre-based gas cell technology, coupled with the optoelectronics from the telecommunications industry, has resulted in a commercial FSI system called FSI Multiline, available from Etalon. The system can operate up to 100 individual fibre-fed interferometer channels, each delivering 500 nm accuracy, from a single laser source.

Divergent beam FSI

All FSI systems to date have shared a common limitation - the need to use narrow, collimated beams, limited to one target per (fibre) channel. Recent work by NPL has developed the technique of divergent beam FSI - instead of collimating the output from each fibre, the beam is dispersed using divergent optics, into a cone of light, centred on the fibre end. Rather than a single target per fibre, multiple targets can be illuminated within the cone of light, each returning a small fraction of incident laser intensity to the fibre. Each fibre interferometer then contains a single reference beam and multiple measurement beams, all incident on a single detector. A Fourier transform-based analysis can then be used to separate these multiple signals into a frequency spectrum where the frequency is directly related to the distance from the fibre to the target. In this case, each fibre interferometer is able to measure multiple absolute distances to targets within the cone of illumination. By using omni-directional retro-reflecting targets such as balls of glass with refractive index equal to 2, a multilateration system can be constructed, where absolute 3D positions of multiple targets can be determined using FSI.

Frequency Scanning Interferometry (FSI) research - Figure 2

Applications

NPL is researching potential application areas for FSI in precise 3D coordinate metrology such as monitoring of large structures in the aerospace industry, tracking of robots in 6D (3D position, three rotations), machine tool monitoring and error mapping and large volume ubiquitous metrology networks. Reecent research into FSI was funded under the EMRP LUMINAR project.

Recent publications

Last Updated: 31 Jul 2017
Created: 1 Oct 2014

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