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Jan 04

Publication update - compact and stable laser diode speckle interferometry

figure8

Quantitative shape measurement of small light bulb using the dual-wavelength technique.

Our research on electronic speckle pattern interferometry (ESPI) was recently published in Measurement Science and Technology. Our interferometer uses a simple and compact optical arrangement which acquires a reference wave from a wedged window placed in the path of the illuminating beam. The design has come out of our work investigating the effects of laser speckle in diffusely reflecting gas cells. This offers an advantage over conventional ESPI interferometers which typically have widely separated beam paths and as such are subject to environmental fluctuations which disturb one beam path differently to the other, which leads to degradation of the desired fringes. Such interferometers are fine in laboratory conditions but impractical in field situations where vibration and thermal fluctuations are much higher.

figure3

Compact ESPI using wedged window to acquire a reference wave.

Our interferometer is based on a current and temperature stabilized laser diode, which allows temporal phase stepping to be performed on the diode itself through modulation of the control current and therefore requires no external moving parts. Further, a coarser frequency tuning can be achieved through modulation of the control temperature enabling shifting to adjacent longitudinal modes (mode hopping). This allows shape measurements to be made using the dual-wavelength technique.

figure4

Correlation fringes (a) and wrapped phase fringes (b) obtained from a flat plate out-of-plane displacement using our speckle interferometer.

Measurements were made using an aluminium flat plate into which an out-of-plane displacement could be induced and a small light bulb which demonstrated the interferometer's shape measurement capability. The paper was very successful and maintained a position in the journal's top ten most read articles for over a month.

The paper is open access and available here

A mechanically stable laser diode speckle interferometer for surface contouring and displacement measurement
D Francis, D Masiyano, J Hodgkinson and R P Tatam
Measurement Science and Technology 26(5), 055402 (2015) - Open Access