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Exotic ultrasonics for the real world (Ultrafast project)

The main aim of this project is to dramatically increase the speed of picosecond ultrasonic experiments. Pico second ultrasonics is a well established laser ultrasound technique that is used for material characterisation.

The experiments are based around the pump robe technique where a pump laser pulse generates the acoustic waves and a time delayed probe pulse measured the interaction of the sound wave with the sample. The d=time delay is controlled by scanning a large (and typically slow) mechanical stage. Due to the high frequency waves generated (in the low to hundreds of GHz) the pump beam is modulated at a lower intermediate frequency so that low frequency electronics can be used to capture the signals.

Traditional detector arrangement is to focus the returning probe light onto a photodiode, the output of which goes to a lock in amplifier whose reference is the drive signal for the modulator. This yields a very sensitive detection system capable of measuring modulation depths in the range of 1 part in 106-107. Due to the slow speed of the mechanical delay line and the point measurement system the experiments can take a long time to perform making imaging applications impractical. We have developed custom made detectors that increase the data acquisition speed by more than an order of magnitude. We have achieved this by parallelising the detection by building a custom linear array detector which has very large light capturing ability. This combined with a suitable algorithm can replace the traditional photodiode/lock-in amplifier combination and provide a similar signal to noise ratio. The speed increases we have achieved have allowed us capture both 1D scans and more recently 2D images. Work progresses to improve the detectors and our system still further.

This project has involved many people to date; Richard Smith has built up the main optical system and code to drive the experiment, with help and advice from Steve Sharples. Some of the samples that have been measured were prepared by Teti Stratoudaki. The custom detectors were designed and laid out primarily by Mark Pitter, Roger Light and Nick Johnston. The direction and drive of the project has been managed by Mike Somekh who brought us all together for this very successful project.

Conferences & Publications

Parallel detection of low modulation depth signals: application to picosecond ultrasonics, R J Smith, M G Somekh, S D Sharples, M C Pitter, I Harrison and C Rossignol , Meas. Sci. Technol. 19 (2008) 055301 (8pp)

Parallel Ultrafast ultrasonics, R Smith, M Somekh S Sharples, M Pitter, R Light, N Johnston, Rank prize funds - mini symposia on optical aspects of NDT, 19-22nd May 2008 Grasmere cumbria

Parallel Detection for picosecond ultrasonics, RJSmith MG somekh, S Sharples, M Pitter, R light, N Johnston, Acoustics ’08,Paris, June 2008, article 003508

Parallel detection in picosecond ultrasonics with both commercial and custom array detection,R Smith, M Somekh, S Sharples, M Pitter, R Light, N Johnston, 1st Symposium of Laser ultrasonics, Montreal Canada, July 2008.