Sound into Light

A Laser Doppler Vibrometer shining a laser onto a resonator. The device could detect motion of a membrane just a few pico metres in amplitude

A Laser Doppler Vibrometer shining a laser onto a resonator. The vibrometer could detect motion of a membrane vibrating with an amplitude of less than one thousandth of a millimetre. The inset show the laser spot in detail.

Recently I have encountered two pieces of technology which have left me speechless: gobsmacked in amazement. And as I write about it now, I realise that they both did the same thing – they turned sound into light – but in completely different ways.

The first device was a Laser Doppler Vibrometer – a device that could detect tiny motions of a surface. It worked by shining a low power laser – like a laser pointer – onto a surface and analysing the light which scattered off the surface. What was amazing was how far away it could do this from – and how sensitive it was. We tested it on our resonator – the copper object in the photograph above. The device was about a metre away and yet it could easily detect vibrations of just a thousandth of a millimetre – it was as sensitive as our (very expensive) microphones! The company that makes the device (Polytec) have made a video that explains how it works

The second device was an optical fibre that could sense sound. What? A company (Silixa) have developed a special optical fibre – which can be up to 10 km long – and a device which turns this fibre into the equivalent of 10,000 independent microphones. It can simultaneously listen to the sound at every metre along the fibre.

Imagine hanging the fibre in a room – you could listen anywhere along the fibre and not hear all the other sounds! Silixa have thankfully produced products that are dramatically more useful than a party eavesdropper!

Silixa haven’t disclosed how it works but I think I can guess. I think it is an optical fibre with two cores. Light can travel in each of the two cores of the fibre almost completely independently. But when the fibre is strained – bent – even ever so slightly, light can leak from one core to the other.

I think the device works by shining a short pulse of bright laser light down one fibre. The pulse is only around 1 nanosecond long and so is only around 0.2 metres from start to finish. It travels from the laser to the end of a 10 kilometre fibre in around 50 millionths of a second. If the fibre is unstrained, very little light leaks from this brightly illuminated core to the dark core. But if the fibre is strained – perhaps because a sound wave has bent it microscopically – some light leaks from the bright core to the dark core. It then has to travel back to the source where it can be detected.

  • The time delay between sending the pulse into one core and the detecting the light in the second core allows one to work out where on the fibre the light came from.
  • If one sends several thousand pulses per second, one can evaluate the state of strain of the fibre several thousand times per second.
  • If one just detects the light with a particular delay after the initial pulse, then one is sensitive to vibrations of the fibre a particular distance away from the source
    • If we detect light 10.000 microseconds after the pulse, one is detecting light from 1.000 km away from the source
    • If we detect light 10.005 microseconds after the pulse, one is detecting light from 1.001 km away from the source

What is amazing is not that I can think up an explanation – right or wrong I don’t know – but that a company can make a product which actually does this in the field!

I take my hat off to Polytec and Silixa – they have made products which made my jaw drop. Thanks.

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