How it works, first sketches

A project log for DIY fiber optic sensors, again

Make your own fiber optic sensors

jaromir.sukubajaromir.sukuba 04/21/2018 at 18:333 Comments

The simplest way of making physical measurements with optical fiber (OF) is doing what you are usually explicitly being told to not to do - bending the fiber. It makes sense, as for proper fiber optic communication one is aiming at lowest possible loss of signal in OF; but that is not the case with fiber sensing applications. Here, we are looking for some way of modulating the passing light with the measured quantity, usually in form of amplitude, spectral or phase modulation. The easiest way is amplitude modulation (performed by bending the OF), so that's what I've chosen here.

Bend the fiber!

I will not repeat basics principles behind optical fibers as those are well known, I'll rather focus focus on the less usual aspects of this problematic.

Key parameter of OF is insertion loss - ie. how much of light is attenuated by some amount of length, usually expressed in dB per km. This value is guaranteed by manufacturer, under specific conditions, one of them is bend radius. As bend radius decreases, light in OF core is can't follow the curvature and gets leaked to the fiber cladding, from which it can't enter to the core again and gets attenuated. The smaller bend radius, the more leak to cladding and higher insertion loss. One has to be careful here, as too tight bend can damage or even break the OF.

Provided I can transform the measured quantity into OF bend and measure the insertion loss, I get fiber fiber optic sensor (FOS).

One of possible arrangements is on picture below

OF is placed on compressible material and movable push rod is placed above it. When push rod is pushed into compressible material, it bends the fiber and causes insertion loss. Within some limits, the insertion loss rises as push rod compresses the material; after saturation point (material is compressed, OF can't bend any more) it doesn't change much. The movement of push rod can mean measurement of position or displacement; with help of flexible material it can be transformed to force or weight measurement, or even air/fluid pressure (using flexible bellow) or temperature (with bimetal strip); or whatever else that can be converted to linear motion.

Signal conditioning is not very complicated here.

Led with appropriate wavelength emits light into the fiber, the light gets received with photodiode (PD in this block diagram). The amount of light (=signal from photodiode) is the higher the less attenuation is in path. Electric signal from photodiode is amplified by and ready for processing, in this example is brought to ADC and processed by microcontroller.

As usual, devil is in detail, but that is something for next project log.


Ted Yapo wrote 04/22/2018 at 04:03 point

Mims made a sensitive seismometer with an optical fiber as a pendulum:

I wonder if something similar with 2 fibers coupled through a short air gap could work as a more general-purpose sensor.  Like, maybe couple the fibers in a section of heat-shrink tubing.  When the tubing flexes, the fibers become mis-aligned and the loss increases.

Anyway, cool project! It got me wondering about stuff, which is what I really look for :-)

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jaromir.sukuba wrote 04/22/2018 at 22:22 point

I'm glad you like this project.

Funny enough, I had similar idea for sensor.. Problem with this is the fiber alignment. It would work better with POF, where you have wider core (=much less problems with respect to this). Also, the gap will cause light difraction, which will be - unless the fiber is cleaved at perfectly right angle - directionally non-uniform, that may cause off-coaxial arrangement under measured force - supposedly to have higher loss - to achieve actually lower loss than default straight arrangement. It's going to have also dependance on wavelength and of course gap size.. What more, with really small gaps you need to take care of reflections and it's resonances, you may create Fabry-Perot resonator, being even more wavelength dependant.

All those things (and some other too) can be used for your advantage and open field for experiments, but could ruin your day when unexpected.

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Ted Yapo wrote 04/22/2018 at 22:37 point

Yes, alignment seems critical.  Practical issues seem like they're going to dominate theory in any sensor of this type :-)

What about using a coherent source (laser diode) and detecting speckle noise?  Maybe an AC amplifier could detect the speckle moving and be insensitive to the DC component, if that buys you anything.

Also, for LEDs, TOSLINK fibers and connectors, while bulky and lossy (they're typically POF, although longer ones can be glass) might make the coupling easier.  I see the LED transmitters at surplus places and ebay, and the fibers are cheap and readily available.

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