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How do fiber optic sensors work

A project log for DIY fiber optic sensors

Make your intrisic fiber-optic sensor system at home, using your mbed/arduino, RaspberryPI and cheap optical fibers

jaromir.sukubajaromir.sukuba 06/06/2014 at 11:140 Comments

There are a lot of possible methods to achieve the goal, but I'll focus on sensors being wavelength specific optical devices.

In theory, simplest one is fiber Bragg grating, FBG.

FBG

Quote from wikipedia:

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.


Pretty simple, huh?

In fact, the only thing one needs to create FBG is periodic change in refractive index of core (just to be sure – check out how the optic fiber works). This can be done using laser, for example. Imagine shooting single laser ray on fiber, partially “destroying” the core, then move the fiber by some distance, shooting again, rinse and repeat until a lot of nicks are made. Notice the uncertain words “partially”, “some”, “a lot of”. The distance between the nicks is actually easy to determine – Wikipedia again

The refractive index will typically alternate over a defined length. The reflected wavelength (), called the Bragg wavelength, is defined by the relationship,

where is the effective refractive index of the grating in the fiber core and is the grating period.

The other parameter is usually subject of experiments, especially when it comes to laser power. The needed amount of shots is usually in hundreds or thousands.

While making FBG seems simple, there is a minor problem with positioning system for FBG inscribing. To achieve wavelength of ~1,5 micrometer (suitable wavelength for fiber optic systems), the needed fiber movement is in order of hundreds of nanometers. It could be nice challenge, but there are simpler ways to achieve the goal – long period gratings (LPG).

Long period grating

LPG is done by periodical coupling light of core and cladding (I assume you have still opened the tab with this Wikipedia article). Fortunately, the periodicity has to be much longer compared to FBGs, just in order of fraction of millimeter; the coupling can be done by partially “melting” core and cladding together, using laser, hot wire or electric discharge. What more, the amount of those couplings is a few dozens.

Now we are getting somewhere. In theory, one should just take standard single-mode optical fiber, strip the coating off and apply laser/discharge/high temperature into tiny region of fiber, move on, apply again, move … etc. until a LPG is done.

How does the sensor work, after all?

Now we have single grating on fiber, what to do with it? Well, when you look at the fiber sensor spectrum with spectrometer, you can see peak, characteristic for given sensor geometry. When the geometry is changed, the spectrum also changes. Imagine you have peak wavelength at 1550nm when fiber is resting. Now you can take the fiber and pull it (glass core fibers can be pulled up to few percent), the peak wavelength will move to 1551nm, for example. When you release the fiber, it will move back to 1550nm.

Voila, we have tensometer.

What’s next?

Now I need to find out two things:

1, How to measure spectrum of the fiber, creating optical sensor interrogator

2, How to make the core/cladding joints.

I have some thoughts of how to do it, so stay tuned for updates.

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