Fiber-optic sensors

Electron or Photon sensing?

Nowadays when we talk about sensors, electrons are involved in sensing, transmitting and displaying parts. We know light speed is around 299,792,458 m/s and for hydrogen electron we got approximately 2,200,000 m/s that indicates sensing based on light is 100 times much quicker and clear (with 220 Kev energy electron gets 90% speed of light and for 99.9999992% speed we would need 4 Gev energy). Electron drift velocity could be calculated by :

v= (I/nAq)

For electrons we need conducting materials like copper, Al, Au, … and for light we need wave guides that boundary conditions restricts the light to travel with minimum loss in one direction. Here we use cylinderical waveguides which are made from silica and materials like fluorozirconate, fluoroaluminate, and chalcogenide glasses as well as crystalline materials like sapphire.

green laser trace (coupled in optical fiber)

Sensing media changed to light and fiber optics but we still need semiconductors like GaN, GaP, GaAsInP, … in structures like Quantum cascade lasers to produce laser (highly monochromatic, directional, coherent, … properties) and to capture photons in CCD/CMOS cameras like Si, GaAs, Ge, InGaAs, … .

tcd1304 linear array ccd

Tapered optical fibers application?

To increase interaction of light with environment, optical fiber are tapered using flame (butane+oxygen) , laser (high intensity), hydrogen, nuclear radiations, … . Evanescence field penetration depth is depended to tapering angle, tapered ratio and length of tapering, … . Evanescence field interaction with gas, electric field, magnetic field, … changes coupling of tapered area and changes in intensity, phase, polarity, wavelength, frequency, … are observed in output.

125 µm fiber optics (core+clad) before tapering

In laser base tapering, interference and special masks with linear patterns like grating could make patterns of FBGs (Fiber Bragg Grating) with UV laser on germanium doped silica fibers.

Automated tapering?

Tapering process needs automation with controlling limits. To set automation to specific tapered diameter, image processing is necessary to overcome images aberration and imaging problems. In below image, calibrated system with above image could measure diameter with µs delay in precision system and ms delay in ordinary system using camera. A python program (also a C++ code) reads stream and with a sampling frequency takes frames out of stream and runs modified edge, line, … detection algorithm.
Every frame has at least three layers called RGB layers (as Bayer filters), two algorithms get involved to determine gradient of intensities in 3 layers and gray scaled layer. Standard gray scale layer is made of [0.299 R, 0.587 G, 0.114 B] , this kernel’s output is intensity matrix and differences of fiber upper and lower boundaries could be determined using gradient method and storing grater vertical/horizontal valued lines and filtering them.
In advanced method we could use neural networks like RNN (LSTMs , …) , classification algorithms, object detection haar cascade and mach zehnder interferometer to determine refractive indexes and diameters with grater precision.

tapered optical fiber

DIY Optical Spectrum Analyzer?

For ordinary applications, output intensity measuring with silicon camera is enough but for practical important situation we could analyze the optical spectrum of switchable wavelength lasers or broadband lamps. An easy DIY visible light spectrometer could be made from simple grating (cd, dvd, …), laser pointer, back-illuminated CMOS image sensors and a raspberry Pi unit. Below image is a proof of DIY OSA (Optical spectrum analyzer) practical principles.

diffraction grating behaviour of cd, dvd, … and ordinary light spectrum

If we analyze the spectrum of above image we get below image of spectrum which changes in time of sensing. Python code with tkinter GUI is used to monitor changes and also a Java Script (React.js + Apache server) GUI is used to broadcast and monitor the data in server based application to clients which are connected to specific IP/Domain.

light spectrum from DIY OSA (raspberry pi)

Sensing Materials?

Soon … .

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