General > General Technical Chat
Anyone working in Photonics?
nfmax:
--- Quote from: ajb on March 19, 2021, 08:54:59 pm ---...Also random interesting things like optical fiber-based sensors, where an optical structure at one end of the fiber exhibits optical characteristics that change in response to an external condition like temperature or strain, which can be read by a light source+detector at the other end of the fiber...
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You can also use the optical fibre itself as a sensor. Over the years, I've worked developing systems using laser light pulses, launched into one end of a fibre, which are backscattered as they interact with the molecular scale structure of the glass, returning to the launch site as a delayed 'echo', much like a radar or lidar system. Except that the backscatter signal is continuous, being generated at each point in the fibre as the laser pulse reaches it. Simple Rayleigh scattering returns light at the same wavelength as the input pulse, and forms the basis of the OTDR (Optical Time-Domain Reflectometry) method of measuring attenuation in fibres. However, if you use an extremely narrow linewidth laser (~kHz) and coherently detect the backscatter, using a frequency-shifted copy of the laser output as a local oscillator, you can measure microscopic displacements of each point along the fibre. This lets you use the fibre as a distributed vibration sensor or microphone.
Raman scattering is a non-linear interaction which causes the wavelength of the backscattered light to be different from that of the incident laser pulse. The Raman scattering process is temperature-sensitive, so you can make a distributed thermometer or temperature sensor based on the intensity of Raman backscatter. It isn't easy, as the amount of Raman-scattered light is very much less than the incident pulse, and there are all sorts of complications arising from the variations in fibre attenuation and group velocity at the different wavelengths involved. However, the wavelength shift is large enough to enable the Raman backscatter to be easily separated from the Rayleigh signal using optical filters.
Brillouin scattering is another non-linear process, also sensitive to temperature but sensitive to strain as well. The level of Brillouin scattering is intrinsically much larger than that of Raman scattering (unless the fibre has been specifically designed to suppress it - as some high-performance communication fibres now are) but the wavelength shift is much smaller. Optical filtering is no longer possible, so coherent detection using an optical local oscillator, followed by standard microwave IF technique, is required to recover useful measurements.
These sensors have many applications, mostly measuring long, thin things. Like oil wells, gas pipelines, security fences, railways, and even optical communication cables themselves (using either a spare fibre or 'spare' wavelengths in a traffic-carrying fibre). To extend the range, it is possible in some cases to use intermediate optical amplifiers (EDFAs) which are themselves 'pumped' (i.e. powered) by light sent from the measuring instrument.
It's an involved and fascinating field. The technical problems are non-trivial, and as well as photonics 'proper', can require expertise in analogue electronics, digital signal processing, measurement physics, and materials science & chemistry.
For more information, I recommend finding a copy of An Introduction to Distributed Optical Fibre Sensors by my former colleague Arthur H. Hartog, published by CRC Press Taylor & Francis Group, ISBN 978-1-4822-5957-5
PartialDischarge:
--- Quote from: JohnnyMalaria on March 21, 2021, 10:02:30 pm ---
Indeed. I've used acousto-optic modulators made with it (as well as other that use flint glass or lead molybdate). In my application, I use two to derive a precise, stable and known frequency difference of the order of a few kHz between two laser beams. They combine to create a kind of interferometer to detect nanoparticle motion.
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What I wonder is where are devices bought from, are they all custom made or is there a well known supplier for the easiest configurations?
JohnnyMalaria:
--- Quote from: MasterTech on March 21, 2021, 10:17:01 pm ---
--- Quote from: JohnnyMalaria on March 21, 2021, 10:02:30 pm ---
Indeed. I've used acousto-optic modulators made with it (as well as other that use flint glass or lead molybdate). In my application, I use two to derive a precise, stable and known frequency difference of the order of a few kHz between two laser beams. They combine to create a kind of interferometer to detect nanoparticle motion.
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What I wonder is where are devices bought from, are they all custom made or is there a well known supplier for the easiest configurations?
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Plenty of off-the-shelf solutions. The ones I've used:
https://www.brimrose.com/
http://www.isomet.com/
https://intraaction.com/
thermistor-guy:
--- Quote from: jpanhalt on March 21, 2021, 05:00:23 pm ---More than 50 years ago, I chose photochemistry for an advanced degree. Never regretted it; although, that is not what I ended up doing. I know photonics is different, but if your uni offers it and you have the time, I recommend at least one advanced course in chemistry.
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I've worked on non-contact temperature sensing using fluorescence. I knew nothing about it at first, but picked up what I needed. The application was calibrating thermal cyclers for DNA quantitation (via Polymerase Chain Reaction). Using luminescence for non-contact sensing is a fascinating area, with important bio-tech applications.
Just now I saw an ad from ThermoFisher for "microvolume UV-Vis spectrophotometers" for protein and nucleic acid research. This kind of instrumentation needs a cross-disciplinary design team: optics, electronics, mechanics, biochemistry, photochemistry, software. If you like that kind of development, an optics+electronics background will give you an edge. Extra advantage if your course teaches you MEMs.
mawyatt:
Another very interesting application of photonics was started in 1979 called remote sensing spectral radiometry which involved remote sensing the chemical makeup of the atmosphere at up to 5 miles away, and doing so completely passively (no signal source). The technique relied on the small temperature variations in the atmosphere to create an effective absorption and reradiating spectrum in the 8 to 12 micron region which could be detected remotely with an extremely sensitive interferometer. This instrument involved a tiny HeNe laser controlled moving mirror interferometer with cryogenically cooled (Split-Cycle Sterling Cooler) HgTe sensor producing an interferogram which was capture with an 18 bit ADC synched with the moving mirror position.
After some math the result was the spectrum at a distance which contained the various complex molecular signatures of molecules in the 8-12 micron region. The unique signatures of known chemical agents used in warfare were stored and compared to the captured spectral signatures. The science advisor to US President Regan claimed this passive technique to be impossible without illuminating the atmosphere with a high power CO2 laser, and the program should be terminated. Lighting up a high power laser in the battlefield in not a good idea for obvious reasons, and the concerns were valid!!
A live demo was setup in front of Congressional Selected Scientific Staffers with a non-toxic stimulant agent sprayed from a moving jeep at ~5miles in a battlefield type environment. The result was an outstanding success, the program survived resulting in the continued development of the XM21 Remote Sensing Chemical Agent Detector. Later the XM21 went in full production and protected by early warning the Allied Forces during Desert Storm and made the front page of Newsweek magazine :-+
Best,
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