Fiber optics Flashcards
(10 cards)
What are the general characteristics and typologies of fiber optics sensor technology?
Peculiar characteristics include: EM immunity, small dimensions, embedment capabilities (in composite). Different coatings are available to withstand different conditions, but also convey different outer radii/thickness.
Types of sensors:
* Point-like (FBG) multiplexing capability [size: mm]
* Integral (Interferometric) one or more fibers [size: mm to 10m]
* Distributed (Raman, Rayleigh, Brillouin) entire fiber length [size: km]
What is the working principle of Fiber Optics?
A physical quantity to be measured induces a change in light properties. Fiber optics create a waveguide for light propagation through different refractive index between Core and Cladding, a direct application of snell’s law.
Evaluating the amplitude change of light, phase shift of light, or frequency shift of the light, we may determine what is happening on the material/structure.
What is the manufacturing process of Fiber Optics?
A preform is first made, and then the fed into an inline furnace. After validation with a micrometer, a buffer is applied, and then cured in an oven. The resulting glass fiber then gets another buffer and curing (final layer), and fed into a tractor mechanism to be rolled. Optical properties of the glass can be governed with special treatments (doping). Chemical Vapor Deposition technology to introduce gaseous mixtures of silicon chloride and germanium chloride into a silica tube.
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What are the most common FO architectures and their typologies?
Mach-Zehnder interferometric sensor (integral sensor). Operates via phase shift of light
Michelson interferometric sensor (local sensor). Operates via phase shift of light
Fabry-Perot interferometric sensor (local sensor). Operates via phase shift
FBGS (Fibre Bragg Grating Sensor): local measurements, high accuracy and resolution (3-5με), multiplexing, real time monitoring, compact and reliable optoelectronic device, insensitive to disturbances along the fibre. Operates via frequency shift of light
What are the typical operating windows for Fibre Optics?
I Window: 850nm, II Window: 1300nm, III Window: 1550nm. Operating outside of these will have greater losses. These are all slightly infrared wavelengths.
What is the working principle of a Fiber Bragg Grating Sensor (FBGS)?
Successive gratings will reflect specific wavelengths and let pass others. After enough gratings of the same time, the superimposed interference will be a sharp peak resembling a delta-dirac. When the grating is deformed, pitch (distance between successive adjacent grating lines, Λ) will increase, and the effective refractive index will increase. This results in a shift of reflected wavelength.
This deformation, however, can be both mechanical and thermal. Additionally, also the refractive index may vary based on mechanical or thermal load. Indeed, these aspects are coupled.
The Bragg wavelength λ_B shifts due to:
Δλ_B=λ_B (α+ξ)ΔT+λ_B (1-p_e)ε
Where:
α = thermal expansion coefficient of the fiber
ξ = thermo-optic coefficient (change in refractive index with temperature)
p_e = photoelastic constant
ΔT = temperature change
ε = mechanical strain
Problem: You can’t distinguish whether a shift in λ_B is due to temperature or strain unless one of them is isolated.
What is the capillary tube method to decouple the FBGS?
FBG1 is directly embedded into the composite structure, FBG2 is inscribed at the extremity of an optical fiber which is inserted in a capillary tube which is, in turn, embedded into the composite. As such, FBG2’s reading does not change with strain. That is: K_ε_2=0. This is then functionally a temperature only reference sensor. Removing the contribution of temperature from the coupled sensor, FBG1, we get the strain contribution.
What is the bi-refringent fibers method to decouple the FBGS?
Bi-refringent optical fibers have two main axes that are called fast axis and slow axis, depending on how fast the waves propagate on each one. A proper manufacturing process allow to have two refractive indices through two different axes.
A guided polarized wave travelling on the axis which has a high refractive index (i.e. slow axis) will run more slowly than a guided polarized wave running on the other axis (i.e. fast axis).
This functionally gives us two sensors, which then allows us to solve the system.
What kind of multiplexing is supported by FBGS? How do they work?
Two types of multiplexing are possible: time division or wavelength division.
Time Division Multiplexing (TDM)
Time Division Multiplexing relies on temporal separation. All FBGs can have the same Bragg wavelength, but they are physically located at different points along the optical fiber. A narrowband pulsed laser is used to send short light pulses into the fiber. As the light pulse travels down the fiber, it encounters each FBG sequentially. The reflections from each grating return at different times, depending on the distance between the grating and the light source. By measuring the time delay between the launch of the pulse and the arrival of each reflection, the system identifies which sensor generated which signal.
TDM is highly scalable because it allows many sensors with identical spectral properties to coexist on the same fiber, differentiated purely by their physical location. This makes it ideal for distributed sensing over long distances or high sensor densities. However, it requires precise timing equipment and fast optical detectors capable of distinguishing reflections separated by only nanoseconds. Indeed, there is a minimum distance between sensors imposed by the temporal resolution of the equipment. Pulse broadening in long fibers can also reduce resolution, and the sensors must be read sequentially, which makes the method inherently slower than WDM. Additionally, the system design is more complex and typically more expensive due to the requirement for pulsed laser sources and time-resolved signal processing.
Wavelength Division Multiplexing (WDM)
In Wavelength Division Multiplexing, each Fiber Bragg Grating is written with a different Bragg wavelength. This means that every FBG sensor reflects a distinct portion of the spectrum from a broadband light source, such as a superluminescent LED (SLED) or amplified spontaneous emission (ASE) source. When light passes down the optical fiber, each FBG reflects only its assigned wavelength back to the interrogator, where the return spectrum is analyzed to determine shifts in wavelength caused by strain or temperature. These shifts are used to calculate physical changes in the environment.
WDM allows all sensors to be read simultaneously since they operate in different spectral bands. This makes WDM systems very fast, robust, and well-suited for static or slowly varying sensing applications. The simplicity of implementation, due to the use of passive optical components and no moving parts, is a key advantage. However, the method is limited by the available spectral bandwidth. Since each FBG must have a unique reflection peak with sufficient spacing to prevent overlap, the total number of sensors that can be placed on a single fiber is constrained — typically to a few dozen. Additionally, fabrication precision is critical, and temperature-induced spectral drift can complicate readings if peak overlap occurs.
Explain a chirped/distributed FBGS
A standard FBGS has a uniform pitch, causing a peak wavelength in the measurements. If instead, pitch is variable, it is possible to get a reading for each wavelength within a given range, acting as a nearly continuous sensor. This creates a wider reflection spectrum and introduces a spatial mapping between wavelength and position along the grating. Because they reflect over a continuous wavelength range, the response of a chirped grating can be analyzed to detect spatially varying strain or temperature. For example, if part of the structure to which the fiber is bonded experiences strain while the rest does not, that strain will only shift the corresponding portion of the reflected spectrum. By examining changes in the shape of the reflection spectrum, one can infer how strain or temperature varies along the sensor.
Through numerical tools, the strain field applied to the sensor can be recovered. Modelling of the chirped grating is also not trivial. Starting from the CMT (coupled mode theory), which describes the propagation of light inside the sensor, a series of equations describing the transmission and reflection modes can be established. These, however, also need to be solved numerically. Alternatively, one can employ a Transfer Matrix Method approximating the grating as M uniform adjacent sub-gratings and then solving of the equations for M sub-grating (that become ordinary).
It will also be necessary to perform an ICR (inverse chirp reconstruction) apodization. Apodization in FBGs is the intentional shaping of the grating’s refractive index modulation amplitude (i.e. the strength of the grating) along its length. This procedure is performed in order to reduce the perceived error between simulated spectrum and measured ‘real’ spectrum. The refractive index n_eff of each grating is changed iteratively to minimize the error e, eventually allowing accurate reading.
