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The SoM's FPGA can be programmed via a graphical approach using the LabVIEW platform. The energy of the protons at the beam pipe exit was calculated by means of Monte Carlo simulation. This distance corresponds to the layer to the right of the Bragg peak (or the next layer compared to the incident beam direction).

The final step in the analysis is the calculation of the relationship between the measured range and the ΔE energy lost by the particles.

Radiograph data analysis

Future developments

By exploiting the functions of the described proton imaging system, a new method to quantify the quality of the treatment plan will be investigated. A simple dot pattern in the field of view of the radiographic system, presented in this chapter, was covered by the pencil beam. The PSD measured the centroid, FWHM and fluence of the beam delivered at each position.

The RRD measured the centroid, the FWHM of the range of protons delivered in each pour.


PSD and RRD performance was tested at the CATANA proton therapy facility with energies up to 58 MeV. Additionally, Monte Carlo simulations of the response of the RRD detector and radiography of a calibrated target were measured by the system. From the analysis of the results and from a comparison with the data from the simulations, the architecture and technology were validated.

Future developments concern the real-time qualification of a treatment plan and the comparison of results with those provided by the official dosing system.

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Many fields such as medical, oil and gas, civil industry, automotive industry as well as aerospace industry (structural health monitoring) have benefited from optical fiber grating sensors [1-4]. Fiber gratings are known as intrinsic sensing devices, and therefore the propagation of light inside the fiber is controlled and controlled. Fiber gratings have a perturbation with a certain periodicity which will cause the fiber properties to change.

One type of fiber grating is the long period grating (LPG), which Vengsarkar et al. The operating principle consists of coupling the forward propagation core mode with one or more forward propagation cladding modes [8]. The coupling includes cladding modes, which means that the evanescent field will extend into the surroundings of the fibers.

The FBG promotes the coupling of the propagating core mode with the counter-forming core mode. By appropriately choosing the period of an LPG, it is possible to ensure that the core state will couple to a cladding state operating at the tipping point (TAP) [11], also known as the phase matching tipping point (PMTP) or the dispersion tipping point (DTP). Methods used to improve the sensing capability of LPGs have included methods such as tapering [12] and etching [13]; however, this can weaken the fiber's structure and require more gentle handling or complicated packaging.

The properties of LPGs by PMTP can be further tailored by adding a nanoscale functional coating for chemical and gas sensing [21]. However, many still need to be applied in real situations outside the laboratory [2].

Long-period gratings at phase matching turning point

This chapter aims to provide a more comprehensive coverage of LPGs operating at and around the phase-matching turning point compared to what can be found in existing literature [22]. This central resonance wavelength and the sensitivity of an LPG is affected by the order of the coupled cladding mode;. Optical fiber LPG structures (a three-layer cylindrical waveguide consisting of the core, cladding and surround) can be modeled using coupled mode theory [24–26].

This can be used to describe the power transferred between the modes of the waveguides. 2 k cl−coM Aco e (+i 2 δ cl−coM z ) (4) Where Aco is the amplitude of the core mode along the z-axis, A clM is the amplitude of the cladding mode along the z-axis, the z-axis along the optical fiber axis, k is the coupling constant, m is the induced index edge modulation. Phase matching curves of resonant wavelength against lattice period of an LPG can be generated by calculating the dispersion of the core and cladding modes.

Since γ can be used to generalize the sensitivity of the LPG [11], this suggests that an LPG transmission spectrum produced with a period that closely matches the breakpoint will have the greatest sensitivity to external perturbations [11, 14]. With increasing wavelength, the effective refractive index of the cladding mode will decrease more than the effective refractive index of the core mode [15, 30]; this corresponds to the double bands that become apparent in the LPG transmission spectrum. A much larger development can be observed for the LP021 mode compared to the LP020 mode due to the much smaller gradient of the phase matching curve.

When an external disturbance is applied to the LPG, the mode's two resonant bands around the turning point may move toward or away from each other. Phase-matching curves of the 20-25th cladding modes of an optical fiber with a cut-off wavelength of 670 nm.

Illustration of an LPG transmission spectrum with resonance bands at discrete wavelengths.
Illustration of an LPG transmission spectrum with resonance bands at discrete wavelengths.


  • Fabrication considerations

LPG can also be improved by reducing the coating via hydrofluoric acid (HF) and plasma etching [48] to tailor the coupling strength of the coating mode to the PMTP. Plasma etching via ion bombardment and chemical reaction has been used to etch the fiber coating of an LPG to bring the resonance closer to the turning point. Radiation exposure has also been demonstrated to alter the refractive index of B-Ge co-doped fibers, with an equivalent increase in core refractive index of ca.

Due to the nature of LPG, they can be very sensitive to the surrounding environment. The time of UV exposure may also play a role in the sensitivity of LPG at the break point; spectrum of PMTP LPG with a period of 168.7 μm, inscribed. Transmission spectra of PMTP LPG with a sheath diameter of 34.8 μm and a period of 288.5 μm showing a large wavelength shift with a surrounding refractive index change of 0.001.

By changing the coating diameter but keeping the same period, the double resonance bands will also change accordingly [44]. Hydrogen loading can induce or enhance photosensitivity in a fiber by increasing the effective refractive index difference between the core and cladding [52]. However, hydrogen will diffuse from the fiber gradually over time, causing the LPG spectrum to shift [52, 53].

This rapid removal of hydrogen will still cause the resonance wavelengths to shift due to the changing effective indices, but will remain stable and permanent after the annealing process is complete. This must be taken into account when choosing a period to produce an LPG, at or around the tipping point, using a hydrogen-filled fiber [33, 43].


  • Filters
  • Temperature sensing
  • Strain sensing
  • Refractive index sensing
  • Chemical and gas sensing
  • Sensor limitations

By carefully choosing the grating period to allow coupling close to or at the inflection point, it is possible to improve the temperature sensitivity of an LPG. The separation of the dual bands was calculated to be -33.6 nm/1000 μm, whereas the sensitivity of an LPG can be more than an order of magnitude smaller [16]. This can be from the fiber being bent, or from scattering or absorption due to the fiber's protective sheath.

The refractive index of the local environment will affect the effective refractive indices of the wear modes propagating in the fiber. This allows simple calibration and linear interpolation to determine sensor sensitivity within this refractive index range. The sensitivity of LPG can also be optimized by controlling the optical thickness of the overlay so that the tipping point coincides with the mode transition region and has been proven theoretically and experimentally [ 28 , 64 ].

The refractive index sensitivity of the first arc-induced LPG at the breakpoint increased from 400 to 700 nm/RIU to 887–2146 nm/RIU. By combining these phenomena with reduced-diameter fibers, the sensitivity of LPG can be further increased, which can help improve the resolution of biochemical sensing applications. The high sensitivity of PMTP enables the detection of small amounts and concentrations of various chemicals.

The choice of a specific fiber type can also contribute to the final properties of the manufactured sensor. The broad spectral width of the resonance bands can also limit the multiplexing capabilities of the PMTP LPG.


There are a number of parameters, such as temperature or voltage, that change the refractive index of the material. Sellmeier dispersion of the silica was taken into account for the refractive index of the material. 1), it is possible to relate the resonance wavelength to the effective index of the WGM resonance.

Second, the thermo-optical effect causes a change in the refractive index of the material due to a variation in temperature. With this calibration it is possible to correlate the shifts in wavelength with the increase in temperature in the core of the fiber. It should be noted that the axial resolution of the technique will be greater than the grid period.

Then the average increase of temperature must be similar to that established in the case of the uniformly irradiated fiber, for. On the contrary, in the section after the grid (and even at the last millimeter of the FBG), the increase in temperature is below the detection limit of the technique. According to Kramers-Kronig relations, the change in the refractive index is associated with a variation of the absorption coefficient.

The contribution to the loss through the absorption mechanism was measured using the WGM technique (see Fig. 9b). Both contributions are in the same order of magnitude, but smaller in three of the four FUTs. This technique is based on the use of normal axial modes propagating through the fiber.

The measurements were repeated at 1064 nm, to study the dispersion of the elasto-optical effect.

Figure 6 summarizes the measurements performed for the different doped fibers. A similar trend can be observed in all the cases; the resonances shift fast in wavelength for low pump powers, and, beyond certain pump, heating tends to saturate
Figure 6 summarizes the measurements performed for the different doped fibers. A similar trend can be observed in all the cases; the resonances shift fast in wavelength for low pump powers, and, beyond certain pump, heating tends to saturate

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Figure 1 provides a schematic of an FBG inscribed on a fibre core. As is evident from Eq
Illustration of an LPG transmission spectrum with resonance bands at discrete wavelengths.
Figure 4 shows how the grating period of an LPG approaching the turning point  of the LP 021  mode affects the transmission spectrum of the LP 020  and LP 021  cladding  modes
Figure 6 summarizes the measurements performed for the different doped fibers. A similar trend can be observed in all the cases; the resonances shift fast in wavelength for low pump powers, and, beyond certain pump, heating tends to saturate

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