Tag: FBG system

Fiber Bragg gratings in bogie frame

The application of fiber optic technology as temperature and strain gauges is quite surprising in bogie frames. To be more precise, these fiber optic sensors are applied for examining the carbon fiber bogie, in addition to standard surface-mounted electrical-resistance fiber optic strain gauges.

Optical fibers of 125 micrometers in diameter or 250 micrometers with a coating layer are perfect for this aim. The thing is that the optical fiber is improved to produce fiber Bragg gratings (FBG) in the fiber, efficiently producing a number of semi-reflective mirrors over short but equal intervals.

The operating principle of the FBG system is based on the reflection of the signal (a small amount of the signal at each semi-reflective mirror) when the light is transmitted through a fiber Bragg grating. Herewith, “the originally reflected wavelengths (without the influence of strain) from each Bragg grating are compared to the reflected wavelengths when the structure is loaded.”

It should be noted that in the case of FBG deformation by strain, the spacing between the semi-reflective mirrors is either enlarged (tension) or decreased (compression). Therefore, the change combined with the efficient refractive index and the period of the fiber Bragg gratings leads to a shift in the reflected central Bragg wavelength.

The thing is that the wavelength size demonstrates the strain magnitude. Nevertheless, there is the same effect that happened with temperature change, while the temperature effect is over 10 times the strain effect that is why the fiber optic technology needs to correct for temperature.

The researchers present the techniques applied to compensate for temperature where the fiber Bragg grating is placed close to the end-face of a cleaved optical fiber. The fact is the optical fiber with FBG is put in a capillary tube where one end is fused to the fiber, well away from the grating, and the opposite end is sealed. Finally, the FBG system responds only to temperature.

Nonetheless, it is not enough only to install several strain gauges into the bogie and link them to the instrumentation either. Ir is required to choose the proper fiber, for instance, bend-insensitive optical fibers are suitable. These are optical fibers where the diameter of the core includes 9.5-micrometer fibers with 4.5 mm long fiber Bragg gratings.

Additionally, it is necessary to properly install FBG systems to the bogie so as to act as a homogeneous part of the structure. Fiber Bragg gratings provide such benefits as efficient strain gauge transfer, capable to accommodate localized variations in the surface topology of the composite.

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Fiber Bragg gratings make semiconductor lasers more stable

Researchers from the Czech Republic demonstrate a new technique that allows improving wavelength stability and tunability of semiconductor laser diodes in fiber laser interferometers due to fiber Bragg gratings (FBGs) technology. This simulation technique makes the calculation of arbitrary fiber Bragg grating (apodized, chirp, etc.) with a high level of accuracy by a combination of techniques based on layered dielectric media (LDM) and transfer matrix technology.

Thus, based on the simulations and measurements made by the commercially available FBG technology, it has succeeded in the development of a special 100 mm long fiber Bragg grating with apodization. Herewith, the researchers confirm that the new FBG technology of improved linewidth and mode-hop free tuning range of semiconductor laser systems at the wavelength 760 nm enables to increase the resolution of a laser interferometer. Therefore, the absolute fiber laser interferometer with Vertical Cavity Surface Emitting Laser (VCSEL) to easily apply the FBG system to make the wavelength parameter more stable and monitor the tuning range was produced.

Typically, two types of fiber Bragg gratings are distinguished in the used technology: FBG with a period of ~0.5 µm, and Long-Period Fiber Bragg Gratings (LPFGs) with a period from 100 to 500 µm. Additionally, it should be mentioned that their production requires different methods. Thus, the creation of fiber Bragg gratings is performing by interference patterns, while long-period FBGs are produced by side irradiation of the fiber optic components through an amplitude mask or using the fiber translating technique.

Therefore, the application of FBGs offers high thermal stability, retaining optical properties up to 500 °C. Moreover, the production technique has a dependence on such parameters as the length, the type, and the other factors of the FBG technology. It is possible to determine the necessary parameters on the basis of the FBG spectral profile.

Finally, the simulation technique based on the application of fiber Bragg gratings was developed and tested several types of FBGs. For example, chirped and apodized fiber Bragg gratings (FBGs with modulation of the amplitude and with modulation of the spacing). Herewith, this new FBG system provides numerous benefits in comparison to other types of FBGs.

The main benefit is suppressing the side lobes in the fiber Bragg grating spectral properties. The developed FBG system is a compact reliable one that ensures the operation in an industrial environment where the majority of optical components would be fiber-optic.

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