Sapphire fiber Bragg gratings (SFBG) were used for temperature diagnosis of a commercial inductive heated furnace of a fiber draw tower. In a fiber preform temperature gradients > 10K/mm were detected. In the temperature range from 1000°C to 1900°C the measured temperature inside the furnace is about 140K below the control temperature detected by a pyrometer on a spot of the heating element. The dependence of applied temperature on the furnace heating power was derived that can allow furnace control without pyrometer and therefore extending operation range below 1000°C. No degradation of SFBG was observed.
Fiber Bragg gratings (FBGs) have widespread applications in security, information, structural health monitoring, and biophotonics. In telecom applications, FBG inscription has reached a high level of maturity, but remains mainly limited to germanium doped photosensitive single mode fibers. Special applications, like filtering in light harvesting fibers or resonator mirrors for fiber lasers have to deal with special aspects which make the design and realization of FBGs a challenging task. One aspect is the extended wavelength range of these applications. Another aspect is the increasing demand to inscribe fiber Bragg gratings in non-photosensitive germanium-free fibers. Therefore, novel concepts of photosensitivity are proposed. Finally, to increase the amount of captured light the size of the fiber core and the numerical aperture have also to be increased. This goes along with multimode operation and prevents good filtering properties of Bragg gratings.
Fiber Bragg gratings as key components in telecommunication, fiber lasers, and sensing systems usually rely on the Bragg condition for single mode fibers. In special applications, such as in biophotonics and astrophysics, high light coupling efficiency is of great importance and therefore, multimode fibers are often preferred. The wavelength filtering effect of Bragg gratings in multimode fibers, however is spectrally blurred over a wide modal spectrum of the fiber. With a well-designed all solid multicore microstructured fiber a good light guiding efficiency in combination with narrow spectral filtering effect by Bragg gratings becomes possible.
Sapphire based optical fibers provide an attractive basis for ultra-high temperature stable optical sensor elements. Fiber Bragg gratings can be inscribed in such fibers by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. We have investigated crystalline optical fibers as well as structured sapphire-derived all glass optical fibers with aluminum content in the core of up to 50 mol%. The reflection properties, the index modulation and the attenuation effects will be discussed. Results concerning the temperature and strain sensitivity for use as sensor elements at high temperatures will be presented.
Here we report on the inscription of fiber Bragg gratings with two beam interferometry and deep ultraviolet femtosecond laser exposure. This allows to target first order Bragg grating reflections from 2000 nm down to 660 nm without the need for germanium doping or hydrogen loading. The grating in the visible region is remarkable because it bases on a femtosecond laser induced structural size around 110 nm.
In this paper, improved fabrication and calibration techniques of Fiber Bragg Gratings (FBG) for very high temperature sensing applications up to more than 1500 °C will be presented. The fibers used are single crystalline sapphire fibers, which are applicable in such high temperature ranges due to their high melting point at 2040 °C and their extreme thermal stability. The inscription of the FBGs was performed by the second harmonic wave of a Ti:Sa-femtosecond laser system. With pulses of 400 nm wavelength first order gratings could be achieved. Using a two-beam phase mask interferometer, grating arrays within a wide spectral range have been fabricated with only one phase mask and without additional calibration routine. The inscribed grating arrays were wavelength-calibrated using a reference FBG, and their temperature sensitivity was evaluated.
KEYWORDS: Sapphire, Sensors, Temperature metrology, Fiber Bragg gratings, Data acquisition, Temperature sensors, Fiber optics sensors, Optical fibers, Crystals, Process control
We tested the long term stability of first order Sapphire Fiber Bragg gratings (SFBG) at 1400°C for a period of 28 days in air. During the whole period temperatures detected by the SFBG differed less than ±2K°C from the temperatures measured by a type B thermocouple. The spectra at the beginning and the end of the installation were identical. The reliable practical application of wavelength-multiplexed two-grating SFBG arrays for quasi-distributed sensing at very high temperatures has been demonstrated.
Fiber Bragg gratings (FBG) were inscribed in single crystalline sapphire fibers by fs-laser irradiation. Due to the used multi-mode air clad fiber a sapphire-FBG spectra showa a wide asymmetric peak with a half width of 7 nm. Different mathematical peak functions were tested to determine a fiber Bragg wavelength. It was shown that the shift of the calculated Bragg wavelengths in dependence on temperature is identical for the different peak functions. The determination of the fiber Bragg wavelength shift with a resolution of 10pm allows temperature measurements within an accuracy of ±1°C in the temperature range up to 1500°C. Sapphire FBG were used to measure the temperature distribution and thermal fluctuations within an inductive heated furnace in the range from 100°C to 1500°C.
We demonstrate the inscription of fiber Bragg gratings in single crystalline sapphire using the second harmonic of a Ti:Sa-amplified femtosecond laser system. With the laser wavelength of 400 nm first order gratings were fabricated. The interferometric inscription was performed out using the Talbot interferometer. This way, not only single gratings but also multiplexed sensor arrays were realized. For evaluating of the sensor signals an adapted multimodal interrogation setup was build up, because the sapphire fiber is an extreme multimodal air clad fiber. Due to the multimodal reflection spectrum, different peak functions have been tested to evaluate the thermal properties of the grating. The temperature sensors were tested for high temperature applications up to 1200°C with a thermal sensitivity in the order of 25 pm/K which is more than the doubled of that one reached with Bragg gratings in conventional silica fibers.
We demonstrate volume Bragg gratings inscribed in S-TIH53 glass. S-TIH53 is in the proper meaning not
photosensitive; therefore we used a fs-laser system for the inscription process. The grating structure was formed in a
Talbot interferometer and was investigated with help of the external Bragg reflection method. With this method we could
measure the reflectivity profile and thereto the size of the grating. To ensure that the generated gratings are no surface or
absorption gratings the probes were investigated by a microscope and absorption measurements and heating experiments
were done.
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