Tino Elsmann

Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation

The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.

100 W average power femtosecond laser at 343 nm

We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2 < 1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

Chirped Phase Mask Interferometer for Fiber Bragg Grating Array Inscription

We present a phase mask inscription technique with two beam interferometry using a lateral nonhomogeneous beam splitter to create gratings with nonhomogeneous periods, the so-called chirped fiber Bragg gratings. Inscription experiments with deep ultraviolet excimer and femtosecond laser sources reveal how this inscription method depends on the coherence properties of the inscription laser. Nonhomogeneous beam splitters are shown to provide a method to generate chirped fiber Bragg gratings with great wavelength versatility, even with the ultraviolet femtosecond laser.

Optical sapphire fiber Bragg gratings as high temperature sensors

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.

Fiber Bragg Gratings in the Visible Spectral Range With Ultraviolet Femtosecond Laser Inscription

In this letter, we investigate the inscription of fiber Bragg gratings in the visible spectral range using deep ultraviolet femtosecond laser exposure and two-beam interferometry. The properties of first-order reflection gratings and third-order gratings for use in the visible wavelength range are compared. Stronger gratings have been achieved for first-order reflecting Bragg gratings compared with third-order gratings. We demonstrate a fiber Bragg grating with a grating period of 226 nm and a filtering efficiency of more than 30 dB.

Advanced fabrication and calibration of high-temperature sensor elements based on sapphire fiber Bragg gratings

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.

High-Temperature Strain Sensing Using Sapphire Fibers With Inscribed First-Order Bragg Gratings

Strain sensor designs and strain measurements based on single-crystal sapphire fibers with inscribed first-order fiber Bragg gratings for applications up to 600 °C are presented. We report on all the details of two different sensor designs; for instance, we show that the resolution of multimode sapphire fiber Bragg grating (SFBG) strain sensors is about Δl/l = 10-5 (10 μstrain), which is comparable with state-of-the-art high-temperature sensors. We apply our sensors for the determination of the thermal expansion coefficients of high-temperature steel alloys, showing a good match to known values. Hence, we believe that SFBG sensors may represent a promising alternative to currently used non-optic-based strain-detecting devices.

Fiber Bragg grating inscription in optical multicore fibers

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.

Inscription of first order fiber Bragg gratings in sapphire fibers by 400 nm femtosecond laser pulses

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.