An output dielectric mirror is deposited on the central part (~15 μm diameter) at the end face of a 1-km graded-index fiber and tested as output mirror of multimode Raman laser with highly-multimode (M2~34) 940-nm LD pumping. In the cavity with highly-reflective input FBG, Raman lasing of Stokes wave at 976 nm starts at the threshold pump power of ~80 W. The output beam quality factor measured near the threshold (~1W at 976 nm) M2~2 confirms mode selective properties of such output mirror. The power scaling capabilities at increased pump power together with a more detailed characterization of the output beam (spatial profile, spectrum and its stability) are performed and the obtained characteristics are compared with those for output coupling based on Fresnel reflection from the mirror-free fiber end face.
We present the results of direct laser-induced periodic surface structuring of semiconductors thin films (a-Si, a-Ge) deposited on glass substrate at different ambient environments (air, vacuum, nitrogen) resulting in regular gratings with the period of 600 nm to 900 nm at the laser wavelength of 1026 nm oriented either along (a-Si) or transverse (a-Ge) to the linear laser polarization direction. The processing speed has a different effect on morphology of obtained structures: on a-Si film, an increase of scanning speed leads to the reorientation of gratings and reduction of their period, while on a-Ge, the uniformity degradation and increase of the period are observed. Changing the ambient atmosphere from air to nitrogen and vacuum, when writing structures on a-Ge, helps to minimize the uniformity degradation and obtain highly regular nanogratings.
Recently, highly regular thermochemical laser-induced periodic surface structures (TLIPSS) have become the subject of active studies. TLIPSS are formed in the interference maxima due to the local oxidation of the material irradiated with ultrashort laser pulses and are characterized by the elevation of the relief that forms parallel oxide protrusions. The gas surrounding is expected to affect the morphology and chemical composition of the resulting TLIPSS; however, such effects were rarely studied so far. Here we present the results of the TLIPSS fabrication on glass-supported Si-Ti bilayer films using an astigmatic Gaussian IR femtosecond beam both in air and a nitrogen-rich atmosphere. The formation of ordered TLIPSS with the period of ≈ 920 nm is observed at slow scanning speeds (∼1 μm/s) and low fluences in a nitrogen-rich atmosphere. Raman spectroscopy revealed the presence of TiO2 (rutile) peaks, as well as bands centered at 280 cm-1 and 320 cm-1, which can be related to TiN in amorphous and polycrystalline phase.
We investigate possibilities of using a multicore fiber containing fiber Bragg grating (FBG) in sensing applications. We use the advantages of the femtosecond point-by-point technique to inscribe FBGs in the selected cores of polyimidecoated 7-core fiber. Besides the results on 3D shape sensing we present new approaches for acoustic wave detection and environment refractive index sensing. In particular, we show that spatial division multiplexing with a multicore fiber allows one to create multipoint acoustic sensor based on a single laser source. In addition, to detect the change in environment refractive index by using point-by-point FBG, there is no need in fiber coating removal. Thus, durability of the fiber along with the FBG inscription approach is of great potential for fabrication of next generation multiple parameter sensors.
We present the results of high-ordered periodic surface structure formation by femtosecond laser pulses (λ = 1026 nm, τ = 232 fs) using an astigmatic Gaussian beam with a diameter of ≈100 μm on the surface of metal films (thickness of 15 nm and 150 nm) of Hf sputtered on the glass substrate. Continuous and uniform structures with a period of ≈700 nm are formed at a scanning speed up to 2000 μm/s on the film with a thickness of 15 nm. In addition, the formation of ablative periodic substructures with a period of about 160 nm oriented perpendicularly to the main ones was observed on the oxide protrusions. In the case of 150 nm thick film, defects, cracks and phase shifts accompanied a uniform structure with a period of ≈940 nm formed at a speed up to 500 μm/s.
We present the results on fabrication of 3D fiber Bragg grating (FBG) arrays in specialty 7-core optical fibers with straight or twisted (spun) cores. Femtosecond laser inscription technology allowed us to modify the fibers through the acrylate or polyimide protective coatings that significantly increases the durability of the FBG sensors as compared to conventional UV inscription approach, requiring the coating removal. Custom-made 7-core fiber with polyimide coating opens up new prospects for shape sensors operating in high-temperature environment. Twisted-core fiber makes it possible to measure not only the shape, but also the direction of fiber torsion that is essential for a free-standing sensors. A novel method enabling core-selective FBGs inscription in a 7-core spun optical fiber is presented in this work. By using the created sensors bending radii down to several millimeters can be measured with a high precision. Separation of different core FBGs by wavelength makes it possible to combine several cores during their interrogation, which allows for sensor measurements through a single optical port.
We present the results on fiber Bragg gratings inscription with femtosecond laser pulses in a Fibercore SM- 7C1500(6.1/125) 7-core single-mode optical fiber. By focusing femtosecond pulses into the volume of the fiber and by controlling the transverse spatial position of the pulse absorption region we selectively modify the individual fiber cores of the fiber and at the same time specify geometry of the each grating. We show that different longitudinal profiles of coupling coefficient can be realized for the FBG, including uniform, chirped and apodized ones.
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