Laser-induced periodic surface structures (LIPSS) offer a simple, single-step technique for creating periodic patterns on solids in an ambient air. Combined with metals like gold or silver, LIPSS enhances grating-assisted refractive index (RI) sensors using surface plasmon resonance (SPR). This study analyzes an SPR sensor in the Kretschmann configuration with a periodically modulated silver layer. The modulation is created by LIPSS formed on a titanium-coated glass prism using femtosecond laser pulses, followed by a silver coating. Reflection spectra for wavelengths λ = 600-1700 nm were calculated for various angles of incidence and RI values (1.33- 1.4). Unlike a flat sensor with a single resonance, the LIPSS-based sensor shows multiple resonances (λ = 1000-1700 nm) due to modes on inner and outer interfaces. LIPSS reduces resonance spectral width by an order of magnitude, but also reduces sensitivity similarly, weakly affecting the Figure of Merit. The LIPSS-assisted sensor was also tested experimentally.
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.
The influence of β-radiation exposure (the total dose up to 41.1 MGy and dose rate of 2.5 kGy/s) on the spectral characteristics of high and low reflective FBGs inscribed using femtosecond laser radiation in Ge-doped and pure-silica core fibers with polyimide and metal coating is presented. The largest Bragg wavelength induced shift (BWS) of +55 pm is observed in the case of weak reflective FBG (type I) inscribed in Ge-doped fiber. A comparable red shift in wavelength of + 50 pm is observed in the case of high reflective FBG (type II), which is explained by an increase in the concentration of GeE’-centers and, accordingly, an increase in the effective refractive index. Moreover, a significantly smaller BWS of -10 pm was obtained in the case of high reflective FBGs inscribed in pure-silica core fibers.
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.
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