Gold-coated tilted fiber Bragg gratings (TFBGs) can now be considered as a mature technology for lab-on-fiber sensing based on surface plasmon resonance (SPR) excitation. This sensing architecture brings considerable assets such as easy light injection, temperature fluctuations immunity and remote operation in very small volumes of analytes. Different metal configurations have been used so far, without considerations about their relative performances in terms of surrounding refractive index (SRI) sensing. In this work, we study the impact of the coating on the cladding mode distribution in the TFBG transmitted amplitude spectrum and subsequently on its SRI sensitivity. Different configurations of gold coating are produced and tested, relying on both the sputtering and electroless deposition processes. Interesting spectral features are reported, confirming that the coating thickness and its relative disparity are important design parameters that drive the overall sensing performances.
Surface plasmon resonance (SPR) excitation has been widely studied in the well-known Kretschmann prism configuration, leading to a large variety of refractometric optical sensors. In recent research, this bulky optical device has found a counterpart thanks to the use of metal-coated optical fibers, mainly allowing to considerably reduce the size of the sensors. Some approaches make use of multimode, etched or unclad fibers while the grating- based alternatives are mostly focused on uniform, long period or tilted fiber Bragg gratings (TFBGs). However, plasmonic optical fiber sensing has been pretty much restricted to aqueous solutions due to the remarkable applications of these devices in (bio)chemical sensing. This work gives the roadmap through SPR excitation in air by using a 10° TFBG refractometer. With regard to aforementioned developments, the photo-inscription process is carried out with an excimer laser emitting at 193 nm, which creates the grating planes in a position close to the core-cladding interface. By doing so, it is possible to obtain a cladding mode resonance comb covering the range of the spectrum that corresponds to refractive index values around the one of the air, without the need of using highly tilted FBGs. Indeed, the coupling of cladding modes to the outer medium can be observed in the optical transmitted spectrum of a bare TFBG. In addition, the thickness of the gold thin-film deposited at the grating location is reduced to one third of the one used for SPR excitation in liquids. In this way, all the cladding modes are reflected by the metal when the TFBG is immersed in solution but when it is left in the air a SPR signature appears in the spectrum. The methods described in the present paper are intended to support further developments on plasmonic optical fiber solutions applied to refractive index sensing in gaseous atmospheres.
In this work, 5-mm long TFBGs were inscribed in photosensitive single-mode optical fiber using the direct writing plane-by-plane femtosecond laser inscription method; a flexible inscription approach that enables absolute control of the grating period, length, angle, width and depth of the grating planes. This new fabrication method brings important differences compared to classical inscription methods. Firstly, these gratings exhibit very low photo-induced birefringence (measured ~8pm) and as we rely on a direct writing process, the tilt angle of the inscribed grating does not affect the Bragg wavelength, allowing for precise positioning. In addition, this method enables the high order grating production, allowing a behavioral study of higher order cladding modes located at lower wavelengths in the 1200 – 1600 nm range. 8th order gratings were produced with cladding and Bragg mode resonances in the C+L bands. The temperature and strain sensitivities were measured for both the Bragg and higher order cladding modes, yielding an exceptional performance. The higher order modes exhibit a negative axial strain, up to -1.99nm (more than two times higher than the standard Bragg peaks) and a solid temperature sensitivity of 10.25 pm/°C : At the same time, for the designed order cladding modes (of the 8th) the refractive index sensitivity is measured at 22 nm/RIU. Keywords:
We present results of recent research where we have utilized a femtosecond laser to micro-structure silica and polymer optical fibres in order to realize versatile optical components such as diffractive optical elements on the fibre end face, the inscription of integrated waveguide circuits in the fibre cladding and novel optical fibre sensors designs based on Bragg gratings in the core. A major hurdle in tailoring or modifying the properties of optical fibres is the development of an inscription method that can prove to be a flexible and reliable process that is generally applicable to all optical fibre types; this requires careful matching of the laser parameters and optics in order to examine the spatial limits of direct laser writing, whether the application is structuring at the surface of the optical fibre or inscription in the core and cladding of the fibre. We demonstrate a variety of optical components such as two-dimensional grating structures, Bessel, Airy and vortex beam generators; moreover, optical bridging waveguides inscribed in the cladding of single-mode fibre as a means to selectively couple light from single-core to multi-core optical fibres, and demonstrate a grating based sensor; finally, we have developed a novel femtosecond laser inscription method for the precise inscription of tailored Bragg grating sensors in silica and polymer optical fibres. We also show that this novel fibre Bragg grating inscription technique can be used to modify and add versatility to an existing, encapsulated optical fibre pressure sensor.
We present research into the use of femtosecond lasers to develop optical waveguides inscribed in the cladding of singlemode, silica optical fibre (SMF28). The waveguides are inscribed near to the fibre core, coupling light into them evanescently and so behaving as traditional couplers. By carefully controlling the laser parameters we are able to inscribe cladding waveguides with no evidence of damage through ablation. We show that this flexible inscription method can be used as an enabling technology to couple light from single-core fibres to new multi-core optical fibres, and in this work specifically to 4-core fibre. The SMF28 fibre is fusion spliced to the multi-core fibre and using the femtosecond laser we inscribe bridging waveguides from the centrally located single mode fibre core to a selected offset core of the 4-core fibre. To demonstrate the efficiency of the method and the possibility of making new kinds of optical fibre sensors, we inscribe a fibre Bragg grating (FBG) in one of the four fibre cores. The light reflected from the FBG is coupled back to the SMF28 core via bridging waveguide and we recovered the reflection spectrum of the grating using a commercial high-resolution spectrometer.
We present the results of investigations regarding laser micro-structuring of single mode optical fibres by direct access of the fibre end face and compare this with inscription in planar samples. We combine a high numerical aperture objective and femtosecond laser radiation at visible wavelengths to examine the spatial limits of direct writing and structuring at the surface of the optical fibre. We realise a number of interesting devices from one- and two-dimensional grating structures, to Bessel, Airy and vortex beam generators. We show the versatility of this simple but effective inscription method, where we demonstrate classic multiple slit diffraction patterns and patterns for non-diffracting beams, confirming that the flexible direct write method using femtosecond lasers can be to produce binary masks that can lead to beam shaping using a method that is applicable to all types of planar samples and through fine control of laser parameters to multi-mode and singlemode optical fibres.