Capability for Fine Tuning of the Refractive Index Sensing Properties of Long-Period Gratings by Atomic Layer Deposited Al2O3 Overlays

Abstract

This work presents an application of thin aluminum oxide (Al2O3) films obtained using atomic layer deposition (ALD) for fine tuning the spectral response and refractive-index (RI) sensitivity of long-period gratings (LPGs) induced in optical fibers. The technique allows for an efficient and well controlled deposition at monolayer level (resolution ∼ 0.12 nm) of excellent quality nano-films as required for optical sensors. The effect of Al2O3 deposition on the spectral properties of the LPGs is demonstrated experimentally and numerically. We correlated both the increase in Al2O3 thickness and changes in optical properties of the film with the shift of the LPG resonance wavelength and proved that similar films are deposited on fibers and oxidized silicon reference samples in the same process run. Since the thin overlay effectively changes the distribution of the cladding modes and thus also tunes the device's RI sensitivity, the tuning can be simply realized by varying number of cycles, which is proportional to thickness of the high-refractive-index (n > 1.6 in infrared spectral range) Al2O3 film. The advantage of this approach is the precision in determining the film properties resulting in RI sensitivity of the LPGs. To the best of our knowledge, this is the first time that an ultra-precise method for overlay deposition has been applied on LPGs for RI tuning purposes and the results have been compared with numerical simulations based on LP mode approximation.

Opto-Electrochemical Sensing Device Based on Long-Period Grating Coated with Boron-Doped Diamond Thin Film

“…Deposition of the high-n overlay shifts appearance of this effect, i.e., vanishing of the resonances and higher sensitivity, towards lower external RI. Thin film with high-n modifies effective refractive index of cladding modes [28]. It can be clearly seen shift of both λ 1 and λ 2 towards shorter wavelength as a result of increase in external RI.…”

“…3 as λ 1 and λ 2 is most probably a result of asymmetrical deposition of the overlay around the LPG. For asymmetrical overlay, some additional cladding modes are induced and they couple to the core mode at different wavelengths [28].…”

Arc-Induced Long Period Fiber Gratings

“…Arc-induced LPFGs have been characterized as a function of the variation of physical parameters such as strain and temperature [40,[66][67][68][69][70][71][72], bending and torsion [73], pressure [74,75], and external refractive index [76][77][78][79][80][81][82][83][84][85][86]. They were also exposed to gamma radiation [51].…”

“…LPFGs are intrinsically sensitive to changes of the external refractive index and, therefore, they are used as refractometers [14,18,47,48,61,65,81,. As discussed in [18] standard arc-induced LPFGs are limited to resolutions of about 10 −3 in changes of the refractive index and, therefore, several techniques such as tapering, etching, and bending or by implementing interferometric configurations have been applied in order to improve their resolution.…”

Regeneration of titanium oxide nano-coated long-period grating biosensor

“…In order to improve the LPG sensitivity, a titanium oxide (TiO x ) nano-coating was deposited with atomic layer deposition (ALD) method on LPG surface [3]. The ALD method allows us for deposition thin films of welldefined properties [4]. Such a nanocoated LPG can be further used as a label-free biosensor, where binding of biomolecules is induces changes in optical properties at the surface of the nano-coating [5].…”