Nanodeposition of materials with complex refractive index in long-period fiber gratings

Abstract

An overlay of higher refractive index than the cladding is deposited on a Long Period Fiber Grating (LPFG). This permits to improve in a great manner the sensitivity of the device to ambient and overlay refractive index changes. This causes large shifts of the attenuation bands in the transmission spectrum. To obtain a maximum sensitivity for specific refractive indices of the overlay and the ambient, an optimum overlay thickness must be selected. If the refractive index of the overlay is complex there is an additional phenomenon of vanishing of the attenuation bands in the transmission spectrum. This occurs for specific thickness values of the overlay. The problem is analyzed with a numerical method based on LP mode approximation and coupled mode theory. Experimental results are contrasted with theoretical ones.

Radiation tolerant humidity sensors based on nano-scale TiO<inf>2</inf>-coated LPGs for high-energy physics applications

“…1c also reveals that, by increasing the RH value, the attenuation band visibility also increases. This is an important aspect as, in most cases, LPGs coated with "lossy" overlays experience partial or full resonance fading when operating in transition mode [22].Similar results have also been obtained at 10, 0 and -10°C. Fig.…”

Lossy Mode Resonance (LMR) Based Fiber Optic Sensors: A Review

“…This coupling primarily depends upon the thin film layer thickness. As thickness of the thin film is increased, some of the waveguide modes start to be guided in the thin film due to which a modal redistribution takes place [21], [22]. Therefore, to understand the phenomenon of lossy mode resonance, one can analyze the evolution of modal effective index.…”

High Sensitive Refractometric Sensor Using Symmetric Cladding Modes of an FBG Operating at Mode Transition

“…Transmission loss of attenuation bands of the clad modes are decreased because of imaginary part of the refractive index of the overlay material. Also with increasing thickness of the overlay layer there is a decrement of the coupling coefficient between forward core and backward clad modes which causes a reduction in attenuation dip [19,22]. Response of the LP 0,12 cladding mode as a function of the surrounding refractive index in the range 1.333 to 1.378 is shown in Fig.4.…”

“…We have used the symmetric cladding modes of a standard FBG. It was shown earlier for long period grating sensors that the coupling between the forward propagating core and the cladding modes can be influenced by deposition of a nanometer thin overlay film of high RI material [18,19]. For a particular thickness of the high RI film the cladding mode can be made to transit to its next lower order.…”