Index Sensitivity Of Long-period Gratings Research Articles

Mode Dependence of Refractive Index Sensitivity of Long-Period Gratings Induced by Periodically Embedded Multimode Fiber

Mode Dependence of Refractive Index Sensitivity of Long-Period Gratings Induced by Periodically Embedded Multimode Fiber

Towards refractive index sensitivity of long-period gratings at level of tens of µm per refractive index unit: fiber cladding etching and nano-coating deposition.

In this work we report experimental results on optimizing the refractive index (RI) sensitivity of long-period gratings (LPGs) by fiber cladding etching and thin aluminum oxide (Al2O3) overlay deposition. The presented LPG takes advantage of work in the dispersion turning point (DTP) regime as well as the mode transition (MT) effect for higher-order cladding modes (LP09 and LP010). The MT was obtained by depositing Al2O3 overlays with single-nanometer precision using the Atomic Layer Deposition method (ALD). Etching of both the overlay and the fiber cladding was performed using hydrofluoric acid (HF). For shallow etching of the cladding, i.e., DTP observed at next = 1.429 and 1.439 RIU for an LPG with no overlay, followed by deposition of an overlay of up to 167 nm in thickness, HF etching allowed for post-deposition fine-tuning of the overlay thickness resulting in a significant increase in RI sensitivity mainly at the DTP of the LP09 cladding mode. However, at an external RI (next) above 1.39 RIU, the DTP of LP010 was noticed, and its RI sensitivity exceeded 9,000 nm/RIU. Deeper etching of the cladding, i.e., DTP observed for next above 1.45 RIU, followed by the deposition of thicker overlays (up to 201 nm in thickness) allowed the sensitivity to reach values of over 40,000 nm/RIU in a narrow RI range. Sensitivity exceeding 20,000 nm/RIU was obtained in an RI range suitable for label-free biosensing applications.

Numerical analysis of refractive index sensitivity of long-period gratings in photonic crystal fiber

Abstract The modal coupling properties and resonance spectral response of long-period gratings (LPGs) in index-guiding photonic crystal fiber (PCF) are numerically investigated with respect to its sensitivity to changes in the index of refraction of the measurand in the PCF air channels using a full-vectorial mode solver combined with frequency-domain method. We show that the shift of wavelength resonance of a PCF-LPG structure can be extremely sensitive by appropriately designing the cladding air-channel dimension and geometry as well as the periodicity of the LPG inscribed in an endlessly single mode PCF (ESM-PCF-LPG). Specifically, a sensitivity of 4.3 × 10−8 refractive index unit (RIU) can be obtained in a refractive index range of 1.33 and 1.4 using an ESM-PCF-LPG with hexagonally arrayed 4-ring air channels at the grating periodicity of 1300 μm. A sensitivity of 4.1 × 10−7 RIU can be achieved in the range of 1.0–1.25 employing an ESM-PCF-LPG with similarly arrayed but 5-ring air channels at the grating periodicity of 350 μm. The predicted index sensitivity, coupled with other schemes for selectivity and specificity, opens the ESM-PCF-LPG structure for a variety of sensing and detection applications.