Pulsed CO₂ Laser-Fabricated Cascades of Double Resonance Long Period Gratings for Sensing Applications

We present a detailed theoretical and experimental study of cascaded double resonance long period gratings (C DR LPGs) for fabricated sensing applications. The matrix description of cascaded LPGs is presented, and several important particular cases are considered related to the regular and turn around point (TAP) gratings. A pulsed CO₂ laser was used to fabricate ordinary and cascaded DR LPGs in a photosensitive optical fiber. The responses of the fabricated C DR LPGs to surrounding refractive index (SRI) temperature as well to longitudinal strain have been studied. A statistical comparison of the SRI sensitivities of ordinary and cascaded DR LPGs is presented to outline the capabilities and advantages of cascaded DR gratings. It was experimentally established that the temperature dependence of the wavelength split at the TAP follows a logarithmic dependence and the sensitivity to temperature is inversely proportional to the temperature itself. We evaluate the temperature stability needed for SRI-based sensing application and the importance of fine-tuning to the operational point slightly after the TAP to ensure maximum sensitivity of the sensor.