Direct Engraving of FFPI Strain Sensor on Silica Fibers
Using the 157-nm micromachining system, the first type of MFFPI sensor is fabricated with a micro-rectangular notch structure inside a silica fiber with a typical size of 40-60 lm. The 157-nm source light is projected onto the fiber through a rectangular steel mask that is 25 times larger than the notch. The notch fabricated inside the fiber is an FPI cavity with two smooth and parallel reflective interfaces. An excellent fringe visibility of up to 26 dB was achieved, and a precise strain measurement under high temperatures of up to 800°C was demonstrated (see Figure 3.15). The sensor determines strain by
Figure 3.15 (a) SEM photos of the FP sensor; (b) reflective spectrum of PCF FP sensor; (c) strain characteristics of PCF sensor at different temperatures; and (d) temperature response of the sensor. (From Ran, Z. L. et al. 2007. Optics Letters, 32(21), 3071-3073.)
measuring the cavity length variation (8 = AL/L, where L and AL are the real cavity length and variation to original length, respectively). More importantly, this sensor has the unique feature of temperature self-compensation due to the unique cavity structure. The thermal expansion of the fiber core reduces the cavity length since the fiber core is free along the longitudinal direction due to the air gap, while the thermal expansion of the fiber cladding increases the cavity length due to the existence of the cladding that remains after laser ablation. The two effects cancel each other when temperature varies.