Inline fiber optic power sensor featuring a variable tap ratio based on a tightly focused femtosecond laser inscription

Abstract

We propose and demonstrate an inline fiber optic power sensor (IFPS) resorting to an embedded waveguide tap, which is formed to traverse across the cladding and core of a standard single-mode fiber. The tap was produced via a single-step inscription based on the femtosecond laser direct-writing method. A tightly focused pulsed laser beam has been particularly exploited to suppress the elongation along the laser propagation direction, thereby improving the cross-sectional symmetry of the created tap waveguide. The fabricated fiber optic tap has been stably combined with a photodiode via a compact package. The achieved tap ratio could be tuned from 1.0% to 5.9% at the wavelength of 1550 nm by adjusting the applied laser power, while the induced excess loss was kept below 0.6 dB. The proposed IFPS will be highly suitable for real-time power monitoring in a variety of applications, including optical communication networks and systems.

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“…The length of the reshaped core that serves as the sensing region is L = 14.8 mm, which is approximately equal to the re-imaging distance. Based on the previous femtosecond laser-writing tests [26,44], it is presumed that the RI of the modified core is uniformly the same as that of the cladding ( n mod = n clad ), which corresponds to a negative RI change of −0.005. As indicated in Figure 2b, the fundamental fiber mode at the 1550 nm wavelength is launched into the SMF along the x -direction, and the pathway monitor is placed along the propagation direction.…”

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