Miniature fiber-optic near-surface gap-coupled cladding waveguide Mach-Zehnder interferometric refractive index sensor inscribed by femtosecond laser

A miniature fiber-optic gap-coupled cladding waveguide Mach-Zehnder interferometer (MZI) is proposed and demonstrated for high-sensitive refractive index measurement. Two straight cladding waveguides respectively coupled with the fiber core are inscribed by a femtosecond laser (fs) to form the MZI structure. At their far ends, the two cladding waveguides are very close to the cladding surface, and a short separation was intentionally designed for the coupling of light between them and generating light field with evanescent wave expanded outside the cladding surface, which makes the MZI structure very sensitive to ambient refractive variations. In a refractive index range of 1.34–1.42, a high sensitivity of −59.90238 nm/RIU is achieved simultaneously with good linearity. The sensing structure also shows good characteristic of temperature insensitivity, which may find practical applications in chemical or biological sensing.