| Summary: | Optical fiber sensors based on an interferometer structure play a significant role in monitoring physical, chemical, and biological parameters in natural environments. However, sensors with high-sensitivity measurement still present their own challenges. This paper deduces and summarizes the methods of sensitivity enhancement in interferometer based fiber optical sensors, including the derivation of the sensing principles, key characteristics, and recently-reported applications.The modal coupling interferometer is taken as an example to derive the five terms related to the sensitivity: (1) the wavelength-dependent difference of phase between two modes/arms <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∂</mo><msub><mi>ϕ</mi><mi>d</mi></msub><mo>/</mo><mo>∂</mo><mi>λ</mi></mrow></semantics></math></inline-formula>, (2) the sensor length <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>L</mi><mrow><mi>w</mi><mo>,</mo><mi>A</mi></mrow></msub></semantics></math></inline-formula>, (3) refractive index difference between two modes/arms <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="sans-serif">Δ</mi><msub><mi>n</mi><mrow><mi>e</mi><mi>f</mi><mi>f</mi><mo>,</mo><mi>A</mi></mrow></msub></mrow></semantics></math></inline-formula>, (4) sensing parameter dependent length change <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>, and (5) sensing parameter dependent refractive index change <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>. The research papers in the literature that modulate these terms to enhance the sensing sensitivity are reviewed in the paper.
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