Investigation On Intensity-modulated Fiber Sensors For Multi-Parametric Measurement

This paper focuses on the research of intensity-modulated fiber sensorswhich are based on optical fiber grating and fiber interferometer. The measuredparameters involve load, tilt, strain, curvature, refractive index and magnetic field.The adopted intensity measurement methods can be classified into two classes. Oneclass is the use of chirped fiber Bragg grating, which is connected with the chirpgrating effect. The parameters can be detected through the measurement of thechange of the reflected light power from the fiber Bragg gratings (FBGs). Anotherclass for the demodulation of the measured parameters is the combination of phase orpolarization modulation and a tunable laser source. The main contents are describedas follows.The development of fiber sensor technology, the characteristics and applicationsof fiber sensor are introduced. Kinds of common fiber sensors and their sensingprinciples are also introduced.The sensing principle of the chirped fiber Bragg grating is introduced and itsreflection spectral characteristics, reflection bandwidth, reflected light power havebeen studied theoretically and numerically through the transfer matrix method. Anovel intensity-modulated load sensor and a2-D tilt sensor based on chirped FBGsare proposed and studied. For both of them, special geometric structure is adopted tomake the FBG chirped, hence the applied load or tilt information is encoded intoreflected optical powers of the FBG and the intensity measurement can be realizedby an optical power meter. Since the temperature variation will not change thereflection bandwidths but only the central wavelength shifts of the FBG, themeasurement of load or tilt is insensitive to temperature.A fiber intermodal interference based high extinction ratio senor is proposed anddemonstrated. The sensing principal and the applications in strain, curvature andrefractive index measurement are studied. The intensity measurement is realized bycombinating the phase modulation and a tunable laser source. Since the photoniccrystal fiber birefringence is insensitive to temperature, temperature insensitive measurement can be carried out.A high extinction ratio magnetic field senor based on fiber polarizationinterference is proposed and demonstrated. The intensity measurement is realized bythe combination of polarization modulation and a tunable laser source.