In smart structure monitoring, twist angle is one of the most critical mechanical parameters for infrastructure
deterioration. A compact temperature-insensitive optical fiber twist sensor based on multi-phase-shifted helical long
period fiber grating has been proposed and experimentally demonstrated in this paper. A multi-phase-shifted helical long
period fiber grating is fabricated with a multi-period rotation technology. A π / 2 and a 3π / 2 phase shift is introduced
in the helical long period fiber grating by changing the period. The helical pitch can be effectively changed with a
different twist rate, which is measured by calculating the wavelength difference between two phase shift peaks. Although
the wavelength of the phase shift peak also shifts with a change of the temperature, the wavelength difference between
two phase shift peaks is constant due to two fixed phase shifts in the helical long period fiber grating, which is extremely
insensitive to temperature change for the multi-phase-shifted helical long period fiber grating. The experimental results
show that a sensitivity of up to 1.959 nm/(rad/m) is achieved.
We propose and demonstrate a highly sensitive optical fiber microfluidic refractometer. A microhole is fabricated in the photonic crystal fiber (PCF) by using femtosecond laser beam, which combines the tunable mode coupler and microfluidic channel. The mode field diameter of the guided light is changed with the refractive index in the microfluidic channel, which results in the tunable coupling ratio between the core and the cladding in the PCF. Therefore, the refractive index of the liquid in the microfluidic channel is detected by interrogating the fringe visibility of the reflection spectrum. These experiments results demonstrate that the sensor is insensitive with the temperature and strain, and a RI sensitivity of up to 150.7 dB∕RIU is achieved, establishing the tunable mode coupler as a sensitive and versatile sensor.
A fiber optic white-light interferometry based on cross-correlation calculation is presented. The detected white-light spectrum signal of fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensor is firstly decomposed by discrete wavelet transform for denoising before interrogating the cavity length of the EFPI sensor. In measurement experiment, the cross-correlation algorithm with multiple-level calculations is performed both for achieving the high measurement resolution and for improving the efficiency of the measurement. The experimental results show that the variation range of the measurement results was 1.265 nm, and the standard deviation of the measurement results can reach 0.375 nm when an EFPI sensor with cavity length of 1500 μm was interrogated.
A distributed fiber optic pressure sensor based on a piece of the photonic crystal fiber (PCF) is proposed and
experimentally demonstrated. The sensor is fabricated by splicing the PCF with a single-mode fiber (SMF), and the free
end face of the PCF is filmed with a reflectivity of 99%. A Michelson interferometer is formed in the PCF. The pressure
position can be located by measuring the phase difference of the interferometer, and the pressure can be interrogated by
measuring the amplitude of the optical spectrum. The experimental results show that the pressure and its position along
the PCF can be simultaneously interrogated.
A dispersion coefficient measurement of photonic crystal fiber by using white-light interferometry based technology is
presented. A section of photonic crystal fiber (PCF) is fused in one arm of a Mach-Zehnder interferometer, and the
relative phase, delay time and chromatic dispersion coefficient of PCF are calculated by sweeping the wavelength from
1525nm to 1565nm. A section of PCF with the length of 0.779m is measured and the experimental results are shown.
The result indicates that the proposed method can measure the chromatic dispersion of PCFs with a reliable accuracy.
An optical fiber Fabry-Pérot tunable filter is constructed by fixing two microlensed mirror-coated fibers to the opposite ends of a piezoelectric transducer. A tunable filter with a free spectral range of 70 nm, a finesse of 175, an insertion loss of 1.05 dB, and a tuning frequency exceeding 1 kHz has been experimentally demonstrated. The filter is easy to construct at a low cost, and it is anticipated that it will be used in fiber-optic sensing systems, spectrometers, and tunable optical fiber lasers.
A three-wavelength method is developed to interrogate an extrinsic Fabry-Pérot interferometer (EFPI) for the measurement of a dynamic measurand. Three signals 120 deg out of phase are created by using three fiber Bragg gratings (FBGs). A phase demodulator, which is the same as that used for demodulating a 3×3 coupler-based interferometer, is used to measure the phase change. A minimum detectable phase of 0.43 µrad/Hz1/2 is achieved.
Compensation method is a popular method for testing aspheric surface. In this dissertation, a compensator is designed according to normal aberration compensation method. In order to realize the normals of emergent wavefront of compensator consistent with that of aspheric surface, the concept of virtual glass is adopted. According to the characteristic of aspheric surfaces, general method of determining virtual glass model's parameters in ZEMAX is presented. Single pass layout using virtual glass model for designing compensator is proposed. It reduces the number of surfaces by a half. This simplifies the process of design and optimization and improves the accuracy of design. A design example is given. The maximum optical path difference is less than 0.0001λ(λ=632.8 nm).
A stabilizing technique has been developed for a 3×3-coupler-based Mach-Zehnder interferometer, and the interferometer is used to interrogate a fiber Bragg grating (FBG) sensor. The fluctuation of an output can be removed by using the technique. New arithmetic is also introduced to recover the wavelength shifts of the FBG. The experimental results show that stable wavelength shifts of the FBG were obtained. The accurate calculation of modulation amplitude was also demonstrated.
Aspheric surfaces are widely used in optical systems. Advanced testing technology is needed for the production of aspheric surfaces. Compensation method is a popular method for testing aspheric surfaces. In order to realize the certification of compensator that is composed of multiple lenses, reflective CGH whose diameter is smaller than that of aspheric surface is used as aspherical simulator to calibrate the compensator. The challenge of realizing the accurate certification of the compensator is of high accuracy design of CGH. According to optical propagation theory, wavefront tracing method is proposed in this dissertation. The derivation process of wavefront tracing method is given. By using this method, the phase function of CGH can be directly acquired. Simulators of many different aspheric surfaces are designed. The design results prove the feasibility of wavefront tracing method. Compared to ray tracing method, the wavefront tracing method can simplify the design process and improve the design accuracy.
A four-element fiber optic hydrophone array Based on 3×3 coupler terminated Mach-Zehnder interferometer is
developed. The 3×3 coupler terminated Mach-Zehnder interferometer is constructed to sense the underwater acoustic
signal. Spatial multiplexing of fiber optic hydrophone array is achieved by separating the laser into four channels
utilizing combined couplers. For each hydrophone element, a demodulator is used to recover the acoustic signal. This
configuration is easily implemented and can detect weak signal in a high noisy water environment. A 4-element,
spatially multiplexed system is constructed with a noise equivalent sound pressure of 1.5×10-4Pa/Hz at 1kHz and the
frequency range of 80Hz to 1.5 KHz. It has an almost flat response with an average sensitivity of -133dB,and dynamic
range of 110dB .