Baokai Cheng

Interferogram Reconstruction of Cascaded Coaxial Cable Fabry-Perot Interferometers for Distributed Sensing Application

This paper describes a distributed sensing concept using coaxial cable-based Fabry-Perot interferometers (CCFPIs). Multiple reflectors are implemented along a coaxial cable, where every two consecutive reflectors can be considered as a low finesse CCFPI, which has a relatively weak reflection coefficient and insertion loss. The interferogram in a frequency domain of each individual CCFPI could be reconstructed through the proposed signal processing method, so that the phase detection could be applied to any CCFPIs on one cable to achieve high measurement accuracy. A large capacity sensor network with a relatively high measurement accuracy can be implemented simultaneously. The concept takes advantage of the time-domain multiplexing method and the pure frequency domain measurement, which is herein called a joint-time-frequency demodulation technique. Due to its effectiveness and robustness, the device is especially attractive for structural, downhole, or underwater applications.

Micro-cantilever-based fiber optic hydrophone fabricated by a femtosecond laser

We report an open cavity, cantilever-based fiber optic Fabry-Perot interferometer hydrophone. The hydrophone is made of fused silica material, and its micro-cantilever beam is directly fabricated by femtosecond (fs) laser micromachining. The theoretical analyses and experimental verifications of the frequency response of the sensor are presented.

Distributed torsion sensor based on cascaded coaxial cable Fabry–Perot interferometers

Cascaded coaxial cable Fabry–Perot interferometers (FPI) are studied and demonstrated for distributed torsion measurement. Multiple weak reflectors are implemented on a coaxial cable so that any two consecutive reflectors can form a Fabry–Perot cavity. By fixing the cable sensor in a helical form on a shaft, the distributed torsion of the shaft can be measured by the cascaded Fabry–Perot cavities. A test on a single section shows that the sensor has a linear response with a sensitivity of 1.834 MHz (rad/m)−1 in the range of twisted rate from 0 to 8.726 rad m−1. The distributed torsion sensing capability is useful in drilling process monitoring, structure health monitoring and machine failure detection.

Microwave-assisted frequency domain measurement of fiber-loop ring-down system

A new frequency domain measurement method for a fiber-loop ring-down system is proposed in this Letter. Compared to traditional time domain measurement, this method uses a microwave modulated continuous wave (CW) laser as a light source, making full use of the duty cycle to achieve enhanced measurement efficiency. By measuring the amplitude modulation over a frequency span, this technique can be used to determine the ring-down time in the frequency domain, which will then be used to calculate the loss in the ring.

A Fluidic-Based High-Pressure Sensor Interrogated by Microwave Fabry–Perot Interferometry

A fluidic-based sensor is proposed and demonstrated for high-pressure measurement. The sensor consists of a reservoir and a capillary outlet. The reservoir deforms under pressure manifesting the liquid level change in the capillary. Utilizing the built-in waveguide on the capillary, the liquid level is measured by microwave Fabry–Perot interferometry in the spectral domain. The applied pressure variation is monitored by the spectrum shift of the microwave interferogram. The pressure response of the sensor is tested up to 2000 psi, with a resolution of 2.5 psi, and repeatability within ±20 psi. Benefiting from rigidity in material and flexibility in dimension of the sensor structure, the sensor has good robustness and adjustable sensitivity and range for applications in high-pressure environments.

Stress-induced birefringence and fabrication of in-fiber polarization devices by controlled femtosecond laser irradiations.

Optical birefringence was created in a single-mode fiber by introducing a series of symmetric cuboid stress rods on both sides of the fiber core along the fiber axis using a femtosecond laser. The stress-induced birefringence was estimated to be 2.4 × 10(-4) at the wavelength of 1550 nm. By adding the desired numbers of stressed rods, an in-fiber quarter waveplate was fabricated with a insertion loss of 0.19 dB. The stress-induced birefringence was further explored to fabricate in-fiber polarizers based on the polarization-dependent long-period fiber grating (LPFG) structure. A polarization extinction ratio of more than 20 dB was observed at the resonant wavelength of 1523.9 nm. The in-fiber polarization devices may be useful in optical communications and fiber optic sensing applications.

Coherence-length-gated distributed optical fiber sensing based on microwave-photonic interferometry

This paper presents a new optical fiber distributed sensing concept based on coherent microwave-photonics interferometry (CMPI), which uses a microwave modulated coherent light source to interrogate cascaded interferometers for distributed measurement. By scanning the microwave frequencies, the complex microwave spectrum is obtained and converted to time domain signals at known locations by complex Fourier transform. The amplitudes of these time domain pulses are a function of the optical path differences (OPDs) of the distributed interferometers. Cascaded fiber Fabry-Perot interferometers (FPIs) fabricated by femtosecond laser micromachining were used to demonstrate the concept. The experimental results indicated that the strain measurement resolution can be better than 0.6 µε using a FPI with a cavity length of 1.5 cm. Further improvement of the strain resolution to the nε level is achievable by increasing the cavity length of the FPI to over 1m. The tradeoff between the sensitivity and dynamic range was also analyzed in detail. To minimize the optical power instability (either from the light source or the fiber loss) induced errors, a single reflector was added in front of an individual FPI as an optical power reference for the purpose of compensation.

Microwave Photonic Distributed Sensing in Harsh Environment

We report a new distributed fiber optic sensing technique using optical carrier based microwave interferometry. The concept has been demonstrated using different types of optical fibers including singlemode fiber, multimode fiber, single crystal sapphire fiber and polymer fiber. Using the microwave-photonic technique, many fiber interferometers with the same or different optical path differences were interrogated and their locations could be unambiguously determined. The distributed sensing capability was demonstrated using cascaded low-finesse Fabry-Perot interferometers fabricated by fs laser micromachining. Spatially continuous, fully distributed temperature and strain measurements were used as examples to demonstrate the capability of the proposed concept.

All optical fiber polarization controlling devices fabricated by femtosecond laser irradiation

This paper reports the stress-induced birefringence generated in an optical fiber using femtosecond laser (fs) irradiations and the fabrication of in-fiber waveplates and polarizers. Optical birefringence was created in a single-mode fiber by introducing a series of symmetric cuboid stress rods on both sides of the fiber core and along the fiber axis using a femtosecond laser. The stress-induced birefringence was estimated to be 2.4×10-4 at the optical wavelength of 1550 nm. By controlling the length of the stress rods, waveplates of the desired retardance can be fabricated. The stress-induced birefringence was further explored to fabricate in-fiber polarizers based on the polarization-dependent long-period fiber grating (LPFG) structure. For the in-fiber polarizer based on low order mode LPFG, a polarization extinction ratio of more than 25 dB was observed at the wavelength of 1527.8 nm. A high order mode LPFG based in-fiber polarizer, with a broad bandwidth of 100 nm near 1550 nm, was investigated as well. The in-fiber polarization devices with low insertion loss may be useful in optical communications and fiber optic sensing applications.

Femtosecond Laser Fabricated Multimode Fiber Sensors Interrogated by Optical-Carrier-Based Microwave Interferometry Technique for Distributed Strain Sensing

A multimode fiber (MMF) based cascaded intrinsic Fabry-Perot interferometers (IFPIs) system is presented and the distributed strain sensing has been experimentally demonstrated by using such system. The proposed 13 cascaded IFPIs have been formed by 14 cascaded reflectors that have been fabricated on a grade index MMF. Each reflector has been made by drawing a line on the center of the cross-section of the MMF through a femtosecond laser. The distance between any two adjacent reflectors is around 100 cm. The optical carrier based microwave interferometry (OCMI) technique has been used to interrogate the MMF based cascaded FPIs system by reading the optical interference information in the microwave domain. The location along with the shift of the interference fringe pattern for each FPI can be resolved though signal processing based on the microwave domain information. The multimode interference showed very little influence to the microwave domain signals. By using such system the strain of 10-4 for each FPI sensor and the spatial resolution of less than 5 cm for the system can be easily achieved.