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A high temperature optical link capable of multi-megabits per second data rates at 300°C is presented. The system utilizes wide bandgap optical sources and detectors to achieve extreme temperature operation. Testing was conducted at multiple temperatures between room temperature and 325°C and at multiple light source currents. Light coupling into and out of a UV capable optical fiber was evaluated, and a model was created utilizing the test data of the photodiode dark current and the fiber optic cable insertion loss and attenuation and assess optical communications capability to 325°C and beyond.

We proposed and fabricated a microwave-frequency photonic fiber grating (MPFG) by femtosecond laser micromachining on optical fibers. Illuminated by low coherent light source, the MPFG can be interrogated using proposed microwave photonic system to show the resonant peaks in microwave frequency domain. We studied the working principle and characteristics of this device. After that, we discussed the influence of fiber type, apodization and light source coherence lengths on this device. The device can also respond to ambient temperature change like fiber optic sensors.

The performance of existing gas sensors often degrades in field conditions because of the loss of measurement accuracy in the presence of interferences. Thus, new sensing approaches are required with improved sensor selectivity. We are developing a new generation of gas sensors, known as multivariable sensors, that have several independent responses for multi-gas detection with a single sensor. In this study, we analyze the capabilities of natural and fabricated photonic three-dimensional (3-D) nanostructures as sensors for the detection of different gaseous species, such as vapors and non-condensable gases. We employed bare Morpho butterfly wing scales to control their gas selectivity with different illumination angles. Next, we chemically functionalized Morpho butterfly wing scales with a fluorinated silane to boost the response of these nanostructures to the vapors of interest and to suppress the response to ambient humidity. Further, we followed our previously developed design rules for sensing nanostructures and fabricated bioinspired inorganic 3-D nanostructures to achieve functionality beyond natural Morpho scales. These fabricated nanostructures have embedded catalytically active gold nanoparticles to operate at high temperatures of ≈300 °C for the detection of gases for solid oxide fuel cell (SOFC) applications. Our performance advances in the detection of multiple gaseous species with specific nanostructure designs were achieved by coupling the spectral responses of these nanostructures with machine learning (a.k.a. multivariate analysis, chemometrics) tools. Our newly acquired knowledge from studies of these natural and fabricated inorganic nanostructures coupled with machine learning data analytics allowed us to advance our design rules for sensing nanostructures toward the required gas selectivity for numerous gas monitoring scenarios at room and high temperatures for industrial, environmental, and other applications.

We describe a silicon integrated photonic interrogator with on-chip wavelength and temperature referencing for absolute measurements. We demonstrate fiber Bragg grating interrogation with ±1oC temperature accuracy without thermo-electric coolers.

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.

CO2 leakage is a major concern in a geological carbon sequestration projects due to adverse environmental consequences, where the main leakage risk is identified to be along existing wells through a thick, low-permeable cap rock. To pursue robust and cost effective real-time downhole monitoring technology for CO2 sequestration wellbore integrity, a permanently deployed coaxial cable casing imager is developed and evaluated in laboratory in this paper. The prototype of the casing imager consists of evenly distributed coaxial cable strain sensors helically wrapped around the pipe. The system is deployed on both PVC pipe and steel pipe to test its performance in casing deformation monitoring, including axial compression, radial expansion, bending, and ovalization. The strain sensors are pre-stressed and then helically wrapped onto the pipe with high strength epoxy. Multiple linear variable differential transformers (LVDTs) or strain gauges are used as an independent measurement of the actual pipe deformation in comparison to the casing imager measured pipe deformation. The test results demonstrated the ability of the lab-scale casing imager prototype in real-time casing deformation monitoring, including axial compression, radial expansion, bending, and ovalization, which would prove of great value in evaluating wellbore integrity state and providing early warnings of leakage risk that will contaminate the groundwater during CO2 injection. The low cost and high robustness of the distributed coaxial cable sensors will greatly lower the downhole monitoring cost and increase the system longevity. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

A ferrofluid filled coaxial cable Fabry-Perot interferometer is proposed for magnetic field sensing. Both the interferogram shift and the reflection intensity change of the sensor response are studied when external magnetic field is applied. The sensor is tested to work from 0 G to 160 G, and has a highly linear response above 50 G, with the sensitivity of 459 kHz/G. This sensor can be integrated into a coaxial cable sensing platform for multi-function or distributed sensing applications.

Downhole monitoring plays a crucial part in a geological carbon dioxide (CO 2 ) sequestration project, especially in providing early warnings of failure. However, most downhole monitoring technologies are often low in spatial resolution and time‐consuming, or expensive and have system longevity issues. To address this issue, a robust and cost‐effective distributed coaxial cable Fabry‐Perot interferometer‐based temperature sensor is proposed for real‐time downhole monitoring. The coaxial cable sensor (CCS) was made in house and tested using a high pressure high temperature (HPHT) testing apparatus to study the sensor accuracy, sensitivity, stability, and crosstalk effect in simulated downhole conditions. The laboratory test results indicate that the sensor can work under simulated downhole conditions of pressures up to 1000 psia and temperatures up to 110°C. At 1 ATM, the sensor has an accuracy of about 1%. At 1000 psia, the hysteresis phenomenon is observed, but it is reduced and tends to stabilize after repeated heating and cooling treatments. The pressure crosstalk effect was observed on the flexible cable sensor and minimized on the rigid cable sensor. The temperature and pressure range of the distributed CCS allows long‐term in situ monitoring for a well depth up to 2500 feet, which would prove of great value in detecting temperature change associated with wellbore leakage that may lead to groundwater contamination. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

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.

A metal ceramic coaxial cable (MCCC) sensor containing multiple Fabry-Perot interferometers (FPIs) has been designed, fabricated, and evaluated for measuring distributed high temperatures. The MCCC sensor demonstrated here consists of stainless steel tube and wire conductors, alumina tube insulator, and three pairs of air gap reflectors distributed along the cable. The multipoint sensor operation is realized by the joint time-frequency domain technique, which combines the functions of time-domain reflectometry and frequency division multiplexing to enable the measurement of spatially distributed temperature. The 3-point MCCC-FPI sensor has been demonstrated for distributed temperature measurements between 250 and 550 °C and operability up to 1000 °C. The sensor offers high sensitivity of >0.18 MHz/°C with a measurement resolution of 1 Hz and reasonably small high temperature measurement error range of ±3 °C.

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 µe using a FPI with a cavity length of 1.5 cm. Further improvement of the strain resolution to the ne 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.

This paper reports a highly sensitive distributed optical fiber sensing technique based on resolving the optical interference information in the microwave frequency domain. This technique uses a microwave intensity modulated coherent light through cascaded weak reflectors along a single mode fiber (SMF). The microwave is used to resolve the location of each reflector along the SMF. The coherence length of the optical carrier performs as the time domain gate and segments the optical interference between reflectors in the time domain. Within the coherence length, a small optical path difference change between two reflectors produces a change in the amplitude of the corresponding time domain pulses. The sensitivity of sub-μe for distributed strain sensing has been demonstrated.

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.

An easy way to flexibly fabricate special long-period fiber grating (LPFG) devices by controlled CO2 laser pulses are presented in this paper. By controlling the pulse power and duration, consecutive exposures were made on an optical fiber to fabricate an LPFG. Existing LPFGs can also be erased or modified by applying CO2 laser exposures on specific grating sections. The erasing effect has been studied to fabricate special grating devices for various applications. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1735–1738, 2013

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.

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.

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.

Metal-ceramic coaxial cable Fabry-Pérot interferometric (MCCC-FPI) sensors have been developed using a stainless steel tube and a stainless steel wire as the outer and inner conductors, respectively, a tubular α-alumina insulator, and a pair of air gaps created in the insulator along the cable to serve as weak reflectors for the transmitting microwave (MW) signal. The MCCC-FPI sensors have been demonstrated for high temperature measurements using MW signals in a frequency range of 2–8 GHz. The temperature measurement is achieved by monitoring the frequency shift (Δƒ) of the MW interferogram reflected from the pair of weak reflectors. The MW sensor exhibited excellent linear dependence of Δƒ on temperature, small measurement deviations (±2.7%), and fast response in a tested range of 200–500 °C. The MCCC has the potential for further developing multipoint FPI sensors in a single-cable to achieve in situ and continuous measurement of spatially distributed temperature in harsh environments.

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.

We present a coaxial cable interferometer based distributed sensing system for 3D beam shape estimation. By making a series of reflectors on a coaxial cable, multiple Fabry–Perot cavities are created on it. Two cables are mounted on the beam at proper locations, and a vector network analyzer (VNA) is connected to them to obtain the complex reflection signal, which is used to calculate the strain distribution of the beam in horizontal and vertical planes. With 6 GHz swept bandwidth on the VNA, the spatial resolution for distributed strain measurement is 0.1 m, and the sensitivity is 3.768 MHz me −1 at the interferogram dip near 3.3 GHz. Using displacement-strain transformation, the shape of the beam is reconstructed. With only two modified cables and a VNA, this system is easy to implement and manage. Comparing to optical fiber based sensor systems, the coaxial cable sensors have the advantage of large strain and robustness, making this system suitable for structure health monitoring applications.

Fiber optic probes for chemical sensing based on the extraordinary optical transmission (EOT) phenomenon are designed and fabricated by perforating subwavelength hole arrays on the gold film coated optical fiber endface. The device exhibits a red shift in response to the surrounding refractive index increases with high sensitivity, enabling a reflection-based refractive index sensor with a compact and simple configuration. By choosing the period of hole arrays, the sensor can be designed to operate in the near infrared telecommunication wavelength range, where the abundant sources and detectors are available for easy instrumentation. The new sensor probe is demonstrated for refractive index measurement using refractive index matching fluids. The sensitivity reaches 573 nm/RIU in the 1.333–1.430 refractive index range.

A U-shaped optical fiber inline microchannel was fabricated by femtosecond laser irradiation and subsequent selective chemical wet etching. A high quality micro-cavity embedded inside the channel was obtained to construct a Fabry-Perot interferometer (FPI). A fringe visibility of 20 dB in spectrum domain was achieved. High temperature survivability of this micro device was also demonstrated. The proposed assembly-free optical fiber inline interferometer is attractive for sensing applications in high-temperature harsh environments.

We report a method to fabricate SERS fiber probe by femtosecond laser micromachining on a microfiber tip. Multimode optical fibers are tapered to outer diameter of 4 to 20 μm. Femtosecond laser pulses are used to form nanostructures on the cleaved endface of the microfiber tip. This endface is then activated by silver sputter coating. To confine the excitation and reflection signal within the tapered core of the microfiber tip, the probe is side-coated with silver plating. High quality SERS signal of Rhodamine 6G molecules is detected through back excitation and collection from a lead in fiber of up to several meters long. The small size of the SERS micro-probe is promising for intra-cellular or even single cell detection, while the back excitation and collection setup make it suitable for remote sensing applications.

We report a reflection mode based microfiber half coupler probe for refractive index (RI) sensing. The probe was made by cleaving a fused 2 $\times$ 2 optical fiber coupler at the center of the tapered region. When used for RI sensing, the reflection spectrum blueshifted as the surrounding RI increased. The sensing capability of the probe was tested using sucrose solutions with different concentrations. The relations between the RI sensitivity and the taper waist diameter as well as the RI range were investigated. The RI sensitivity increased quickly as the taper diameter decreased.

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.

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.