A distributed sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (FBGs) was designed and fabricated for temperature measurements in commercial boilers under field conditions. Three cascaded FBGs with Bragg wavelengths ranging from 1540 nm to 1570 nm were inscribed in a commercial air-clad single-crystal sapphire fiber, which were packaged inside high purity alumina sheath tubes. The packaged sensor was deployed in a commercial boiler through an existing view port during operation. The tests were performed in a coal-fired boiler, whose temperature varied based on power demands in the local area. The reflected signal from the sensor was simultaneously recorded by an onsite optical interrogation system and converted into temperature information, which was monitored remotely in real time through the Virginia Tech network. The performance of the sensor was consistent during the entire test duration of 42 days, over the course of which it measured temperatures up to 700°C, showing the survivability of the sensing system in a field environment. The system has a demonstrated temperature measurement range from 25°C to 1200°C.

The sensing range of Brillouin distributed fiber sensors (BDFS) is typically in the order of tens of kilometers due to the attenuation of the optical fiber and restricted input pump power. This limits the use of BDFS in certain long range applications such as oil and gas pipeline monitoring; where maintenance and safety monitoring requires sensing lengths up to hundreds of kilometers. This deterioration in the sensing performance cannot be counteracted by indefinitely increasing the pump power injected into the sensing fiber; as nonlinear effects such as modulation instability, self-phase modulation, and significant pump depletion occurs within the sensing fiber. In this paper, we demonstrate an extended sensing range system for pipeline monitoring using Brillouin optical time domain reflectometry (BOTDR) combined with Raman amplification and inline erbium-doped fiber amplifier (EDFA). Variations in pump light power, propagation direction, and injection location are explored to allow full control over the signal amplification in any particular section of the total sensing fiber length. Thus, the signal-to-noise ratio (SNR) for a certain location along the length of the fiber can be enhanced to provide more useful localized information. By using a continuous wave 1480nm Raman laser, and 980nm-pumped inline EDFA, the proposed system is theoretically validated over 150 km sensing fiber.

Nonlinear laser wavelength tuning effects appear as phase noise in wavelength swept interferometry. A new method is proposed to compensate tuning nonlinear effects in optical frequency domain reflectometry (OFDR). The proposed method is simpler in configuration, and unlike conventional nonlinear compensation methods, it does not require separate auxiliary interferometer detection, which eliminates the need for an extra photo detector and an acquisition channel for the auxiliary interferometer. In the proposed method, an intentional beating signal is introduced in the beginning of the OFDR spectrum which is treated as an auxiliary interferometer to acquire tunable laser phase information for post signal processing. The proposed method can reduce overall OFDR system cost, reduce the data acquisition time and computational load by half, and make system configuration simpler by eliminating the need for extra components. Feasibility of the proposed method was demonstrated by compensating for tuning nonlinearity effects in an optical fiber approximately 35 m long with a measured spatial resolution of ~30 μm. To confirm performance of the proposed method, a comparison was carried out with a conventional nonlinear tuning compensation method, which requires the separate auxiliary interferometer. Moreover, distributed sensing using the proposed method was also demonstrated in an optical fiber approximately 35 m in length by performing strain sensing with 3 cm sensing resolution.

Simultaneous distributed strain and temperature measurement based on Brillouin frequency shift (BFS) was demonstrated using a dual-Brillouin-peak single-mode (SM) optical fiber. The fiber is designed and fabricated with enhanced Brillouin gain of higher-order acoustic modes so that the gains of the multiple Brillouin peaks are at a similar level. In a 25-kilometer sensing length with 5-meter spatial resolution, the achieved temperature resolution is 2°C and the strain resolution is 40 microstrain.

Optical Frequency Domain Reflectometry (OFDR) is notable for its ability to characterize an optical fiber’s distributed Rayleigh scatter reflection profile with high spatial resolution and high sensitivity in a single laser optical frequency sweep, allowing for the measurement of strain and temperature with millimeter spatial resolution. We investigate manipulating the polarization state of the light launched into the fiber and the use of polarization diverse detection to capture the full polarization response of the scatter pattern. This enables measurement of fiber birefringence amplitude and orientation with high spatial resolution, and a wide range of additional sensing capabilities.

Internal corrosion can occur when aqueous electrolytes are present inside natural gas transmission pipelines. Despite upstream gas dehydration treatments, liquid water can form through condensation of water vapor or may be introduced from plant upsets. With dissolved salts and acidic gases such as CO₂ and H₂S, aqueous electrolytes become very corrosive with increased conductivity and lower pH. Since water provides the electrolytes that initiate and sustain corrosion, detection of water can locate the spots for potential internal corrosion inside the pipelines. In this work, a simple optical fiber-based sensor for fully distributed water monitoring has been demonstrated and studied. The system consists of an unmodified off-the-shelf single mode (SM) optical fiber and an optical backscatter reflectometer (OBR) capable of measuring the spatial profile of strain changes along the fiber. The polymer jacket coating of the SM fiber is hydroscopic and serves as the water sensing layer due to expansion/swelling from water absorption. The swelling induced strain change is interrogated with the OBR to enable fully distributed water monitoring. This strain-based H₂O sensor is sensitive to H₂O molecules regardless of the phase (liquid or vapor) or the surrounding media. Strain changes were measured at different relative humidity levels from 0% to 100% to demonstrate reversibility and linear correlation between humidity and strain. This sensor has the advantages of fully distributed sensing, low cost, simple preparation, easy operation, and good sensitivity.

Since the early days of flight gyroscopes have acted as a key enabling technology for stabilization and guidance of aircraft and rockets. Mechanical gyros were the first to be used. They suffered from repeated failures and were aggressively replaced by ring laser gyros beginning in the late 1970s. Almost in parallel with the introduction of ring laser gyros, the first solid state gyros based on fiber optics began their development, at about the same time I began working on fiber sensors at McDonnell Douglas. This paper summarizes thoughts on the past, present and future of fiber optic gyros.

New developments in LIDAR and atmospheric sensing experiments highlight the need for studies of the optical bandwidth and wavelength dependence of multi-watt, large bandwidth, high dynamic range polarization-maintaining optical amplifiers in the 2—2.1 μm band. Recently we have demonstrated a hybrid single clad-double clad Tm-doped fiber amplifier with greater than 20 W output and a dynamic range of <20 dB in the 2 μm band, and a <25W output PM hybrid HDFA/TDFA with a dynamic range of 34 dB. Both demonstrations were carried out at a single input wavelength of 2051 nm. In this paper we extend our experimental studies to the signal wavelength dependence of a PM hybrid HDFA/TDFA with a single clad Hodoped preamplifier [4-7] and a double clad Tm-doped power amplifier. We have studied the performance of the amplifier from 2004 to 2108 nm, and in this paper, we report first experimental results for this wavelength region. We find that our hybrid Ho-Tm-doped design provides a PM fiber amplifier with a combination of large output optical signal-to-noise ratio, broad operating bandwidth, and high P_out of 28.5 W at λs = 2069 nm.

We simulate supercontinuum generation for various shapes of a dispersion varying As₂S₃ waveguides on MgF₂ substrate with air cladding. The supercontinuum generation is simulated for pulses of 2000 W peak power and 50 fs pulse width centered at 1.55 μm wavelength. For a uniform waveguide the generated spectrum is considerably narrower than that for a non-uniform waveguide. This is because the non-uniform waveguide allows a continuous phase matching of the generated waves through nonlinear interaction. We have demonstrated that a high coefficient of nonlinear refractive index is necessary for generating supercontinuum with large bandwidths. Larger supercontinuum bandwidth is predicted for waveguides with increasing As₂S₃ thickness along the propagation direction.

An embeddable, robust and cost-effective optical interferometric strain sensor with nano-scale strain resolution is reported in this paper. The principal structure of the sensor consists of an optical fiber, a quartz rod coated with a thin gold layer, and two metal shells employed to transfer the strain, orient and protect the optical fiber and quartz rod. The optical fiber endface, combining with the gold-coated surface forms an extrinsic Fabry-Perot interferometer. When the sensor is subjected to an external compressive/tensile stress, slide between the two metal shells will occur, resulting in a cavity length variation of the interferometer. A temperature compensation design is employed in the structure to minimize the temperature crosstalk of the sensor. The sensor was firstly calibrated, and the result showed that our prototype sensor can realize a measurement resolution of 30 nanostrain (nε) and a sensitivity of 10.01 με/μm over a range of 1000 με. After calibration of the sensor, monitoring the shrinkage strain of a cubic brick of mortar in real time during the drying process was conducted. The strain sensor was compared with a commercial linear variable displacement transducer, and the comparison results in four weeks demonstrated that our sensor had much higher measurement resolution and gained more detailed and useful information. Due to the advantages of the extremely simple, robust and cost-effective configuration, it is believed that the sensor is significantly beneficial to practical applications, especially in structural health monitoring.

We present a review of recent progress in quantitative performance assessment of optical interferometry. Specifically, a three-level framework is introduced for evaluating the sensitivity of optical pathlength measurement. Its application is demonstrated for whitelight interferometry and wavelength shifting interferometry. It shows that there is a performance gap, potentially significant, between currently achieved measurement sensitivity and the sensitivity limit of the system. The framework allows one to accurately assess hardware performance, identify sources of degradation, and devise optimization strategy. It may offer guidance to lower sensor cost and improve commercial competitiveness.

This paper studies the use of Microsphere Photolithography (MPL) as an alternative to Focused Ion Beam milling or e-beam lithography to pattern plasmonic fiber-optic based sensors. In the MPL approach, silica microspheres are self-assembled to form a Hexagonal Close-Packed (HCP) array on top of a layer of photoresist. The microspheres serve as an optical element and focus collimated UV radiation to an array of photonic jets inside the photoresist layer. The exposed region is dependent on the angle of incidence of the UV radiation which facilitates hierarchical patterning. Pattern transfer can be accomplished using either etching or lift-off with the size of the features dependent on the exposure dose. While low-cost and very scalable, the use of MPL influences the design and performance of the fiber probe in several ways. Specifically, the entire cleaved face is patterned without alignment to the fiber core and any defects in the self-assembled microsphere lattice are transferred to the surface. This paper presents the low-cost fabrication of Extraordinary Optical Transmission (EOT) type plasmonic fiber-optic based sensors. The sensors consist of a thin aluminum film on the cleaved face of single mode optical fiber, perforated with a HCP hole-array. At resonance, Extraordinary Optical Transmission (EOT), decreases the reflection from the fiber tip. The conditions for resonance are dependent on the local environment surrounding the fiber tip and the resonant wavelength can be used to measure the index of refraction of a liquid. Experiments show that viable sensors can be created with MPL. The reflection spectra of the sensors was measured in various concentrations of sugar water with a measured sensitivity of 8.33 mg/mL/nm. These results are compared to simulation results which provides a sensitivity analysis of the sensors.

In order to characterize energetic materials, it is highly desirable to be able to measure velocity, position, pressure, and temperature throughout the event. This paper looks at approaches associated with extracting these parameters from data obtained using high speed fiber grating sensor systems. Prior work has demonstrated that velocity, position, and local pressure data may be obtained using chirped fiber grating sensors (CFBGs). Uniform fiber grating sensors have been used to quantify local pressures beyond the range associated with CFBGs. A main challenge faced today is the separate of these parameters from temperature. A novel approach that may be used to isolate temperature and pressure measurements using a high-speed fiber grating sensor system is presented in this paper.

This presentation reports the use of nanostructured sapphire optical fiber (NSOF) with Au nanorods entrapped in anodized aluminum oxide (AAO) cladding for surface-enhanced Raman spectroscopy (SERS) sensing at elevated temperatures. Specifically, Au nanorods were fabricated in AAO pore channels by in-situ electroless reduction aided with Au nanoparticle catalysts. We show that the AAO pore channels endow the Au nanorods with enhanced thermal stability through geometrical confinement at up to 1000°C. The NSOF with entrapped Au nanorods was used for SERS measurements at temperatures ranging from room temperature to 800°C. The capacity of the novel fiber structure for sensing of hot gases in harsh environments was demonstrated.

Although single crystal sapphire fiber has been fabricated extensively for decades, many details surrounding the impacts of growth conditions on fiber quality are still unreported. Traditional fiber quality measurements require stopping the fiber growing process, cutting the fiber into short pieces, and measuring the transmission which is time consuming and highly variable. We developed a very simple method to monitor the fiber quality in real-time. During the fiber growth process, the melting pool shape becomes stable and incandescence from the molten zone can be used as an active light source. By connecting the other end of the growing fiber to a spectrometer, we can monitor the light intensity as fiber length increases continuously. Not only we are able to deduce the current fiber quality being grown, but also to identify the optimum growing conditions include the growth rate, rod-to-fiber ratio, and position of the crystallization interface, as well as monitoring impacts from laser power instability. These measurements can be further compared to cutback style measurement, or loss measurements using Raman interrogation. Different single crystal fibers, including sapphire and YAG are grown while measuring throughput during growth.

Optical fiber-based probes are promising candidates for many applications in radiotherapy dosimetry and quality assurance due to their unique properties including the ability to perform in vivo, real-time, and intracavitary measurements with high spatial resolution. However, a significant issue related to fiber optic dosimetry of photon and proton therapies is Čerenkov radiation generated in the optical fibers. We have designed, fabricated, and characterized fiber probes composed of solid core and hollow core fibers in conjunction with scintillators for radiotherapy dosimetry. Using a spectroscopic method to subtract the Čerenkov contamination, we showed good agreement between our fiber probe and standard ionization chamber measurements.

The effect of two-photon absorption (TPA) on all-optical logic operation in quantum-dot semiconductor optical amplifier (QD-SOA) has been carried out. The rate equation was modeled with the TPA effect for the logic XOR gate, AND gate, and, for pseudo-random bit sequence (PRBS) generation. The output Q-factor (quality) has increased due to the implementation of TPA induced pumping. The results show that the quality of the output depends on the input pulse width and the speed of operation. The PRBS system has been shown to operate at 250 Gb/s and 320 Gb/s and the Q-factor decreases with an increase in pulse width.

The absence of an effective and stable cladding has been a major hurdle in utilizing single crystal fibers for harsh environment sensing applications despite the promise of sapphire for temperatures as high as 1800°C. This work discusses the development of a high temperature cladding for sapphire fibers using wet chemical methods. Magnesium aluminate spinel has been chosen as the material for the cladding as it has a lower refractive index compared to sapphire and does not undergo significant interdiffusion with sapphire at temperatures below approximately 1200°C. Different sol-gel based approaches have been pursued to develop polycrystalline cladding layers with thicknesses greater than a micron, as required to ensure adequate confinement of the guided electromagnetic radiation within the fiber core. For sapphire fibers, high temperature stability of the cladded fibers as well as the effect of the cladding layer on optical characteristics under different application relevant gas environments at elevated temperatures has been investigated.

The detection range of Distributed Acoustic Sensor (DAS) systems is limited by signal attenuation to approximately 75 km. The ability to increase the detection range is of great commercial interest to the offshore wind farm operators interested in structural health monitoring of their subsea cables. In most cases, the operators are interested in monitoring 200km~400km subsea cable where the fibres can be accessed at two ends of the cable. In this paper, we present a new, commercially viable, ultra low-loss sensing element, comprising of discrete broadband reflectors. Optical time domain reflectometry measurements were performed on a 100 m sample of the fibre. The sample contained reflectors placed at 3 m intervals. At reflector sites, the recorded trace revealed increases in the backscatter signal two hundred times that of the unmodified regions of the fibre. Theoretically, the spatial resolution of a system utilising this new element is only restricted by the ability to resolve two reflector points. Therefore, the ultra low loss fibre also offers the potential for high spatial resolution measurements over large distances, as long as sufficient data acquisition and processing techniques are employed. The significant enhancement means no amplification of the reflected signal is required, further reducing the cost of the system. To verify the long distance capability of the fibre, the sample was subjected to optical side scattering radiometry measurements. The largest side-scattered loss from a reflector point was 10^-4 dB per reflector. If a reflector was placed every meter, the total fibre attenuation is predicted to be 0.3 dB per km.

Here we propose and demonstrate an optical time-domain reflectometry (OTDR) system that uses multimode fiber to make quantitative acoustic measurements that are immune to signal fading. The vast majority of OTDR fiber sensors use single mode fiber, which are limited by signal fading and by low light levels. Sensing with multimode fiber has the potential to overcome both of these limitations. First, multimode fiber sensors are immune to signal fading by simultaneously detecting many spatial modes. Second, multimode fiber sensors can use increased light levels due to a higher threshold for non-linear effects. Combined with a higher capture fraction of Rayleigh scattered light, this enables multimode fiber sensors to detect higher light levels and exhibit lower noise. We demonstrate new methods that use off-axis holography to recover quantitative strain information from the time-dependent backscattered speckle field. The multimode fiber sensor exhibits a linear response, a bandwidth of 20 kHz, and has a noise level better than 6 pε/√Hz. This work represents the first multimode fiber OTDR systems that take full advantage of the unique features of multimode fiber for acoustic sensing.

A hollow coaxial cable Fabry-Perot resonator for displacement and strain measurement up to 1000 °C is presented here. Inspired by optical fiber Fabry-Perot interferometers, a Fabry-Perot resonator is implemented on a homemade hollow coaxial cable by introducing two highly-reflective reflectors along the cable. By tracking the shift of the amplitude reflection spectrum of the microwave resonator, the displacement and strain can be determined. The displacement measurement experiment showed the sensor could function properly up to 1000 °C. The sensor was also employed to measure the thermal strain of a steel plate during the heating process from 100 to 900 °C.

Inspired by Rayleigh backscattering based sensing methodology on an optical fiber, we present a novel sensing concept based on the random inhomogeneities on a coaxial cable. As an analogy of Rayleigh backscattering along an optical fiber length, “backscattering” also exists from a commercial cable due to its inherent defects along a cable length which induce a local variation. The accumulated back-scattered signals along the cable can be obtained using frequency domain reflectometry. By analyzing the shift in the local back-scattered signal, the local perturbations can be determined, so that truly distributed sensing capability using a coaxial cable can be achieved.

A scheme to generate 50 GHz pulse train using rational harmonic mode locking technique is proposed and studied experimentally. The modulation frequency is adjusted to achieve a 50 GHz pulse train which has a pulse width of 5.25 ps. Numerical simulation results show good agreement with the experimental results.

Fiber-optic flow sensor based on a laser-heated silicon Fabry-Pérot interferometer (FPI) exhibits a strong directivity owing to the cylindrical shape of the sensor head. In this work, a new sensor structure has been designed to effectively reduce the directivity. The proposed sensor embeds the laser-heated silicon FPI in a Tin microsphere (diameter ~1mm). Due to the circular shape of the outer metal layer, a more symmetric response to flow from different directions is achieved. In the meantime, the high thermal conductivity and small footprint of the metal sphere helps maintain the good responsivity of the silicon FPI to the flow. Directivity of the newly designed sensor has been tested in water flow. Experimental results suggest that deviation in the directional response is reduced to 4% at a speed of ~1.4 ms^-1 , in comparison to the 44% for the original sensor without the metal shell. The directivity can be reduced further by improving the fabrication techniques for the metal sphere.

Magnetostrictive fiber sensors combine the phase sensitivity of interferometry with the magnetically induced strain of ferromagnetic materials. Configurations include fiber wrapped around mandrel halves separated by a magnetostrictive rod, fiber bonded to a magnetostrictive ribbon and fiber jacketed with a magnetostrictive film. Processing advances in the deposition of dense, uniform films on the cylindrical surface of the fiber offer the advantage of reduced demagnetization of the magnetostrictive material. In this paper, we investigate the design of a sensor based on a magnetostrictive film jacketing the fiber. We analyze the system resolution and minimum film thickness of the fiber sensor using nickel, Metglas and Terfenol-D films. For each of these magnetostrictive films, we present simulation results on the resolution as a function of the film-fiber interaction length. In our analysis, we assume a phase generated carrier demodulation scheme. We next analyze the magnetostrictive strain of the compound film-fiber system as a function of film thickness. This analysis sets the minimum film thickness for effective strain on the system. Finally, we propose a geometry which allows a compact sensor package with a reasonable film-fiber interaction length.