Distributed temperature sensing (DTS) is demonstrated using an enhanced scattering optical fiber (ESF). The temperature measurement is based on measuring the intensities of the backscattered light with two broadband lights. The spectra of the broadband light partially overlap with the backscattered spectrum of the ESF. Since the spectrum of the ESF shifts with temperature, the intensities of the backscattered light will change. The temperature coefficients of the system are -0.011 dB/°C and 0.022 dB/°C. Using the system, a low cost and high speed DTS system can be accomplished.
Established techniques for measuring the transmission matrix (TM) of a multimode fiber (MMF) allow for spot scanning at the distal end of the fiber through phase control at the proximal end, enabling ultrathin medical endoscopes and other applications that benefit from controllable light fields in MMF. Adding this capability to fibers utilized for other applications allows imaging to be performed within these areas. One outstanding limitation of this technology is the need to re-calibrate the fiber upon bending or other environmental perturbation. Here, we demonstrate a modified shape-sensor fiber that allows both shape sensing and imaging within the same fiber. In addition to permitting an image at the end of a shape sensor probe, the unification of these two technologies opens up the possibility of using the reconstructed fiber shape to mathematically update the calibration of the imaging waveguide in a dynamic environment, as has been proposed in the prior literature. Creating a robust method for maintaining knowledge of the fiber’s TM as the fiber is manipulated is critical for clinical deployment of this technology.
KEYWORDS: Signal to noise ratio, Acoustics, Optical fibers, Metals, Single mode fibers, Optical sensing, Ferroelectric materials, Structural health monitoring, Fiber optics sensors, Data acquisition
Pipeline infrastructure monitoring based on distributed fiber-optic acoustic sensing is gaining significant attention aimed at real-time rapid detection of leakages, third-party intrusion, geo-hazards, corrosion, and other structural damages. Typical fibers installations are external to a pipeline, however retrofitting of existing pipelines through internal installation is desirable despite deployment challenges. Highly sensitive distributed acoustic sensing integrated within new pipelines or retrofit in existing pipelines can enable early detection of damage and degradation. In this work, we demonstrate pipeline integrity monitoring using distributed acoustic sensing and the Rayleigh backscattering-enhanced optical fibers deployed internal to the pipeline for high sensitivity detection of acoustic events. More specifically, traditional and backscattering-enhanced optical fibers are interrogated using bench-top phase-sensitive optical time-domain reflectometry (Φ-OTDR). The distributed acoustic sensing characteristics of two types of backscattered-enhanced fibers, Type A and Type B, are experimentally investigated. Our measurement analysis shows that the SNR of the acoustic event detection enhances ~2-fold and ~3-fold using the Type A and Type B fiber, respectively than that of the traditional SMF for pipeline monitoring. The presented investigation is a first validation for in-pipe deployed distributed acoustic sensing with high SNR and provides useful insight for diverse pipeline monitoring applications in the oil and gas distribution industry.
Multimode fibers (MMFs) have a very large number of propagating modes per unit area and therefore allow for imaging with a very large number of pixels relative to their diameter. This makes MMFs perfect candidates for ultrathin endoscopes in applications such as deep brain imaging. However, the accuracy of the input-output relation that is needed, e.g., for distal spot scanning without moving parts, requires a new calibration after the fiber position or temperature has been significantly altered.
While neural networks have been used before to attempt to solve these challenges, we present an MMF-based imaging method that tolerates and classifies different fiber positions, using two single-layer fully-connected neural networks that only require the optical intensity without measuring the optical phase. One network learns the nonlinear relation between the input and output intensities and allows for image reconstruction in the presence of position changes, while the other network classifies that position change for different images. We show that our method is superior to memory-effect-based position sensing, both for small position changes where the relation between position change and output specklegram rotation angle is linear, as well as for larger position changes where this linearity and uniqueness break down. We also show that the position classification results are robust to temperature and polarization perturbations, and that our position classifier is able to effectively generalize. Likewise, we show that our imaging network also is robust to 30°C perturbations in temperature and 10° in polarization.
Transmission matrix measurements relating the electric field at the ends of standard step-index and graded-index multimode fibers promise to enable next generation miniaturized endoscopes. Relatively few measurements of specialty fibers and components have been demonstrated. Here, we present transmission matrix measurements and distal control through a variety of specialty fibers, including fibers for harsh environments, a polarization maintaining fiber, coreless fibers, a rectangular core fiber, multicore fibers, and a pump signal combiner. The calibration of these fibers and structures enables their dual-use for imaging and their original design application and allows control of the spatial profile of the light used in sensing, power delivery, and amplification.
We review the progress of BDFA development for O-band amplification. Currently, BDFAs can provide up to 35 dB small signal gain, less than 5.5 dB noise figure, and up to 29 dBm output power over 1270-1360 nm. We discuss amplifier design issues including pump wavelength and bandwidth allocation, as well as components performance and availability. Recent transmission experiments including LAN-WDM and CWDM modules reach extension over the spooled fiber and installed cables will be presented. Future milestones in BDF/BDFA development would also be suggested. Since with the implementation of BDFAs the transmission over O-band would no longer be power budget limited, we will briefly discuss the options of chromatic dispersion compensation over O-band.
We developed a family of silica-based BDFAs operating over telecom O-band (1260-1360 nm). We demonstrated that 80 meters long single pump single stage amplifier can provide up to 19 dB gain, 20 dBm output power with 5 dB noise figure and 20% power conversion efficiency over 80 nm bandwidth (6-dB). The amplifier gain peak can be flexibly centered over 1305-1325 nm by pump wavelength selection. We designed simple BDFA operating over IEEE standardized part of the O-band (1272-1310 nm) and demonstrated that it can extend 425 Gb/s 400GBASE-LR8 transmission (eight 26.6 Gbaud/s PAM-4 channels) beyond 50 km of G.652 fiber.
Higher-order mode (HOM) fibers have been engineered to allow propagation of linearly polarized symmetric modes LP0,N in a robust way. Compared with the fundamental mode LP(0,1), HOMs exhibits an effective area that can be larger by over two order magnitude, and thus propagating light in these modes could greatly suppress the effect of nonlinear effects. HOM fibers could also be doped with rare earth ions in order to amplify light propagating in these modes, which offers the enormous potential for generating high-intensity pulses. Excitation of HOM gain fiber using cladding pumping with multimode pump source is attractive for ytterbium based amplifiers, because of the availability of low-cost multimode pump diodes in the 975nm wavelength range. One problem associated with cladding pumping which leads to excitation of the large doped core (over 100 μm diameter) is that it could result in a large amount of amplifiedspontaneous- emission (ASE) noise, particularly when the input signal is weak. Optimization of amplifier design is critical in order to suppress ASE and achieve high gain and pump-to-signal conversion efficiency. We conducted numerical modeling of a cladding pumped HOM-amplifier, which revealed that this problem could be mitigated by using a relatively long gain-fiber that allowed reabsorption of the forward propagating ASE resulting in a further amplification of the signal. We demonstrate efficient amplification of a LP0,10 mode with an effective area 3140μm2 in an Yb-doped HOM amplifier cladding pumped at 975nm. We have successfully obtained a 20.2dB gain for 0.95 W 1064 nm input seed signal to more than 105W.
Scaling the power-level of fiber sources has many practical advantages, while also enabling fundamental studies on the light-matter interaction in amorphous guiding media. In order to scale the power-level of fiber-sources without encountering nonlinear impairments, a strategy is to increase the effective-area of the guided optical-mode. Increasing the effective-area of the fundamental mode in a fiber, however, presents the challenges of increased susceptibility to mode-distortion and effective-area-reduction under the influence of bends. Therefore, higher-order-mode (HOM) fibers, which guide light in large effective-area (Aeff) Bessel-like modes, are a good candidate for scaling the power-level of robust fiber-sources. Many applications of high-power fiber-sources also demand a deterministic control on the polarization-state of light. Furthermore, a polarization-maintaining (PM)-type HOM fiber can afford the added possibility of coherent-beam combination and polarization multiplexing of high-power fiber-lasers. Previously, we reported polarization-maintaining operation in a 1.3 m length of PM-HOM fiber that was held straight. The PM-HOM fiber guided Bessel-like modes with Aeff ranging from 1200-2800 μm2. In this work, we report, for the first time, that the polarization-extinction-ratio (PER) of the HOM exceeds 10 dB in an 8 m long fiber that is coiled down to a diameter of 40 cm. This opens a path towards compact and polarization-controlled high-power fiber-systems.
We demonstrate a 1km long optical fiber with continuous grating enhanced back scattering and attenuation close to standard single mode fiber. Scattering was observed to be more than 10dB above the Rayleigh back scattering of the optical fiber over a 10nm bandwidth between 1542 and 1552nm. The fiber attenuation was estimated to be 0.4dB/km. Our result was enabled through the fabrication of a standard single mode fiber with a UV transparent coating and reel to reel continuous UV grating inscription over more than 1km. We anticipate that enhanced scattering fiber will have impact in many sensor systems that rely on optical back scatter, including distributed acoustic sensing, security applications and structural health monitoring.
We describe the fabrication and performance of a continuously grated twisted multicore fiber sensor array. The grated fiber sensor comprises nearly continuous Bragg gratings along its entire length. The gratings are inscribed over lengths in excess of 10m in fibers with UV transparent coating using a flexible and scalable reel to reel processing system. The arrays are tested using optical frequency domain reflectometry (OFDR). We report on automated analysis routines applied to these OFDR measurements that allow for characterization of 100s of individual grating exposures that make up a continuously grated fiber length. We also report on the spectral loss of the continuously grated fiber, showing that it is suitable for applications with sensors in excess of 100m. Finally, we report on the fiber sensing characteristics by performing measurements of fiber bend using a fiber shape reconstruction algorithm on OFDR traces obtained from four of the fiber cores.
We report on the optical and sensor performance characteristics of meter long continuous twisted multicore optical fiber gratings. We describe a method to analyze the optical performance of all the cores in the multicore array. We also report on the sensitivity of our arrays to local changes such as bend and twist. Our analysis provides guidance for the proper operating range of multicore fiber sensing arrays.
We report on improved spatial uniformity of sensor grating arrays in offset and multicore fibers. We show improvement over conventional side writing in such fibers, in which cores offset from the center of the fiber exhibit grating strength variations due to lensing at the fiber surface. Such strength variations can degrade the performance of sensing systems that rely on continuous scattering from offset cores along a fiber. Our improved system uses multicore fibers whose coating is UV transparent and applies index matching materials to mitigate lensing aberrations. We show that it is capable of continuously inscribing gratings over any length of fiber.
In this work we report on a fiber grating fabrication platform suitable for parallel fabrication of Bragg grating arrays over arbitrary lengths of multicore optical fiber. Our system exploits UV transparent coatings and has precision fiber translation that allows for quasi-continuous grating fabrication. Our system is capable of both uniform and chirped fiber grating array spectra that can meet the demands of medical sensors including high speed, accuracy, robustness and small form factor.
Mode conversion at the output of a higher-order mode fiber amplifier is proposed and demonstrated with an axicon for the first time. M2 of 1.25 is achieved for 82% conversion efficiency.
KEYWORDS: Polarimetry, Polarization, Calibration, Sensors, Light scattering, Data acquisition, Signal detection, Scattering, Field programmable gate arrays, Sensing systems
We report on an all fiber polarimetric optical detector system suitable for sensor applications that rely on fast polarization measurements. Our device exhibits an RF bandwidth of 500 MHz and an optical calibration bandwidth of over 30nm in the S-C-L-bands (1460-1625 nm) and above, with minimal PMD, PDL, and return loss when used in-line. A set of automated self-calibration procedures ensure high accuracy without the need for external polarization optics or reference polarimeters. We integrated our polarimeter into an acquisition systems capable of measuring fast polarization rotation events over long period of time with 250 MS/s peak sampling rate.
In this paper we report on the development of a complete integrated optical fiber assembly suitable for shape sensing.
Our shape sensor module consists of a length (>1m) of twisted multicore optical fiber with fiber Bragg gratings inscribed
along its length. Our fiber has a compact 180 micron coated diameter, a twist of 50 turns per meter and grating
reflectivities greater than 0.01% per cm of array, suitable for high efficiency scatter measurements over many meters of
fiber. Single core to multicore fanouts and low reflectivity fiber termination are used to terminate the end of the array.
We present the first direct measurements of enhanced nonlinearities in large-mode-area fibers due to
bend induced reductions in effective area. Both Raman scattering and self-phase modulation are
observed to increase in tightly coiled fibers. The measured increase in nonlinearity compares well with
predictions from simulations of the modal effective area.
Line scan cameras are used for rapidly monitoring a moving web or sheet of material. Lighting for line scan inspection
should illuminate a long narrow rectangle, which is imaged onto the linear array of pixels in a line scan camera. This
distributed light source should provide a uniform power density at the desired wavelengths. Tungsten halogen lamps and
LED arrays can meet many of these objectives, but not in a highly directional beam with minimal thermal issues. We
have developed a new distributed light source that is based on diffracting light from a highly blazed grating written in the
core of a single mode fiber. The grating is blazed such that out-coupling is 90 degrees to the fiber axis. The fiber is
bonded to a cylindrical optic that collimates the azimuthal power distribution. Connecting a single laser diode to the
fiber can generate 1 milliwatt per square centimeter over a 10 cm by 0.5 cm rectangular region. Longer gratings and/or
multiple segments can be connected to illuminate longer regions. The distributed power density, spatial uniformity,
degree of collimation, and spectral bandwidth of these illuminated rectangles are reported. This highly directional
distributed source will enhance the utility of line scan cameras in multiple applications.
We present a source of high power femtosecond pulses at 1550 nm generating compressed pulses at the end of a single mode fiber pigtail. The system generates sub 35 femtosecond pulses at a repetition rate of 50 MHz, with average powers greater than 400 mW. The pulses are generated in a passively modelocked, erbium doped fiber laser, and amplified in a short, erbium doped amplifier. The output of the fiber amplifier consists of highly chirped picosecond pulses. These picosecond pulses are then compressed in standard single mode fiber. While the compressed pulses in the SMF pigtail do show a low pedestal that could be avoided with the use of bulk-optic compression the desire to compress the pulses in SMF is motivated by the ability to splice the single mode fiber to a nonlinear fiber, for continuum generation applications. We demonstrate that with highly nonlinear dispersion shifted fiber (HNLF) fusion spliced directly to the amplifier output, we generate a supercontinuum spectrum that spans more than an octave, with an average power 350 mW. Such a high power, all-fiber supercontinuum source has many important applications including frequency metrology and biomedical imaging.
The fiber grating fabrication based on use of the phase masks is the most stable and accurate manufacturing technology. This paper presents a brief overview of holographic methods of phase masks and fiber Bragg gratings (FBGs) writing and characterization with emphasis on the chirped gratings. We discuss the range of FBG parameters enabled by current technological methods, as well as the relation between the accuracy of FBG parameters and the performance of FBG-based dispersion compensators. While holographic phase mask and FBG writing principles have much in common, the phase mask and FBG production is a unified technology where the quality of the FBG is determined by numerous factors in the process of fabrication. As one of the significant factors, we study the effect of mirror non-flatness on the group delay ripple of chirped FBG. The quality of phase masks and FBGs is often important to characterize directly. In this paper we consider holographic side-diffraction methods of their characterization, which are very accurate and provide the information that is not simple to obtain from spectroscopic measurements.
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