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.
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 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.
A novel approach for stripping cladding light from double clad fibers is demonstrated. This is achieved by index matching the cladding of the fiber with a glass capillary collapsed onto the fiber, allowing the cladding modes to expand in a larger volume of the capillary before they are dissipated through a high-index heat sink material into a metal package. We minimize the signal quality degradation by using a lower melting point capillary glass. We demonstrate a device with 100W cladding power removed with 99% (20dB) extinction. Continuous operation for an hour without any power degradation is demonstrated.
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.
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.
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