Adaptive inverse mapping: a model-free semi-supervised learning approach towards robust imaging through dynamic scattering media.

Imaging through scattering media is a useful and yet demanding task since it involves solving for an inverse mapping from speckle images to object images. It becomes even more challenging when the scattering medium undergoes dynamic changes. Various approaches have been proposed in recent years. However, none of them are able to preserve high image quality without either assuming a finite number of sources for dynamic changes, assuming a thin scattering medium, or requiring access to both ends of the medium. In this paper, we propose an adaptive inverse mapping (AIP) method, which requires no prior knowledge of the dynamic change and only needs output speckle images after initialization. We show that the inverse mapping can be corrected through unsupervised learning if the output speckle images are followed closely. We test the AIP method on two numerical simulations: a dynamic scattering system formulated as an evolving transmission matrix and a telescope with a changing random phase mask at a defocused plane. Then we experimentally apply the AIP method to a multimode-fiber-based imaging system with a changing fiber configuration. Increased robustness in imaging is observed in all three cases. AIP method's high imaging performance demonstrates great potential in imaging through dynamic scattering media.

Optical Fiber Sensors: introduction to the feature issue.

This special issue contains a collection of papers on optical fiber sensors that were originally presented and published in a more succinct form in conjunction with the 27th International Conference on Optical Fiber Sensors (OFS) held in Alexandria, Virginia, United States, from 29th August to 2nd September, 2022.

Long-range multicore optical fiber displacement sensor.

In this Letter, a long-range optical fiber displacement sensor based on an extrinsic Fabry-Perot interferometer (EFPI) built with a strongly coupled multicore fiber (SCMCF) is proposed and demonstrated. To fabricate the device, 9.2 mm of SCMCF was spliced to a conventional single-mode fiber (SMF). The sensor reflection spectrum is affected by super-mode interference in the SCMCF and the interference produced by the EFPI. Displacement of the SMF-SCMCF tip with respect to a reflecting surface produces quantifiable changes in the amplitude and period of the interference pattern in the reflection spectrum. Since the multicore fiber is an efficient light collecting area, sufficient signal intensity can be obtained for displacements of several centimeters. By analyzing the interference pattern in the Fourier domain, it was possible to measure displacements up to 50 mm with a resolution of approximately 500 nm. To our knowledge, this is the first time that a multicore fiber has been used to build a displacement sensor. The dynamic measurement range is at least seven times larger than that achieved with an EFPI built with a conventional SMF. Moreover, the SMF-SCMCF tip is robust and easy to fabricate and replicate.

Temperature-insensitive curvature sensor based on Bragg gratings written in strongly coupled multicore fiber.

A novel temperature-insensitive optical curvature sensor has been proposed and demonstrated. The sensor is fabricated by inscribing fiber Bragg gratings with short lengths into a piece of strongly coupled multicore fiber (SCMCF) and spliced to the conventional single-mode fiber. Due to the two supermodes being supported by the SCMCF, two resonance peaks, along with a deep notch between them, were observed in the reflection spectrum. The experimental results show that the depth of the notch changes with the curvature with a sensitivity up to 15.9dB/m-1 in a lower curvature range. Besides, thanks to the unique property of the proposed sensor, the notch depth barely changes with temperature. Based on the intensity demodulation of the notch depth, the temperature-insensitive curvature sensor is achieved with the cross sensitivity between the temperature, and the curvature is as low as 0.001m-1/∘C.

Mode-selective few-mode Brillouin fiber lasers based on intramodal and intermodal SBS.

Mode-selective fiber lasers have advantages in a number of applications. Here we propose and experimentally demonstrate a transverse mode-selective few-mode Brillouin fiber laser using the mode-selective photonic lantern. We generated the lowest three orders of linearly polarized (LP) modes based on both intramodal and intermodal stimulated Brillouin scattering (SBS). Their slope efficiencies, optical spectra, mode profiles, and linewidths were measured.

Single-mode, low loss hollow-core anti-resonant fiber designs.

In this paper, we numerically investigate various hollow-core anti-resonant (HC-AR) fibers towards low propagation and bend loss with effectively single-mode operation in the telecommunications window. We demonstrate how the propagation loss and higher-order mode modal contents are strongly influenced by the geometrical structure and the number of the anti-resonant cladding tubes. We found that 5-tube nested HC-AR fiber has a wider anti-resonant band, lower loss, and larger higher-order mode extinction ratio than designs with 6 or more anti-resonant tubes. A loss ratio between the higher-order modes and fundamental mode, as high as 12,000, is obtained in a 5-tube nested HC-AR fiber. To the best of our knowledge, this is the largest higher-order mode extinction ratio demonstrated in a hollow-core fiber at 1.55 µm. In addition, we propose a modified 5-tube nested HC-AR fiber, with propagation loss below 1 dB/km from 1330 to 1660 nm. This fiber also has a small bend loss of ~15 dB/km for a bend radius of 1 cm.

Optics at CREOL: introduction to the feature issue.

This feature issue highlights some current applied optics and photonics related research activities taking place at CREOL, The College of Optics & Photonics at the University of Central Florida. The issue includes contributions from various CREOL research groups, showing diversity and particular focus areas at our Center for Excellence in Optics.

Wavefront shaping optical elements recorded in photo-thermo-refractive glass.

The paper presents an overview of the benefits of recording phase masks into the bulk of photo-thermo-refractive glass. We demonstrate that both binary and gray-scale phase masks can be encoded into the medium, and that such masks can be used for mode conversion and beam shaping with near-theoretical efficiency. We further demonstrate that by encoding the phase mask profile into a transmitting volume Bragg grating, it is possible to create tunable and achromatic phase masks without requiring a complex phase pattern.

A path to high-quality imaging through disordered optical fibers: a review.

In this paper, we review recent progress in disordered optical fiber featuring transverse Anderson localization and its applications for imaging. Anderson localizing optical fiber has a transversely random but longitudinally uniform refractive index profile. The strong scattering from the transversely disordered refractive index profiles generates thousands of guiding modes that are spatially isolated and mainly demonstrate single-mode properties. By making use of these beam transmission channels, robust and high-fidelity imaging transport can be realized. The first disordered optical fiber of this type, the polymer Anderson localizing optical fiber, has been utilized to demonstrate better imaging performance than some of the commercial multicore fibers within a few centimeters transmission distance. To obtain longer transmission lengths and better imaging qualities, glass-air disordered optical fibers are desirable due to their lower loss and larger refractive index contrast. Recently developed high air-filling fraction glass-air disordered fiber can provide bending-independent and high-quality image transport through a meter-long transmission distance. By integrating a deep-learning algorithm with glass-air disordered fiber, a fully flexible, artifact-free, and lensless fiber imaging system is demonstrated, with potential benefits for biomedical and clinical applications. Future research will focus on optimizing structural parameters of disordered optical fiber as well as developing more efficient deep-learning algorithms to further improve the imaging performance.

Transverse mode-switchable fiber laser based on a photonic lantern.

We propose and experimentally demonstrate an intra-cavity transverse mode-switchable fiber laser based on a mode-selective photonic lantern and a few-mode Er-doped fiber amplifier. The six lowest-order LP modes can lase independently and are switchable by changing the input port of the photonic lantern. We measured the slope efficiency, mode intensity profile, and optical spectrum of each lasing mode. In addition, we demonstrate donut-shaped LP11 and LP21 modes using incoherent superposition and simultaneous lasing of the two degenerate modes.

Single-frequency blue laser fiber amplifier.

An all-fiber amplifier for a single-frequency blue laser was demonstrated for the first time, to the best of our knowledge. Over 150 mW continuous-wave single-transverse-mode blue laser output was obtained with a 10 m 1000 ppm thulium-doped fluoride fiber pumped by a 1125 nm fiber laser at a power of 2 W. The output power was limited due to the onset of the competitive lasing at 783 nm. Photodarkening and photo-curing of the thulium-doped fiber amplifier were also studied and analyzed.

Interferometer based on strongly coupled multi-core optical fiber for accurate vibration sensing.

We report on the use of a simple interferometer built with strongly-coupled core optical fiber for accurate vibration sensing. Our multi-core fiber (MCF) is designed to mode match a standard single-mode optical fiber (SMF). The interferometer consists of a low insertion loss SMF-MCF-SMF structure where only two super-modes interfere. The polymer coating of the MCF was structured and the interferometer was sandwiched between a flat piece and a V-groove. In this manner our device is highly sensitive to force with sensitivity reaching -4225 pm/N. To make the MCF interferometer sensitive to vibrations the flat piece was allowed to move, thus, its periodic movements exert cyclic localized pressure on the MCF which makes the interference pattern to shift periodically. Our sensors can be used to monitor vibrations in a broad frequency range with the advantage that the measurements are unaffected by temperature changes.

Miniature multicore optical fiber vibration sensor.

We demonstrate a compact and versatile interferometric vibration sensor that operates in reflection mode. To build the device, a short segment of symmetric strongly coupled multicore optical fiber (MCF) is fusion spliced to a single-mode optical fiber (SMF). One end of the MCF segment is cleaved and placed in a cantilever position. Due to the SMF-MCF configuration, only two supermodes are excited in the MCF. Vibrations induce cyclic bending of the MCF cantilever which results in periodic oscillations of the reflected interference spectrum. In our device, the MCF itself is the inertial mass. The frequency range where our device is sensitive can be easily tailored from a few hertz to several kilohertz through the cantilever dimensions.

Ultrasensitive vector bending sensor based on multicore optical fiber.

In this Letter, we demonstrate a compellingly simple directional bending sensor based on multicore optical fibers (MCF). The device operates in reflection mode and consists of a short segment of a three-core MCF that is fusion spliced at the distal end of a standard single mode optical fiber. The asymmetry of our MCF along with the high sensitivity of the supermodes of the MCF make the small bending on the MCF induce drastic changes in the supermodes, their excitation, and, consequently, on the reflected spectrum. Our MCF bending sensor was found to be highly sensitive (4094 pm/deg) to small bending angles. Moreover, it is capable of distinguishing multiple bending orientations.

Modal analysis of antiresonant hollow core fibers using S2 imaging.

We analyze the higher-order core mode content in various designs of antiresonant hollow core fibers using spatially and spectrally resolved imaging. Hollow core fibers have great potential for a variety of applications, and understanding their mode content is crucial for many of these. Two different designs of hollow core fibers are considered, the first with eight nontouching rings and the second with eight touching rings forming a closed boundary core. The mode content of each fiber is measured as a function of length and bending diameter. Low amounts of higher-order modes were found in both hollow core fibers, and mode specific and bending-dependent losses have been determined. This study aids in understanding the core modes of hollow core fibers and possible methods of controlling them.

Bending sensor combining multicore fiber with a mode-selective photonic lantern.

A bending sensor is demonstrated using the combination of a mode-selective photonic lantern (PL) and a multicore fiber. A short section of three-core fiber with strongly coupled cores is used as the bend sensitive element. The supermodes of this fiber are highly sensitive to the refractive index profiles of the cores. Small bend-induced changes result in drastic changes of the supermodes, their excitation, and interference. The multicore fiber is spliced to a few-mode fiber and excites bend dependent amounts of each of the six linearly polarized (LP) modes guided in the few-mode fiber. A mode selective PL is then used to demultiplex the modes of the few-mode fiber. Relative power measurements at the single-mode PL output ports reveal a high sensitivity to bending curvature and differential power distributions according to bending direction, without the need for spectral measurements. High direction sensitivity is demonstrated experimentally as well as in numerical simulations. Relative power shifts of up to 80% have been measured at radii of approximately 20 cm, and good sensitivity was observed with radii as large as 10 m, making this sensing system useful for applications requiring both large and small curvature measurements.

Mode-resolved gain analysis and lasing in multi-supermode multi-core fiber laser.

Multi-core fibers (MCFs) with coupled-cores are attractive large-mode area (LMA) specialty fiber designs that support the propagation of a few transverse modes often called supermodes (SMs). Compared to other LMA fibers, the uniqueness of MCF arises from the higher degrees of design space offered by a multitude of core-array geometries, resulting in extended flexibility to tailor SM properties. To date, the use of MCF as gain media has focused on lasers that operate in only one selected SM, typically the lowest order in-phase SM, which considerably limited the potential of these multi-core structures. Here, we expand the potential of MCF lasers by investigating multi-SM amplification and lasing schemes. Amplifier and laser systems using a 7 coupled-cores Yb-doped MCF as gain medium were successfully designed and assembled. Individual SM could be decomposed using the correlation filter technique mode analysis and the modal amplification factors (γi) were recorded. With access to amplification characteristics of individual transverse modes, a monolithic MCF laser was demonstrated that operates simultaneously on the two SMs carrying the highest optical gain.

Optimization of multicore fiber for high-temperature sensing.

We demonstrate a novel high-temperature sensor using multicore fiber (MCF) spliced between two single-mode fibers. Launching light into such fiber chains creates a supermode interference pattern in the MCF that translates into a periodic modulation in the transmission spectrum. The spectrum shifts with changes in temperature and can be easily monitored in real time. This device is simple to fabricate and has been experimentally shown to operate at temperatures up to 1000°C in a very stable manner. Through simulation, we have optimized the multicore fiber design for sharp spectral features and high overall transmission in the optical communications window. Comparison between the experiment and the simulation has also allowed determination of the thermo-optic coefficient of the MCF as a function of temperature.

Multicore fiber sensor for high-temperature applications up to 1000°C.

A novel high temperature sensor based on customized multicore fiber (MCF) is proposed and experimentally demonstrated. The sensor consists of a short, few-centimeter-long segment of MCF spliced between two standard single-mode fibers. Due to interference effects, the transmission spectrum through this fiber chain features sharp and deep notches. Exposing the MCF segment to increasing temperatures of up to 1000°C results in a shift of the transmission notches toward longer wavelengths with a slope of approximately 29 pm/°C at lower temperatures and 52 pm/°C at higher temperatures, enabling temperature measurements with high sensitivity and accuracy. Due to its compact size and mechanical rigidity, the MCF sensor can be subjected to harsh environments. The fabrication of the MCF sensor is straightforward and reproducible, making it an inexpensive fiber device.

Short monolithic dual-wavelength single-longitudinal-mode DBR phosphate fiber laser.

We propose and demonstrate a 5-cm-long monolithic dual-wavelength single-longitudinal mode distributed Bragg reflector (DBR) all-phosphate fiber laser. Strong UV-induced fiber Bragg gratings are directly written in highly Er/Yb codoped phosphate fiber. The separation between gratings is selected as 1 cm to only excite two longitudinal modes in the DBR cavity. By exploiting the spatial hole burning effect and the polarization hole burning effect, stable narrow-linewidth dual-wavelength lasing emission with 38 pm wavelength spacing and a total emitted power of 2.8 mW is obtained from this DBR fiber laser. A microwave signal at 4.58 GHz is generated by the heterodyne detection of the dual-wavelength laser.