Amy Van Newkirk

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.

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.

Multicore Fiber Sensors for Simultaneous Measurement of Force and Temperature

We demonstrate a force and temperature sensor consisting of a multicore fiber (MCF) spliced between two single-mode fibers. Increasing of the sensitivity to applied force is achieved through etching of the MCF cladding, with an overall increase of 7× compared with a 125-μm MCF device, and 12× compared with a standard fiber Bragg grating (FBG). Simultaneous decoupling of force and temperature with high accuracy is demonstrated using two MCF sections with different outer diameters. This device has robust operation up to 1000 °C, making it more suitable than FBGs for extreme environments.

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.

Modal analysis of antiresonant hollow core fibers using S 2 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.

Optical Stress-sensing Alumina Nanocomposite Coatings for Aerospace Structures

Alpha alumina (α-Al2O3) nanocomposites have a multi-functional stress sensing ability via their photoluminescence (PL) spectral peak shifts defined by Piezospectroscopic (PS) relationships. This has prompted the development of α-Al2O3 nanocomposite coatings to enable real-time stress measurements and damage assessment of structures with the potential for unparalleled spatial resolution and sensitivity. Here, two types of stress-sensing coatings were evaluated, including an atmospheric plasma spray (APS) coating on metallic substrates and an epoxy nanocomposite coating on a composite substrate. The stress sensitivity of the APS coating, represented by the slope of the peak shifts against stress or PS coefficient, varied inversely with the thickness of the substrates between 3.2 and 1.7 cm−1/GPa. These values decreased by more than 50% after tensile cycling for all substrate thicknesses due to microstructural damage in the APS coating indicating the need for repeatability and durability studies. The epoxy nanocomposite coating successfully captured the stress gradients associated with an open hole tension (OHT) composite substrate revealing damage initiation at 77% of failure load, earlier than visual appearance of a surface crack (93%). The findings validate the successful development of quantitative and multiscale spatial resolution stress-sensing coatings, capable of detecting subsurface damage of composite structures, that will take structural testing and integrity monitoring to the next level.

High Temperature Sensor based on Supermode Interference in Multicore Fiber

A high temperature fiber optic sensor based on multicore fiber is presented. Experimental results show the sensor operating stably at temperatures up to 1000°C with the capability to multiplex sensors in a single chain.

Advances in Multi-Core Fiber Lasers

Multi-core fiber lasers and their potential for power scaling of fiber laser systems will be discussed. Recently developed concepts will be presented including supermode selection techniques and simultaneous multi-supermode laser operation.

Coherent Perfect Absorption in a Weakly Absorbing Fiber

Coherent perfect absorption refers to the interferometric enhancement of absorption in a partially lossy medium upto 100%. This can be achieved without modifying the absorbing medium itself, instead of engineering its photonic environment. Ion-doped fibers are one of the most technologically relevant absorbing materials in optics, which are widely employed in fiber amplifiers and lasers. Realizing complete optical absorption of an incident field in short-length moderately-doped fibers remains a challenge for the cost-effective design of compact fiber lasers. Here, we exploit the concept of coherent perfect absorption to overcome this challenge, whereby two appropriately designed fiber Bragg gratings define a short-length erbium-doped-fiber cavity that enforces complete absorption of an incident field on resonance—-independently of the doped-fiber intrinsic absorption. This approach applies to any spectral window and guarantees the efficient utilization of the fiber dopants along its length, thus, suggesting the possibility of next-generation efficient single-longitudinal-mode fiber lasers for applications in optical communication, sensing, and metrology.

Toward single-mode UV to near-IR guidance using hollow-core anti-resonant silica fiber

Hollow-core anti-resonant (HC-AR) fibers with a “negative-curvature” of the core-cladding boundary have been extensively studied over the past few years owing to their low loss and wide transmission bandwidths. The key unique feature of the HC-AR fiber is that the coupling between the core and cladding modes can be made anti-resonant (strongly inhibited) by suitably arranging the anti-resonant tubes in the cladding, which results in low loss and broad spectral bandwidths. HC-AR fibers have been fabricated aimed at visible, near-or mid-IR transmission [1-4]. Here we fabricate and characterize a silica HC-AR fiber having a single ring of 7 non-touching capillaries, designed to have effectively single-mode operation and low loss from UV to near-IR.