Hollow-core Optical Fiber Research Articles

Long-length 3D printed hollow-core polymer optical fiber for wideband light guidance

A hollow-core microstructured polymer optical fiber containing a six-pointed star cladding design is directly drawn from a 3D printed preform for wideband light guidance. Fabrication of a complex geometry-based preform is enabled using commercially available translucent polymer filaments in a 3D printer. Light guidance from visible to infrared regions through the core of 57.8 cm long fiber is achieved with a minimum loss of 0.11 dB/cm in the near-infrared spectrum. The bending loss of the proposed design is also investigated to determine the light guidance under a mechanical effect. The modified fabrication process of the polymer optical fiber with novel design could be an alternative to conventional optical fibers for visible and infrared light guidance.

Study of backward Brillouin scattering in gas-filled anti-resonant fibers

Brillouin scattering in gas shows unmatched gain properties in hollow-core optical fibers filled at high pressure. Here, the gain characteristics are studied for two common gases, namely, N2 and CO2, which show distinct features and are compared to expected responses deduced from conventional thermodynamic models for gases. This is realized—for the first time to our knowledge—in anti-resonant hollow-core optical fibers, demonstrating their full suitability for generating and exploiting Brillouin amplification in fluidic media. The potential of Brillouin scattering in gases is manifested in a distributed temperature sensor that is totally immune to strain and benefits from the absence of shear stress in the gaseous medium. The experimental results presented indicate that gases with smaller molecular masses show a higher temperature sensitivity than gases with larger masses. This inverse proportionality between the temperature sensitivity and the molecular mass of a gas shown in the experiment qualitatively agrees with the ideal gas model.

Azimuthal confinement: the missing ingredient in understanding confinement loss in antiresonant, hollow-core fibers

Antiresonant, hollow-core optical fibers are currently challenging or even exceeding the loss performance of conventional solid-core fibers. Despite this progress, there are aspects of the guidance mechanism in these fibers that are still not understood. For example, a physical mechanism to explain why negative curvature of the core surround is correlated with low loss remains elusive. It is shown that the glass elements of the cladding structure with an approximately radial orientation play a crucial role in determining the confinement loss by strongly shaping the wave fields in the azimuthal coordinate. This shaping, described as azimuthal confinement, can result in an evanescent field in the radial direction through the cladding, and this leads to a confinement loss that is substantially lower than would be the case without azimuthal confinement. A comprehensive theory of azimuthal confinement is developed, yielding an expression for the confinement loss of any fiber structure with a single antiresonant glass layer between the core and the outer glass jacket. This is tested by comparison with large-scale numerical simulations on two types of cladding structure. It is shown that negative curvature of the core surround has little or no intrinsic role in reducing confinement loss in fibers with a nodeless cladding structure. The power of azimuthal confinement is demonstrated in model structures where the confinement loss drops by more than two orders of magnitude as the radial width of the cladding is increased. It is anticipated that the concept of azimuthal confinement will be valuable in interpreting confinement loss in a wide range of existing antiresonant, hollow-core fibers and in the design of novel, low loss cladding structures.

Parameter optimization of hollow-core optical fiber phase modulators.

We studied the effect of varying gas concentration, buffer gas, length, and type of fibers on the performance of optical fiber photothermal phase modulators based on C2H2-filled hollow-core fibers. For the same control power level, the phase modulator with Ar as the buffer gas achieves the largest phase modulation. For a fixed length of hollow-core fiber, there exists an optimal C2H2 concentration that achieves the largest phase modulation. With a 23-cm-long anti-resonant hollow-core fiber filled with 12.5% C2H2 balanced with Ar, phase modulation of π-rad at 100 kHz is achieved with a control power of 200 mW. The modulation bandwidth of the phase modulator is 150 kHz. The modulation bandwidth is extended to ∼1.1 MHz with a photonic bandgap hollow-core fiber of the same length filled with the same gas mixture. The measured rise and fall time of the photonic bandgap hollow-core fiber phase modulator are 0.57 µs and 0.55 µs, respectively.

Long and flexible atmospheric pressure plasma jet probes for operation in humid environments

Herein, we describe a new flexible atmospheric pressure plasma jet device composed of hollow-core optical fibers and introduce two potential applications of the device: endoscopic plasma treatment and decomposition of aqueous phosphorus compounds. The proposed device is 1.6 m long and highly flexible, has a small probe size of several hundred micrometers, and spatially separates the plasma jet from the electrical input, making it very suitable for treatment through the biopsy channel of a conventional endoscope. Because the wire electrode of the fabricated atmospheric pressure plasma jet device is thoroughly isolated inside the hollow of the optical fiber, the device produces a plasma column in an identical discharge state regardless of external environmental conditions. As a result, the device can operate safely and steadily in highly humid environments, such as underwater, which can be utilized to decompose phosphorus compounds in fresh water.

Giant Enhancement of Raman Scattering by a Hollow-Core Microstructured Optical Fiber Allows Single Exosome Probing.

Microstructured optical fibers (MOFs) provide solutions for breaking through the bottlenecks in areas of high-power transmission and high-efficiency optical waveguides. Other than transporting light waves, MOFs can synergistically combine microfluidics and optics in a single fiber with an unprecedented light path length not readily achievable by planar optofluidic configurations. Here, we demonstrate that hollow-core anti-resonant optical fibers (HcARFs) can significantly enhance Raman scattering by over three orders of magnitude (EF ≈ 5000) compared with a planar setup, due to the joint mechanisms of strong light-matter interaction in the fiber core and the cumulative effect of the fiber. The giant enhancement enables us to develop the first optical fiber sensor to achieve single cancer exosome detection via a sandwich-structured strategy. This enables a multiplexed analysis of surface proteins of exosome samples, potentially allowing an accurate identification of the cellular origin of exosomes for cancer diagnosis. Our findings could expand the applications of HcARF in many exciting areas beyond the waveguide.

Maintaining Microwave Discharge in Hollow Core Optical Fibers for Gas Fiber Lasers

Maintaining Microwave Discharge in Hollow Core Optical Fibers for Gas Fiber Lasers

Measurement of resonant bend loss in anti-resonant hollow core optical fiber: erratum.

An error on the part of the authors in drafting resulted in Eq.(3) being incorrect in the published paper [Opt. Express25, 20612 (2017)10.1364/OE.25.020612]. We present a corrected version of the equation. It should be noted that this does not affect the presented results or conclusions of the paper.

Loss in hollow-core optical fibers: mechanisms, scaling rules, and limits

Over the past few years, progress in hollow-core optical fiber technology has reduced the attenuation of these fibers to levels comparable to those of all-solid silica-core single-mode fibers. The sustained pace of progress in the field has sparked renewed interest in the technology and created the expectation that it will one day enable realization of the most transparent light-propagating waveguides ever produced, across all spectral regions of interest. In this work we review and analyze the various physical mechanisms that drive attenuation in hollow-core optical fibers. We consider both the somewhat legacy hollow-core photonic bandgap technology as well as the more recent antiresonant hollow-core fibers. As both fiber types exploit different guidance mechanisms from that of conventional solid-core fibers to confine light to the central core, their attenuation is also dominated by a different set of physical processes, which we analyze here in detail. First, we discuss intrinsic loss mechanisms in perfect and idealized fibers. These include leakage loss, absorption, and scattering within the gas filling the core or from the glass microstructure surrounding it, and roughness scattering from the air–glass interfaces within the fibers. The latter contribution is analyzed rigorously, clarifying inaccuracies in the literature that often led to the use of inadequate scaling rules. We then explore the extrinsic contributions to loss and discuss the effect of random microbends as well as that of other perturbations and non-uniformities that may result from imperfections in the fabrication process. These effects impact the loss of the fiber predominantly by scattering light from the fundamental mode into lossier higher-order modes and cladding modes. Although these contributions have often been neglected, their role becomes increasingly important in the context of producing, one day, hollow-core fibers with sub-0.1-dB/km loss and a pure single-mode guidance. Finally, we present general scaling rules for all the loss mechanisms mentioned previously and combine them to examine the performance of recently reported fibers. We lay some general guidelines for the design of low-loss hollow-core fibers operating at different spectral regions and conclude the paper with a brief outlook on the future of this potentially transformative technology.

Surface Plasmon Resonance-Based Gold-Coated Hollow-Core Negative Curvature Optical Fiber Sensor.

The hollow-core fiber-based sensor has garnered high interest due to its simple structure and low transmission loss. A new hollow-core negative-curvature fiber (HC-NCF) sensor based on the surface plasmon resonance (SPR) technique is proposed in this work. The cladding region is composed of six circular silica tubes and two elliptical silica tubes to reduce fabrication complexity. Chemically stable gold is used as a plasmonic material on the inner wall of the sensor structure to induce the SPR effect. The proposed sensor detects a minor variation in the refractive indices (RIs) of the analyte placed in the hollow core. Numerical investigations are carried out using the finite element method (FEM). Through the optimization of structural parameters, the maximum wavelength sensitivity of 6000 nm/RIU and the highest resolution of 2.5 × 10-5 RIU are achieved in the RI range of 1.31 to 1.36. In addition, an improved figure of merit (FOM) of 2000 RIU-1 for Y-polarization and 857.1 RIU-1 for X-polarization is obtained. Because of its simple structure, high sensitivity, high FOM, and low transmission loss, the proposed sensor can be used as a temperature sensor, a chemical sensor, and a biosensor.

Design and analysis of Hollow-core fiber with anti-resonant structure for Ethylene detection

An optical fiber for mid-infrared (mid-IR) guidance has long attracted great interest due to its wide range of applications especially in gas sensing. Ethylene gas is one of an important gas, which release from the fruit and flower, owing to their ripening process. Thus, the controlling and detection of ethylene gas is of interest for the storage and exportation of fruits and flowers. Gas detection using the hollow-core optical fiber (HCF) is a promising technique due to it intrinsic micro-channel structure that allows the gas to flow through the HCF for detection propose. Here, a hollow-core anti-resonant fiber (HC-ARF) is designed and optimized for mid-IR guidance for the purpose of ethylene detection. The HC-ARF includes 6 inner tubes which is optimized to achieve a low confinement loss at a wavelength of 3.2 µm (ethylene absorption wavelength). The simulation result shows that a confinement loss of 2 dB/m at the wavelength of 3.2 µm can be achieved when the fiber core diameter is 128 µm and the thickness and the diameter of inner tubes are 2 µm and 92 µm, respectively.

Utilizing broadband wavelength-division multiplexing capabilities of hollow-core fiber forquantum communications.

One of the major challenges in the deployment of quantum communications (QC) over solid-core silica optical fiber is the performance degradation due to the optical noise generated with co-propagating classical optical signals. To reduce the impact of the optical noise, research teams are turning to new and novel architectures of solid-core and hollow-core optical fiber. We studied the impact when co-propagating a single-photon level (850nm) and two classical optical signals (940nm and 1550nm) while utilizing a nested antiresonant nodeless fiber (NANF) with two low-loss windows. The 940nm signal was shown to impact the single-photon measurement due to the silicon detector technology implemented; however, multiplexing techniques and filtering could reduce the impact. The 1550nm signal was shown to have no detrimental impact. The results highlight that both high bandwidth optical traffic at 1550nm and a QC channel at 850nm could co-propagate without degradation to the QC channel.

Broadband birefringence hollow-core anti-resonant optical fiber with elliptical air holes

We present a novel hollow-core anti-resonant optical fiber (ARF) with nested elliptical and nested circular air holes, where these cladding air holes form into the elliptical core. The simulation results show the fiber has low loss, high birefringence, broadband, and bend-resistance properties by optimizing its structure parameters. Besides, it also exhibits that the fundamental mode (FM) losses of x-polarization and y-polarization, high birefringence, and broad transmission band are 0.03 dB/m, 0.043 dB/m, 1.33 × 10−4, 460 nm, respectively.

Fundamental thermal noise in antiresonant hollow-core fibers

To understand the effect of fundamental thermal noise in hollow-core optical fibers, the authors propose an adapted noise model and compare it to careful measurements. They find that the use of evacuated hollow cores could allow for an order-of-magnitude improvement in phase sensitivity with respect to standard fiber, enabling better precision in interferometric applications such as fiber gyroscopes and laser frequency stabilization.

Internal Gas Composition and Pressure in As-drawn Hollow Core Optical Fibers

We present a study into the gas pressure and composition within a hollow core optical fiber immediately after fabrication. Results from three different experimental techniques indicate that the initial absolute pressure inside of the hollow core is significantly lower than atmospheric pressure. By measuring the equilibrium height to which water ingresses into the hollow core, we estimate the absolute internal gas pressure to be 20 kPa < <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{P}}_{\boldsymbol{i}}}$</tex-math></inline-formula> < 29 kPa for the fibers reported here that were fabricated using standard techniques. The initial gas composition within the hollow core was studied using Raman and absorption spectroscopy and the evolution of the gas composition provides indirect information about the condition of the silica surfaces inside the fiber. The measurements indicate that these internal surfaces become saturated with atmospheric water vapor as this is drawn into the open-ended fiber, initially due to a pressure gradient post-fabrication. Our findings are an essential foundation for the study of long-term optical and mechanical performance of hollow core fibers and important for accurate characterization of these specialty fibers. The first is becoming increasingly important as commercial applications of these fibers expand.

Low-loss microwave photonics links using hollow core fibres

There are a host of applications in communications, sensing, and science, in which analogue signal transmission is preferred over today’s dominant digital transmission. In some of these applications, the advantage is in lower cost, while in others, it lies in superior performance. However, especially for longer analogue photonics links (up to 10 s of km), the performance is strongly limited by the impairments arising from using standard single-mode fibres (SSMF). Firstly, the three key metrics of analogue links (loss, noise figure, and dynamic range) tend to improve with received power, but this is limited by stimulated Brillouin scattering in SSMF. Further degradation is due to the chromatic dispersion of SSMF, which induces radio-frequency (RF) signal fading, increases even-order distortions, and causes phase-to-intensity-noise conversion. Further distortions still, are caused by the Kerr nonlinearity of SSMF. We propose to address all of these shortcomings by replacing SSMFs with hollow-core optical fibres, which have simultaneously six times lower chromatic dispersion and several orders of magnitude lower nonlinearity (Brillouin, Kerr). We demonstrate the advantages in this application using a 7.7 km long hollow-core fibre sample, significantly surpassing the performance of an SSMF link in virtually every metric, including 15 dB higher link gain and 6 dB lower noise figure.

Hollow-core optical fibre sensors for operando Raman spectroscopy investigation of Li-ion battery liquid electrolytes

Improved analytical tools are urgently required to identify degradation and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochemical processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chemistry of a carbonate-based liquid electrolyte during electrochemical cycling in Li-ion batteries with a graphite anode and a LiNi0.8Mn0.1Co0.1O2 cathode. By embedding a hollow-core optical fibre probe inside a lab-scale pouch cell, we demonstrate the effective evolution of the liquid electrolyte species by background-free Raman spectroscopy. The analysis of the spectroscopy measurements reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage and show the potential to track the lithium-ion solvation dynamics. The proposed operando methodology contributes to understanding better the current Li-ion battery limitations and paves the way for studies of the degradation mechanisms in different electrochemical energy storage systems.

Review on All-Fiber Online Raman Sensor with Hollow Core Microstructured Optical Fiber

Raman spectroscopy is widely used for qualitative and quantitative analysis of trace components in scientific fields such as food safety monitoring, drug testing, environmental monitoring, etc. In addition to its demonstrated advantages of fast response, non-destructive, and non-polluting characteristics, fast online Raman detection is drawing growing attention for development. To achieve this desirable capability, hollow core optical fibers are employed as a common transmission channel for light and fluid in the Raman sensor. By enhancing the interaction process between light and matter, the detection sensitivity is improved. At the same time, the Raman spectroscopy signal light collection efficiency is significantly improved. This article summarizes enhancement techniques reported for Raman sensors, followed by a detailed review on fiber-based Raman sensor techniques including theoretical analyses, fabrication, and application based on hollow core photonic crystal fibers and capillary-based hollow core fibers. The prospects of using these fibers for Raman spectroscopy are discussed.

Hollow-core fiber delivery of broadband mid-infrared light for remote spectroscopy.

High-resolution multi-species spectroscopy is achieved by delivering broadband3-4-μmmid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from H37Cl, H35Cl, H2O and CH4 present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C3H6O and C3H8O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the intra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters.

Laser-induced graphene coated hollow-core fiber for humidity sensing

Laser-induced graphene (LIG) is a porous material that can be coated on an antiresonance reflecting hollow-core optical fiber (HCF) for humidity sensing. In this study, a humidity sensor was constructed by dip coating a polyimide (PI) film on the surface of an HCF, followed by rearranging and graphitizing the PI film through laser irradiation, which led to the formation of LIG. The absorption of water vapor by the LIG film changed its refractive index, which varied the reflectivity of the HCF and produced resonance intensity changes in the transmission spectrum. Humidity experiments were conducted with LIG films under different laser parameters, that is, power. The transmission visibility of the resonance dip was evaluated. A sensitivity of 0.187 dB/% RH was obtained in an ultrawide relative humidity (RH) range of 5–95% RH. In addition, The sensor has short response and recovery times. Moreover, the proposed sensor has a wide operating temperature range and low cost, and its temperature cross-sensitivity issue can be solved using different demodulation methods. The results suggest that LIG-coated HCF is a tunable and promising solution for humidity sensing.