Hollow-core Optical Fiber Research Articles

Optical absorption spectrum reveals gaseous chlorine in anti-resonant hollow core fibers

We have observed unexpected spectral attenuation of ultraviolet light in freshly drawn hollow core optical fibers. When the fiber ends are left open to atmosphere, this loss feature dissipates over time. The loss matches the absorption spectrum of gaseous (molecular) chlorine and, given enough time, the transmission spectrum of the fiber recovers to that expected from the morphological structure of the fiber. Our measurements indicate an initial chlorine concentration of 0.45 µmol/cm3 in the hollow core, equivalent to 1.1 mol% Cl2 at atmospheric pressure.

Development of a novel humidity sensor for breath pattern recognition using large dislocation hollow core fiber infused with nano-carbon quantum water gel microfiber

This study presents a humidity sensor based on a large-core-offset hollow core optical fiber (LD-HCF) combined with polyvinyl alcohol carbon quantum dots (PVA-CQDs) nanocomposite microfiber for monitoring human respiratory. PVA-CQDs was introduced into the hollow core structure of LD-HCF to develope the respiratory sensor. The sensor has been integrated into a respiratory monitoring mask, exposed to ultraviolet (UV) light and demonstrated with a humidity sensitivity of 0.1721 nm/%RH as well as response and recovery times of 0.232 s and 0.252 s, respectively. The mask provides the real-time monitoring of human respiratory rates during various activities, including walking, standing, coughing, running, apnea 1 time and apnea 3 times. The typical respiratory patterns have been classified with a remarkable accuracy of 97.62 % using a WOA-1D CNN-LSTM neural network. The respiratory monitoring device is proposed by optical fiber, offering a promising potential in the real-time monitoring for healthcare process.

Polarization purity and dispersion characteristics of nested antiresonant nodeless hollow-core optical fiber at near- and short-wave-IR wavelengths for quantum communications

Advancements in quantum communication and sensing require improved optical transmission that ensures excellent state purity and reduced losses. While free-space optical communication is often preferred, its use becomes challenging over long distances due to beam divergence, atmospheric absorption, scattering, and turbulence, among other factors. In the case of polarization encoding, traditional silica-core optical fibers, though commonly used, struggle with maintaining state purity due to stress-induced birefringence. Hollow core fibers, and in particular nested antiresonant nodeless fibers (NANF), have recently been shown to possess unparalleled polarization purity with minimal birefringence in the telecom wavelength range using continuous-wave (CW) laser light. Here, we investigate a 1-km NANF designed for wavelengths up to the 2-μm waveband. Our results show a polarization extinction ratio between ~−30 dB and ~−70 dB across the 1520 to 1620 nm range in CW operation, peaking at ~−60 dB at the 2-μm design wavelength. Our study also includes the pulsed regime, providing insights beyond previous CW studies, e.g., on the propagation of broadband quantum states of light in NANF at 2 μm, and corresponding extinction-ratio-limited quantum bit error rates (QBER) for prepare-measure and entanglement-based quantum key distribution (QKD) protocols. Our findings highlight the potential of these fibers in emerging applications such as QKD, pointing towards a new standard in optical quantum technologies.

Design of hollow-core anti-resonant optical fiber based on bragg elements with low-loss and wide bandwidth

A type of anti-resonant hollow-core optical fiber consisting of Bragg elements is proposed. The confinement and bending losses are numerically simulated using the finite element method. It is found that the introduction of concentric rings to form anti-resonant structure can reduce the mode transmission loss by 2 ∼ 4 orders of magnitude compared with the anti-resonant element composed of only one dielectric layer, therefore it can achieve ultra-low loss optical transmission. The influence of the structural parameters on the confinement loss was investigated. Furthermore, it is demonstrated that each dielectric layer independently contributes to the confinement of the core modes. It is also found that the Bragg reflection structure can effectively suppress the coupling between core and cladding modes, and achieve broadband low-loss optical transmission under small bending radius. In particular, it can achieve a low transmission loss of less than 0.1 dB km−1 over a wide wavelength range of 600 nm at the bending radius of 3 cm.

Distribution of telecom entangled photons through a 7.7 km antiresonant hollow-core fiber

State of the art classical and quantum communications rely on standard optical fibers with solid cores to transmit light over long distances. However, recent advances have led to the emergence of antiresonant hollow-core optical fibers (AR-HCFs), which, due to the novel fiber geometry, show remarkable optical guiding properties, which are not as limited by the material properties as solid-core fibers. In this paper, we explore the transmission of entangled photons through a novel 7.7 km AR-HCF in a laboratory environment at 1550 nm, presenting the first successful demonstration of entanglement distribution via a long AR-HCF. In addition to showing these novel fibers are compatible with long distance quantum communication, we highlight the low latency and low chromatic dispersion intrinsic to AR-HCF, which can increase the secure key rate in time-bin-based quantum key distribution protocols.

Low-loss, compact, fibre-integrated cell for quantum memories

We present a low-loss, compact, hollow core optical fibre (HCF) cell integrated with single mode fibre (SMF). The cell is designed to be filled with atomic vapour and used as a component in photonic quantum technologies, with applications in quantum memory and optical switching. We achieve a total insertion loss of 0.6(2) dB at 780 nm wavelength via graded index fibre to ensure efficient mode matching coupled with anti-reflection coatings to minimise loss at the SMF-HCF interfaces. We also present numerical modelling of these interfaces, which can be undertaken efficiently without the need for finite element simulation. We encapsulate the HCF core by coupling to the SMF inside a support capillary, enhancing durability and facilitating seamless integration into existing fibre platforms.

Transient gas-induced differential refractive index effects in as-drawn hollow core optical fibers

When a hollow core fiber is drawn, the core and cladding holes within the internal cane geometry are pressurized with an inert gas to enable precise control over the internal microstructure of the fiber and counteract surface tension forces. Primarily by considering the temperature drop as the fiber passes through the furnace and the geometrical transformation of the internal microstructure from preform-to-fiber, we recently established that the gas pressure within the final ‘as-drawn’ fiber is substantially below atmospheric pressure. We have also established that slight changes in the gas refractive index within the core and surrounding cladding holes induced by changes in gas pressure are sufficient to significantly affect both the modality and loss of the fiber. Here we demonstrate, through both simulations and experimental measurements, that the combination of these effects leads to transient changes in the fiber’s attenuation when the fibers are opened to atmosphere post-fabrication. It is important to account for this phenomenon for accurate fiber characterization, particularly when long lengths of fiber are drawn where it could take many weeks for every part of the internal microstructure to reach atmospheric pressure.

A curvature fiber optic sensor with expandable measurement points based on anti-resonant hollow-core optical fiber

This work presents an expandable fiber optic curvature sensor. The sensor achieves multi-point curvature measurements by cascading different lengths of anti-resonant hollow-core fiber sensing units. The sensing unit is formed by connecting the anti-resonant hollow-core fiber between two single-mode fibers. The highest curvature sensitivity achieved by a single sensing unit is −11.86 nm/m−1, with a low temperature sensitivity of only 3 pm/°C. This sensing unit is readily manufacturable and exhibits good curvature sensitivity, temperature insensitivity, and stability. In contrast to traditional multi-point distributed sensors, this sensor can be flexibly installed at various positions, providing curvature sensing at desired locations. Moreover, it demonstrates the potential for multi-point curvature measurements, offering a promising solution for practical applications such as structural monitoring and robotic posture detection.

Hollow core optical fiber enabled by epsilon-near-zero material

Abstract Hollow core optical fibers of numerous guiding mechanisms have been studied in the past decades for their advantages on guiding light in air core. This work demonstrates a new hollow core optical fiber based on a different guiding mechanism, which confines light with a cladding made of epsilon-near-zero (ENZ) material through total internal reflection. We show that the addition of a layer of ENZ material coating (e.g. indium tin oxide layer) significantly reduces the loss of the waveguide compared to the structure without the ENZ layer. We also show that the propagation loss of the ENZ hollow core fiber can be further improved by integrating ENZ materials with lower loss. This study presents a novel type of hollow core fiber, and can find advanced in-fiber photonic applications such as laser surgery/spectroscopy, novel gas-filled/discharge laser, in-fiber molecular/gas sensing, and low-latency optical fiber communication.

Hollow microsphere probes formed by hollow core optical fiber discharging for monitoring gas pressure and temperature.

A hollow core fiber (HCF) is spliced with a single-mode fiber, and then, the end face of the HCF is etched to form a microsphere interferometer for measuring gas pressure and environmental temperature. The total length of each microsphere is less than 200μm. We fabricated two such structures and used femtosecond laser pulses to drill micro-holes on the HCF walls of both structures. One of the structures is directly used to measure air pressure, achieving a sensitivity of up to 2.857 nm/MPa while being almost insensitive to temperature. This structure is capable of assessing pressure down to 3.4kPa within the range of 0-0.5 MPa. Another structure is filled with thermally sensitive material dimethyl silicone oil through a micro-hole, and then, it is sealed with AB adhesive to form a harmonic Vernier effect temperature sensor, with a sensitivity of up to -5.16 nm/°C. This structure is capable of assessing temperature down to 0.38 °C within the range of 30-60 °C. Additionally, the sensors have good repeatability and stability and compact structure and simple manufacturing and can be used as a sensing probe for monitoring gas pressure and temperature under extreme environments.

Guidance of ultraviolet light down to 190 nm in a hollow-core optical fibre.

We report an anti-resonant hollow core fibre with ultraviolet transmission down to 190 nm, covering the entire UV-A, UV-B and much of the UV-C band. Guidance from 190 - 400 nm is achieved apart for a narrow high loss resonance band at 245 - 265 nm. The minimum attenuation is 0.13 dB/m at 235 nm and 0.16 dB/m at 325 nm. With an inscribed core diameter of ∼12 µm, the fibre's bend loss at 325 nm was 0.22 dB per turn for a bend radius of 3 cm at 325 nm.

Polarization sensitive optical side leakage radiometry for distributed characterization of anti-resonant hollow-core fibers.

A novel technique referred to as optical side leakage radiometry is proposed and experimentally demonstrated for non-destructive and distributed characterization of anti-resonant hollow-core optical fibers with high spatial resolution. Through in-depth analysis of the leakage light collection, we discover a unique polarization dependence, which is validated by our experiment. By leveraging this effect and employing Fourier filtering, this method enables accurate quantification of propagation attenuations for fundamental and higher order modes (with the uncertainty of <1 dB/km), identification of localized defects (with the resolution of ∼5 cm), and measurement of ultra-low spectral phase birefringence (at the level of 10-7) in two in-house-fabricated nested antiresonant nodeless hollow-core fibers. Such a fiber characterization approach, boasting unprecedently high accuracy and a potentially wide dynamic range, holds the potential to become an indispensable diagnosis tool for monitoring and assisting the manufacture of high-quality anti-resonant hollow-core fiber.

Three stage HCF fabrication technique for high yield, broadband UV-visible fibers.

Hollow-core optical fibers can offer broadband, single mode guidance in the UV-visible-NIR wavelength range, with the potential for low-loss, solarization-free operation, making them desirable and potentially disruptive for a wide range of applications. To achieve this requires the fabrication of fibers with <300nm anti-resonant membranes, which is technically challenging. Here we investigate the underlying fluid dynamics of the fiber fabrication process and demonstrate a new three-stage fabrication approach, capable of delivering long (∼350m) lengths of fiber with the desired thin-membranes.

Design of Three-Core Structure Broadband Coupler Based on Hollow-Core Anti-Resonant Optical Fiber

Design of Three-Core Structure Broadband Coupler Based on Hollow-Core Anti-Resonant Optical Fiber

Optical solitons in hollow-core fibres

I review the historical observation and subsequent research on optical soliton dynamics in gas-filled hollow-core optical fibres. I include both large-core hollow capillary fibres, and hollow-core photonic-crystal or microstructured fibres with smaller cores, in particular photonic bandgap and antiresonant guiding fibres. I discuss how the optical guidance properties of these different fibre structures influence the soliton dynamics that can be obtained. The dynamics I consider include: soliton propagation at peak power levels ranging from the megawatt to terawatt level, and pulse energies from sub-microjoule to millijoule range; pulse self-compression, leading to sub-cycle and sub-femtosecond pulse duration; soliton self-frequency shifting due to both the Raman effect, and the influence of photoionisation and plasma formation; and resonant dispersive wave emission, leading to the generation of tuneable few-femtosecond pulses across the vacuum and deep ultraviolet, visible, and near-infrared spectral regions.

High refractive index sensing highly sensitive and low loss SPR sensor based on hollow-core D-shaped optical fiber

A hollow-core D-shaped optical fiber-based surface plasmon resonance (SPR) sensor for low-loss and highly sensitive liquid analytes detection is theoretically investigated. The gold (Au) metal nanolayer is coated on the cladding etched D-shaped flat surface to develop the plasmonic effect. The nanolayer coating on the outer flat surface is very easy compared to the inside hollow-core, so our hollow-core sensor manufacturing is too easy compared to other hollow-core refractive indices (RIs) detection sensors. The resonance effect between analytes filled fundamental guided core mode and surface plasmon polariton mode of the D-shaped hollow-core optical fiber sensor is used to obtain the detections of analytes RIs variations. We have found good linear results ([Formula: see text]) in analytes RIs versus resonance wavelength for gold layers thicknesses for the analytes RIs range of 1.45–1.52. This hollow-core D-shaped optical fiber sensor achieves the maximum wavelength sensitivity of 23500[Formula: see text]nm RIU[Formula: see text] and a corresponding resolution of [Formula: see text] RIU. We have obtained the maximum figure of merit (FOM) of 228 1/RIU. The proposed sensor may be highly active in detecting the biological and chemical liquid analytes.

Wavelength modulation spectroscopy of oxygen inside anti-resonant hollow-core fiber-based gas cell

Optical detection of oxygen in A-band (near 13140 cm−1 or 761 nm) with tunable laser diode spectroscopy has been investigated and demonstrated in the past. However, as the demand for miniaturization of gas sensors increases, the hollow-core optical fibers emerge as a promising alternative to conventional large-volume gas absorption cells. In this paper we present a highly convenient, robust and scalable all-fiber setup that utilizes a 1.2 m silica anti-resonant hollow-core fiber as a small-footprint gas cell. Using tunable laser diode, the absorption spectra of oxygen in range from 13133.44 to 13148.12 cm−1 have been recorded. We also performed wavelength modulation spectroscopy of oxygen for samples with various oxygen concentrations and achieved a detection limit of approximately 170 ppm for an averaging time of 5 s. Furthermore, the versatility of the sensing approach is demonstrated by showing its ability to simultaneously detect oxygen at 761 nm and methane at 1651 nm inside the same hollow-core fiber.

Hollow-Core Optical Fibers for Telecommunications and Data Transmission

Hollow-core optical fibers (HCFs) have unique properties like low latency, negligible optical nonlinearity, wide low-loss spectrum, up to 2100 nm, the ability to carry high power, and potentially lower loss then solid-core single-mode fibers (SMFs). These features make them very promising for communication networks and similar applications. However, this class of fibers is still in development. Current applications are almost exclusively limited to low-latency data links for High-Speed Trading (HST); other uses are in the trial stage now. In this paper, we comprehensively review the progress in the development of HCFs including fiber design, fabrication and parameters (with comparisons to conventional single-mode fibers) and support technologies like splicing and testing. A variety of HCF applications in future telecom networks and systems is analyzed, pointing out their strengths and limitations. Additionally, we review the influence of filler gas and entry of contaminants on HCF attenuation, and propose a new fusion splicing technique, avoiding the destruction of the fiber’s photonic cladding at high temperature.

Dual-Pass Hollow-Core Fiber Gas Spectroscopy Using a Reflective Configuration With Heterodyne-Based Signal Detection

Hollow-core optical fibers have been used for laser gas sensing for almost two decades. However, to date, the vast majority of sensing systems have used hollow-core fiber only as a replacement for bulky gas cells (single- or multi-pass). Here, we investigate a reflective, dual-pass configuration in which light enters and exits hollow-core fiber from the same side. The advantage of this approach is the fact that the hollow-core fiber can be used not only as a cell but also as an endoscope-like probe, which does not need any additional tubing for sample delivery. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we identify unwanted reflections from various fiber-to-fiber connections in the setup as the main challenge limiting the performance of the sensor in reflective configuration. We propose and experimentally demonstrate that the problem of reflections can be addressed using heterodyne-based detection, which allows for identifying and extracting useful signals in the radio frequency domain. Absorption and dispersion spectroscopy of methane are performed as a proof-of-concept with a detection limit of methane concentration below 10 ppm. Discussion insights into further development are provided, including strategies for miniaturization and detection limit improvement.

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.