Low-loss Kagome hollow-core fibers operating from the near- to the mid-IR

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

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.

Design of hollow core step-index antiresonant fiber with stepped refractive indices cladding

With the benefits of low latency, wide transmission bandwidth, and large mode field area, hollow-core antiresonant fiber (HC-ARF) has been a research hotspot in the past decade. In this paper, a hollow core step-index antiresonant fiber (HC-SARF), with stepped refractive indices cladding, is proposed and numerically demonstrated with the benefits of loss reduction and bending improvement. Glass-based capillaries with both high (n = 1.45) and low (as low as n = 1.36) refractive indices layers are introduced and formatted in the cladding air holes. Using the finite element method to perform numerical analysis of the designed fiber, results show that at the laser wavelengths of 980 and 1064 nm, the confinement loss is favorably reduced by about 6 dB/km compared with the conventional uniform cladding HC-ARF. The bending loss, around 15 cm bending radius of this fiber, is also reduced by 2 dB/km. The cladding air hole radius in this fiber is further investigated to optimize the confinement loss and the mode field diameter with single-mode transmission behavior. This proposed HC-SARF has great potential in optical fiber transmission and high energy delivery.

Ultrawide bandwidth single-mode polarization beam splitter based on dual-hollow-core antiresonant fiber

An ultrawide bandwidth and single-mode polarization beam splitter (PBS) based on an air-gap type of dual-hollow-core antiresonant fiber (DHC-ARF) is proposed. Nested tubes are introduced into two cladding tubes between two cores to weaken the wavelength dependence of coupling length in DHC-ARF for obtaining ultrawide bandwidth. By tuning the cladding tube sizes, higher-order core modes with the lowest loss can be coupled with cladding tube modes, and thus, effectively, single-mode operation is achieved. Numerical results demonstrate that an 8.15 cm long DHC-ARF can be used to develop a PBS with an operating bandwidth of 370 nm ranging from 1.28 to 1.65 µm, where a polarization extinction ratio is below -20dB and a high-order mode extinction ratio exceeds 100.

Core opportunities for future optical fibers

Hair-thin strands of glass, intrinsically transparent and strong, of which many millions of kilometers are made annually, connect the world in ways unimaginable 50 years ago. What could another 50 years bring? That question is the theme of this Perspective. The first optical fibers were passive low-loss conduits for light, empowered by sophisticated sources and signal processing; a second advance was the addition of dopants utilizing atomic energy levels to promote amplification, and a third major initiative was physical structuring of the core-clad combinations, using the baseline silica material. Recent results suggest that the next major expansions in fiber performance and devices are likely to utilize different materials in the core, inhomogeneous structures on different length scales, or some combination of these. In particular, fibers with crystalline cores offer an extended transparency range with strong optical nonlinearities and open the door to hybrid opto-electronic devices. Opportunities for future optical fiber that derive from micro- and macro-structuring of the core phase offer some unique possibilities in ‘scattering by design’.

Light guidance up to 6.5 µm in borosilicate soft glass hollow-core microstructured optical waveguides

Limited operating bandwidth originated from strong absorption of glass materials in the infrared (IR) spectral region has hindered the potential applications of microstructured optical waveguide (MOW)-based sensors. Here, we demonstrate multimode waveguide regime up to 6.5 µm for the hollow-core (HC) MOWs drawn from borosilicate soft glass. Effective light guidance in central HC (diameter ∼240 µm) was observed from 0.4 to 6.5 µm despite high waveguide losses (0.4 and 1 dB/cm in near- and mid-IR, respectively). Additional optimization of the waveguide structure can potentially extend its operating range and decrease transmission losses, offering an attractive alternative to tellurite and chalcogenide-based fibers. Featuring the transparency in mid-IR, HC MOWs are promising candidates for the creation of MOW-based sensors for chemical and biomedical applications.

Fabrication of tubular anti-resonant hollow core fibers: modelling, draw dynamics and process optimization

The fabrication of hollow core microstructured fibers is significantly more complex than solid fibers due to the necessity to control the hollow microstructure with high precision during the draw. We present the first model that can recreate tubular anti-resonant hollow core fiber draws, and accurately predict the draw parameters and geometry of the fiber. The model was validated against two different experimental fiber draws and very good agreement was found. We identify a dynamic within the draw process that can lead to a premature and irreversible contact between neighboring capillaries inside the hot zone, and describe mitigating strategies. We then use the model to explore the tolerance of the draw process to unavoidable structural variations within the preform, and to study feasibility and limiting phenomena of increasing the produced yield. We discover that the aspect ratio of the capillaries used in the preform has a direct effect on the uniformity of drawn fibers. Starting from high precision preforms the model predicts that it could be possible to draw 100 km of fiber from a single meter of preform.

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.

Laser frequency stabilization and spectroscopy at 2051 nm using a compact CO2-filled Kagome hollow core fiber gas-cell system

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO₂ gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in ¹²C¹⁶O₂: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO₂ partial pressure and we compare the observed CO₂ lineshape features with those calculated using HITRAN, to quantify the properties of the CO₂-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the ¹²C¹⁶O₂ R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

Nonlinear dynamic of picosecond pulse propagation in atmospheric air-filled hollow core fibers

Atmospheric air-filled hollow core (HC) fibers, representing the simplest yet reliable form of gas-filled hollow core fiber, show remarkable nonlinear properties and have several interesting applications such as pulse compression, frequency conversion and supercontinuum generation. Although the propagation of sub-picosecond and few hundred picosecond pulses are well-studied in air-filled fibers, the nonlinear response of air to pulses with a duration of a few picoseconds has interesting features that have not yet been explored fully. Here, we experimentally and theoretically study the nonlinear propagation of ~6 ps pulses in three different types of atmospheric air-filled HC fiber. With this pulse length, we were able to explore different nonlinear characteristics of air at different power levels. Using in-house-fabricated, state-of-the-art HC photonic bandgap, HC tubular and HC Kagomé fibers, we were able to associate the origin of the initial pulse broadening process in these fibers to rotational Raman scattering (RRS) at low power levels. Due to the broadband and low loss transmission window of the HC Kagomé fiber we used, we observed the transition from initial pulse broadening (by RRS) at lower powers, through long-range frequency conversion (2330 cm^-1) with the help of vibrational Raman scattering, to broadband (~700 nm) supercontinuum generation at high power levels. To model such a wide range of nonlinear processes in a unified approach, we have implemented a semi-quantum model for air into the generalized nonlinear Schrodinger equation, which surpasses the limits of the common single damping oscillator model in this pulse length regime. The model has been validated by comparison with experimental results and provides a powerful tool for the design, modeling and optimization of nonlinear processes in air-filled HC fibers.

Optimized inhibited-coupling Kagome fibers at Yb-Nd:Yag (8.5 dB/km) and Ti:Sa (30 dB/km) ranges

We report on the development of hypocycloid core-contour inhibited-coupling (IC) Kagome hollow-core photonic crystal fibers (HC-PCFs) with record transmission loss and spectral coverage that include the common industrial laser wavelengths. Using the scaling of the confinement loss with the core-contour negative curvature and the silica strut thickness, we fabricated an IC Kagome HC-PCF for Yb and Nd:Yag laser guidance with record loss level of 8.5 dB/km associated with a 225-nm-wide 3-dB bandwidth. A second HC-PCF is fabricated with reduced silica strut thickness while keeping the hypocycloid core contour. It exhibits a fundamental transmission window spanning down to the Ti:Sa spectral range and a loss figure of 30 dB/km at 750 nm. The fibers' modal properties and bending sensitivity show these HC-PCFs to be ideal for ultralow-loss, flexible, and robust laser beam delivery.