Hollow-core Negative Curvature Fiber Research Articles

MIR-Pump NIR-Probe Fiber-Optic Photothermal Spectroscopy With Background-Free First Harmonic Detection

We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS- $1{f}$ ) detection of formaldehyde, one of the most common volatile organic compounds (VOCs), in a silica hollow-core negative curvature fiber (HC-NCF). The photothermal gas sensor adopts a mid-infrared interband cascade pump laser at $3.6~\mu \text{m}$ and a near-infrared fiber probe laser at $1.56~\mu \text{m}$ . At the optimal modulation frequency (8 kHz) and modulation index (1.8) of the pump laser, we obtained a normalized noise equivalent absorption (NNEA) coefficient of $4\times 10^{-9}$ cm−1WHz $^{-1/2}$ . The use of HC-NCF with an inner diameter of $65~\mu \text{m}$ enables the sensitive photothermal detection even for a very low pump power of micro-watts. The background-free PTS- $1{f}$ detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic ( $2{f}$ ) detection. A theoretical model was established in this work to interpret the experimental results.

Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber.

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO2 laser, which enables pump propagation in the fundamental LP01 mode to achieve maximum photothermal phase modulation while exciting both the LP01 and LP11 modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

Negative curvature hollow-core anti-resonant fiber for terahertz sensing.

Hollow-core fibers are advantageous for chemical sensing as they facilitate liquid infiltration into the core over conventional porous core fiber. In addition, the requirement of less bulk material significantly decreases the effective material loss (EML). In this paper, a six circular cladding tube negative curvature hollow-core fiber (NC-HCF) is proposed for chemical sensing. Five different chemicals including chloroform, polylactic acid, CCL3, glycerin, and benzene are proposed to fill the core of the NC-HCF, and sensitivities are evaluated by full vector finite element method-based COMSOL software. Numerical results reveal that the proposed sensor exhibits very high relative sensitivity in a wide range of frequency. The fabrication of the proposed fiber is feasible by existing fabrication facilities as it contains realistic fabrication parameters. Hence, the proposed sensor can potentially be used as a chemical sensor especially in the medical, food, and industrial sectors as the five chemicals mentioned above carry great medical and food significance.

Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain.

Negative curvature hollow core fiber (NCHCF) is a promising candidate for sensing applications; however, research on NCHCF based fiber sensors starts only in the recent two years. In this work, an all-fiber interferometer based on an NCHCF structure is proposed for the first time. The interferometer was fabricated by simple fusion splicing of a short section of an NCHCF between two singlemode fibers (SMFs). Both simulation and experimental results show that multiple modes and modal interferences are excited within the NCHCF structure. Periodic transmission dips with high spectral extinction ratio (up to 30 dB) and wide free spectral range (FSR) are produced, which is mainly introduced by the modes coupling between HE11 and HE12. A small portion of light guiding by means of Anti-resonant reflecting optical waveguide (ARROW) mechanism is also observed. The transmission dips, resulting from multimode interferences (MMI) and ARROW effect have a big difference in sensitivities to strain and temperature, thus making it possible to monitor these two parameters with a single sensor head by using a characteristic matrix approach. In addition, the proposed sensor structure is experimentally proven to have a good reproducibility.

Ultrafast 1 MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers

Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of∼40meV. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust∼10Wfiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission.

Single-polarization single-mode hollow-core negative-curvature fiber with silicon-coated cladding

We propose a hollow-core negative-curvature fiber with a silicon-coated double-ring cladding structure. Simulation results show that three broadband single-polarization single-mode regions of 1514–1529 nm, 1536–1546 nm, and 1549–1579 nm are achieved in this fiber. The polarization extinction ratio is up to 4803 and higher order mode extinction ratio reaches 308 at the wavelength of 1550 nm with an x-polarization loss of 0.11 dB/m. The highest polarization extinction ratio is up to 14,232 at the wavelength of 1520 nm. Furthermore, the minimum bend radius that can maintain single-polarization single-mode guidance at the wavelength of 1550 nm is as low as 3 cm.

Design of Negative Curvature Hollow Core Fiber Based on Reinforcement Learning

In negative curvature hollow core fibers (NCHCFs), light guidance is based on the capillary structure in the cladding. To achieve desirable fiber propagation properties, various designs of the capillary structure have been proposed in literature. However, the design process so far depends more or less on experience. In this article, we propose a reinforcement learning (RL) based method of systematically optimizing the capillary structure to achieve low average confinement loss (CL) for a given operating wavelength range and core radius. We use a recurrent neural network (RNN) to interactively study the properties of different capillary structures. The wavelength averaged CLs of the resulting designs are more than one order of magnitude lower than the lowest average CL of prior designs in literature. The same approach can be applied to search for optimum capillary structures in terms of other fiber propagation properties such as bending loss (BL), higher order modes extinction ratio (HOMER), overlap of the optical mode with the capillary structure, or a trade-off among these properties.

A New Method to Achieve Single-Polarization Guidance in Hollow-Core Negative- Curvature Fibers

We propose a new method to achieve single-polarization guidance in a hollow-core negative-curvature fiber. Simulation results show that coating a layer of high-index material with a suitable thickness on the inner surface of one vertical cladding tube in a hollow-core negative-curvature fiber can effectively suppress $y$ -polarized fundamental mode. Silicon is chosen as the high-index material in this paper. In the five-nested-tube silicon-coated negative-curvature fiber, the single-polarization region is located in the wavelength range of 1512–1587 nm with a broad bandwidth of 75 nm. The polarization extinction ratio is up to 1732 at the wavelength of 1550 nm. Moreover, the mode evolutions of $x$ - and $y$ -polarized fundamental modes in the five-nested-tube silicon-coated negative-curvature fiber are investigated to explain why $y$ -polarized fundamental mode is suppressed. Compared with the traditional method of using different tube thicknesses to achieve single-polarization guidance in a negative-curvature fiber, this method possesses the advantages of broad single-polarization bandwidth and high error tolerance.

基于负曲率空芯光纤的光泵太赫兹光纤激光器的理论研究

Based on the compact and efficient optically pumped terahertz laser(OPTL) technology, an optically pumped terahertz fiber laser(OPTFL) based on a negative curvature hollow core fiber was designed. This OPTFL used a hollow-core fiber with polymethylpentene(PMP) material as operation gas chamber and was filled with methanol gas and pumped by 9P(36) continuous-wave(CW) CO2 laser. Based on rate equations and the transmission theory in hollow core fiber, factors affecting the output characteristics of OPTFL were analyzed. By investigating inner microstructure of hollow-core fiber, a negative curvature hollow-core fiber for efficiently transmitting terahertz waves was proposed. Considering the designed negative curvature hollow-core fiber, the feasibility of long cavity OPTFL was analyzed. Theoretical calculations showed that by appropriately increasing the cavity length of the proposed OPTFL, the terahertz output power was expected to reach the order of 100 milliwatts with optimal operating conditions. The results provide a new method for the OPTFL with high power and high performance.

Silica Hollow-Core Negative Curvature Fibers Enable Ultrasensitive Mid-Infrared Absorption Spectroscopy

Ultrasensitive mid-infrared absorption spectroscopy is demonstrated by the use of a novel silica-based hollow-core negative curvature fiber (HC-NCF). The HC-NCF used in this article consists of a single ring of six nontouching cladding capillaries around the hollow core, thus forming a unique core boundary with a negative curvature. Such a silica HC-NCF enables the broadband single-mode transmission in the mid-infrared. By using the HC-NCF as a compact gas cell, a proof-of-principle experiment is conducted to detect the N2O line at 2778.37 cm−1 with a distributed-feedback interband cascade laser emitting at 3.6 μm. A minimum detectable absorbance of 3 × 10−5 is achieved for a fiber length of 120 cm, corresponding to a noise equivalent absorption (NEA) coefficient of 2.5 × 10−7 cm−1. Silica HC-NCFs offer a new opportunity of developing sensitive and compact gas sensors using mid-infrared absorption spectroscopy.

Sub-ppm CO detection in a sub-meter-long hollow-core negative curvature fiber using absorption spectroscopy at 2.3 μm

We demonstrated the sensitive CO detection in a novel hollow-core negative curvature fiber (HC-NCF) with infrared laser absorption at 2.3 μm. The HC-NCF consists of a single ring of eight nontouching silica capillaries around the air core, providing a single-mode light delivery of the 2.3-μm distributed feedback laser. A high coupling efficiency of 90% was achieved with the aid of optimal free-space coupling optics. The hollow-core fiber was used as a gas cell for gas absorption measurement of a total path length of 85 cm. Both direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) were adopted to demonstrate the sensor performance by detecting the CO line R(10) at 4297.7 cm−1. In scanned-wavelength DAS, we obtained a minimum detection limit (MDL) of 13 ppm CO, which was limited mainly by the existing mode noise in the HC-NCF. By applying a pressure difference of 0.8 bar between the two ends of the fiber, we demonstrated a very short gas loading time of only 5 s. Finally, we achieved a MDL of 0.4 ppm CO using the WMS technique, corresponding to a noise equivalent absorption of 1.6 × 10-7 cm−1.

White light continuum in negative hollow-core fiber filled with inert gases

As of the writing of this work, generation of white light continuum (WLC) in hollow-core photonic crystal fibers (HCPCF) filled with gases is being thoroughly researched. These structures allow the possibility of adjusting light generation properties by changing the fill gas pressure. Kagomé hollow-core PCF and silica capillaries have been used to study the nonlinear effects in gases as they facilitate exploring these weak properties. In this paper, we numerically analyze the generation of WLC in a new type of hollow-core PCF, called negative curvature hollow-core fiber (NCHCF), itself filled with different inert gases. We show that this type of fiber is a better alternative to the Kagomé and capillary fibers because it exhibits low losses and the WLC spectrum generated presents a relatively flat wavelength span from around 400[Formula: see text]nm to 2400[Formula: see text]nm under certain circumstances.

Tunable Single-Polarization Single-Mode Negative-Curvature Fiber With an Asymmetrical Refractive Index Cladding for Mid-Infrared Region

We propose a hollow-core negative-curvature fiber implementing tunable single-polarization single-mode operation at mid-infrared region. This fiber owns a symmetrical geometry structure and asymmetrical refractive index cladding composed by two kinds of mid-infrared glass with different refractive indexes. Single-polarization single-mode can be achieved for any wavelength within the wavelength range from 2.0 to 4.0 μm by linear stretching fiber structure in the scale to wavelength, which means that this fiber structure possesses an excellent tunability. We mainly focus on analyzing the properties of the negative-curvature fibers for the wavelength of 2.0, 3.0, and 4.0 μm. Numerical results show that the main single-polarization single-mode regions for these three fibers are 2.020–2.033 μm, 2.998–3.016 μm and 3.998–4.018 μm, respectively. The widest bandwidth reaches 20 nm. The polarization extinction ratios at the wavelength of 2.0, 3.0, and 4.0 μm are 3180, 5085, and 2870, respectively. In addition, these fibers can maintain good single-polarization single-mode performance under bending state. The critical bend radius of the fiber for 3.0 μm is as low as 6 cm.

Photothermal CO detection in a hollow-core negative curvature fiber.

We demonstrate the first, to the best of our knowledge, photothermal carbon monoxide (CO) sensor using a hollow-core negative curvature fiber. The hollow-core fiber features a typical structure of one ring cladding containing eight nontouching capillaries to form a negative curvature core-surround. The photothermal effect in a 40-μm hollow core is induced by CO absorption at 2327nm and detected by a Mach-Zehnder interferometer operating at 1533nm. By using wavelength modulation spectroscopy, we achieve a normalized noise equivalent absorption coefficient of 4.4×10-8 cm-1 WHz-1/2. As CO has a very slow vibrational-translational relaxation process, we enhance the photothermal signal by enhancing the relaxation with the water vapor additive.

Preclinical evaluation of porcine colon resection using hollow core negative curvature fibre delivered ultrafast laser pulses.

Ultrashort pulse lasers offer great promise for tissue resection with exceptional precision and minimal thermal damage. Surgery in the bowel requires high precision and minimal necrotic tissue to avoid severe complications such as perforation. The deployment of ultrashort lasers in minimally invasive or endoscopic procedures has been hindered by the lack of suitable optical fibres for high peak powers. However, recent developments of hollow core microstructured fibres provide potential for delivery of such pulses throughout the body. In this study, analysis of laser ablation via a scanning galvanometer on a porcine colon tissue model is presented. A thermally damaged region (<85 μm) and fine depth control of ablation using the pulse energies 46 and 33 μJ are demonstrated. It is further demonstrated that such pulses suitable for precision porcine colon resection can be flexibly delivered via a hollow core negative curvature fibre (HC-NCF) and again ablation depth can be controlled with a thermally damaged region <85 μm. Ablation volumes are comparable to that of early stage lesions in the inner lining of the colon. This study concludes that the combination of ultrashort pulses and flexible fibre delivery via HC-NCF present a viable route to new minimally invasive surgical procedures.

Optical Amplification in Hollow-Core Negative-Curvature Fibers Doped with Perovskite CsPbBr3 Nanocrystals.

We report a hollow-core negative-curvature fiber (HC-NCF) optical signal amplifier fabricated by the filling of the air microchannels of the fiber with all-inorganic CsPbBr3 perovskite nanocrystals (PNCs). The optimum fabrication conditions were found to enhance the optical gain, up to +3 dB in the best device. Experimental results were approximately reproduced by a gain assisted mechanism based on the nonlinear optical properties of the PNCs, indicating that signal regeneration can be achieved under low pump powers, much below the threshold of stimulated emission. The results can pave the road for new functionalities of the HC-NCF with PNCs, such as optical amplification, nonlinear frequency conversion and gas sensors.

Hollow-core negative curvature fibers for application in optical gas sensors

We demonstrate the potential of applying hollow core and negative curvature optical fibers (HC-NCF) as efficient sensors to monitor the concentration of three high-toxicity gases: methane (CH4), carbon monoxide (CO), and nitrogen monoxide (NO). Numerical simulations demonstrate that the insertion of holes in such fibers guarantees the entry of these gases into their hollow core and allows strong interaction of these gases with the low-loss mode propagating in the HC-NCF. This interaction between light and gas in spectral regions with high gas absorption allows one to monitor reduced concentrations of these gases present in the environment simply by monitoring the optical power at the fiber output. The results show a linear behavior of propagation losses as a function of concentrations of 0% to 100% of CO and NO gas, and for concentrations of 0% to 5% of CH4 gas. This linearity between the propagation losses and the variation of the concentrations of gases in the environment can promote its application in healthcare and environment, to monitor low concentrations of gases ensuring high speed and accuracy.

Higher-Order Mode Suppression in Antiresonant Nodeless Hollow-Core Fibers.

Negative curvature hollow-core fibers (NC-HCFs) are useful as gas sensors. We numerically analyze the single-mode performance of NC-HCFs. Both single-ring NC-HCFs and nested antiresonant fibers (NANFs) are investigated. When the size of the cladding tubes is properly designed, higher-order modes (HOMs) in the fiber core can be coupled with the cladding modes effectively and form high-loss supermodes. For the single-ring structure, we propose a novel NC-HCF with hybrid cladding tubes to enable suppression of the first two HOMs in the core simultaneously. For the nested structure, we find that cascaded coupling is necessary to maximize the loss of the HOMs in NANFs, and, as a result, NANFs with five nested tubes have an advantage in single-mode guidance performance. Moreover, a novel NANF with hybrid extended cladding tubes is proposed. In this kind of NANF, higher-order mode extinction ratios (HOMERs) of 105 and even 106 are obtained for the LP11 and LP21 modes, respectively, and a similar level of 105 for the LP02 modes. Good single-mode performance is maintained within a broad wavelength range. In addition, the loss of the LP01 modes in this kind of NANF is as low as 3.90 × 10−4 dB/m.

负曲率空芯光纤限制损耗的影响因素

负曲率空芯光纤限制损耗的影响因素

Multiphoton fluorescence excitation and detection with a single negative curvature hollow core fibre

In this letter, we propose a single fiber-based sensor setup allowing for simultaneous excitation and detection of multiphoton fluorescence. Presented sensor’s key element is the negative curvature hollow core fiber (NCHCF) with three transmission bands in the visible spectral range (414–423 nm, 510–552 nm and 680–784 nm), allowing for nearly dispersion-free guidance of 160 fs-long laser pulses at 730 nm photon wavelength. Total temporal broadening of a laser pulse propagating in the proposed sensor setup is only (7 ± 1) fs. The NCHCF output beam was additionally focused with a microlensed, multimode fiber tip, increasing the efficiency of multiphoton absorption. The usefulness of the sensor for the multiphoton spectroscopy experiments is tested on the solutions of fluorescein and flavin adenine dinucleotide. This optical fiber sensor combines simplicity, minimal size, and good optical properties, and can be found an interesing solution for the non-linear optical methods used in chemistry, biology and medicine.