Hollow-core Photonic Bandgap Fiber Research Articles

Investigation of A Slow-Light Enhanced Near-Infrared Absorption Spectroscopic Gas Sensor, Based on Hollow-Core Photonic Band-Gap Fiber.

Generic modeling and analysis of a slow-light enhanced absorption spectroscopic gas sensor was proposed, using a mode-tuned, hollow-core, photonic band-gap fiber (HC-PBF) as an absorption gas cell. Mode characteristics of the un-infiltrated and infiltrated HC-PBF and gas absorption enhancement of the infiltrated HC-PBF were analyzed. A general rule of microfluidic parameters for targeting different gas species in the near-infrared was obtained. Ammonia (NH3) was used as an example to explore the effects of slow light on gas detection. The second harmonic (2f) signal and Allan deviation were theoretically investigated based on the derived formulations.

Nondestructive determination of the core size of a hollow-core photonic bandgap fiber using Fabry-Perot interference.

In this Letter, we propose a simple and nondestructive method for the determination of the core size of a hollow-core photonic bandgap fiber (HC-PBF) and its axial uniformity based on a Fabry-Perot cavity induced by a pair of opposite silica-air interfaces within the hollow core. The experimental results indicate that the core size test of the HC-PBF has a nanometer-level precision, and its axial uniformity test has an ultimate spatial resolution of tens of microns. The method provides an effective and precise tool for the investigation of the hollow-core size and its longitudinal evolution.

Diaphragm-free gas-pressure sensor probe based on hollow-core photonic bandgap fiber.

A diaphragm-free probe-type gas-pressure sensor is proposed and experimentally demonstrated based on a hollow-core photonic bandgap fiber (HC-PBF) with a quartz capillary. The section of the HC-PBF acts as a Fabry-Perot cavity, and the quartz capillary acts as a microfluidic channel for a gas inlet. An inner diameter of the quartz capillary (∼2 μm) smaller than the HC-PBF (∼10.9 μm) ensures a mirror reflection and a microfluidic channel simultaneously. The sensor probe has a minimal size (∼125 μm) and can function at gas pressures as high as 8MPa. A higher pressure test is limited by our gas-pressure generation devices. Excellent stability of the sensor is observed in a long timescale, and repeatability of the sensor is confirmed by tests of six different samples. Compared with conventional optical fiber gas-pressure sensors, the proposed sensor involves a simple fabrication process and can acquire probe measurements with high sensitivity (∼4.17 nm/MPa), excellent linearity (0.9999), fast response, and no hysteresis. The proposed sensor can also function at temperatures as high as 800°C, which is beneficial for high pressure measurements in extreme conditions. Moreover, the fast response of the sensor is attractive for dynamic pressure measurements, which needs further study and characterization.

Unique loss characteristics in TE01 modes of conventional photonic bandgap fibers.

We report modeling that demonstrates the reduction of transmission loss and broadening of the bandwidth of a conventional hollow-core photonic bandgap fiber (PBF). Numerical investigation reveals that transmission loss of the high-order TE01 mode is lower than that of the fundamental HE11 mode in fibers with thick cladding walls. By comparing dispersion curves of PBFs with different core-wall thicknesses, we show that the TE01 mode has weaker coupling strength to a surface mode than the HE11 mode. This result opens the way for a wider transmission band and lower transmission loss in PBFs that are subject to the detrimental effects of surface modes.

Phase sensitivity of fundamental mode to external atmospheric pressure for hollow-core photonic bandgap fiber

Abstract Hollow-core photonic bandgap fibers (HC-PBFs) are suitable for spaceborne fiber optical gyroscopes owing to their excellent environmental adaptability. However, hundreds of small holes full of air at one atmosphere of pressure can make the HC-PBF sensitive to external atmospheric pressure. In this study, we investigated the phase sensitivity of the fundamental mode to external atmospheric pressure for the HC-PBF, and the experimental result indicates that the phase sensitivity is approximately 1.6 × 10−5 ppm/Pa, which is mostly contributed by the change in the pressure-induced length. Through the choice of coating, the phase sensitivity to external atmospheric pressure can be reduced by about a factor of five compared to current HC-PBFs, and the excellent temperature performance can be maintained at the same time.

Mode Couplers and Converters Based on Dual-Core Hollow-Core Photonic Bandgap Fiber

We propose mode couplers and converters based on dual-core hollow-core photonic bandgap fiber (HC-PBGF) structures, which might be potential solutions for integrated all-fiber multiplexers or demultiplexers of a mode-division multiplexed (MDM) transmission system using HC-PBGFs. Coupling properties of ${\text{LP}}_{01}$ and ${\text{LP}}_{11}$ mode in the mode coupler are investigated and applied to mode separators and polarization separators. Mode transition from ${\text{LP}}_{01}$ to ${\text{LP}}_{11}$ and ${\text{LP}}_{21}$ mode are demonstrated in the mode converter whose smaller core is expected as input end and matched with single-mode fiber. The coupling between fundamental mode and high-order modes is accomplished through the resonant effect, which is fulfilled by modifying the structure parameter of the smaller core. Such mode converters based on dual-core HC-PBGF could be a promising substitute for the free-space coupling device, which is costly and not suitable for integration design in MDM systems and other hollow-core fiber experiments and applications.

Investigation of Interstice Collapse for Hollow-Core Photonic Bandgap Fiber Fabrication

Hollow-core photonic bandgap fibers (PBGF) have low loss in theory; hence, they have great potential for many applications, such as high-speed big-data fiber communication and fiber sensors. They are fabricated using a two-step fiber drawing method. For PBGF optical properties, fabrication is an important factor. Most studies for microstructure fiber fabrication consider gas pressure, tension, and temperature control in the fiber drawing progress without considering the hexagon fillet in the honeycomb cladding, which could have a significant effect on the bandgap width. This letter focuses on the interstice's impacts among capillaries in the hexagon fillet forming process via the hydrodynamic mode. Research shows that small interstices could collapse totally when the heating temperature is high enough, and the collapse of interstice can help the honeycomb holes restrain collapse slightly. Interstices might be advantageous for controlling the honeycomb holes and hexagon fillets.

All-fiber gas sensor with intracavity photothermal spectroscopy.

We present an all-fiber intracavity photothermal (IC-PT) spectroscopic gas sensor with a hollow-core photonic bandgap fiber (HC-PBF) gas cell. The gas cell is placed inside a fiber-ring laser cavity to achieve higher laser light intensity in the hollow core and hence higher PT modulation signal. An experiment with a 0.62-m-long HC-PBF gas cell demonstrated a noise equivalent concentration of 176ppb acetylene. Theoretical modeling shows that the IC-PT sensor has the potential of achieving sub-ppb (parts-per-billion) acetylene detection sensitivity.

Polarization-Maintaining Hollow-Core Photonic Bandgap Few-Mode Fiber in Terahertz Regime

We propose a terahertz (THz) hollow-core photonic bandgap fiber (HC-PBGF) supporting few-mode operation with large birefringence. The periodic arrangement of square lattice with round corners in fiber cladding offers bandgap guidance. Numerical simulation indicates that a 21-cell HC-PBGF could support six vector modes in the bandgap. Characteristics of guided modes are comprehensively investigated, suggesting modal confinement loss of the order of 10−3 cm−1, group velocity dispersion under 1 psTHz−1cm−1, and modal birefringence higher than 10−4 for all modes at around 0.92 THz. Moreover, 24-cell and 32-cell HC-PBGFs are discussed to explore the influence of HC geometry on guided mode number and modal birefringence.

Theoretical and experimental investigation of light guidance in hollow-core anti-resonant fiber

The inherent material imperfections of solid core optical fiber, for example, Kerr nonlinearity, chromatic dispersion, Rayleigh scattering and photodarkening, set fundamental limitations for further improving the performances of fiber-based systems. Hollow-core fiber (HCF) allows the light to be guided in an air core with many unprecedented characteristics, overcoming almost all the shortcomings arising from bulk material. The exploitation of HCF could revolutionize the research fields ranging from ultra-intense pulse delivery, single-cycle pulse generation, nonlinear optics, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. Therefore, the investigations into the guidance mechanism and the ultimate limit of HCF have become a hot research topic. In the past two decades, scientists and engineers have fabricated two types of high-performance HCFs with loss figures of 1.7 dB/km and 7.7 dB/km for hollow-core photonic bandgap fiber (HC-PBGF) and hollow-core anti-resonant fiber (HC-ARF) respectively. In comparison with the twenty-years-old HC-PBGF technology, the HC-ARF that recently appeared outperforms the former in terms of broadband transmission and high laser damage threshold together with a quickly-improved loss figure, providing an ideal platform for many more challenging applications. While the guidance mechanism and fabrication technique in HC-PBGF have been well recognized, the HC-ARF still has a lot of room for improvement. At the birth of the first generation of broadband HC-ARF, the guidance mechanism was unclear, the fiber design was far from perfect, the fabrication was immature, and the optical properties were not optimized. In the past five years, we have developed an intuitive and semi-analytical model for the confinement loss of HC-ARF and managed to fabricate high-performance nodeless HC-ARF. We further employ our theoretical model and fabrication technique to well control and design other interesting properties, such as polarization maintenance and bending loss in HC-ARF. For a long time, the anti-resonant theory of light guidance has been regarded as being qualitative, and the leaky-mode-based HC-ARF have been considered to have worse performances than the guided-mode-based HC-PBGF. Our investigations in theory and experiment negative these prejudices, thus paving the way for the booming development of HC-ARF technologies in the near future.

Distributed gas sensing with optical fibre photothermal interferometry

We report the first distributed optical fibre trace-gas detection system based on photothermal interferometry (PTI) in a hollow-core photonic bandgap fibre (HC-PBF). Absorption of a modulated pump propagating in the gas-filled HC-PBF generates distributed phase modulation along the fibre, which is detected by a dual-pulse heterodyne phase-sensitive optical time-domain reflectometry (OTDR) system. Quasi-distributed sensing experiment with two 28-meter-long HC-PBF sensing sections connected by single-mode transmission fibres demonstrated a limit of detection (LOD) of ∼10 ppb acetylene with a pump power level of 55 mW and an effective noise bandwidth (ENBW) of 0.01 Hz, corresponding to a normalized detection limit of 5.5ppb⋅W/Hz. Distributed sensing experiment over a 200-meter-long sensing cable made of serially connected HC-PBFs demonstrated a LOD of ∼ 5 ppm with 62.5 mW peak pump power and 11.8 Hz ENBW, or a normalized detection limit of 312ppb⋅W/Hz. The spatial resolution of the current distributed detection system is limited to ∼ 30 m, but it is possible to reduce down to 1 meter or smaller by optimizing the phase detection system.

Performance optimization of hollow-core fiber photothermal gas sensors.

We report performance optimization of hollow-core photonic bandgap fiber (HC-PBF) photothermal (PT) gas sensors. The PT phase modulation efficiency of a C2H2 filled HC-PBF (HC-1550-02) is found independent of the pump modulation frequency for up to ∼330 kHz, but starts to drop quickly to 10% of the maximum value at a couple of megahertz. With a 1.1m long HC-PBF gas cell with angle-cleaved single-mode fiber/HC-PBF joints to reduce reflection and a modified 3×3 Sagnac interferometer with balanced detection for phase demodulation, a noise equivalent concentration of ∼67 ppbC2H2 is achieved with a 1s time constant, and it goes down to ∼18 ppb with 145s integration time. The system has good long-term stability and exhibits signal fluctuations of <1% over a ∼5 h period.

Temperature sensing property of hollow-core photonic bandgap fiber filled with CdSe/ZnS quantum dots in an UV curing adhesive

Abstract A temperature sensor based on the hollow-core photonic bandgap fiber filled with the CdSe/ZnS QDs dissolved in an ultraviolet (UV) curing adhesive is reported. The sensor shows a linear variation of the photoluminescence (PL) peak wavelength for a temperature range from 40 °C to 140 °C, with a correlation factor of 0.99263 and a sensitivity of 0.05744 nm/°C. Although the peak intensity of emission spectrum increased exponentially with the temperature, a linear temperature-dependence result with a correlation factor of 0.99917 and a slope of 2.04 × 10 −3 °C −1 can be obtained with a self-reference spectral intensity method. The linear variation characteristics of the peak wavelength and the self-reference intensity of PL spectrum indicates the designed fiber temperature sensor is feasible in the practical application.

Application of Negative Curvature Hollow-Core Fiber in an Optical Fiber Sensor Setup for Multiphoton Spectroscopy

In this paper, an application of negative curvature hollow core fiber (NCHCF) in an all-fiber, multiphoton fluorescence sensor setup is presented. The dispersion parameter (D) of this fiber does not exceed the value of 5 ps/nm × km across the optical spectrum of (680–750) nm, making it well suited for the purpose of multiphoton excitation of biological fluorophores. Employing 1.5 m of this fiber in a simple, all-fiber sensor setup allows us to perform multiphoton experiments without any dispersion compensation methods. Multiphoton excitation of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) with this fiber shows a 6- and 9-fold increase, respectively, in the total fluorescence signal collected when compared with the commercial solution in the form of a hollow-core photonic band gap fiber (HCPBF). To the author’s best knowledge, this is the first time an NCHCF was used in an optical-fiber sensor setup for multiphoton fluorescence experiments.

Long Period Fiber Grating Inscribed in Hollow-Core Photonic Bandgap Fiber for Gas Pressure Sensing

In this paper, we have proposed and experimentally prepared an improved gas pressure sensing device. It is mainly constructed by a short hollow silica tube segment and a CO2-laser-induced long period grating in hollow-core photonic bandgap fiber (HC-PBF). To effectively enhance the interaction between light in the air-core of HC-PBF and external surroundings, thus to achieve the best possible gas pressure sensitivity, a microchannel is introduced in the middle of the hollow silica tube segment with the femtosecond laser processing technique. The corresponding gas pressure experiments demonstrate that the resonant wavelength of the LPFG shows a blue shift up to −1.3 nm/MPa. Moreover, the temperature response sensitivity of this sensor is as low as 5.3 pm/°C and enable it possible as a temperature-insensitive gas pressure measure apparatus.

Applications of optical technology in gas concentration detection

Seven common optical methods for gas concentration detection are described. The basic principles, advantages and disadvantages of each method are given in detail. The improvement work and some novel ideas are presented. The applications of combined methods are discussed. These optical methods include some con-ventional gas concentration detection technologies, such as optical interferential method, photoacoustic detection (PAS), correlation spectroscopy, and some novel gas concentration detection technologies, such as tunable diode laser absorption spectroscopy (TDLAS), evanescent wave field sensing technology, hollow core photonic bandgap fiber (HC-PBF) sensing technology and fiber loop ring-down spectroscopy (FLRDS). The prospect of optical gas sensing is listed at the end of the paper, which mainly refers to miniaturization, intelligence, portability, low power consumption, high accuracy, fast response and distributed multi-component telemetry technology.

Robust hollow-fiber-pigtailed 930 nm femtosecond Nd:fiber laser for volumetric two-photon imaging.

We demonstrate a robust high power 930 nm femtosecond Nd:fiber laser system with hollow-core photonic bandgap fiber (HC-PBGF) as the output delivery, which can be easily integrated into compact two-photon microscopy system for bio-imaging. The whole laser system can deliver up to 17.4 nJ, 220-fs pulses at 930 nm with repetition rate of 46 MHz. In this paper, this laser was demonstrated as the light source for volumetric imaging of zebrafish blood vessel.

In-Situ Measurement of Backscattering in Hollow-Core Fiber Based Resonant Cavities

We measure the light backscattered by resonant optical cavities based on two types of hollow-core photonic bandgap fibers, namely, 7-cell and 19-cell fibers. The measurement of the intensity backscattered by the cavity at resonance permits to deduce the value of the fiber backscattering coefficient. We find backscattering coefficients of the order of $2.0\times 10^{-6}$ and $1.0\times 10^{-6}\;\mathrm{m}^{-1}$ for the 7-cell and 19-cell fibers, respectively, two orders of magnitude larger than the one for standard solid-core single-mode fiber.

1-m tunable optical delay line using microfluid sliding in a hollow-core fiber: Feasibility study

A novel variable optical delay line based on a hollow-core photonic bandgap fiber is proposed. The device incorporates microfluid, the end surface of which serves as an optical reflector, in the hollow-core of the fiber. The position of the fluid end is controlled by a syringe pump to change the optical delay of the reflected beam. We demonstrate wide tunability of the optical delay up to 1 m with a scan speed of several mm/s. The return loss and beat pattern in the reflected signal is studied and the potential of the device as an ultra-long delay line is discussed.

Optimizing pulse compressibility in completely all-fibered Ytterbium chirped pulse amplifiers for in vivo two photon laser scanning microscopy.

A simple and completely all-fiber Yb chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor is applied in nonlinear optical microscopy. This stretching-compression approach improves compressibility and helps to maximize the fluorescence signal in two-photon laser scanning microscopy as compared with approaches that use standard single mode fibers as stretcher. We also show that in femtosecond all-fiber systems, compensation of higher order dispersion terms is relevant even for pulses with relatively narrow bandwidths for applications relying on nonlinear optical effects. The completely all-fiber system was applied to image green fluorescent beads, a stained lily-of-the-valley root and rat-tail tendon. We also demonstrated in vivo imaging in zebrafish larvae, where we simultaneously measure second harmonic and fluorescence from two-photon excited red-fluorescent protein. Since the pulses are compressed in a fiber, this source is especially suited for upgrading existing laser scanning (confocal) microscopes with multiphoton imaging capabilities in space restricted settings or for incorporation in endoscope-based microscopy.