Hollow-core Photonic Bandgap Fiber Research Articles

Investigation of Long-Period Grating Resonances in Hollow-Core Photonic Bandgap Fibers

The mode-coupling processes of long-period gratings (LPGs) fabricated in hollow-core photonic bandgap fibers are investigated. The LPGs are formed by periodic structural deformations induced by local heating generated by use of a pulsed CO2 laser. Highly polarization-dependent grating resonances are observed in the transmission spectrum of the LPGs and found to be due to coupling from the fundamental to higher order core modes. The mode-coupling process is understood based on coupled local-mode theory: A single deformation of the holey cladding modifies the local-mode field profiles and induces fractional energy transfer between the fundamental and the higher order modes. Resonant coupling between a phase-matched fundamental/higher order mode pair is excited by having multiple such deformations arranged periodically along the fiber. The LPG spectrums are numerically modeled by solving the coupled local-mode equations at different wavelengths and found in agreement with the experimentally measured results.

Investigation of the guided-mode characteristics of hollow-core photonic band-gap fibre with interstitial holes

This paper investigates the guided-mode characteristics of hollow-core photonic band-gap fibre (HC-PBGF) with interstitial holes fabricated by an improved twice stack-and-draw technique at visible wavelengths. Based on the simulation model with interstitial holes, the influence of glass interstitial apexes on photonic band-gaps is discussed. The existing forms of guided-mode in part band gaps are shown by using the full-vector plane-wave method. In the experiment, the observed transmission spectrum corresponds to the part band gaps obtained by simulation. The fundamental and second-order guided-modes with mixture of yellow and green light are observed through choosing appropriate fibre length and adjusting coupling device. The loss mechanism of guided-modes in HC-PBGF is also discussed.

Tunable Fabry-Perot filter using hollow-core photonic bandgap fiber and micro-fiber for a narrow-linewidth laser

A novel tunable fiber Fabry-Perot (FP) filter is proposed and demonstrated by using a hollow-core photonic bandgap fiber (HC-PBF) and a micro-fiber. The interference cavity is a hollow core of HC-PBF. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other reflection mirror is a gold-coated end of micro-fiber that uses chemical etching process to obtain the similar diameter as the core of HC-PBF. Hence the movable mirror can be adjusted with long distance inside the hollow core of HC-PBF. Tunable FP filter is used as a mode selecting component in the reflection mode to implement stable single longitudinal mode (SLM) operation in a ring laser. With FP cavity length of 0.25 ± 0.14 mm, the wavelength of SLM laser can be tuned over 1554-1562 nm with a tuning step of 0.2-0.3 nm, a side-mode suppression ratio (SMSR) of 32-36 dB and a linewidth of 3.0-5.1 kHz. With FP cavity length of 2.37 ± 0.37 mm, the SLM laser can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm, a SMSR of 44-51 dB and a linewidth of 1.8-3.0 kHz.

AN ALL-FIBER GAS SENSING SYSTEM USING HOLLOW-CORE PHOTONIC BANDGAP FIBER AS GAS CELL

An all-fiber compact gas sensing system using a hollow-core photonic bandgap fiber (HC-PBF) as a gas cell is proposed in this paper. Compared with the present reported microstructured optical-fiber gas cells, the HC-PBF gas cell proposed here has relatively lower transmission loss and much simpler construction. The total transmission loss through the HC-PBF gas cell is demonstrated experimentally to be as low as 1.5 dB and the time taken for gas to get into the 90-cm-cell, under the free diffusion condition, is approximately 11 min. Combining the HC-PBF gas cell with a tunable fiber laser, an all-fiber gas sensing system is developed. The properties of the proposed system are demonstrated experimentally by detection of carbon monoxide (CO) and acetylene (C2H2). Approximately linear relationships between the system responses and the concentrations of the detected gases are experimentally demonstrated. The minimal detectable concentration of CO of 300 ppm and C2H2 of 5 ppm are also achieved respectively by the experiments.

High average power, high energy 155 μm ultra-short pulse laser beam delivery using large mode area hollow core photonic band-gap fiber

We demonstrate high average power, high energy 1.55 μm ultra-short pulse (<1 ps) laser delivery using helium-filled and argon-filled large mode area hollow core photonic band-gap fibers and compare relevant performance parameters. The ultra-short pulse laser beam-with pulse energy higher than 7 μJ and pulse train average power larger than 0.7 W-is output from a 2 m long hollow core fiber with diffraction limited beam quality. We introduce a pulse tuning mechanism of argon-filled hollow core photonic band-gap fiber. We assess the damage threshold of the hollow core photonic band-gap fiber and propose methods to further increase pulse energy and average power handling.

A hollow-core photonic bandgap fiber polarization controller

A polarization controller was built by pressurizing laterally three segments of a commercial hollow-core photonic bandgap fiber. By varying the magnitudes of the applied pressures, the output state of polarization showed a good coverage of all the possible polarization states on the surface of the Poincaré sphere.

Experimental study of an elliptical-core photonic bandgap fiber with thin core wall and high aspect ratio and its birefringence characteristics

We fabricate an elliptical hollow-core photonic bandgap fiber (EC-PBGF) with a core-wall thickness of 65 nm and an aspect ratio of 1.4. The core is expanded by applying positive pressure in the hollow-core region during the fiber-drawing process. Both the reduction of core-wall thickness and the increase of aspect ratio result in the enhancement of optical properties in terms of a low attenuation loss and group birefringence. The minimum loss and group birefringence of the fabricated EC-PBGF2 are measured to be 0.1 dB/m and 0.012, respectively.

Characterization of self-phase modulation in liquid filled hollow core photonic bandgap fibers

We experimentally and theoretically study self-phase modulation by Kerr effect in a liquid filled hollow core photonic crystal fiber. We perform a complete characterization of the linear optical properties of the hollow core photonic bandgap fiber filled with deuterated acetone to determine all the characteristics of the propagation mode. The nonlinear coefficient of the fiber is determined by fitting the output spectra broadened by self-phase modulation with a new analytical expression giving the spectra of a hyperbolic secant pulse transmitted through a Kerr medium. The experiment allows a precise determination of the nonlinear index change nI2 of acetone-d6 equal to (1.15&#xb1;0.17)&#xd7;10&#x2212;19&#x2009;m2&#x2009;W&#x2212;1.

Hollow core fiber with an octave spanning bandgap

We thoroughly compare the out-of-plane bandgaps generated by three realistic two-dimensional lattices: a triangular and a square arrangement of holes and a triangular arrangement of rods. We demonstrate that, for any given hole-diameter-to-pitch ratio d/Lambda, the triangular arrangement of interconnected resonators generates the widest possible bandgap along the air line, and we propose a physical interpretation explaining these results. The design of a hollow core photonic bandgap fiber based on such a lattice and able to transmit light with sub-decibel-per-meter losses over an octave of frequencies is presented for the first time, to the best of our knowledge.

Sensing with hollow-core photonic bandgap fibers

This paper examines the unique properties of hollow-core photonic bandgap fibers and discusses potential sensing applications of such fibers. Guidance of light in air instead of silica reduces the effect of material on light propagation and is advantageous to applications such as fiber gyroscopes. An air–silica microstructure has novel mechanical and thermal properties and can be beneficial to applications such as acoustic pressure sensors. Holey cladding provides extra flexibility for geometry modification through post thermal treatment or selective filling of the air holes and can be exploited for developing novel devices such as in-line polarizers/polarimeters, modal interferometer, wavelength filters and sensors. The confinement of gas and liquid phase materials and optical guided modes simultaneously within the hollow core allows strong light/sample interaction over an extended length and may be useful for the development of highly sensitive gas and liquid sensors.

Polymer-composite fibers for transmitting high peak power pulses at 155 microns

Hollow-core photonic bandgap fibers (PBG) offer the opportunity to suppress highly the optical absorption and nonlinearities of their constituent materials, which makes them viable candidates for transmitting high-peak power pulses. We report the fabrication and characterization of polymer-composite PBG fibers in a novel materials system, polycarbonate and arsenic sulfide glass. Propagation losses for the 60 microm-core fibers are less than 2dB/m, a 52x improvement over previous 1D-PBG fibers at this wavelength. Through preferential coupling the fiber is capable of operating with over 97% the fiber's power output in the fundamental (HE(11)) mode. The fiber transmitted pulses with peak powers of 11.4 MW before failure.

Transmission properties of hollow-core photonic bandgap fibers in relation to molecular spectroscopy

The transmission properties of five types of hollow-core photonic bandgap fibers (HC-PBFs) are characterized in the telecom wavelength range around 1.5 microm. The variations in optical transmission are measured as a function of laser frequency over a 2 GHz scan range as well as a function of time over several hours. The influence of these variations on spectroscopy of molecules in a HC-PBF is simulated.

Numerical Analysis of Propagating and Radiating Properties of Hollow Core Photonic Band Gap Fibres for THz Applications

Hollow core photonic bandgap fibres (HC-PBGFs) are numerically investigated in order to obtain low loss wave-guiding and good aperture field distribution in the terahertz region (0.1-10 THz) of the electromagnetic spectrum. The purity of the aperture field distribution at the HC-PBGF section combined with low loss propagation and high coupling efficiency with free-space propagating Gaussian beams suggest a possible employment of such a structure as aperture antennas, for possible feed systems in THz applications and in THz wireless sensing.

All-optical modulation of four-wave mixing in an Rb-filled photonic bandgap fiber

We demonstrate efficient all-optical modulation using Rb vapor confined to a hollow-core photonic bandgap fiber. The intensity of a signal field participating in the four-wave-mixing process is modulated using a weak switching field. We observe 3 dB of attenuation in the signal field with only 3600 photons of switching energy, corresponding to 23 photons per atomic cross section lambda(2)/(2pi). Modulation bandwidths as high as 300 MHz are observed.

Slow-light enhanced absorption in a hollow-core fiber

Light traversing a hollow-core photonic band-gap fiber may experience multiple reflections and thereby a slow-down and enhanced optical path length. This offers a technologically interesting way of increasing the optical absorption of an otherwise weakly absorbing material which can infiltrate the fibre. However, in contrast to structures with a refractive index that varies along the propagation direction, like Bragg stacks, the translationally invariant structures studied here feature an intrinsic trade-off between light slow-down and filling fraction that limits the net absorption enhancement. We quantify the degree of absorption enhancement that can be achieved and its dependence on key material parameters. By treating the absorption and index on equal footing, we demonstrate the existence of an absorption-induced saturation of the group index that itself limits the maximum absorption enhancement that can be achieved.

Influence of strain and pressure to the effective refractive index of the fundamental mode of hollow-core photonic bandgap fibers

We investigate the phase sensitivity of the fundamental mode of hollow-core photonic bandgap fibers to strain and acoustic pressure. A theoretical model is constructed to analyze the effect of axial strain and acoustic pressure on the effective refractive index of the fundamental mode. Simulation shows that, for the commercial HC-1550-02 fiber, the contribution of mode-index variation to the overall phase sensitivities to axial strain and acoustic pressure are respectively approximately -2% and approximately -17%. The calculated normalized phase-sensitivities of the HC-1550-02 fiber to strain and acoustic pressure are respectively 1 epsilon(-1) and -331.6 dB re microPa(-1) without considering mode-index variation, and 0.9797 epsilon(-1) and -333.1 dB re microPa(-1) when mode-index variation is included in the calculation. The latter matches better with the experimentally measured results.

Fiber-Optic Technologies in Laser-Based Therapeutics: Threads for a Cure

In the past decade, novel fiber structures and material compositions have led to the introduction of new diagnostic and therapeutic tools. We review the structure, the material composition and the fabrication processes behind these novel fiber systems. Because of their structural flexibility, their compatibility with endoscopic appliances and their efficiency in laser delivery, these fiber systems have greatly extended the reach of a wide range of surgical lasers in minimally invasive procedures. Much research in novel fiber-optics delivery systems has been focused on the accommodation of higher optical powers and the extension to a broader wavelength range. Until recently, CO2 laser surgery, renowned for its precision and efficiency, was limited to open surgeries by the lack of delivery fibers. Hollow-core photonic bandgap fibers are assessed for their ability to transmit CO2 laser at surgical power level and for their applications in a range of clinical areas. Current fiber-delivery technologies for a number of laser surgery modalities and wavelengths are compared.

Design of Topas photonic bandgap fiber with high birefringence and low confinement loss

A highly birefringent hollow-core photonic bandgap fiber based on Topas cyclic olefin copolymer is designed. The rhombic hollow-core with rounded corners is formed by omitting four central air holes of the cladding structure. The guided modes, birefringence and confinement loss of the fiber are investigated by using the full-vector finite element method. A high phase birefringence of the order of 10−3, a group birefringence of the order of 10−2 and confinement loss less than 0.1dB/km are obtained at the central wavelength (1.55μm) range of the bandgap for fiber with seven rings of air holes in the cladding region.

Spectroscopy of Rb atoms in hollow-core fibers

Recent demonstrations of light-matter interactions with atoms and molecules confined to hollow waveguides offer great promise for ultralow-light-level applications. The use of waveguides allows for tight optical confinement over interaction lengths much greater than what could be achieved in bulk geometries. However, the combination of strong atom-photon interactions and nonuniformity of guided light modes gives rise to spectroscopic features that must be understood in order to take full advantage of the properties of such systems. We use light-induced atomic desorption to generate an optically dense Rb vapor at room temperature inside a hollow-core photonic band-gap fiber. Saturable-absorption spectroscopy and passive slow-light experiments reveal large ac Stark shifts, power broadening, and transit-time broadening, that are present in this system even at nanowatt powers.

Near-infrared Raman spectroscopy using hollow-core photonic bandgap fibers

We report a NIR Raman spectroscopy system incorporating hollow-core photonic bandgap fibers (HC-PBFs) in both the excitation and collection paths. Raman excitation was achieved at NIR laser wavelength of 785nm. We demonstrate that using HC-PBFs, Raman spectroscopy can be performed without the use of an additional longpass filter on the collection side. A narrow bandpass filter on the excitation side is also not required. These results provide a framework where HC-PBF based Raman probes can be developed and used in space restricted biomedical and sensing applications.