Hollow-core Photonic Bandgap Fiber Research Articles

Effects of selectively thickening core wall on birefringence and loss in polarization-maintaining hollow core photonic bandgap fiber

Effects of selectively thickening core wall on birefringence and loss in polarization-maintaining hollow core photonic bandgap fiber

Near-Infrared Dual Greenhouse Gas Sensor Based on Hollow-Core Photonic Crystal Fiber for Gas-Cell In-Situ Applications.

A greenhouse gas sensor has been developed to simultaneously detect multiple gas species within a hollow-core photonic bandgap fiber (HC-PBF) structure entirely composed of fibers. To enhance sensitivity, the gas cell consists of HC-PBF enclosed between two single-mode fibers fused with a reflective end surface to double the absorption length. The incorporation of side holes for gas diffusion allows for analysis of the relationship between gas diffusion speed, number of drilled side holes, and energy loss. As the number of drilled holes increases, the response time decreases to less than 3 min at the expense of energy loss. Gas experiments demonstrated detection limits of 0.1 ppm for methane and 2 ppm for carbon dioxide, with an average time of 50 s. In-situ testing conducted in rice fields validates the effectiveness of the developed gas detection system using HC-PBF cells, establishing all-fiber sensors with high sensitivity and rapid response.

A 20 MHz Repetition Rate, Sub-Picosecond Ti-Sapphire Laser for Fiber Delivery in Nonlinear Microscopy of the Skin.

Nonlinear microscopy (NM) enables us to investigate the morphology or monitor the physiological processes of the skin through the use of ultrafast lasers. Fiber (or fiber-coupled) lasers are of great interest because they can easily be combined with a handheld, scanning nonlinear microscope. This latter feature greatly increases the utility of NM for pre-clinical applications and in vivo tissue imaging. Here, we present a fiber-coupled, sub-ps Ti-sapphire laser system being optimized for in vivo, stain-free, 3D imaging of skin alterations with a low thermal load of the skin. The laser is pumped by a low-cost, 2.1 W, 532 nm pump laser and delivers 0.5-1 ps, high-peak-power pulses at a ~20 MHz repetition rate. The spectral bandwidth of the laser is below 2 nm, which results in a low sensitivity for dispersion during fiber delivery. The reduction in the peak intensity due to the increased pulse duration is compensated by the lower repetition rate of our laser. In our proof-of-concept imaging experiments, a ~1.8 m long, commercial hollow-core photonic bandgap fiber was used for fiber delivery. Fresh and frozen skin biopsies of different skin alterations (e.g., adult hemangioma, basal cell cancer) and an unaffected control were used for high-quality, two-photon excitation fluorescence microscopy (2PEF) and second-harmonic generation (SHG) z-stack (3D) imaging.

The Structural Defects and Optical Performance of Polarization-Maintaining Hollow-Core Photonic Bandgap fiber

Hollow core photonic bandgap fiber (HC-PBGF) has potential in the application of fiber optics gyroscopes (FOGs) for its properties of low nonlinearity, and excellent environmental adaptability. However, due to the structural complexity, the HC-PBGF is prone to defects during fabrication process, which will cause degradation of longitude uniformity and optical properties of fiber. The longitudinal non-uniformity of the structure limits the preparation length of HC-PBGF, while high-precision FOGs need to be made with long-distance and high-performance fibers. To facilitate the application of HC-PBGF in high-precision FOG, we studied the instability factors encountered in the fabrication process of polarization-maintaining (PM) HC-PBGF and the induced structural defects, including the fiber core, core-side hole, and fiber diameter defects. According to analyzing the optical performance variation to structural deformation rate, we conducted that the diameter defect has the greatest impact on the fiber properties, while the core-side hole defect has the weakest impact. Based on the fluctuations of optical performance, we revealed the structural deviation tolerance of PM HC-PBGF and provided theoretical reference for fiber fabrication.

Radiation-induced attenuation of hollow-core photonic bandgap fiber for space applications

Radiation-Induced Attenuation (RIA) is a crucial parameter for the applications of fiber optic gyroscopes in space environments. Under the irradiation of a 60Co source with a radiation rate of 10 rad/s and a total radiation dose of 138 krad, we make real-time measurements of RIA and evaluate the recovery process of homemade 19-cell HC-PBGF and commercial PPMF. Experimental results demonstrate that there is only a 0.45 dB/km loss increment for the RIA of homemade HC-PBGF at the wavelength of 1550 nm, which is only about 1/27 of that of the PPMF. After the radiation source is turned off, the loss of HC-PBGF returns to the initial level before radiation in the tenth hour, but the RIA of PPMF has only a 20% recovery on the seventh day. We first theoretically investigate the kinetics and phenomenological equation of RIA in the Hollow-Core Photonic Band-Gap Fiber (HC-PBGF) and commercial Panda Polarization Maintaining Fiber (PPMF) in the γ-rays environment. Theoretical results proved that the initial defect concentration and RIA increase rate of HC-PBGF are only 1/3 and 1/6 of that of the PPMF. This proves that HC-PBGF can significantly reduce the RIA and exhibits a strong capacity of irradiation resistance, which is of great significance to promote fiber optic gyroscopes for space applications.

Thermal Sensitivity of Birefringence in Polarization-Maintaining Hollow-Core Photonic Bandgap Fibers

Polarization-maintaining (PM) fiber is the core sensitive component of a fiber optic gyroscope (FOG); its birefringence temperature stability is crucial for maintaining accuracy. Here, we systematically investigated the structural thermal deformation and the resulting birefringence variation in typical PM hollow-core photonic bandgap fibers (HC-PBGFs) for FOG according to varying fiber structure parameters. To verify the application potential of PM HC-PBGFs in FOG, we compared the thermal sensitivity of birefringence (TSB) with that of the commonly used Panda PM fiber, which was tested to 5.07 × 10−5/100 °C. For rhombic-core fibers, the TSB was determined by the structure of the cladding and could be tuned as low as low as 10−7/100 °C, two orders of magnitude smaller than that of the panda PM fibers. For hexagonal-core fibers, the birefringence variation depended mainly on the drift of the surface modes (SMs) caused by the deformation of the core. A slight drift in SMs could cause a dramatic birefringence variation in hexagonal-core fiber, and the TSB could be as high as 10−4/100 °C, much higher than that of panda PM fiber. This study lays the foundation for the development of high birefringence temperature-stable HC-PBGFs and their applications in FOG.

Temperature-insensitive Quasi-distributed Fiber-optic Fabry-Perot High-pressure sensing based on microwave interference system

A temperature-insensitive quasi-distributed fiber-optic high-pressure sensing technique based on cascaded extrinsic Fabry-Perot (EFP) sensing units and an optical carrier based microwave interference system (OCMI) is proposed and demonstrated. The EFP unit formed by hollow-core photonic bandgap fiber has advantages of low-insertion loss and low temperature–pressure cross sensitivity, and the OCMI system based on a tunable single-wavelength light can simultaneously reconstruct optical interferograms of multiple EFP units. Experimental results show the optical interferograms reconstructions of three cascaded EFP units are well agreement with those using an optical spectrum analyzer. Moreover, the EFP unit has a wide measurement range of 0 ∼ 100 MPa and a low temperature–pressure cross-sensitivity of –0.02 MPa/℃, which can be comparable to that of a point fiber-optic pressure sensor. This technique provides a new idea of high-performance distributed pressure sensing.

Propagation Properties of Vortex Beams in a Helically Twisted Photonic Bandgap Fiber

We propose a helically twisted hollow-core photonic bandgap fiber (PBGF) for orbital angular momentum (OAM) generation. Results demonstrate that this fiber can effectively separate and transmit higher-order OAM modes while reducing crosstalk. The mode effective refractive index curves as a function of wavelength and the transmission characteristics of OAM are investigated in this letter. We show that this fiber has the potential to be used in OAM generators, light-controlling devices, and all-fiber optical communication applications.

Hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers.

A hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers (HC-PBFs) is theoretically investigated for the first time, to the best of our knowledge. Due to the topological effect, twisting of the fibers changes the effective refractive index and lifts the degeneracy of the photonic bandgap ranges of the cladding layers. This twist-induced hybrid photonic bandgap effect shifts up the center wavelength and narrows the bandwidth of the transmission spectrum. A quasi-single-mode low-loss transmission is achieved in the twisted 7-cell HC-PBFs with a twisting rate α = 7-8 rad/mm, which has a loss < 30 dB/km and higher-order mode extinction ratio > 15 dB. The twisted HC-PBFs could be suitable for applications such as spectral and mode filters.

Hybrid structure polarization-maintaining hollow-core photonic bandgap fiber with anti-resonant tubes and silicon layers.

A novel-hybrid structure polarization-maintaining 19-cell hollow-core photonic bandgap fiber (HC-PBGF) is proposed. Robust single-mode characteristic is achieved by introducing six anti-resonant tubes into the core of 19-cell HC-PBGF. A high birefringence at the level of 10-3 is achieved by adding silicon layers into the y-direction tubes. The higher-order mode extinction ratio (HOMER) is greater than 4.71 × 107, and the high birefringence can be improved to 5 × 10-3. In the waveband from 1530 nm to 1595 nm, the single-mode, high birefringence performance can be effectively maintained even under a tight bending radius of 5 mm.

Accurate Control of Surface Modes in a Hollow-Core Photonic Bandgap Fiber

Accurate Control of Surface Modes in a Hollow-Core Photonic Bandgap Fiber

Compact, Hybrid III-V/Silicon Vernier Laser Diode Operating From 1955–1992 nm

The 2 µm waveband is capable of enabling pervasive applications. The demonstration of the hollow-core photonic bandgap fiber and the thulium-doped fiber amplifier has highlighted the fiber propagation and amplification aspects of fiber communications, indicating its potential as an adjunct to present communication infrastructure at the O/C bands. The above is especially imperative given the current concerns with regards to the upper bandwidth limit of the single-mode fiber. Furthermore, the waveband could facilitate many more applications such as LIDAR and free-space communication. However, water absorption (OH-) is high at most of the 2 μm waveband and this will impact the optical insertion loss of applications implemented in the wavelength region. The relative low water absorption region of the waveband falls within 1950 – 2000 nm. As such, the development of a hybrid/heterogeneous III-V/silicon laser source that operates within the region is important for 2 µm silicon photonics. In this work, we demonstrate a III-V/Si hybrid tunable laser operating from 1955 - 1992 nm for the first time. Room temperature continuous wave operation is achieved with a maximum laser output power of 8.1 mW. This wavelength-tunable laser operates specifically within the low water absorption window, indicating good wavelength suitability for applications at the 2 μm waveband.

Investigation of longitudinal uniformity of the core structure in a hollow-core photonic bandgap fiber.

In this study, a low-noise Fabry-Perot interference-based method is promoted to measure the longitudinal uniformity of the distance between six pairs of opposite silica-air interfaces within the core of a seven-cell hollow-core photonic bandgap fiber. The experimental results demonstrate that the precision of the method is improved to the subnanometer scale. Based on the test results, a model is established to study the effect of the longitudinal uniformity of the core structure on the fiber loss, and the simulation results indicate that the fiber loss could reach ∼22.73 dB/km, which is consistent with the practical loss value.

Single-polarization single-mode hollow-core photonic-bandgap fiber with thin slab waveguide.

A novel single-polarization single-mode hollow-core photonic bandgap fiber with thin slab waveguide (TSW) was designed and simulated. Single-polarization guidance is achieved by the high loss of a polarized fundamental mode (FM) induced by mode coupling with a higher-order TE/TM mode of TSW and low loss of another polarized FM. We achieve a polarization loss ratio ∼ 46.9 dB, birefringence Δn ∼ 2.4 × 10-4, loss ∼ 35.9 dB/km and minimum higher-order mode extinction ratio > 15 dB. Moreover, the performance could be maintained when the guidance wavelength λ = 1.44 ∼ 1.56 µm and bending radius rc > 9 mm. The proposed model will be suitable for application as resonator sensing paths of miniaturized resonator fiber optic gyroscopes, high-performance interferometers, fiber lasers, frequency metrology, quantum communications, and laser-based gas sensing, etc.

Wideband, large mode field and single vector mode transmission in a 37-cell hollow-core photonic bandgap fiber

Stable generation and propagation of ultrafast high-order mode beams has become an important research direction. A core diameter not more than 10 times the wavelength is regarded as the upper limit for single mode transmission. However, a high-power laser requires a core diameter 20 to 40 times the wavelength to achieve high-power and stable output, which exceeds the design limit of the traditional fiber. In this paper, a novel 37-cell hollow core photonic bandgap fiber (HC-PBF) that only supports pure TE01 mode over a bandwidth of 50 nm with the lowest loss of 0.127 dB/km is proposed. The HC-PBF has a core diameter of more than 40 μm. Single mode guidance is achieved by adjusting the lattice size in a particular of the cladding. The best single mode performance with a loss ratio as high as 150,000 between TE01 mode and other modes with minimum loss is obtained. The fiber also has low bend-loss and thus can be coiled to a small bend radius of 1 cm having 1.6 dB/km bend loss. The tunability of the single-mode window and the manufacturing feasibility of the proposed fiber are also discussed.

High-Order Mode Characteristics of a 7-Cell Hollow-Core Photonic Bandgap Fiber

For many years, the number and losses of modes have restricted the large-scale application of hollow core photonic bandgap fibers (HC-PBFs) in data transmission. Researchers have successfully reduced fiber loss to an extremely low level; however, little attention has been given to the specific characteristics of the higher-order modes (HOMs) in HC-PBF. In this paper, the high-order mode characteristics of a 7-cell HC-PBF are investigated in detail, and the non-degenerate phenomenon of HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> is observed. Through an in-depth theoretical analysis by discussing the influence of the structural deformation of the fiber near the core on the high-order mode characteristics, the cause of the phenomenon is described. Thus, an optimized 7-cell HC-PBF for supporting 6 modes is developed. Additionally, the bending losses of all the HOMs in the fiber are measured. Marked bending loss is not observed even when the bending radius is as small as 0.5 cm, which is different from a solid core four-mode fiber. These findings will provide a reference for HC-PBF as a new type of few-mode fiber for the mode division multiplexing field.

Loading and spatially resolved characterization of a cold atomic ensemble inside a hollow-core fiber

We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially-resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1 \times 10^5$ atoms with a transfer efficiency of 2.1 % during the course of 50 ms and a peak loading rate of $4.7 \times 10^3$ atoms ms$^{-1}$ resulting in a peak atomic number density on the order of $10^{12}$ cm$^{-3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400$ $\mu$K, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.

Realization of low confinement loss acetylene gas sensor by using hollow-core photonic bandgap fiber

Realization of low confinement loss acetylene gas sensor by using hollow-core photonic bandgap fiber

Guiding Pure Vector Mode in Hollow Core Fiber Based on a Momentum Selection Theory

The attraction of hollow core fibers (HCF) lies in the long-distance mode retention ability and the transmission speed close to physical limit. Long-term efforts have been made to get a balance between large core and pure mode. However, the unique light guide mechanism of HCFs makes it a difficult question to decrease the number of the allowed modes. New theory of mode control in such fibers is desired. In this paper, we propose a new space for observing optical states based on the principle of representation transformation, so-called the wave vector space (k-space). The mode observation in the k-space provides a different perspective where the mechanical properties of the light are more obvious. We observe typical fiber modes in the k-space and clarify differences of their momentum characteristics, based on which a mode selection principle is proposed. As a theoretical proof of the principle, a novel 19-cell hollow-core photonic bandgap fiber (HC-PBGF) structure that only supports single pure vector mode is exhibited. The designed optical fiber supports only TE01 mode over a bandwidth of 80 nm with the lowest loss of 0.8 dB/km. This example proves the practicality of the proposed momentum selection theory and will inspire breakthroughs in other optical studies.

Crossings in photonic crystal fiber with hybrid core and design of broadband dispersion compensating photonic crystal fiber

A photonic crystal fiber (PCF) with sub-wavelength air inner core and Ge-doped concentric ring outer core is proposed. The full vector finite element method with perfectly matched layer is used for simulation. Mode crossing phenomenon which can be controlled through the hollow core diameter is investigated. A PCF with an effective compensated dispersion range of ±1.38 ps/nm/km from 1460 nm to 1637 nm wavelength is designed. The dispersion compensating PCF (DCPCF) can compensate for the dispersion and dispersion slope of a standard single mode fiber (SMF) at the same time. The kappa value matches very well with that of the commercial single mode fiber. Moreover, the figure of merits inside air core region of the proposed DCPCF is improved by 21.3 dB at 1550 nm wavelength compared to a hollow core photonic bandgap (PBG) fiber. This field enhancement can effectively reduce the material loss and the nonlinear accumulation in dispersion compensation fibers.