Air-core Photonic Bandgap Fiber Research Articles

Two photon imaging probe with highly efficient autofluorescence collection at high scattering and deep imaging conditions

In this paper, we present a 2-photon imaging probe system featuring a novel fluorescence collection method with improved and reliable efficiency. The system aims to miniaturize the potential of 2-photon imaging in the metabolic and morphological characterization of cervical tissue at sub-micron resolution over large imaging depths into a flexible and clinically viable platform towards the early detection of cancers. Clinical implementation of such a probe system is challenging due to inherently low levels of autofluorescence, particularly when imaging deep in highly scattering tissues. For an efficient collection of fluorescence signals, our probe employs 12 0.5 NA collection fibers arranged around a miniaturized excitation objective. By bending and terminating a multitude of collection fibers at a specific angle, we increase collection area and directivity significantly. Positioning of these fibers allows the collection of fluorescence photons scattered away from their ballistic trajectory multiple times, which offers a system collection efficiency of 4%, which is 55% of what our bench-top microscope with 0.75 NA objective achieves. We demonstrate that the collection efficiency is largely maintained even at high scattering conditions and high imaging depths. Radial symmetry of arrangement maintains uniformity of collection efficiency across the whole FOV. Additionally, our probe can image at different tissue depths via axial actuation by a dc servo motor, allowing depth dependent tissue characterization. We designed our probe to perform imaging at 775 nm, targeting 2-photon autofluorescence from NAD(P)H and FAD molecules, which are often used in metabolic tissue characterization. An air core photonic bandgap fiber delivers laser pulses of 100 fs duration to the sample. A miniaturized objective designed with commercially available lenses of 3 mm diameter focuses the laser beam on tissue, attaining lateral and axial imaging resolutions of 0.66 µm and 4.65 µm, respectively. Characterization results verify that our probe achieves collection efficiency comparable to our optimized bench-top 2-photon imaging microscope, minimally affected by imaging depth and radial positioning. We validate autofluorescence imaging capability with excised porcine vocal fold tissue samples. Images with 120 µm FOV and 0.33 µm pixel sizes collected at 2 fps confirm that the 300 µm imaging depth was achieved.

Molecular dispersion spectroscopy in a CO2-filled all-fiber gas cells based on a hollow-core photonic crystal fiber

CO2 sensing using self-fabricated all-fiber gas cells based on a hollow-core photonic crystal fiber and chirped laser dispersion spectroscopy (CLaDS) is presented. I show that CLaDS enables significant reduction of optical fringes arising from the mutlimode nature of the air-core photonic bandgap (PBG) fiber, hence improving the gas sensing system’s stability and performance. By carefully selecting modulation frequency, it is possible to match the period of interference signal (resulting from coupling between fiber’s guided modes) and, therefore, suppress its contribution to the retrieved signal. CLaDS enables successful use of hollow-core PBG fibers in gas sensing applications.

Reduced thermal sensitivity of hybrid air-core photonic band-gap fiber ring resonator

A novel hybrid air-core photonic band-gap fiber (PBF) ring resonator with twin 90° polarization-axis rotated splices is proposed and demonstrated. Frist, we measure the temperature dependent birefringence coefficient of air-core PBF and Panda fiber. Experimental results show that the relative temperature dependent birefringence coefficient of air-core PBF is 1.42×10-8/°C, which is typically ~16 times less than that of Panda fiber. Then, we extract the geometry profile of air-core PBF from scanning electron microscope (SEM) images. Numerical modal is built to distinguish the fast axis and slow axis in the fiber. By precisely setting the length difference in air-core PBF and Panda fiber between two 90° polarization-axis rotated splicing points, the hybrid air-core PBF ring resonator is constructed, and the finesse of the resonator is 8.4. Environmental birefringence variation induced by temperature change can be well compensated, and experimental results show an 18-fold reduction in thermal sensitivity, compared with resonator with twin 0° polarization-axis rotated splices.

Analysis of the errors in free space coupling setup used in an air-core photonic bandgap fiber ring resonator for rotation sensing

The fiber ring resonator is the key component of resonator fiber optic gyros. The configuration of an air-core photonic bandgap fiber (air-core PBF) ring resonator is proposed based on a kind of free space coupling setup (FSCS). The Mathematical model of such kind of ring resonator is derived. With relevant model derivation, the effects of different parameters, including fiber coupling efficiency, alignment of fiber polarization axis and backscattering of fiber end faces, on the performance of ring resonator are simulated. It is clarified that the performance of air-core PBF ring resonator can be greatly influenced by radial linear alignment of air-core PBFs, angular alignment of air-core PBFs, the difference of incident polarization angles and the angle of air-core PBF end faces. According to the simulation, the theoretical model of the FSCS has been established. Furthermore, the solutions to three errors in the FSCS have been advanced, respectively.

Measurement of the Verdet Constant of Polarization-Maintaining Air-Core Photonic Bandgap Fiber.

We propose a method based on the white-light interference technique for measuring the Verdet constant of a polarization-maintaining air-core photonic bandgap fiber (PM-PBF). The experimental results show that the Verdet constant of the PM-PBF is ~3.3 mrad/T/m for the broadband light with a spectral width of ~38 nm and a mean wavelength of ~1550 nm, which is ~124 times less than that of a conventional stress-induced birefringent fibers called PANDA fibers (~0.41 rad/T/m for the same broad-spectrum light). The results indicate that the nonreciprocal error induced by the Faraday effect in a fiber optic gyroscope (FOG) made of the PM-PBF is theoretically ~25 times less than that of a conventional FOG made of the PANDA fiber when other conditions, such as the fiber twist, fiber coil area, and so on, are the same.

Experimental investigation of backreflection at air-core photonic bandgap fiber terminations

Backreflection from the termination of air-core photonic bandgap fibers (PBFs) is experimentally investigated based on a range-extended Mach-Zehnder and Michelson hybrid (M2) interferometer. For primary waves generated by the fundamental modes, the reflectivity is about −90dB; for secondary waves caused by other modes, the reflectivity is less than −80dB when compared to the intensity of the primary wave and −20 to −50dB when compared to their own incident intensity. To suppress the reflection, ~3-centimeter PBF at the end is filled with alcohol through the capillary effect, and this proposed method is shown to be much more convenient and effective than the conventional angle cleaving method.

Measurement of the reflection and loss of the hybrid air-core photonic-bandgap fiber ring resonator.

A fiber ring resonator is the key element in a resonant fiber optic gyroscope (RFOG). Both reflection and loss characteristics can severely decrease the accuracy of the RFOG. This paper first implements optical frequency domain reflectometry (OFDR) to measure the reflection and loss coefficients of the hybrid air-core photonic-bandgap fiber (PBF) ring resonator. Compared with the traditional measurement method of the resonant curve, OFDR can clearly distinguish the two junctions between the air-core PBF and the solid-core fiber. The measured reflection coefficients at the two splicing points are 1.77% and 2.65%, respectively. The excess losses are 2.28dB and 3.22dB, respectively. Thus, the measurement of the junctions in the fiber ring resonator is realized.

Measurement and suppression of secondary waves caused by high-order modes in a photonic bandgap fiber-optic gyroscope.

Air-core photonic bandgap fiber (PBF) is a good choice for fiber-optic gyroscopes (FOGs) owing to the fact that it can be adapted to a wide variety of environments. However, its multimode properties are disadvantageous for the application to FOGs. An interference-based method is proposed to precisely determine the secondary waves caused by the high-order modes and their coupling. Based on the method, two groups of secondary waves have been found, having optical path differences (OPDs) of ~1.859 m and ~0.85 m, respectively, relative to the primary waves in a PBFOG that consists of a 7-cell PBF coil, approximately 180 m in length. Multi-turn bends of the PBF at both ends of the PBF coil after the fusion splicing points are shown to suppress the intensity of these secondary waves by approximately 10 dB.

Reflection-induced bias error in an air-core photonic bandgap fiber optic gyroscope.

Analysis of the bias error induced by reflections in an air-core photonic bandgap fiber gyroscope is performed by both simulation and experiment. The bias error is sinusoidally periodic under modulation, and its intensity is related to the relative positions of the reflection points. A simple and effective method for the suppression of the error is proposed, and it has been verified experimentally.

An investigation of polarization cross-coupling in air-core photonic bandgap fibers

Polarization cross-coupling is one of the most important problems in air-core photonic bandgap fibers (PBF). In this research, polarization cross-coupling is investigated for PBFs of different lengths. The analyzing and simulation results show that the orientation of the birefringent axes induced by residual core ellipticity fluctuates with an average period of ~2.5cm and random angles uniformly distributed over approximately [−7.5°, 7.5°]. The birefringent orientation in PBF varies much more frequently and strongly than that in any conventional fiber because of the difference in drawing process, and this is the most important factor causing the strong polarization cross-coupling in PBFs.

Temperature dependence of beat-length and confinement loss in an air-core photonic band-gap fiber

The temperature dependence of polarization-maintaining (PM) property and loss in a highly-birefringent air-core photonic band-gap fiber (PBF) is investigated. The effects of temperature variation on the effective index, beat-length and confinement loss are studied numerically by using the full-vector finite element method (FEM). It is found that, the PM property of this PBF is insensitive to the temperature, and the temperature-dependent beat-length coefficient can be as low as 2.86×10−8m/°C, which is typically 200 times less than those of conventional panda fibers, the PBF has a stable confinement loss of 0.01dB/m over the temperature range of −30 to 20°C for the slow axis at the wavelength of 1.55μm. The PBF with ultra-low temperature-dependent PM property and low loss can reduce the thermally induced polarization instability apparently in interferometric applications such as resonant fiber optic gyroscope (RFOG), optical fiber sensors, and so on.

Comparative analysis of fiber ring resonator based on different fibers

Abstract A fiber ring resonator (FRR) is the core element in a resonant fiber optic gyro (RFOG) and its resonant depth, finesse and sensitivity determine the performance of RFOG. A configuration of FRR based on air-core photonic bandgap fiber (PBF), photonic crystal fiber (PCF) and panda polarization-maintaining fiber (PMF) is proposed, and the resonant depth, finesse and sensitivity of the FRR are comparative analyzed fully and in depth. It's found that: the FRR using PCF and PBF have better resonant depth than that of using PMF; the FRR using PCF has nearly the same performance as that using PMF, and may be a trade-off used in FRR before the existence of better PBF; the key factor that whether the FRR resonates in optimal resonant state is whether αc and kc of directional coupler satisfy the optimal resonant condition. These theoretical results are helpful to further study and optimize the RFOG based on different fibers.

Effect of Fresnel reflections in a hybrid air-core photonic-bandgap fiber ring-resonator gyro.

A novel hybrid polarization-maintaining (PM) air-core photonic bandgap fiber (PBF) ring resonator is firstly demonstrated by using a conventional solid-core PM fiber optical coupler formed by splicing a section of PM air-core PBF into the resonator. Due to Fresnel reflections exist at the two junctions between the air-core PBF and the solid-core fiber, the forward output signal of this hybrid ring resonator is the normal resonant curve with the superposition of the lightwaves that experienced even numbers of Fresnel reflections and the backward output signal is composed of lightwaves that experienced odd numbers of Fresnel reflections. Rigorous derivations of the forward and backward output signals are given out. The biggest resonant depth and finesse of the hybrid air-core PBF ring resonator predicted are 0.352 and 6.3 respectively by assuming a splice loss of 1.8 dB per junction. These predictions are finally confirmed by testing both the forward and backward output signals of the hybrid ring resonator. With the countermeasures against the influences of the odd numbers of Fresnel reflections, a bias stability of 0.007°/s is successfully demonstrated in a hybrid PM air-core PBF ring-resonator gyro.

Gas Pressure Sensor Based on CO <sub>2</sub>-Laser-Induced Long-Period Fiber Grating in Air-Core Photonic Bandgap Fiber

We reported a gas pressure sensor based on CO 2 -laser-induced long-period fiber grating (LPFG) in an air-core photonic bandgap fiber (PBF). The LPFG was inscribed in an air-core PBF by the use of an improved CO 2 laser system with an ultraprecision 2-D scanning technique, which induced periodic collapses of air holes along the axis of the PBF. Such an LPFG could be used to develop a promising gas pressure sensor with a sensitivity of -137 pm/MPa. Moreover, a simplified fiber model with a relatively similar elastic response was developed to qualitatively study the gas pressure response of the LPFG inscribed in the air-core PBF. A simulated stress distribution along the LPFG revealed that the gas pressure leads to a stress concentration at the collapsed area of the air holes in the fiber cladding, which finally results in a resonant wavelength shift of the LPFG through an elastooptical effect.

Broad-band fluorescent all-fiber source based on microstructured optical fibers

The present work demonstrates a miniature all-fiber optic fluorescence source on the basis of organic dyes. A capillary of fused silica has been used to keep the Rhodamine 6G dissolved in glycerine at one end of which a tapered optical fiber is inserted to receive the useful signal. The fluorescence medium is side excited with a CW laser radiation at 532 nm by a fiber taper. The functioning of the system is demonstrated using a conventional optical fiber, a photonic crystal fiber and a hollow-core fiber. Full Text: PDF References S. James, R. Tatam, "Optical fibre long-period grating sensors: characteristics and application", Meas. Sci. Technol. 14(5), R49, (2003). CrossRef Y.P. Wang, L. Xiao, D.Wang, W. Jin, "Highly sensitive long-period fiber-grating strain sensor with low temperature sensitivity", Opt. Lett. 31(23), 3414, (2006). CrossRef Y. Wang, W. Jin, J. Ju, H. Xuan, H. Ho, L. Xiao, D. Wang, "Long period gratings in air-core photonic bandgap fibers", Opt. Express 16(4), 2784, (2008). CrossRef L. Rindorf, O. Bang, "Highly sensitive refractometer with a photonic-crystal-fiber long-period grating", Opt. Lett. 33(6), 563, (2008). CrossRef G. Durana, J. Gomez, G. Aldabaldetreku, J. Zubia, A. Montero, I. Saez de Ocariz, "Assessment of an LPG mPOF for Strain Sensing", IEEE Sens. J. 12(8), 2668, (2012). CrossRef D.Vezenov, B.Mayers, D.Wolfe, G.Whitesides, "Integrated fluorescent light source for optofluidic applications", App. Phys. Lett. 86(4), 041104, (2005). CrossRef B.Mayers, D.Vezenov, V.Vullev, G.Whitesides, "Arrays and Cascades of Fluorescent Liquid?Liquid Waveguides: Broadband Light Sources for Spectroscopy in Microchannels", Anal. Chem. 77(5), 1310, (2005). CrossRef J.M. Lim, S-H. Kim, J-H. Choi, S.-M. Yang, "Fluorescent liquid-core/air-cladding waveguides towards integrated optofluidic light sources", Lab Chip 8(9), 1580, (2008) CrossRef V. Vladev, T. Eftimov, Union of Scientists in Bulgaria-Plovdiv, 16, 73, (2014).

Improved gas sensor with air-core photonic bandgap fiber

The propagation loss of a fiber can be increased by coupling core mode and surface mode which will deteriorate the performance of photonic bandgap fiber (PBGF). In this paper, we presented an aircore PBGF for gas sensing applications. By designing Λ = 2.63 µm, d = 0.95 Λ, and Rcore= 1.13 Λ, where Λ is the distance between the adjacent air holes, the fiber was single-mode, no surface mode was supported with fiber, and more than 90% of the optical power was confined in the core. Furthermore, with optimizing the fiber structural parameters, at wavelength of λ = 1.55 µm that is in acetylene gas absorption line, significant relative sensitivity of 92.5%, and acceptable confinement loss of 0.09 dB/m, were simultaneously achieved.

空芯光子晶体光纤谐振腔的设计与优化

空芯光子晶体光纤谐振腔的设计与优化

An investigation of numerical aperture of air-core photonic bandgap fiber

Air-core photonic bandgap fiber (PBF) is the perfect choice of the next-generation fiber optical gyroscope (FOG), with excellent temperature, electromagnetism and radiation adaptability. Numerical aperture is an important optical parameter of PBF for application in FOG. The PBF’s maximum theoretical numerical aperture (NAmax) is calculated and compared with the far-field numerical aperture (NAeff) through experiments. The result indicates that the relationship between NAmax and NAeff has much stronger dependence on wavelength than that of the conventional fiber, and they get close at wavelengths near the middle of the photonic bandgap with the error less than 5%. Furthermore, photonic bandgap fiber optical gyroscope (PBFOG) with no fusion splicing points is proposed, and the optimization method and results of the PBF’s structure parameters for application in PBFOG are given from the aspect of numerical aperture.

Investigation of residual core ellipticity induced nonreciprocity in air-core photonic bandgap fiber optical gyroscope

Air-core photonic bandgap fiber (PBF) is an excellent choice for fiber optic gyroscope owing to its incomparable adaptability of environment. Strong and continuous polarization mode coupling is found in PBFs with an average intensity of ~-30 dB, but the coupling arrives at the limit when the maximum optical path difference between the primary waves and the polarization-mode-coupling-induced secondary waves reaches ~10mm, which is corresponding to the PBF length of ~110 m according to the birefringence in the PBF. Incident light with the low extinction ratio (ER) can suppress the birth of the polarization-mode-coupling-induced secondary waves, but the low-ER light obtained by the conventional Lyot depolarizers does not work here. Consequently, a large nonreciprocity and a bias error of ~13°/h are caused in the air-core photonic bandgap fiber optical gyroscope (PBFOG) with a PBF coil of ~268 m.

A 5-mm piezo-scanning fiber device for high speed ultrafast laser microsurgery.

Towards developing precise microsurgery tools for the clinic, we previously developed image-guided miniaturized devices using low repetition rate amplified ultrafast lasers for surgery. To improve the speed of tissue removal while reducing device diameter, here we present a new 5-mm diameter device that delivers high-repetition rate laser pulses for high speed ultrafast laser microsurgery. The device consists of an air-core photonic bandgap fiber (PBF) for the delivery of high energy pulses, a piezoelectric tube actuator for fiber scanning, and two aspheric lenses for focusing the light. Its inline optical architecture provides easy alignment and substantial size reduction to 5 mm diameter as compared to our previous MEMS-scanning devices while realizing improved intensity squared (two-photon) lateral and axial resolutions of 1.16 μm and 11.46 μm, respectively. Our study also sheds light on the maximum pulse energies that can be delivered through the air-core PBF and identifies cladding damage at the input facet of the fiber as the limiting factor. We have achieved a maximum energy delivery larger than 700 nJ at 92% coupling efficiency. An in depth analysis reveals how this value is greatly affected by possible slight misalignments of the beam during coupling and the measured small beam pointing fluctuations. In the absence of these imperfections, self-phase modulation becomes the limiting factor for the maximum energy delivery, setting the theoretical upper bound to near 2 μJ for a 1-m long, 7-μm, air-core PBF. Finally, the use of a 300 kHz repetition rate fiber laser enabled rapid ablation of 150 µm x 150 µm area within only 50 ms. Such ablation speeds can now allow the surgeons to translate the surgery device as fast as ~4 mm/s to continuously remove a thin layer of a 150 µm wide tissue. Thanks to a high optical transmission efficiency of the in-line optical architecture of the device and improved resolution, we could successfully perform ablation of scarred cheek pouch tissue, drilling through a thin slice. With further development, this device can serve as a precise and high speed ultrafast laser scalpel in the clinic.