Fiber End Face Research Articles

Fiber-optic ultrasound generator using periodic gold nanopores fabricated by a focused ion beam

Ultrasound generation from an optical fiber, based on the photoacoustic principle, is a promising approach to many ultrasonic applications, specifically those requiring wide bandwidth and compact size in order to achieve high resolution as well as the capability of being operated in limited space. A fiber-optic ultrasound generator using gold nanopores is reported. The gold nanopores, having high absorption efficiency, were fabricated using a focused ion beam (FIB) on the fiber endface, which was excited by a nanosecond laser in order to generate ultrasound signals via the photoacoustic principle. Experimental results demonstrate that these wide bandwidth ultrasound signals can be generated by this compact fiber-optic ultrasound generator fabricated using a FIB.

Integration of large-area metallic nanohole arrays with multimode optical fibers for surface plasmon resonance sensing

Here, we report an implementation of enhanced surface plasmon resonance sensing on optical fibers through integration of large-area metallic nanohole arrays on the endfaces of multimode optical fibers by a template transfer method. The Au nanohole array integrated optical fiber sensors show a high refractive index sensitivity 559 nm per refractive index unit, narrow resonant peak width less than 10 nm in aqueous solution, and consistent optical responses with peak position variation less than 0.5 nm for various core diameters of optical fibers at same dielectric conditions. These specifications demonstrates that the nanohole array based optical fibers can serve as a high-performance, portable and lensless plasmonic platform with superior robustness and accessibility for surface plasmon resonance sensing.

Fiber-optic project-fringe interferometry with sinusoidal phase modulating system

A fiber-optic sinusoidal phase-modulating (SPM) interferometer for fringe projection is presented. The system is based on the SPM technique and makes use of the Mach–Zehnder interferometer structure and Young’s double pinhole interference principle to achieve interference fringe projection. A Michelson interferometer, which contains the detection of Fresnel reflection on its fiber end face and interference at one input port of a 3 dB coupler, is utilized to achieve feedback precise control of the fringe phase, which is sensitive to phase drifting produced by the nature of the fiber. The phase diversity for the closed-loop SPM system can be real-time measured with a precision of 3 mrad. External disturbances mainly caused by temperature fluctuations can be reduced to 57 mrad for the fringe map. The experimental results have shown the usefulness of the system.

Generation of a compact high-power high-efficiency normal-dispersion pumping supercontinuum in silica photonic crystal fiber pumped with a 1064-nm picosecond pulse

Broadband normal dispersion pumping supercontinuum (SC) generation in silica photonic crystal fiber (PCF) is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.

New Scheme of Hyperboloid Microlens for High-Average and High-Yield Coupling High-Power Lasers to Single-Mode Fibers

A new scheme of hyperboloid microlens (HM) employing automatic grinding and precise fusing techniques to achieve high-average and high-yield coupling efficiency from high-power 980-nm lasers into single mode fibers is proposed and demonstrated. The fiber endface of the HM exhibited a double-variable curvature in the major and minor axes that was characterized as a hyperboloid. By selecting half transverse length of the hyperbola and using fusing process to precise and quantitative controlling the required minor radius of curvature within 2.4-2.8 μ m and offset within 0.8 μ m, the HMs exhibited a high-average coupling efficiency of 83%. For fabricated high-coupling HMs, the result was also found that the tolerance of minor radius of curvature decreased as the offset increased. This study demonstrates that the proposed HMs through both automatic grinding and precise fusing techniques can achieve high-average and high-yield coupling efficiency better than any other grinding techniques to form asymmetric microlenses for utilizing in many low-cost lightwave interconnection applications.

Microcavity Fiber Fabry–Pérot Interferometer With an Embedded Golden Thin Film

We propose an ultracompact microcavity fiber Fabry-Pérot interferometer (MCFFPI) based on an ultrathin film of gold (Au) embedded in a single-mode fiber endface. The embedded Au thin film splits the input light into two paths for interference. The interference fringes of the proposed configuration with different cavity lengths are observed experimentally, and we verify that the interference patterns are from a combination of the Au-embedded reflector and Fresnel reflection of fiber endface. The MCFFPIs are further applied to measure the external refractive index (RI) of surrounding and for high temperature ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> ) sensing. Experimental results show that an excellent sensitivity of the RI sensing and a good linearity response of high temperature measurement are achieved.

NUMERICAL SIMULATION OF FIBER-OPTIC PHOTOACOUSTIC GENERATOR USING NANOCOMPOSITE MATERIAL

Photoacoustic generation using an optical fiber is a promising approach to meet the challenges in most advanced ultrasonic non-destructive testing (NDT) applications. The energy-absorption layer coated on the fiber endface plays an important role in converting the laser energy into heat and exciting the acoustic wave. The selection of the absorption material and the optimization of the structure dimension are the keys for achieving high energy-conversion efficiency. A 2D-axisymmetric finite element analysis (FEA) model was established to simulate the photoacoustic generation process using a newly developed absorption material, polydimethylsiloxane/gold nanoparticle (PDMS/Au NPs) nanocomposites, with different thicknesses and laser pulse durations. The experimental results obtained using a similar set-up show a similar trend predicted by this FEA simulation. The FEA results provide practical clues to the design of the fiber-optic photoacoustic generator.

Novel field installable optical fiber splice and connector for optical access networks

We have developed two types of field installable connection techniques. One is mechanical splicing, which is used to connect coated optical fibers without the need for stripping or cleaning procedures. The other is a field assembly connection technique, which employs a new type of field installable connector that makes it possible to realize a physical contact connection with chamfer grinding. Mechanical splicing is achieved by precisely aligning and directly connecting coated fibers with a capillary. The assembled splice is installed with 1.3-μm single-mode fibers that have an 80-μm cladding and a 125-μm coating and they exhibit good optical performance with a low average insertion loss of 0.2dB. Connection is achieved with our developed field installable connector by using a chamfered fiber endface and the compression force of the buckled fiber. The assembled connectors achieve physical contact with the chamfered fiber endface, which provides good optical performance with a low insertion loss of 0.11dB.

Generation of harmonics and supercontinuum in nematic liquid crystals

Nonlinear optical properties of nematic liquid crystals (NLC) have been investigated. A technique for efficient laser frequency conversion in a microscopic NLC volume deposited on an optical fibre end face is experimentally demonstrated. An efficient design of a compact NLC-based IR frequency converter with a fibre input and achromatic collimator is proposed and implemented. Simultaneous generation of the second and third harmonics is obtained for the first time under pumping NLC by a 1.56-mm femtosecond fibre laser. The second-harmonic generation efficiency is measured to be about 1 %, while the efficiency of third-harmonic generation is several tenths of percent. A strong polarisation dependence of the third-harmonic generation efficiency is revealed. When pumping NLC by a cw laser, generation of spectral supercontinua (covering the visible and near-IR spectral ranges) is observed. The nonlinear effects revealed can be due to the light-induced change in the orientational order in liquid crystals, which breaks the initial symmetry and leads to formation of disclination structures. The NLC optical nonlinearity is believed to be of mixed orientationalelectronicnature as a whole.

Subwavelength optical trapping with a fiber-based surface plasmonic lens

We demonstrate three-dimensional (3D) optical trapping of subwavelength polystyrene beads and bacteria with a surface plasmonic lens fabricated on the endface of an optical fiber. To the best of our knowledge, this is the first demonstration of 3D trapping of subwavelength particles with single fiber optical tweezers. The optical power for achieving a stable 3D trap is smaller compared with conventional optical tweezers, indicating a stronger trap. Compared with surface plasmon tweezers, the trap enabled by our fiber tweezers is located ≈6 wavelengths away from the fiber endface, reducing thermal effects due to the metal absorption and preventing physical contact with the trapped objects.

Ice type detection using an oblique end-face fibre-optic technique

This paper reports on a novel, compound fibre-optic ice sensor consisting of two source fibre bundles and two signal ones. All the bundles had oblique fibre end-faces that were not perpendicular to the fibre axis. Dynamically icing experiments in an icing wind tunnel showed that with increasing thickness of ice accreted on the sensor, one group, comprising the first source and signal fibre bundles, output decreasing signal for glazed ice, increasing signal for rime ice, and constant signal for mixed ice, while the other group, comprising the second source and signal bundles, output constant signal for rime ice and decreasing signal for glazed or mixed ice. The different ice-growth curves are caused by different optical intensity distributions in the fibres for different ice types and thus can be used to accurately measure the ice type and thickness. This sensor is competitive in ice detection, especially identification of ice type in the aviation industry.

The Effect of the Broken Edge on the Concentricity of the Micro-Polished Optical Fiber End-Face

The concentricity of the optical fiber micro lens affects the coupling efficiency of a laser module a lot. In this paper, the effect of the perpendicularity of the broken edge on the concentricity of the micro optical fiber end-face during the micro polishing process is studied. A mathematical model is derived to describe the variation of the material removal rate (MRR) when the tilt angles and amount of feed of the optical fiber are changed. An experiment is built to measure the concentricity of optical fiber with different tilt angles and feeds. The result shows that both of the dimensionless factor QAB and measured concentricity have the same tendency. The concentricity is better when the tilt angle is smaller and the amount of feed is larger. An improved polishing process is suggested. A good concentricity within 1μm can be achieved when fabricate the double-variable curvatures end-face of the optical fiber.

Rapid fabrication of micro-nanometric tapered fiber lens and characterization by a novel scanning optical microscope with submicron resolution

In numerous applications of optical scanning microscopy, a reference tapered fiber lens with high symmetry at sub-wavelength scale remains a challenge. Here, we demonstrate the ability to manufacture it with a wide range of geometry control, either for the length from several hundred nanometers to several hundred microns, or for the curvature radius from several tens of nanometers to several microns on the endface of a single mode fiber. On this basis, a scanning optical microscope has been developed, which allows for fast characterization of various sub-wavelength tapered fiber lenses. Focal position and depth of microlenses with different geometries have been determined to be ranged from several hundreds of nanometers to several microns. FDTD calculations are consistent with experimental results.

Fabry-Perot Vapor Microsensors Fabricated onto Fibre Endface by Multiphoton Polymerization Technique

We report on the fabrication of two different Fabry-Perot optical resonators on the endface of a SMF28 fibre, by employing Direct Laser Writing by multi-photon polymerization [1, 2]. We explore these endface fibre sensing probes for tracing common organic solvents [3]. The devices developed have been optimised for operation at the spectral region lying between 1415nm to1655nm, while being interrogated in reflection mode using a simple 50/50 fibre coupler. The laser used was a Ti:sapphire unit emitting at 800nm, 200fs pulses at a repetition rate of 50–80 MHz. The beam projection system was based on an inverse microscope; beam steering was performed using a x–y galvanometric mirror scanning the beam through a high magnification objective. Lateral resolution was of the order of 250nm [1]. The material that used for the fabrication of the endface microstructures is a zirconium-silicon, organic-inorganic hybrid photosensitive material [4]. The first microstructure consists of a thin, flat membrane suspended on four pillars attached onto the silica fibre endface (Fig. 1a). This architecture allows the formation of a small air cavity between the end face of the fibre and the suspended thin membrane, which acts as a multilayer Fabry-Perot resonator, providing interrogation capabilities of the media inside the air cavity section; or attached on the membrane endface [5]. Liquid or gaseous media can be trapped between the empty gap of the microdrum resonator, detecting refractive index or absorption changes in reflection mode. The specific cavity demonstrated herein consists of a 14.4um air gap formed between the fibre endface and the photopolymerised structure, while the membrane thickness is 10.4um , resulting in the periodic modulation of the reflected optical spectrum by notches of ~29dB in amplitude strength with a free spectral range of ~87nm.

Optical coatings on f iber for relative-humidity sensing applications

A fiber-optic relative-humidity (RH) sensor composed of multilayer of porous dielectric films is proposed. The transducer deposited on fiber end-face is multilayer coating consisting of nano-porous TiO2 and SiO2 films, which forms a low-fineness Fabry-Perot (F-P) filter with one of minimum reflections at about 1 350-nm wavelength. The dielectric thin films realized by e-beam evaporation without ion-source assistance have columnar and porous structures, which exhibit sensitivity to RH change of environments. When the sensor is exposed to an environment of RH change from 10.9% to 92.8%, experimental results demonstrate 77.9-nm shift of characteristic wavelength.

Filmed extrinsic Fabry–Perot interferometric sensors for the measurement of arbitrary refractive index of liquid

Abstract A filmed extrinsic Fabry–Perot interferometric (FEFPI) sensor for the measurement of arbitrary refractive index (RI) is experimentally demonstrated. The FEFPI sensor was constructed by aligning two fiber endfaces, on which the multilayer dielectric mirrors are deposited. There is a fixed reflection at the filmed fiber endface no matter how much the RI is, and we can obtain a clear interferogram with a high visibility. A resolution of up to 1.64 × 10 −5 refractive index unit is achieved in our experiment.

Physical Contact Connection Between SC Connector and Raw Cleaved SM-Fiber End

We achieve a physical contact (PC) connection by using simple butt jointing between an SC connector and a raw cleaved single-mode (SM) fiber end without index matching material. We show that there are two requirements for a cleaved fiber endface for PC connection. One is that the fiber endface inclination is less than 0.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , and the other is that there is no mist on the endface. The cleaving condition is also discussed to allow us to consistently obtain an appropriate endface. A crack depth of less than 5 μm and a bending radius of 100 mm before breaking are the optimum values in the conventional fiber cleaving process. The average insertion and return losses are 0.14 and 51.1 dB, respectively, and these values are similar to those for a conventional SC connector.

Fabrication of micro-optical devices at the end of a multimode optical fiber with negative tone lift-off EBL

A fabrication method based on negative tone lift off EBL is developed for constructing nano-structures at end faces of multimode optical fibers. With this new approach, precise and robust nano-structures with high spatial resolutions can be fabricated with minimum damage to the optical fiber face during the fabrication process. Based on this approach, high numerical aperture micro Fresnel zone plates (MZP) with focal lengths ∼3 µm were fabricated on the face of an optical fiber. The focusing characteristics of the fabricated MZP showed good consistency with the numerical simulations at the specified wavelength (∼405 nm).

1:61 photonic lanterns for astrophotometry: a performance study

In this paper, we study how the spectral information is transmitted through a 1:61 photonic lantern (PL) when illuminating the multimode (MM) end with a point source. We found that the spectrum undergoes two major effects between the MM input and the independent single-mode (SM) outputs of the PL. First, at resolution R = 775 (at 1550 nm), the overall output spectrum envelope strongly differs from SM to SM outputs. The spectra measured at the 61 SM outputs also vary when the small point object (10 μm in diameter, compared with the 100 μm diameter MM core) is moved across the MM fibre end-face of the PL. However, the sum of all the spectra measured at the SM outputs is very close to the spectrum of the point source, and is insensitive to the position of the point source. Secondly, at resolution R = 77 500 a fine 150 pm periodic structure was observed at the SM outputs. We found that a PL in the MM–SM configuration (i.e. MM input/SM outputs) is a very stable system regarding variations of the transmitted spectra when it is left mechanically non-perturbated and only submitted to environment-related changes, compared to a conventional step-index MM fibre of same core diameter. The latter clearly shows characteristic modal noise. Furthermore, we showed that at resolution R = 77 500 the sum of the spectra measured at 61 the SM outputs is very close to the spectrum of the point source, and does not present the 150 pm periodic oscillations. We also studied two PLs spliced together in the MM–SM–MM configuration and the results showed that the spliced PLs perform very well as mode scramblers, especially under conditions where conventional step-index MM fibres usually fail at performing effective scrambling.

Three-Dimensional Shape Reconstruction System Based on Fiber-Optic Interference Fringe Imaging

In this article, we describe a three-dimensional shape reconstruction system based on the Mach–Zehnder interferometer structure and Young's double pinhole interference principle, while utilizing Fresnel reflection on fiber end face and interference at the fourth port of 3-dB coupler to achieve closed-loop precise control of fringe phase. A root-mean-square phase stability of 4 mrad is measured with the system. The shape of the object is determined by analyzing the fringe pattern. A new algorithm called rotating rectangular window autoselection is used as the band-pass filter. The measuring time of the whole system is less than 200 ms, and the error of system is 0.27 mm. Meanwhile, the overall complexity of the measuring algorithm is O(nlogn). © 2012 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 22, 208–213, 2012 © 2012 Wiley Periodicals, Inc.