Fiber End Face Research Articles

Optofluidic microvalve-on-a-chip with a surface plasmon-enhanced fiber optic microheater.

We present an optofluidic microvalve utilizing an embedded, surface plasmon-enhanced fiber optic microheater. The fiber optic microheater is formed by depositing a titanium thin film on the roughened end-face of a silica optical fiber that serves as a waveguide to deliver laser light to the titanium film. The nanoscale roughness at the titanium-silica interface enables strong light absorption enhancement in the titanium film through excitation of localized surface plasmons as well as facilitates bubble nucleation. Our experimental results show that due to the unique design of the fiber optic heater, the threshold laser power required to generate a bubble is greatly reduced and the bubble growth rate is significantly increased. By using the microvalve, stable vapor bubble generation in the microchannel is demonstrated, which does not require complex optical focusing and alignment. The generated vapor bubble is shown to successfully block a liquid flow channel with a size of 125 μm × 125 μm and a flow rate of ∼10 μl/min at ∼120 mW laser power.

Tunable Characteristics and Mechanism Analysis of the Magnetic Fluid Refractive Index With Applied Magnetic Field

The tunable refractive index of the magnetic fluid (MF) is a unique optical property, which has attracted a lot of research interest in recent years. In this paper, a method based on the Fresnel reflection at the fiber end face is presented. Experimental measurements are carried out to investigate the magnetic field (intensity and direction) and temperature-dependent refractive index of the MF. For a given concentration, with the increase of the magnetic field intensity, the nMF increases gradually when H//Light, while decreases when H⊥Light. The effect of temperature on nMF is relatively insignificant with the sensitivity of -8 × 10 -5 /°C. In addition, the mechanism is analyzed from the point of the microstructure by the Monte Carlo method.

Intrinsic Fabry-Pérot Interferometer Based on Concave Well on Fiber End

miniature fiber-optic Fabry-Perot cavity interferometerA miniature fiber-optic Fabry-Perot cavity interferometer is presented. The interferometer is fabricated at the fiber tip by wet chemical etching using hydrofluoric acid. A concave well is created on the fiber end face. Interference happens between the reflections from the fiber core and the end of the cladding. We demonstrate applications of the interferometric sensor for temperature and gas refractive index sensing.

Comparison of Silica and Sapphire Fiber SERS Probes Fabricated by a Femtosecond Laser

Different types of fibers were compared for construction of reflection-based surface-enhanced Raman-scattering (SERS) fiber probes. The probes were made by direct femtosecond (fs) laser micromachining of nanometer structures on the fiber endface and subsequent chemical plating of a thin layer of silver. Rhodamine 6G solutions were used to evaluate the performance of the SERS probes. In comparison with the silica fibers, the single-crystal sapphire fiber has much lower background Raman scattering. The fs laser is found effective to fabricate high-quality sapphire fiber SERS probes for detection of weak Raman signals in a reflection configuration.

A daylight experiment for teaching stellar interferometry

We discuss the design of a simple experiment that reproduces the operation of the Michelson stellar interferometer. The emission of stellar sources has been simulated using light emerging from circular end-faces of step-index polymer optical fibers and from diffuse reflections of laser beams. Interference fringes have been acquired using a digital camera, coupled to a telescope obscured by a double aperture lid. The experiment is analogous to the classical determination of stellar sizes by Michelson and can be used during the day. Using this experimental set-up, we can determine the size of extended sources, located at a distance of about 75 m from our telescope, with errors less than 25%.

A plasmonic optical fiber patterned by template transfer as a high-performance flexible nanoprobe for real-time biosensing.

Surface plasmon resonance (SPR) on metal nanostructures offers a promising route for manipulation and interrogation of light in the subwavelength regime. However, the utility of SPR structures is largely limited by currently used complex nanofabrication methods and relatively sophisticated optical components. Here to relieve these restrictions, plasmonic optical fibers are constructed by transferring periodic metal nanostructures from patterned templates onto endfaces of optical fibers using an epoxy adhesive. Patterned metal structures are generally extended from two-dimensional (2D) nanohole arrays to one-dimensional (1D) nanoslit arrays. By controlling the viscosity of the adhesive layer, diverse surface topographies of metal structures are realized with the same template. We design a special plasmonic fiber that simultaneously implements multimode refractive index sensing (transmission and reflection) with remarkably narrow linewidth (6.6 nm) and high figure of merit (60.7), which are both among the best reported values for SPR sensors. We further demonstrate a real-time immunoassay relying on our plasmonic fiber integrated with a special flow cell. Plasmonic optical fibers also take advantages of excellent stability during fiber bending and capability of spectrum filtering. These features enable our plasmonic fibers to open up an alternative avenue for the general community in biosensing and nanoplasmonics.

Dielectric multilayer-based fiber optic sensor enabling simultaneous measurement of humidity and temperature.

A multilayer-based fiber optic sensor enabling simultaneous measurement of humidity and temperature is proposed and demonstrated. The sensitive elements were multilayer coatings consisting of nano-porous TiO(2) and SiO(2) films, which were deposited on fiber end-face to form a Fabry-Perot (F-P) filter structure. Relative-humidity (RH) sensing is correlated with the shift of interference fringe due to the change of effective refractive index of porous coatings when exposed to different RH environments. The sensor is sealed in a glass tube in case of temperature measurement. Experimental results show that the average sensitivity are 0.43nm/%RH and 0.63nm/°C respectively when environmental RH changes from 1.8%RH to 74.7%RH and temperature changes from 21.4°C to 38.8°C. The proposed sensors present high repeatability, and especially highly sensitive to lower moisture measure.

Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face

An optical fiber relative-humidity sensor (OFRHS) with evaporated dielectric coatings is proposed and demonstrated. The sensitive coatings, composed of multilayers of Ti3O5 and SiO2, form an extrinsic Fabry–Perot cavity on the distal end of the multimode fiber. As the effective refractive index of the porous coatings were correlated with the change of ambient Relative-humidity (RH), which will at last result in the shift of interference fringe. By monitoring the drift of reflected interference fringe under different RH levels, the information about RH of the environment under test can be extracted. Experimental results show that the average sensitivity is 0.43nm/%RH when environmental RH changes from 1.8%RH to 74.7%RH. The proposed sensor was proved to be high repeatability, little hysteresis and especially highly sensitive to lower moisture measure.

High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams

The fabrication method of the high-quality fiber microaxicons (FMAs) on the endface of the optical fiber was developed. Using several types of the commercially available optical fibers we experimentally demonstrated the fabrication of a high-quality FMA focusing a laser beam into a tiny spot with a FWHM≈0.6λ and Bessel-like field distribution. It was also demonstrated that choosing the appropriate chemical composition of the etching solution makes it possible to change the shape of the FMA tip from conical to hemispherical. This allows one to change the spatial distribution of the output laser beam, which can represent both the Bessel-like beam with a depth of focus of up to 49λ and a very tiny focal spot close to the diffraction limit size. Experimentally measured focusing characteristics of the fabricated FMAs obtained using a homemade collection-mode scanning near-field optical microscope setup demonstrate good agreement with numerical simulations based on the 3D finite-difference time-domain simulations.

A Feasibility Study of the Material Removal Rate of the Conical End-Face of the Optical Fiber under Polishing

The surfaces of the microlenses at the conical end-faces of optical fibers require high quality to keep the best performance of laser beam delivery. The polishing parameters play an important role to get the best quality of the microlenses. Most of the past researches study on the polishing parameters of flat surfaces. In this study, the polishing area of the conical end-face of the optical fiber is varied during fabrication. The relation between removed volume and removed thickness is built to determine the material removal rate. An experiment is carried out and the results show that the first two spinning turns are much effective for polishing under the parameters of the experiment.

Efficient delivery of 60 J pulse energy of long pulse Nd:YAG laser through 200 μm core diameter optical fibre

Most of today’s industrial Nd:YAG lasers use fibre-optic beam delivery. In such lasers, fibre core diameter is an important consideration in deploying a beam delivery system. Using a smaller core diameter fibre allows higher irradiances at focus position, less degradation of beam quality, and a larger stand-off distance. In this work, we have put efforts to efficiently deliver the laser output of ‘ceramic reflector’-based long pulse Nd:YAG laser through a 200 μm core diameter optical fibre and successfully delivered up to 60 J of pulse energy with 90% transmission efficiency, using a GRADIUM (axial gradient) plano-convex lens to sharply focus down the beam on the end face of the optical fibre and fibre end faces have been cleaved to achieve higher surface damage thresholds.

拉锥光纤端面耦合效率的研究

光纤端面形状对光纤耦合的相关指标影响较大。光纤传输系统的速率越高，反射光对系统的影响越大。反射光会沿着光纤反馈到系统的激光器光源中，引起系统的不稳定和噪声，使得光纤系统的传输信息不可靠。因此我们提出通过拉锥将光纤端面制成曲面，而锥体曲面与光纤系统匹配，可以与光纤线路进行低损耗连接，具有极低的后向反射，得到更高的回波损耗。基于光纤熔接机的熔接功能，在放电熔融时使夹具反向移动，使光纤在熔融状态下获得径向拉力，可以改变光纤径向尺寸分布，光纤平端面可形成具有一定锥体的曲面。本文测量了不同放电时间和放电强度条件下，相应形成的光纤端面的插入损耗和回波损耗值，并与光纤平端面对比, 得到回波损耗可达到36 dB，并通过测试和分析，我们首次提出插入损耗随耦合端面距离改变的变化规律的结论。The shape of fiber end has a relatively vital effect on related indices of fiber coupling. The higher the fiber transmission system is, the greater the effect of reflected light is on the system. The reflected light can feed back into the laser source, causing instability and noise of system, which leads to unreliable transmission information. Then we put forward the idea that the end face can be made into curved surface by tapering to match with fiber system and be connected with low loss which has extremely low back reflection and gets higher return loss. On the basis of fusion function of fiber fusion splicer, the fixture moves reversely in the melt when it discharges and the fiber obtains radial tension in the molten state that can change the fiber radial size distribution. Then fiber end face can form a curved surface with a certain cone. In this paper we measure return loss and insert loss of fiber end faces respectively corresponding to different discharge time and intensity. In comparison with fiber flat end, we conclude that return loss of tapered fiber can reach 36 dB and to our knowledge, we first time put forward the rule that insert loss changes when the coupling end distance changes.

Rotation function development of four-core fiber optic tweezers

We demonstrate a new four-core fiber optic tweezers by polishing the end face of a four-core fiber into a truncated pyramid that is asymmetric to fiber cores. Optical intensity propagation and distribution are modeled numerically, and rotation function is developed by means of calculating three-dimensional trapping force and torque in ray optics regime. Simulation results show our tweezers is a feasible approach that can trap and rotate microscopic spheroid objects stably.

Reflection based extraordinary optical transmission fiber optic probe for refractive index sensing

Fiber optic probes for chemical sensing based on the extraordinary optical transmission (EOT) phenomenon are designed and fabricated by perforating subwavelength hole arrays on the gold film coated optical fiber endface. The device exhibits a red shift in response to the surrounding refractive index increases with high sensitivity, enabling a reflection-based refractive index sensor with a compact and simple configuration. By choosing the period of hole arrays, the sensor can be designed to operate in the near infrared telecommunication wavelength range, where the abundant sources and detectors are available for easy instrumentation. The new sensor probe is demonstrated for refractive index measurement using refractive index matching fluids. The sensitivity reaches 573nm/RIU in the 1.333–1.430 refractive index range.

Development of a Fiber-Optic Probe Hydrophone for a Cryogenic Liquid

We report the progress of developing a fiber-optic probe hydrophone (FOPH) system applicable to the measurement of local density/pressure fluctuations in a cryogenic liquid. The measurement principle is based on a law of classical optics known as “Fresnel’s reflection loss.” We use this principle at the end-face of a single-mode optical fiber that is immersed in liquid helium. Since the refraction index of liquid 4He is a function of density, and the refraction index of the core of the optical fiber is constant, the pressure can be obtained by measuring the reflectivity. We have succeeded in measuring the temperature dependence of the static density, and we go on to discuss the possibility of application to acoustic pressure measurements.

A localized surface plasmon resonance-based optical fiber sensor with sub-wavelength apertures

An optical fiber refractive index sensor based on localized surface plasmon (LSP) resonances within resonant subwavelength apertures in a gold film is demonstrated. The excitation of LSPs within the apertures in the film on the optical fiber endfaces provided the basis of a compact refractive index sensor for liquids. A sensitivity of 755 ± 12 nm per refractive index unit was demonstrated for this sensor.

Dengue immunoassay with an LSPR fiber optic sensor

Dengue fever is a viral disease that affects millions of people worldwide. Specific tests for dengue are not usually performed due to high costs, complicated procedures and, in some cases, long time to yield a result. For widespread use of specific tests to be possible, fast, reliable and fairly simple methods are needed. In this paper, we present a new dengue diagnostic method for the acute phase of the infection. The method proposed uses an all-optical fiber sensor based on Localized Surface Plasmon Resonance (LSPR) and specular reflection from gold nanoparticles (AuNPs). Dengue anti-NS1 antibody was immobilized on AuNPs deposited on the endface of a standard multimode fiber (62.5 µm/125 µm). The sensor is able to detect NS1 antigen at different concentrations, with limit of quantification estimated to be 0.074 μg/ml = 1.54 nM. These results indicate that the sensor could potentially be used for dengue diagnosis in the acute phase of the infection.

Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications

We present high average power picosecond and nanosecond pulse delivery at 1030 nm and 1064 nm wavelengths respectively through a novel hollow-core Negative Curvature Fiber (NCF) for high-precision micro-machining applications. Picosecond pulses with an average power above 36 W and energies of 92 µJ, corresponding to a peak power density of 1.5 TWcm⁻² have been transmitted through the fiber without introducing any damage to the input and output fiber end-faces. High-energy nanosecond pulses (>1 mJ), which are ideal for micro-machining have been successfully delivered through the NCF with a coupling efficiency of 92%. Picosecond and nanosecond pulse delivery have been demonstrated in fiber-based laser micro-machining of fused silica, aluminum and titanium.

Magnetic Field Sensor Based on Backscattered Intensity Using Ferrofluid

A magnetic field sensor based on backscattered intensity of a single mode fiber immersed in a ferrofluid is experimentally demonstrated. An OTDR system is used to interrogate the reflected power at the fiber end face, revealing >2 dB of modulation for magnetic field changes up to 1525 Gauss. The sensor shows the same response when used in remote and multiplexed configurations, demonstrating the high potential and versatility of this sensing approach.

Optimal FBG Sensor Installation Process for Crane SHM System

In order to improve the accuracy of measurement results, we should do our best to reduce the optical loss while welding FBG Sensors and adopt appropriate bonding technology. Taking simple tools and suitable approach can also achieve good result. We take three steps while welding FBG sensors, the steps are manufacture end face, welding, and heat shrinkable. Manufacturing end face of a fiber is the key step among them. It is important to adopt the sequence diagonal welding method and use a damp cloth to cool the other parts of the sensor while welding which can increase the accuracy of measurement. Use pen-made red light to check the fiber joints.