Tapered Fiber Tip Research Articles

Fiber-based optrode with microstructured fiber tips for controlled light delivery in optogenetics

Objective. Optogenetic modulation of neuronal activity requires precise and flexible light delivery to deep brain regions. Flat cleaved optical fibers combined with electrodes are widely used in implantable optogenetic devices for light delivery and electrical monitoring of neural activity. However, the flat fiber tip geometry induces serious tissue damage upon insertion, and makes it difficult to adjust and control the spatial extent of illumination within the brain. With their strongly increased tissue-compatibility and the possibility of spatial illumination control, tapered fibers outperform cleaved fibers in targeted neural photo-stimulation. Approach. In this work, we describe our device concept, and present a novel approach for reproducible fabrication of tapered fiber tips via grinding. Furthermore, we characterize recording electrodes by commenting data obtained from electrochemical impedance spectroscopy (EIS). We also investigate the impact of different cone angles (14°, 30°, 60°, and 90°) on the illumination profile and optical throughput. Main results. We fabricated a fiber-based optrode with cone tip and two deposited electrodes. Custom grinding setup for fabrication of tapered fiber tips with various cone angles is developed as a part of our research. Microscope images showed very good optical quality of cone tips. The results of transmitted optical power measurements performed with integrating sphere suggest that, compared to the flat cleaved optical fiber, transmitted power decreases exponentially with cone angle reduction. Obtained emission profiles (as induced fluorescence in Rhodamine 6G water solution) indicate very strong effect of cone angle on shape and size of illumination volume. Results obtained from EIS show the effect of electrode size on its recording capability. Significance. Compared to optrodes with flat cleaved optical fiber, the demonstrated fiber-based optrode with cone tip allows controlled light delivery with reduced invasiveness. The possibility to fabricate reproducible fiber tips with various cone angles enables control of light delivery in optogenetic experiment. The results presented here give neuroscientists the possibility to choose the appropriate tissue-compatible cone geometry depending on their stimulation requirements.

Experimental investigations of underwater laser propulsion microspheres based on a tapered fiber propulsion system

An underwater propulsion microsystem is proposed in this work, which employs a nanosecond laser pulse out from the tapered fiber tip. Noteworthily, the system can generate a directional shock wave (or plasma) to propel the polystyrene (PS) microsphere. Through simulation, the shock wave propagation characteristics and the bubble dynamic are investigated. Experimentally, high-speed photography method is employed to obtain the motion image of microsphere. The results show that the propulsion efficiency is dependent on the laser energy. Meanwhile, we explain the role of the bubble dynamic process in propelling microsphere, and find that the bubble diameter increases with the laser energy. In addition, an experiment is performed to separate and remove the PS microsphere clusters in water at fixed point. Compared with conventional technology, this new method has advantages of high controllability, directional and non-contact, and can be used for directional manipulation of underwater microstructures and removal of contaminated microspheres in water environments.

Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band.

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO2 film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10−6 RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.

Light-induced thermal convection for collection and removal of carbon nanotubes

Carbon nanotubes (CNTs) have exhibited immense potential for applications in biology and medicine, and once their intended purpose is fulfilled, the elimination of residual CNTs is essential to avoid negative effects. In this study, we demonstrated the effective collection and simple removal of CNTs dispersed in a suspension via thermal convection. First, a tapered fiber tip with a cone angle and end diameter of 10° and 3 μm, respectively, was fabricated via a heating and pulling method. Further, a laser beam with a power and wavelength of 100 mW and 1.55 μm, respectively, was launched into the tapered fiber tip, which was placed in a CNT suspension, resulting in the formation of a microbubble on the fiber tip. The temperature gradient on the microbubble and suspension surface induced thermal convection in the suspension, which resulted in the accumulation of CNTs on the fiber tip. The experimentally formed CNT cluster possessed a circular top surface with a diameter of 87 μm and an arched cross-section with a height of 19 μm. Furthermore, this CNT cluster was firmly attached to the fiber tip. Therefore, the removal of CNT clusters can be realized by simply removing the fiber tip from the suspension. Moreover, we simulated the thermal convection that caused CNT aggregation. The obtained results indicate that convection near the fiber tip flows toward it, which pushes the CNTs toward the fiber tip and enables their attachment to it. Further, the flow velocity is symmetrically distributed as a Gaussian function, which results in the formation of a circular top surface and arched cross-sectional profile for the CNT cluster. Our method may be applied in biomedicine for the collection and removal of nano-drug residues.

A whispering-gallery-mode microlaser based on dual erbium-doped microspheres

We demonstrate a low-cost whispering gallery mode tunable microlaser based on dual erbium-doped microspheres, which is created by directly doping erbium ions when melting the tapered fiber tip by electrode discharge of a fusion splicer. The incident light from the tapered fiber is diverged into the microsphere and stimulates whispering gallery mode. The light field of whispering gallery mode interacts with the internal gain material, erbium ions, and ends up with lasing. The quality factor (Q) value of the device is up to 1.4 × 104. When the pumping laser power changes continuously from 4.6 mW to 7.5 mW, the lasing wavelength shifts from 1560.14 nm to 1560.49 nm, the lasing emission power can be tuned from 0.25 µW to 0.41 µW.

Micro- and nano-fiber probes for optical sensing, imaging, and stimulation in biomedical applications

The flexibile nature of optical fiber enables it to offer remote-access capabilities, which could be used in many biomedical applications. This review focuses on different micro- and nano-structured fiber probes for applications in biosensing, imaging, and stimulations. The modifications to fiber could extend design freedom from waveguide optimization to functional material integration. Fiber probes with optimized waveguide structures or integrated functional materials could achieve enhanced optical mode interaction with biosamples, and hence obtain ultrasensitive biosensors with a remarkably low limit of detection. Furthermore, bioimaging with a high spatial resolution can be obtained by engineering dispersion and nonlinearity of light propagation in the fiber core or designing a metal-coated tapered fiber tip with a sub-wavelength aperture. Flat metasurfaces can be assembled on a fiber tip to achieve a large depth of focus and remove aberrations. Fiber is also a compact solution to realize the precise delivery of light for in vivo applications, such as deep brain stimulation. The optical beam size, shape, and direction could be steered by the probe parameters. Micro- and nano-technologies integrated with fiber contribute to various approaches to further improve detection limit, sensitivity, optical resolution, imaging depth, and stimulation precision.

Sensitive Handheld Refractometer by Using Combination of a Tapered Fiber Tip and a Multimode Fiber

This work presents a sensitive handheld refractometer using a tapered fiber tip, a multimode fiber, and a tiny mirror. The taper fiber tip was made using a modified wet etching method. The angular intensity distribution of the transmission light from the tapered fiber is sensitive to the refractive index of the surrounding solution. The low angular wave was reflected and collected by a multimode fiber. By setting a small collection angle and 10-mm distance between the fibers and the mirror, the maximum intensity sensitivity reaches up to 7.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> %/RIU (refractive index unit). The sensor resolution of the sensor probe can achieve 8.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU under 0.25% laser intensity stability. The simple handheld refractometer can be used to determine the properties of various liquids. For the glucose solution measurement under handheld condition, 0 to 50 °Bx can be distinguished by monitoring the intensity change. Considering the hand vibration and environmental disturbance, the sensor resolution was 1.22 °Bx.

Surface plasmon coupled nano-probe for near field scanning optical microscopy.

Near-field scanning optical microscopy (NSOM) is a powerful tool for study of the nanoscale information of objects by measuring their near-field electric field distributions. The near-field probe, which determines NSOM system performance, can be either a scattering-type or an aperture-type. Both types have strengths and weaknesses. Here we propose and study a surface plasmon-coupled type nano-probe, which works as a hybrid scheme and could potentially combine the advantages of the two NSOM probe types. The key element of the proposed probe is a nanoparticle-on-film structure designed on a tapered fiber tip. On the one hand, the probe can yield the signals scattered in the near field by a nanoparticle with a scattering mechanism; on the other hand, the scattered signals can be transmitted by the metal film and coupled into the fiber via surface plasmon coupled emission, thus providing a collection mode similar to an aperture-type NSOM. This will lead to signal enhancement, while greatly suppressing background noise. This surface plasmon-coupled nano-probe thus has great potential for near-field optical microscopy applications.

All-optical airflow sensor enables simultaneous speed and direction measurement.

Aiming to achieve simultaneous measurement of airflow speed and direction with high accuracy, a new all-optical airflow sensor based on triangulation concept is proposed. It consists of a suspending tapered fiber tip and a multicore fiber, acting as the flow interaction element and the localization reference, respectively. During operation, the detection light emitted from the tapered fiber will be coupled to the multicore fiber. Upon exposure to airflow, the induced deflection of the tapered fiber tip will change the individual receiving optical powers of the three cores arranged into triangular pattern, which are dependent on both of the flow speed and direction. Through properly designing structure parameters as core spacing, suspending length, tip diameter and axial fiber distance, the omnidirectional airflow with flow speed within 90 mm/s has been experimentally measured with high fidelity.

Quantum dot decorated nano-pyramid fiber tip for scanning near-field optical microscopy

We report a new type of nano-optical tip, the quantum dot decorated nano-pyramid fiber tip, for scanning near-field optical microscopy. The nano-pyramid tip has a large apex angle (70.5 degree), and consequently exhibits a significant improvement (17-fold) of the signal collection efficiency compared to conventional tapered fiber tips. Moreover, thanks to the template-based fabrication method and the surface treatment technique, the quantum-dot decorated nano-pyramid tip enjoys a clean and smooth surface, and can be produced with high accuracy and high reproducibility. Experimental results show that, with the nano-pyramid tip, the optical field distribution of a photonic nanostructure can be mapped with a 20 nm resolution (λ/20). It provides a powerful and reliable tool for near-field optical measurements.

Living Nanospear for Near-Field Optical Probing.

Optical nanoprobes, designed to emit or collect light in the close proximity of a sample, have been extensively used to sense and image at nanometer resolution. However, the available nanoprobes, constructed from artificial materials, are incompatible and invasive when interfacing with biological systems. In this work, we report a fully biocompatible nanoprobe for subwavelength probing of localized fluorescence from leukemia single-cells in human blood. The bioprobe is built on a tapered fiber tip apex by optical trapping of a yeast cell (1.4 μm radius) and a chain of Lactobacillus acidophilus cells (2 μm length and 200 nm radius), which act as a high-aspect-ratio nanospear. Light propagating along the bionanospear can be focused into a spot with a full width at half-maximum (fwhm) of 190 nm on the surface of single cells. Fluorescence signals are detected in real time at subwavelength spatial resolution. These noninvasive and biocompatible optical probes will find applications in imaging and manipulation of biospecimens.

Thulium fiber laser-induced vapor bubble dynamics using bare, tapered, ball, hollow steel, and muzzle brake fiber optic tips

This study characterizes laser-induced vapor bubble dynamics for five different distal fiber optic tip configurations, to provide insight into stone retropulsion commonly experienced during laser ablation of kidney stones. A thulium fiber laser with 1908-nm wavelength delivered 34-mJ energy per pulse at 500-μs pulse duration through five different fibers such as 100-μm-core / 170-μm-OD bare fiber tip, 150- to 300-μm-core tapered fiber tip, 100-μm-core / 300-μm-OD ball tip fiber, 100-μm-core / 340-μm-OD hollow steel tip fiber, and 100-μm-core / 560-μm-OD muzzle brake fiber tip. A high-speed camera with 10-μm-spatial and 9.5-μs-temporal resolution was used to image the vapor bubble dynamics. A needle hydrophone measured pressure transients in the forward (0 deg) and side (90 deg) directions while placed at a 6.8 ± 0.4 mm distance from the distal fiber tip. Maximum bubble dimensions (width/length) averaged 0.7/1.5, 1.0/1.6, 0.5/1.1, 0.8/1.9, and 0.7 / 1.5 mm, for bare, tapered, ball, hollow steel, and muzzle brake fiber tips, respectively (n = 5). The hollow steel tip exhibited the most elongated vapor bubble shape, translating into increased forward pressure in this study and consistent with higher stone retropulsion in previous reports. Relative pressures (a.u.) in (forward/side) directions averaged 1.7/1.6, 2.0/2.0, 1.4/1.2, 6.8/1.1, and 0.3/1.2, for each fiber tip (n = 5). For the hollow steel tip, forward pressure was 4 × higher than for the bare fiber. For the muzzle brake fiber tip, forward pressure was 5 × lower than the bare fiber. Bubble dimensions and pressure measurements demonstrated that the muzzle brake fiber tip reduced forward pressure by partially venting vapors through the portholes, which is consistent with the observation of lower stone retropulsion in previous reports.

Strain and temperature discrimination by using temperature-independent FPI and FBG

We present a compact fiber sensor with a hybrid structure including a temperature-independent Fabry-Perot interferometer (FPI) and a fiber Bragg grating (FBG) for strain and temperature discrimination. The temperature-independent FP interferometer which used as a strain gauge is formed by inserting a segment of tapered fiber tip in the capillary and subsequently splicing the other end of the capillary to a single mode fiber (SMF), the tapered fiber end-face and the spliced face form the FP cavity. Because the inserted taper tip has the degree of freedom along the fiber axial, when the FP interferometer is subjected to temperature variation, the thermal expansion of the fiber taper tip will resist the FP cavity length change caused by the variation of capillary length, and when the inserted tapered fiber length is selected as 160 μm, the temperature sensitivity of the whole FP sensor can be entirely eliminated. The FBG which acts as a thermometer is fabricated using the standard phase masks technique. The experimental results demonstrate that the sensor prototypes can realize the strain and temperature discrimination by combining a FBG and the temperature-independent FP interferometer.

Intensity-modulated lateral microdisplacement sensor enabled by cleaved tapered fiber tips

A lateral microdisplacement sensor based on tapered fiber tips, which is suitable for measuring lateral displacement with high sensitivity, is proposed. A jacket-off single-mode fiber (SMF) is tapered down to tens of microns and then cleaved in the middle of the waist region to get two identical tapered fiber tips. The lateral microdisplacement can be measured by monitoring the transmission intensity of a tip-tip structure. The beam propagation characteristics of such tapered fiber tips with different waist diameters are analyzed in detail. The tip-tip structure, together with the structure composed by one tip and one cleaved SMF, is investigated both experimentally and numerically. Experimental and numerical results show that this kind of lateral microdisplacement sensing structure can provide remarkable performance. A sensitivity of about −1.0 dB/μm with a measuring range of 28 μm is obtained experimentally.

Simultaneous measurement of temperature and refractive index using focused ion beam milled Fabry-Perot cavities in optical fiber micro-tips.

Optical fiber micro-tips are promising devices for sensing applications in small volume and difficult to access locations, such as biological and biomedical settings. The tapered fiber tips are prepared by dynamic chemical etching, reducing the size from 125 μm to just a few μm. Focused ion beam milling is then used to create cavity structures on the tapered fiber tips. Two different Fabry-Perot micro-cavities have been prepared and characterized: a solid silica cavity created by milling two thin slots and a gap cavity. A third multi-cavity structure is fabricated by combining the concepts of solid silica cavity and gap cavity. This micro-tip structure is analyzed using a fast Fourier transform method to demultiplex the signals of each cavity. Simultaneous measurement of temperature and external refractive index is then demonstrated, presenting sensitivities of - 15.8 pm/K and -1316 nm/RIU, respectively.

Sensitive Detection of Small Particles in Fluids Using Optical Fiber Tip with Dielectrophoresis.

This work presents using a tapered fiber tip coated with thin metallic film to detect small particles in water with high sensitivity. When an AC voltage applied to the Ti/Al coated fiber tip and indium tin oxide (ITO) substrate, a gradient electric field at the fiber tip induced attractive/repulsive force to suspended small particles due to the frequency-dependent dielectrophoresis (DEP) effect. Such DEP force greatly enhanced the concentration of the small particles near the tip. The increase of the local concentration also increased the scattering of surface plasmon wave near the fiber tip. Combined both DEP effect and scattering optical near-field, we show the detection limit of the concentration for 1.36 μm polystyrene beads can be down to 1 particle/mL. The detection limit of the Escherichia coli (E. coli) bacteria was 20 CFU/mL. The fiber tip sensor takes advantages of ultrasmall volume, label-free and simple detection system.

Investigation on spectral response of micro-cavity structure by symmetrical tapered fiber tips

We proposed and experimentally demonstrated a micro-cavity structure made of symmetrical tapered fiber tips. The waist of a conventional fiber taper fabricated from heating and stretching technique is symmetrically cleaved, and the aligned fiber tips with air gap constitute a Fabry–Perot micro-cavity due to the reflection at the tip facet. The spectral responses of such micro-cavity structure have been investigated both in beam propagation models and experiments. The multibeam interference in the micro-cavity and the impact of the waist diameter and cavity length on the spectral response has been successfully demonstrated. And a micro-cavity structure with 45μm waist diameter was experimentally achieved, the measured spectra agree well with the simulation ones, indicating that the spectral response of the micro-cavity structure is contributed by both the multibeam interference and the Fabry–Perot micro-cavity.

Fiber Fabry-Perot interferometer with controllable temperature sensitivity.

We proposed a fiber taper based on the Fabry-Perot (FP) interferometer structure with controllable temperature sensitivity. The FP interferometer is formed by inserting a segment of tapered fiber tip into the capillary and subsequently splicing the other end of the capillary to a single-mode fiber (SMF), the tapered fiber endface, and the spliced face form the FP cavity. Through controlling the inserted tapered fiber length, a series of FP interferometers were made. Because the inserted taper tip has the degree of freedom along the fiber axial, when the FP interferometer is subjected to temperature variation, the thermal expansion of the fiber taper tip will resist the FP cavity length change caused by the evolution of capillary length, and we can control the temperature sensitivity by adjusting the inserted taper length. In this structure, the equivalent thermal expansion coefficient of the FP interferometer can be defined; it was used to evaluate the temperature sensitivity of the FP interferometer, which provides an effective method to eliminate the temperature effect and to enhance other measurement accuracy. We fabricated the FP interferometers and calibrated their temperature characters by measuring the wavelength shift of the resonance dips in the reflection spectrum. In a temperature range of 50°C to 150°C, the corresponding temperature sensitivities can be controlled between 0 and 1.97 pm/°C when the inserted taper is between 75 and 160 μm. Because of its controllable temperature sensitivity, ease of fabrication, and low cost, this FP interferometer can meet different temperature sensitivity requirements in various application areas, especially in the fields which need temperature insensitivity.

Single tapered fiber tip for simultaneous measurements of thickness, refractive index and distance to a sample.

We demonstrate the capability of an air cavity Fabry-Perot interferometer (FPI), built with a tapered lead-in fiber tip, to measure three parameters simultaneously, distance, group refractive index and thickness of transparent samples introduced in the cavity. Tapering the lead-in fiber enhances the light coupling back efficiency, therefore is possible to enlarge the air cavity without a significant deterioration of the fringe visibility. Fourier transformation, used to analyze the reflected optical spectrum of our FPI, simplify the calculus to determine the position, thickness and refractive index. Samples made of 7 different glasses; fused silica, BK7, BalF5, SF2, BaF51, SF15, and glass slides were used to test our FPI. Each sample was measured nine times and the results for position, thickness and refractive index showed differences of ± 0.7%, ± 0.1%, and ± 0.16% respectively. The evolution of thickness and refractive index of a block of polydimethylsiloxane (PDMS) elastomer due to temperature changes in the range of 25°C to 90°C were also measured. The coefficients of the thermal expansion and thermo-optic estimated were α = 4.71x10(-4)/°C and dn/dT = -4.66 x10(-4) RIU/°C, respectively.

Long-Range Interferometric Displacement Sensing With Tapered Optical Fiber Tips

A Fabry-Perot interferometer built with a properly selected tapered optical fiber tip and a flat reflecting target is proposed for long range displacement sensing. By scaling down the diameter of the core and cladding of the optical fiber that forms the interferometer, the divergence of the output beam is reduced. The tapered fiber tip also couples the reflected light from the target more efficiently. As a result, well-defined interference patterns are observed even for long cavities which allows to sense displacements as long as 80 mm. The fabrication of the fiber tips is simple and reproducible and the interrogation of the interference patterns is fast as a conventional fiber Bragg grating interrogator can be used. The possibility of multiaxis displacement sensing is discussed.