Coreless Fiber Research Articles

Etching-enabled extreme miniaturization of graded-index fiber-based optical coherence tomography probes.

.We introduced and validated a method to miniaturize graded-index (GRIN) fiber-based optical coherence tomography (OCT) probes down to in diameter. The probes consist in an assembly of single-mode (SM), coreless (CL), and graded-index (GRIN) fibers. We opted for a probe design enabling controlled size reduction by hydrogen fluoride etching. The fabrication approach prevents nonuniform etching for both the GRIN and SM fiber components, while it requires no probe polishing postetching. We found that the miniaturized probes present insignificant loss of sensitivity () compared to their thicker () counterparts. We also showed that their focusing capabilities remain tunable and highly predictable. The fabrication process is simple and can be carried out by using inexpensive telecom equipment. Both the fabrication process and the developed probes can benefit the prototyping of minimally invasive endoscopic tools.

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

AbstractWhispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm−2 and ≈1.39 mJ cm−2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm−1 and a high net modal gain coefficient of ≈485 cm−1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.

Compact Mach–Zehnder interferometer based on processed hollow-core fiber for gas pressure sensing

A Mach–Zehnder interferometer based on processed hollow-core fiber (HCF) is proposed and experimentally demonstrated for gas pressure sensing. The fiber sensor is fabricated by splicing a segment of HCF between the two coreless fibers. One end of the HCF is processed into an open cavity, which allows gas pressure of the air-core to be equivalent to that of the external environment. Gas pressure change induces refractive index change of the air-core, thus the measurement of gas pressure can be achieved by monitoring wavelength shift of the interference pattern. The proposed fiber sensor exhibits a high gas pressure sensitivity of 9.35 nm / MPa in the range from 0.1 to 0.04 MPa and a temperature sensitivity of 30 pm / ° C in the range from 30°C to 90°C. Furthermore, the fiber sensor processes the advantages of compactness, low cost, and all-fiber configuration, which makes it suitable for gas pressure measurement.

Tailoring the response of single-mode–multimode–single-mode structures fabricated with a special low-index fiber to changes in the surrounding refractive index

An optical fiber refractive index (RI) sensor based on a single-mode–multimode–single mode (SMS) fiber structure fabricated with a coreless low-index multimode fiber (CLIMF) tapered to several diameters is presented. Both the response to changes in the external RI and its spectral dependence were strongly influenced by the length and diameter of the CLIMF. Different SMS sensors with diameters of 120, 100, 80, 60, 40 μm and the corresponding reimaging lengths at 1550 nm are immersed in RI of 1.32, 1.34, 1.36, 1.39, and 1.42 to study the response to changes in the external RI. The diameter of 80 μm yields the best sensitivity (540.7 nm / RIU) with a resolution of 6 × 10 − 5 in the RI range 1.32 to 1.39. By duplicating the reimaging length at 1550 nm for this diameter, the RI range increased to 1.42 and a maximum sensitivity of 1022 nm / RIU is achieved in the RI range from 1.39 to 1.42. Also the simulation results corroborate the wavelength positions of the peaks of the experimental spectrums. We show that the response range to changes in the external RI is controlled with the SMS geometry. This advantage makes the SMS sensor very suitable for biological applications.

Nanoliter liquid refractive index sensing using a silica V-groove fiber interferometer

A unique all-fiber interferometric sensor was proposed and successfully demonstrated efficient low-refractive-index liquid sensing in the range from 1.33 to 1.37, which is compatible with those of bio-liquids. A special silica coreless optical fiber with an open V-groove was used as an optical sensing medium, which provided a high sensitivity for a minute liquid volume in the nanoliter scale. The V-groove fiber (VGF) was serially concatenated between two single-mode fibers (SMFs). The LP01 mode guided along the input SMF excited the higher-order modes in the VGF to generate multimode interference, whose spectrum was transmitted through the output SMF. A single liquid droplet with volume of ∼80 nanoliters wet the entire hydrophilic surface of the VGF, and the transmission spectra shifted corresponding to its refractive index in a very linear manner. The sensor also showed a negligible temperature cross-sensitivity in the range 25°C–75°C, which overlaps with the biological temperature window such that the sensitivity of 159.696 nm per refractive index unit (nm/RIU) remained independent of the temperature variation. Modal properties of VGF were thoroughly analyzed numerically, and detailed processes for the sensor fabrication and sensing experiments were reported.

Multimode-Coreless-Multimode Fiber-Based Sensors: Theoretical and Experimental Study

This paper presents a complete study on the spectral behavior of a multimode-coreless-multimode fiber-optic structure, as well as its application as a refractometer and a liquid level sensor. The combination of two standard multimode fibers fused to a coreless fiber segment allows generating narrow interferometric bands in the optical spectrum, whose sensitivity can be improved by an adequate selection of the dimensions of the device (the coreless segment length and the diameter of the sensing area). A second way to improve the performance of the device is to deposit a thin film of SnO2, which allows increasing the sensitivity up to 314 nm/RIU. This widens the number of applications where this structure can be used. As an example, a liquid level sensor with 0.73 nm/mm sensitivity is presented.

2 × 2 microparticles curvilinear transport channel based on dual self-accelerating beams

We propose and demonstrate a 2 × 2 microparticles curvilinear transport channel based on a dual self-accelerating beam generator. The device is composed of a dual-core fiber, a hollow capillary fiber, and a coreless silica fiber. The dual self-accelerating beams produced by the device propagate along the curvilinear trajectory due to the transverse accelerating property and then cross at the front of the fiber probe. The experimental results show that the yeast cell is transported along the curvilinear trajectory. By adjusting the optical power ratio of dual beams, we may control and ensure the yeast cell steers at the cross point. The proposed 2 × 2 curvilinear microparticles transport channel based on dual self-accelerating beams can realize microparticles sorting and obstacle avoidance, which means that it would be a useful tool in biology and colloidal science.

Fiber Reshaping-Based Refractive Index Sensor Interrogated through Both Intensity and Wavelength Detection.

A fiber reshaping-based refractive index (RI) sensor is proposed relying on both optical intensity variation and wavelength shift. The objective of this study is to completely reshape the core and to ultimately mimic a coreless fiber, thereby creating a highly efficient multimode interference (MMI) coupler. Thus, propagation modes are permitted to leak out into the cladding and eventually escape out of the fiber, depending on the surrounding environment. Two interrogation mechanisms based on both the intensity variation and wavelength shift are employed to investigate the performance of the RI sensor, with the assistance of leaky-mode and MMI theories. By monitoring the output intensity difference and the wavelength shift, the proposed RI sensor exhibits high average sensitivities of 185 dB/RIU and 3912 nm/RIU in a broad range from 1.339 to 1.443, respectively. The operating range and sensitivity can be adjusted by controlling the interaction length, which is appealing for a wide range of applications in industry and bioscience research.

Surface plasmon resonance refractive index sensor based on fiber-interface waveguide inscribed by femtosecond laser.

A novel surface plasmon resonance (SPR) configuration based on fiber-interface waveguide was proposed and realized by combining the technology of femtosecond laser writing waveguide with SPR effect for measuring refractive index (RI) of analyte. A U-shaped waveguide is inscribed in the coreless fiber and its bottom is very close to the fiber surface, which can produce strong evanescent field being sensitive to ambient media. When the fiber surface is coated with a layer of gold film, the strong evanescent field can excite the SPR effect on the fiber surface. Most importantly, different from some types of fiber SPR sensors with a fragile physical structure, the fiber-interface waveguide SPR sensor exhibits an excellent mechanical strength. Such a SPR sensor exhibits a high sensitivity of ∼3352 nm/RIU at the RI value of ∼1.395, which may have important practical applications in medicine, environmental monitoring, and food safety.

Large diameter coreless fiber as a saturable absorber for erbium doped fiber laser cavity based on multimode interference

• Easily handling C-band passively Q-switched erbium doped fiber laser nm has been demonstrated. • Easily spliced large core diameter coreless fiber is served as a saturable absorber for Q-switching ignition. • High pulse repetition rate of 73.3–92 KHz without any intra-cavity tuning element has been obtained. • Stable output pulses of SNR of 50 dB is generated. We demonstrated a Q-switched erbium-doped fiber laser (EDFL) based on large diameter coreless fiber without intra-cavity tuning element for the first time. A single mode fiber –coreless fiber –single mode fiber (SNCFS) structure was employed as a passive Q-switcher relies on Kerr nonlinearity associated with multi-mode interference (MMI) effect. By splicing 10 cm of 125 µm in diameter coreless fiber (NCF) between two single mode fibers (SMFs), C-band Q-switched EDFL with 3-dB bandwidth of 0.138 nm was successfully obtained, making it a simple, compact and easily handling modulation device. A stable Q-switched pulses were achieved with a pump threshold of 188 mW. Pulse repetition rate ranges from 73.3 to 92.0 kHz, minimum pulse width of 3.5 µs and maximum energy of 50.9 nJ were recorded.

Surface plasmon resonance sensor based on coreless fiber for high sensitivity

A compact and simple surface plasmon resonance sensor based on coreless fiber for refractive index measurement is developed. Compared with common SPR sensors, the proposed SPR sensor has a shorter length of 1 cm, avoids a taper and can be used to detect a wider range of 1.335–1.4019. The resonant wavelength shifts ∼200 nm to the longer wavelength in the refractive index range of 1.3335–1.4019. The average refractive sensitivity of 2836 nm/RIU is obtained in the range of 1.3335–1.4019, and the highest sensitivity can reach 4328.2 nm/RIU. Furthermore, our device has the advantages of small size, wide measuring range, excellent stability and short response time, which is appropriate for biological sensing.

Multimode-coreless-multimode fiber biosensor based on surface plasmon resonance

This study presents a facile, rapid and effective method for the fabrication of optical fiber surface plasmon resonance biosensors. A section of coreless fiber is spliced between two multimode fibers to construct a multimode-coreless-multimode (MCM) fiber structure. We take full advantage of the transmission characteristics of the coreless fiber, and the experimental results confirm that this MCM after coating with a thin gold film is able to excite surface plasmon wave effectively. Three sensors with different lengths of coreless optical fibers of 1 cm, 1.6 cm and 2 cm were constructed. And when the refractive index (RI) of the sucrose solution changes from 1.33 to 1.3883, the RI sensitivity was measured to be 2129.66 nm/RIU, 2249.33 nm/RIU and 2307.93 nm/RIU, respectively. After a comprehensive comparison, we chose the MCM optical fiber sensor with a coreless fiber length of 1.6 cm for biological experiments. After directly immobilized the antibody on the gold film, the biosensor was able to selective detect human immunoglobulin G (HIgG) with a sensitivity of 215 nm/ (mg ml−1). The limit of detection is 0.465 µg ml−1. The biosensor exhibited effective antibody immobilization ability and high sensitivity for HIgG detection, which fully demonstrates its potential applications in the broader biomedical or chemical field.

Numerical analysis of circularly polarized modes in coreless photonic crystal fiber

Abstract In this paper, the coreless photonic crystal fiber (CO-PCF) is investigated using finite element. The absence of core in the PCF structure is achieved by applying the permanent twist on its cladding boundary and hence the light propagation path tends to be circularly polarized. The other modes can also be exerted other than the fundamental modes is known as cladding filled modes or super modes.

Fiber-optic multipoint laser-ultrasonic excitation transducer using coreless fibers.

Photoacoustic ultrasound excitation has great potential in structural nondestructive testing and applications for medical treatments as a promising alternative to electrical ultrasound. This study proposes and demonstrates a multipoint optical fiber laser-ultrasonic transducer system, wherein the fiber-optic ultrasonic transducer is fabricated by a coreless fiber segment's fusion with single-mode fibers at each end. Simulation and experiment results show that the transducer coupling ratio is dependent on the coreless fiber's length. The structure of such an ultrasonic transducer is easily manufactured. Thus, the structures of these optical fiber ultrasonic transducers with different coupling ratios are connected in the order of small to large coupling ratios. In this manner, multipoint ultrasonic excitation with equal intensities at each excitation point can be obtained using this simple and low-cost method. Using laser guidance through the optical fiber to generate ultrasound can efficiently solve some shortcomings of traditional ultrasonic transducers, such as large volume, small bandwidth, and electromagnetic interference. Moreover, this type of fiber-optic ultrasound transducer has higher mechanical strength than other fiber-optic ultrasound transducers and is expected to be useful in structural health-monitoring of buildings.

A diaphragm-free fiber Fabry-Perot gas pressure sensor.

A diaphragm-free fiber gas pressure sensor based on the extrinsic Fabry-Perot interferometer (EFPI) is proposed and experimentally demonstrated. The sensor is fabricated by fusion splicing a sandwich structure of a single mode fiber, a hollow core fiber (HCF), and a coreless fiber. A femtosecond (fs) laser is employed to drill a micro-channel on the side wall of the HCF to allow gas entering/exiting the HCF. The refractive index of the gas has a linear relationship with the gas pressure. Thus, the gas pressure can be detected by investigating the optical path difference of the EFPI. The experimental results show that the proposed sensor exhibits a high gas pressure sensitivity of ∼1.80 µm/MPa from vacuum to 10 MPa at room temperature. Pressure responses under high temperatures are also investigated. The proposed sensor has properties of robust structure, miniature size, large measuring range, and high sensitivity.

Dual-Cavity Fabry–Perot Interferometric Sensors for the Simultaneous Measurement of High Temperature and High Pressure

A fiber-optic hybrid sensor based on dual-cavity Fabry–Perot interferometers is proposed and demonstrated for the simultaneous measurement of high temperature and high pressure. The proposed sensor is fabricated by fusion splicing of two single-mode fibers (SMFs), a hollow core fiber (HCF), and a coreless fiber (CF), forming a four-section structure of leading SMF-SMF-HCF-CF. A micro-hole is fabricated at the end of the leading SMF using a femtosecond laser, forming an air gap between two SMFs. The CF is thinned and roughened by the femtosecond laser to form a diaphragm. The SMF-based cavity is used as the temperature sensor and the HCF-based cavity as the pressure sensor. Lengths of two cavities are interrogated simultaneously by using the fast Fourier transform-based white-light interferometry. Experimental results show that the proposed sensor exhibits a temperature sensitivity of 19.8 nm/°C and a pressure sensitivity of ~98 nm/MPa, within the temperature range of 20 °C–800 °C and the pressure range of 0–10 MPa. The maximum measurement errors of temperature and pressure are 5 °C and 0.2 MPa, respectively. As far as we know, it is the first time to detect the temperature of 800 °C and the pressure of 10 MPa simultaneously.

Relative Humidity Fiber Sensor Based on Multimode Interferometer Coated with Agarose-Gel

In this work, a relative humidity (RH) sensor based on a structure with multimode interference is proposed and experimentally demonstrated. The multimode sensor is fabricated by fusion splicing a coreless fiber section to a single mode fiber. A hydrophilic agarose gel is coated on the coreless fiber, using the dip coating technique. By changing the surrounding RH, the refractive index of the coated agarose gel will change, causing a wavelength shift of the peak in the reflection spectra. For RH variations in the range between 60.0%RH and 98.5%RH, the sensor presents a maximum sensitivity of 44.2 pm/%RH, and taking in consideration the interrogation system, a resolution of 0.5%RH is acquired. This sensor has a great potential in real time RH monitoring and can be of interest for applications where a control of high levels of relative humidity is required.

Optical Fiber Tip Sensor for the Measurement of Glucose Aqueous Solutions

A reflective fiber optic sensor based on multimode interference for the measurement of refractive index variations in glucose aqueous solutions is proposed. The sensor is fabricated by splicing a short section of coreless silica fiber to standard single mode fiber. The influence of the coreless fiber dimensions on the sensor performance is analyzed. By changing the sensor length, no significant impact is observed. However, the reduction of the sensing head diameter leads to a large improvement of the sensitivity. The smaller sensor, with a length of 5 mm and a diameter of 24 μm, presents a maximum sensitivity of 1467.59 nm/RIU, for the refractive index range between 1.364 and 1.397 RIU. Taking into account the acquisition system, a maximum theoretical resolution of 6.8 × 10 -5 RIU is achieved.

Femtosecond laser-inscribed fiber interface Mach-Zehnder interferometer for temperature-insensitive refractive index measurement.

A new fiber interface Mach-Zehnder interferometer has been fabricated, to the best of our knowledge, in coreless fiber by femtosecond laser-inscription for temperature-insensitive refractive index measurement. A straight waveguide was inscribed along the central axis of the coreless fiber as the reference arm, and the other curved waveguide (interface waveguide) was then inscribed bending toward the cladding interface to obtain a strong evanescent field sensitive to ambient refractive index. This fiber interface Mach-Zehnder interferometer exhibits a high refractive index (RI) sensitivity of ∼3000 nm/RIU at an RI value of 1.432. Moreover, with the significant advantages of high mechanical strength and temperature independence, such a fiber Mach-Zehnder interferometer may find many potential applications in biochemical sensing.

Determination of thermo-optic coefficient of ethanol-water mixtures with optical fiber tip sensor

Abstract In this work, the thermo-optic coefficient (TOC) of ethanol-water mixtures, through refractive index and temperature measurements are determined using an etched optical fiber tip based on a multimode interferometer. The proposed probe is fabricated by fusion-splicing a 5.2 mm long coreless fiber section to a single mode fiber. To reduce the sensor dimensions and improve its sensitivity towards external medium variations, the fiber tip is subjected to wet chemical etching using a solution of 40% hydrofluoric acid, presenting a final diameter of 24.4 μm. The TOC of each solution is estimated and, in the case of deionized water and pure ethanol, its value is of −1.128 × 10−4 °C−1 and −3.117 × 10−4 °C−1, respectively.