Microfiber Coil Resonator Research Articles

WITHDRAWN: The periodic evolution of the resonant mode in a lossy microfiber coil resonator with three turns

The periodic evolution of the resonant mode in a lossy microfiber coil resonator with three turns is unraveled, and two types of loss conditions are assumed to investigate the characteristics of resonant mode with respect to the coupling state by means of numerical modeling. Under a given loss condition, the resonant mode exhibits periodic evolution between normal resonance, white-light cavity effect and resonance mode splitting as the change of the phase shift and coupling state. When the coupling state exceeds a certain threshold under a specific loss coefficient, the microfiber coil resonator predominantly exhibits normal resonance with characteristics akin to a single-ring resonator. Once the coupling state decreases to this threshold and matches the loss, white-light cavity effect will be activated. Moreover, as the coupling state falls below this threshold, the degeneracy of the resonance mode is raised and the resonant phase undergoes a bifurcation. The excitation conditions for each resonant mode have been derived, and the critical coupling conditions exist for both normal resonance and mode splitting in the case of relatively small losses. It’s found that there exists a threshold loss at which the critical coupling states of normal resonance and mode splitting coincide with the condition of white-light cavity effect. The characteristics of each mode have also been discussed.

Ultra-high-resolution detection of Pb2+ ions using a black phosphorus functionalized microfiber coil resonator

A black phosphorus (BP) functionalized optical fiber sensor based on a microfiber coil resonator (MCR) for Pb2+ ion detection in an aquatic environment is presented and experimentally demonstrated. The MCR-BP sensor is manufactured by winding a tapered microfiber on a hollow rod composed of a low-refractive-index polycarbonate (PC) resin with the BP deposited on the internal wall of the rod. Based on the propagation properties of the MCR, the chemical interaction between the Pb2+ ions and the BP alters the refractive index of the ambient environment and thus results in a detectable shift in the transmission spectrum. The resonance wavelength moves towards longer wavelengths with an increasing concentration of Pb2+ ions, and the sensor has an ultra-high detection resolution of 0.0285 ppb (parts per billion). The temperature dependence is 106.95 pm/°C due to the strong thermo-optic and thermal-expansion effect of the low-refractive-index PC resin. In addition, the sensor shows good stability over a period of 15 days. The local pH also influences the sensor, with the resonance wavelength shift increasing as pH approaches a value of 7 but then decreasing as the pH value increases further due to the effect of the BP layer by H+ and OH− ions. The sensor shows the potential for high-resolution detection of Pb2+ ions in a liquid environment with the particular advantages of having a simple structure, ease of fabrication, low cost, low loss, and simple interrogation.

High-sensitivity salinity sensor based on optical microfiber coil resonator

A simple, compact, and high-sensitivity optical sensor for salinity measurement is reported based on an optical microfiber coil resonator (MCR). The MCR is manufactured by initially wrapping microfiber on a polymethylmethacrylate (PMMA) rod, which is dissolved to leave a hollow cylindrical fluidic channel within the coil for measurement. Based on the light propagation through the MCR, the device's spectrum moves to long wavelengths with increased salinity in the fluid. The MCR device's sensitivity can reach up to 15.587 nm/% with a resolution of 1.28 × 10-3%. It is also confirmed that the temperature dependence is 79.87 pm/°C, which results from the strong thermal-expansion coefficient of the low refractive index epoxy. The experimental results indicate that the device can be widely used as a high sensitivity salinity sensor in water and other liquids due to its stability, compactness, electromagnetic immunity, and high sensitivity.

Theoretical and numerical studies of microfiber coil resonator for ambient refractive index sensing

Theoretical and numerical studies of microfiber coil resonator for ambient refractive index sensing

Investigation of Temperature Dependence of Microfiber Coil Resonators

The temperature-sensitive performance of a microfiber coil resonator (MCR) is thoroughly investigated. The MCR is fabricated by wrapping a microfiber on a PMMA rod coated with a UV-curable low refractive index epoxy. The temperature sensitivity is measured by investigating the correlation between the shift of the resonant wavelength and the surrounding temperature. It is determined that a range of parameters of the MCRs, including the gap between two adjacent rings, the diameter of the supporting rod, the number of rings, and the diameter of the microfiber, have a great influence on the temperature sensitivity of the MCRs. By optimizing the fabrication parameters of MCRs, such as the gap of the adjacent microrings and the diameter of supporting rod, etc., the maximum temperature sensitivity obtained is 237.31 pm/°C, which is about 2.3 times higher than that of MCR embedded in EFIRON UV-373 polymer and 23 times higher than that of MCR embedded in Teflon because of the strong thermo-optic and thermal expansion effects of the low refractive index epoxy and the supporting rod used in the experiments. Theoretical (numerical) simulation and experiment results are considered in the assessment of the optical performance improvement of MCR-based optical fiber temperature sensors.

A Review of Microfiber-Based Temperature Sensors.

Optical microfiber-based temperature sensors have been proposed for many applications in a variety of industrial uses, including biomedical, geological, automotive, and defense applications. This increasing demand for these micrometric devices is attributed to their large dynamic range, high sensitivity, fast-response, compactness and robustness. Additionally, they can perform in-situ measurements remotely and in harsh environments. This paper presents an overview of optical microfibers, with a focus on their applications in temperature sensing. This review broadly divides microfiber-based temperature sensors into two categories: resonant and non-resonant microfiber sensors. While the former includes microfiber loop, knot and coil resonators, the latter comprises sensors based on functionally coated/doped microfibers, microfiber couplers, optical gratings and interferometers. In the conclusions, a summary of reported performances is presented.

All-optical tunable delay line based on nonlinearities in a chalcogenide microfiber coil resonator

We propose an all-optical tunable delay line based on nonlinear effects in a two-turn chalcogenide microfiber coil resonator (MCR). Continuous rotation of light due to coupling between the two uniform MCR turns at the resonance induces a time delay. Using numerical simulation, we show the possibility of designing a continuously tunable all-optical and real-time delay line, taking advantage of the high Kerr nonlinear effect in a chalcogenide glass-based MCR. The threshold power for making the Kerr nonlinearity in the chalcogenide sulfide glass (Ge20Ga5Sb10S65, also known as 2S2G) MCR is less than 0.1 W. By selecting an appropriate pump power, one can adjust the time delay in the proposed MCR delay line as desired. Results of numerical simulations show that the time delay in an appropriately designed two-turn MCR can be tuned in the range of 24–147 ps with a bandwidth of ∼4.1 GHz. Pulse broadening and distortion related to group velocity dispersion (GVD) and the self-phase modulation (SPM) effect of the proposed microfiber coil delay line (MCDL) are also studied.

Dynamic analysis of optical microfiber coil resonators.

We present transient time analysis of a two-turn optical microfiber coil resonator (MCR). Our dynamic model is based on two sets of equations, coupled mode and nonlinear Schrödinger equations. The pulse response of this device is obtained by numerically solving the modified sets of equations in a dynamic regime. The results show that if the input pulse of the MCR is set at an off-resonance wavelength, this resonator operates as an all-pass filter with neither loss nor time delay. But in the case of resonance, the output pulse may have loss and a relatively long time delay, according to the continuous rotation of light between the first and the second turns of the MCR. Tunable time delays up to td=320 ps are obtained by choosing different values of the coupling coefficients. Furthermore, the material and structural dispersions of the MCR are studied, and it is shown that strong dispersive effects can occur even in this millimeter dimensions photonic device. Pulse broadening and distortion effects of the MCR are studied in the dynamic regime. The results show that, for high bit rate applications, the dispersion effects of the MCR should be carefully considered. Finally, fundamental soliton solution and its conditions in the MCR are investigated.

PMMA-rod-assisted temperature sensor based on a two-turn thick-microfiber resonator

We demonstrate a simple temperature sensor based on an optical microfiber coil resonator which is wrapped on a PMMA rod and embedded in Teflon. The microfiber is thick, strong and easy to be handled, which is 4~5 μm in diameter and more than 1 cm in waist region length. The PMMA rod has a diameter of ~2 mm. The temperature sensitivity is ~80 pm/°C thanks to the large thermal expansion coefficient of PMMA.

Optical electrical current sensor utilizing a graphene-microfiber-integrated coil resonator

A graphene-based electrical current sensor is proposed utilizing a microfiber coil resonator. Monolayer graphene sheet with a large sheet resistance is transferred onto the surface of a glass capillary rod. A microfiber is spirally wrapped around the graphene sheet to form a coil resonator. Heat generated from electrical current shifts the resonant wavelength because of the thermal effect in the microfiber resonator. The sensor exhibits a very good performance with a high sensitivity of 67.297 μm/A2, which is two orders of magnitude higher than that reported earlier. Our results show that microfiber-graphene-integrated devices have great potential for miniature and highly sensitive fiber sensors for monitoring electrical current.

Theoretical and experimental study of structural slow light in a microfiber coil resonator

In this paper, a compact slow-light microfiber coil resonator (MCR) is fabricated and the slow-light properties of it are analyzed and tested. Based on coupled-wave theory, a theoretical model for describing the slow-light propagation in the MCR is established. Experimentally, the MCR slow-light element is fabricated and its relative slow-light time delay is measured. The group velocity of the light pulse in the MCR slow-light element can be reduced to about 0.47c (c is the speed of light in vacuum) and the shape of the light pulse passing through the MCR is well preserved.

A Two-Layer Microcoil Resonator With Very High Quality Factor

We show that a compact microfiber coil resonator (MCR) can obtain very high quality factors by using two-layer structure with just a few turns of fiber. A record high quality factor, nearly 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> , is achieved from a three-turn two-layer MCR.

Inspection of electrical wires for insulation faults and current surges using sliding temperature sensor based on optical Microfibre coil resonator

We present a compact and robust temperature sensor based on an optical microfiber coil resonator packaged around a Teflon tube. The probe can efficiently slide along electrical wires to map the local temperature for locating insulation faults and reporting high current surges. A temperature responsivity of 95 pm/°C up to ~80 °C was demonstrated. This sensor is potentially low cost and simple to fabricate, making it attractive for domestic and industrial applications.

Development of a small-size embedded optical microfiber coil resonator with high Q

We fabricate a small-size embedded optical microfiber coil resonators (OMCRs) with a high Q-factor and investigate the dependence of the transmission characteristics of the OMCRs on the microfiber diameter and the OMCR diameter. Various microfibers with different diameters of 2.0, 2.1, 2.2, and 2.3 µm are fabricated by using a micro-tapering technique and are then coiled onto silica support rods. OMCRs are embedded by using a polymer with a low refractive index. When a 2.1-µm-diameter microfiber is coiled around a 10-mm-diameter silica support, a Q-factor of 1.0 × 105 is obtained, and the free spectral range, the full width at half maximum, and the extinction ratio are measured to be 50 pm, 15 pm, and 3.8 dB, respectively. The Q-factor also has a strong dependence on the OMCR diameter.

Wavelength-Selective Optical Amplifier Based on Microfiber Coil Resonators

In this paper, we present a new and compact structure for optical amplifiers based on erbium-doped microfiber coil resonators (MCR). The performance of the device is modeled by solving numerically the MCR coupled-mode equations and the rate equations in the steady-state regime. Characteristics of amplifier are analyzed for two-, three-, and four-turn MCRs. The signal gain spectrum is studied in terms of amplifier length, signal wavelength, and signal, and pump power. Simulation results demonstrate that such a device selectively amplifies specific wavelengths due to its resonance nature. It is possible to obtain optical gain as high as 30 dB using a 10 mW pump power and a 0.1 μW signal power. The amplified peaks show very narrow full-width at half-maximum that are very promising for microlasers.

Millimeter-Sized Microfiber Coil Resonators With Enhanced Quality Factors by Increasing Coil Numbers

Microfiber coil resonators (MCRs) with quality factors as high as ~470 000 and small size (<; 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) were fabricated and characterized. Both simulated and experimental results showed that the average full-width at half-maximum of the resonant dips decreased with the number of rings, providing a proof of the dependence of increasing quality factor upon the number of rings, even when the gaps among adjacent rings were not strictly uniform. Compared to the previous reports, a multi-ring MCR can thus offer a new choice of obtaining a microfiber resonator with high quality factor.

Transmission characteristic of multi-turn microfiber coil resonator

A transmission characteristic of multi-turn microfiber coil resonator (MCR), which is assembled from an adiabatic microfiber, is demonstrated. A comb spectrum was obtained by wrapping a microfiber on a low refractive indexed rod. A resonance with a free spectral range of 0.8nm was observed with a single turn on a 0.5mm diameter low-indexed rod and the change in transmission spectrum of the MCR was evident as the number of turns increased. For every additional microfiber turn, there was an addition of an eigenmode to the system which could be observed from the addition of a new resonance for every fringe in the transmission spectrum. The FSR for every added eigenmode was measured to be close to 0.8nm.

Teflon-coated microfiber resonator with weak temperature dependence

A temperature insensitive three-turn microfiber coil resonator (MCR) is demonstrated by embedding it in Teflon with opposite thermo-optic coefficient. The temperature dependence of a MCR strongly depends on the microfiber size which controls the ratio of thermal effect contributions from silica and polymer. Fabricated from a ~3μm-diameter microfiber, the temperature dependence of our MCR is compensated to less than 6pm/°C. Further suppression of the temperature dependence can be realized with ideal microfiber radius.

In-line absorption sensor based on coiled optical microfiber

We fabricated and tested an evanescent-wave absorption sensor consisting of an optical microfiber coil resonator embedded in fluidic channel walls. Low concentrations of flowing analyte show optical losses in agreement with a modified Beer–Lambert law. Higher concentration causes a limit value of the measured optical losses arising from adsorption mechanisms.

Embedded Optical Microfiber Coil Resonator With Enhanced High-$Q$

We successfully fabricate a uniform cylindrical microfiber coil resonator by wrapping a very thin strand of microfiber on a support rod and embedding/packaging it with a low refractive index polymer. An improved <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor in excess of 220 000 and an extinction ratio of 11.5 dB are achieved due to nearly ideal coupling and phase matching between the adjacent turns. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor is the highest reported value to date for any microfiber-based resonators.