Section Of Multimode Fiber Research Articles

Super-low-power optical trapping of a single nanoparticle.

We propose and demonstrate a simple approach for noncontact, three-dimensional, and stable trapping of a single nanoparticle with a super-low incident laser power (0.7mW) via the single-fiber optical tweezers. We splice a section of single-mode fiber and a section of multimode fiber to construct a Bessel-like beam, which produces narrow output laser beams. We integrate a high-refractive-index glass microsphere on the tip of the multimode fiber to focus the narrow output laser beams. The focused beams provide a nanoscale optical trap for a single nanoparticle (polystyrene sphere, diameter of 200nm). This optical fiber probe has the advantages of high laser transmission efficiency, high spatial resolution, and minimum joule heating. The proposed approach extends the application potential of fiber-based optical manipulations, such as nanoparticle sorting, single-cell organelle analysis, and bio-sensing.

A Self-Referencing Intensity-Based Fabry–Perot Cavity for Curvature Measurement

In this article, a self-referencing intensity-based fiber optic sensor relying on the principle of Fabry-Perot interference is proposed and demonstrated to measure curvature. The sensor is manufactured producing an air bubble cavity between two sections of multimode fiber. By detecting optical power variations at specific wavelengths, it was possible to measure curvature, enabling this sensor as a self-referencing system. For this setup, the achieved curvature sensitivity was 0.561 ± 0.014 dB/m -1 , with a correlation factor up to 0.997, within the measurement range of 0.0-0.8 m -1 . The proposed system has several features, including the self-referencing characteristic and its structure simplicity in terms of measuring procedure, making it a useful system.

Highly sensitive fiber-optic accelerometer using a micro suspended-core fiber.

A compact fiber-optic accelerometer was proposed and demonstrated experimentally based on Fabry-Perot interference (FPI). The device consists of a suspended-core fiber embedded in a hollow-core fiber, forming an enclosed cavity structure. A short section of multi-mode fiber (MMF) was spliced on the leading-in single-mode fiber (SMF), which worked as a micro lens to focus the light to decrease the transmission loss. A well-defined interference spectrum was achieved by a low-fitness FP interferometer formed by both the end-face of lead-in fiber and the end-face of suspended-core fiber. Thanks to the outstretched FP cavity by suspended-core fiber, the sensor is highly sensitive to vibration along the fiber axis. Moreover, a one-dimensional mechanical transducer was used to improve the frequency band of the sensor. By the side-band filtering technology, the vibration was detected and analyzed by a simple intensity interrogation technology.

High-temperature sensor based on suspended-core microstructured optical fiber.

We propose a high-temperature sensor based on a suspended-core microstructured optical fiber (SCMF). The sensor is constructed by fusion splicing a piece of SCMF between two sections of multimode fibers (MMFs) which act as light beam couplers. The multimode interference is formed by the air cladding modes and the silica core modes in the SCMF. Fast Fourier transform is adapted to filtering the raw transmission spectra of the MMF-SCMF-MMF structure. The wavelength shift of the dominant spatial frequency is monitored as the temperature varies from 50 °C to 800 °C. The sensitivities of 31.6 pm/°C and 51.6 pm/°C in the temperature range of 50 °C-450 °C and 450 °C-800 °C are respectively achieved. Taking advantage of the compact size, good stability and repeatability, easy fabrication, and low cost, this proposed high-temperature sensor has an applicable value.

In-fiber temperature sensor based on green up-conversion luminescence in an Er3+-Yb3+co-doped tellurite glass microsphere.

A novel, to the best of our knowledge, in-fiber temperature sensor based on green up-conversion (UC) luminescence in an Er3+-Yb3+ co-doped tellurite glass microsphere is described. The tellurite glass microsphere is located firmly inside a suspended tri-core hollow-fiber (STCHF) structure. The pump light launched via a single-mode fiber (SMF) is passed through a section of multimode fiber, which is fusion spliced between the SMF and the STCHF into the cores suspended inside the hollow fiber and coupled into the microsphere. Green and red UC emissions of the Er3+ ions are observed using 980nm pump excitation. The temperature-sensing capability of the tellurite glass microsphere is based on the thermally coupled effect between the upper energy levels responsible for green emissions at 528nm and 549nm. The resulting fluorescence intensity ratio, depending on the surrounding temperature range from 303K to 383K, is experimentally determined, and a maximum sensitivity of 5.47×10-3 K-1 is demonstrated. This novel in-fiber microsphere-resonator-based device is highly integrated and has the additional advantages of ease of fabrication, compact structure, and low fabrication cost and therefore has great application potential in integrated optical sources including lasers.

A high sensitivity gas pressure sensor based on femtosecond laser micro/nano processing

A compact and novel Mach-Zehnder interferometer used for gas pressure sensing is proposed and demonstrated experimentally. The sensor is constructed by fusion splicing a short piece of dual side-hole fiber (DSHF) in between two sections of multimode fibers (MMF). Interference occurs as the optical path length difference (OPLD) between light beams along air holes and silica of the DSHF. With the opening drilled on one air hole by femtosecond laser micro/nano processing, the filled gas directly affects light beam along this air hole, which results in the interference spectrum shift. Experimental results show that the sensor exhibits a high sensitivity of −8.58 nm/MPa with the increasing gas pressure. The sensor has potential application in gas pressure sensing for its advantages of high sensitivity, low temperature crosstalk and easy fabrication.

Strain sensing based on a core diameter mismatch structure

AbstractIn this paper, it is proposed and experimentally investigated a sensor structure based on a core diameter mismatch technique for strain sensing. An uncoated short section of multimode fiber is spliced between two standard single mode fibers forming the sensor structure, called the SMS (single‐mode‐multimode‐single‐mode) sensor. The proposed SMS sensor, in the strain analysis, generates destructive or constructive interference patterns as load or strain are applied, changing the power of the optical signal only, without varying the wavelength. The sensor configuration offers several motivating attributes, such as easy fabrication, low‐cost, high‐efficiency and high sensitivity, moreover allows interrogation of the optical signal using the transmitted or reflected total optical power. The results and these advantages indicate that the proposed SMS sensor device is suitable for strain measurement, which is helpful in a broad range of applications, for instance, structural health monitoring.

Graphene oxide-film-coated splitting ratio-adjustable Mach-Zehnder interferometer for relative humidity sensing.

In this paper, a splitting ratio-adjustable Mach-Zehnder interferometer (MZI) for the measurement of relative humidity (RH) is proposed and experimentally demonstrated. The sensing head contains three sections of single mode fiber (SMF) and two sections of multimode fiber (MMF), in which the two MMFs are spliced among the three SMFs. The MMFs are corroded with hydrofluoric acid and act as mode couplers to split and recombine light owing to the core diameter mismatch with the SMF. A layer of graphene oxide (GO) is coated on the MMFs by dip-coating and natural evaporation. The effective refractive index of the GO will vary when it absorbs the water molecules. As a result, the intensity of the transmission light in the core and cladding of the single mode fiber can be adjusted. Thus, the intensity of the resonant dip will vary when the relative humidity changes. The experimental results show that a humidity sensitivity of 0.263 dB/RH% with a linear correlation coefficient of 99% can be achieved in a relative humidity range of 35% to 85%.

Fiber in-line magnetic field sensor based on Mach-Zehnder interferometer integrated with magnetic fluid

In this paper, magnetic fluid (MF), a new type of optical functional nanomaterial with interesting optical characteristics under the external magnetic field, is adopted to form a novel fiber-optic magnetic field sensor. The proposed sensor is based on Mach-Zehnder interferometer (MZI) and has a multimode-singlemode-multimode (MSM) fiber structure. The MSM structure was fabricated by splicing a section of uncoated single mode fiber (SMF) between two short sections of multimode fibers (MMFs) using a fiber fusion splicer. The magnetic field sensing probe was made by inserting the fiberoptic structure in an MF-filled capillary tube. Variations in an external magnetic field is seen to cause changes in the refractive index of MF. This tunable change in the refractive index with magnetic field strengths between 0.6 mT to 21.4 mT produces a shift in the peak position of the wavelength. The shift of the valley wavelength with magnetic field intensity has a good linearity of up to 99.6%. The achieved sensitivity of the proposed magnetic field sensor is 0.123 nm/mT, which is improved by several folds compared with those of most of the other reported MF-based magnetic field sensors. Furthermore, we build the corresponding circuit-based measurement system, and the experimental results show that the voltage change indirectly reflects the change of the external magnetic field strength. Therefore, this provides the potential to fiber-based magnetic field sensing applications.

Simultaneous Strain and Temperature Sensor Based on a Fiber Mach-Zehnder Interferometer Coated with Pt by Iron Sputtering Technology.

We demonstrated a fiber in-line Mach-Zehnder interferometer (MZI) coated with platinum (Pt) for the simultaneous measurement of strain and temperature. The sensor was fabricated by splicing a section of multimode fiber (MMF) between two single mode fibers (SMFs) and the Pt coating was prepared by iron sputtering technology. Fine interference fringes of over 20 dB with a compact size of 20 mm were achieved. The experimental results of the two different resonant dips showed strain sensitivities of −2.06 pm/με and −2.21 pm/με, as well as temperature sensitivities of 55.2 pm/°C and 53.4 pm/°C, respectively. Furthermore, it was found that the Pt coating can improve the strain sensitivity significantly, resulting in an increase of about 54.5%. In addition, the sensor has advantages of easy fabrication, low cost, and high sensitivity, showing great potential for the dual-parameter sensing of strain and temperature.

A novel optical fiber magnetic field sensor based on Mach-Zehnder interferometer integrated with magnetic fluid

In this paper, a novel Magnetic fluid-filled optical fiber sensor consisting of a multimode-single mode-multimode (MM-SM-MM) fiber structure has been proposed and experimentally demonstrated. The proposed sensor is based on a Mach-Zehnder interferometer, which is fabricated by splicing a section of uncoated single mode fiber between two short sections of multimode fibers with a fiber fusion splicer. The micro-structure is sealed in a Magnetic fluid-filled capillary tube and the magnetic field sensing probe is formed. Variations in an external magnetic field is seen to cause changes in the refractive index of MF. This tunable change in the refractive index with magnetic field strengths between 0.6 mT to 21.4 mT produces a shift in the position of the peak of the wavelength. The shift of the valley wavelength with magnetic field intensity has a good linearity of up to 99.586%. The achieved sensitivity of the proposed magnetic field sensor is 0.12306 nm/mT. Furthermore, we also proposed the corresponding building methods of the measurement system based on the MM-SM-MM structure. It turned out that a voltage change indirectly reflects the change of the external magnetic field strength. This therefore provides the potential to fiber-based magnetic field sensing applications.

High sensitivity and low loss open-cavity Mach-Zehnder interferometer based on multimode interference coupling for refractive index measurement

An open-cavity Mach-Zehnder interferometer (MZI) liquid refractive index (RI) sensor based on multimode interference (MMI) coupling is proposed and demonstrated. The sensor is fabricated by embedding a small section of no core fiber (NCF) into two sections of multimode fiber (MMF) with large lateral-offset splicing. In term of MMI effect, the lengths of the MMFs are optimized to enhance the exciting and coupling efficiencies of the interference lights. Therefore, this coupling method reduces the insertion loss obviously and allows bigger tolerance of offset distance. The simulated and experimental results all prove it has the most optimized loss compared with other coupling ways that we know. A sensor with 1 mm sensing length is fabricated and its loss is only −11.9 dB which is prominently reduced compared with other open cavity MZIs with the similar cavity length. This sensor has a high RI sensitivity of −1364.343 nm/RIU and shows a good linear response in the measurement range. The temperature sensitivity is about 33 pm/°C, so the cross-sensitivity is only 2.419 × 10−5 RIU/°C. The proposed sensor can promote the development of the open-cavity MZI to satisfy practical demands.

A simple all-fiber comb filter based on the combined effect of multimode interference and Mach-Zehnder interferometer

A polarization-dependent all-fiber comb filter based on a combination effect of multimode interference and Mach-Zehnder interferometer was proposed and demonstrated. The comb filter was composed with a short section of multimode fiber (MMF) fusion spliced with a conventional single mode fiber on the one side and a short section of a different type of optical fiber on the other side. The second type of optical fiber is spliced to the MMF with a properly designed misalignment. Different types and lengths of fibers were used to investigate the influence of fiber types and lengths on the performance of the comb filter. Experimentally, several comb filters with free spectral range (FSR) values ranging from 0.236 to 1.524 nm were achieved. The extinction ratio of the comb filter can be adjusted from 6 to 11.1 dB by varying polarization states of the input light, while maintaining the FSR unchanged. The proposed comb filter has the potential to be used in optical dense wavelength division multiplexing communication systems.

Intensity Demodulated Refractive Index Sensor Based on Front-Tapered Single-Mode-Multimode-Single-Mode Fiber Structure.

A novel intensity demodulated refractive index (RI) sensor is theoretically and experimentally demonstrated based on the front-tapered single-mode-multimode-single-mode (FT-SMS) fiber structure. The front taper is fabricated in a section of multimode fiber by flame-heated drawing technique. The intensity feature in the taper area is analyzed through the beam propagation method and the comprehensive tests are then conducted in terms of RI and temperature. The experimental results show that, in FT-SMS, the relative sensitivity is −342.815 dB/RIU in the range of 1.33~1.37. The corresponding resolution reaches 2.92 × 10−5 RIU, which is more than four times higher than that in wavelength demodulation. The temperature sensitivity is 0.307 dB/°C and the measurement error from cross-sensitivity is less than 2 × 10−4. In addition, fabricated RI sensor presents high stability in terms of wavelength (±0.045 nm) and intensity (±0.386 dB) within 2 h of continuous operation.

Tunable and Switchable Erbium-Doped Fiber Laser Using a Multimode-Fiber Based Filter

We demonstrated a compact tunable and switchable single/dual-wavelength erbium-doped fiber laser. The fiber laser can be tuned and switched from single-wavelength to dual-wavelength oscillation by using our recently proposed tunable comb filter. The comb filter consists of a section of multimode fiber (MMF) coiled into a polarization controller and two sections of single mode fibers (SMFs) to form a SMF/MMF/SMF structure, serving as a simple tunable all-fiber Mach-Zehnder interferometer. Due to the insertion of the MMF-based polarization controller (PC), an additional phase shift is introduced from the difference of the birefringence intensity in different dominant modes, which can be used to tune the fiber laser. In the experiment, by properly adjusting the PC, a tuning range of 9.3 nm can be achieved for the single-wavelength operation. Moreover, dual-wavelength operation with different free-spectral-ranges can be obtained. The tunable and switchable fiber lasers are of great importance for their applications in optical testing, optical fiber sensing, and signal processing.

Temperature-independent hygrometry using micromachined photonic crystal fiber.

An in-fiber Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated for relative humidity (RH) and temperature measurements. The MZI is formed by a grapefruit-shaped photonic crystal fiber (G-PCF) cascaded with a short section of multimode fiber that serves as a mode coupler. To enhance sensitivity to humidity, femtosecond laser micromachining was performed to remove a portion of cladding of the G-PCF to expose its core to the ambient medium. The output interference spectrum is fast Fourier transformed to produce a spatial frequency spectrum that describes the intensity composition of the cladding modes in the MZI. In our investigation, it was observed that the interference dip intensity has a sensitivity of -0.077 dB/% RH to the change of RH in the range of 25%-80%RH, whereas the dip wavelength has a temperature sensitivity of ∼3.3 pm/°C in the range of 25°C-70°C. In addition, the dip intensity was insensitive to temperature. These characteristics have provided convenience in eliminating temperature cross talk and achieving accurate humidity measurement.

Refractive index sensing characteristics of carbon nanotube-deposited photonic crystal fiber SPR sensor

Abstract In this paper, the carbon nanotubes (CNTs)-deposited Au film photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor (CNTs/Au-PCF sensor) and CNTs-deposited Ag film PCF SPR sensor (CNTs/Ag-PCF sensor) were developed and utilized to conduct a series of experiments for the refractive index sensing characteristics study of the CNTs-deposited SPR sensors. The PCF, spliced between two sections of multimode fibers (MMFs), was coated with a metal (Au or Ag) film and then deposited with CNTs for further sensing. CNTs coating can enhance the confined electric field intensity surrounding the sensing layer, making the SPR sensor more sensitive to the changes in the ambient medium. Compared with conventional Au film PCF SPR sensor (Au-PCF sensor), the sensitivity of CNTs/Au-PCF sensor increases by 1016.09 nm/RIU. Compared with conventional Ag film PCF SPR sensor (Ag-PCF sensor), the sensitivity of CNTs/Ag-PCF sensor increases by 709.22 nm/RIU. Therefore, we find that CNTs have a more significant effect on the Au-PCF sensor than the Ag-PCF sensor. The experimental measurements results agreed well with the simulation results. Furthermore, CNTs have high surface-to-volume ratio and extremely excellent biocompatibility. Bovine serum albumin (BSA) was employed as the target analyte to evaluate the feasibility of the CNTs/Au-PCF sensor for the detection of biomolecules, and the sensor exhibits higher sensitivity (8.18 nm/(mg/mL)), lower limit of detection (LOD) (2.5 µg/mL), and faster response time (8 s) than the Au-PCF sensor. Such CNTs-deposited SPR sensors with high sensitivities and fast response present highly promising potential for application in the field of biochemistry.

Optical Fiber Fabry–Perot Tip Sensor for Detection of Water–Glycerin Mixtures

A fiber sensor based on a Fabry–Perot cavity is reported for measuring mixtures of water and glycerin. The sensor is fabricated by producing an air bubble near the end face of a multimode fiber section, and reshaping the tip in order to produce a thin silica diaphragm. It is observed that there is dependence between diaphragm dimensions and the structure sensitivity. The sensor with a 20 μm thick diaphragm presents a sensitivity of 7.81 pm/wt.% regarding the variation of water mass fraction in glycerin. With this sensing head, an experimental resolution of 2.5 wt.% is estimated. By converting the mass fraction into refractive index variations, a maximum sensitivity of 5.49 nm/RIU is obtained. Moreover, given the low-temperature sensitivity (1.6 pm/ °C), the proposed cavity should be adequate to perform temperature independent measurements. The purity degree of glycerin is one of the most important parameters to be determined in applications such as in pharmaceutical or cosmetic area. The proposed sensor can be an alternative to the previously developed ones.

Dual-Parameters Optical Fiber Sensor With Enhanced Resolution Using Twisted MMF Based on SMS Structure

A compact and low cost optical fiber sensor by introducing higher order modes interference with fiber twisting based on single-mode-multimode-single-mode structure is proposed and demonstrated for simultaneous measurement of strain and temperature. The sensor is fabricated by heating and twisting a section of multimode fiber (MMF), which is spliced between two single-mode fibers. By adjusting the heating temperature and rotate speed during fabrication, the twisted region is introduced in MMF, which is able to couple more power of light into the cladding and introduce higher order modes in modal interferences. With this method, strain sensitivity of -7 and -2.19 pm/με as well as temperature sensitivity of 17.33 and 13 pm/°C are obtained. By spatial frequency demodulation method, measurement resolution reaches ±2.14 με and ±0.89 °C, which has great potential in dual-parameters measurement of temperature and strain with high resolution.

Temperature Sensing Characteristics Based on Up-Taper and Single Mode–Multimode Fiber Structure

A novel temperature sensor based on up-taper and single mode–multimode fiber has been presented and demonstrated in this letter. The sensor is fabricated by fusing an up-taper in a single mode fiber (SMF) and splicing the SMF with a section of multimode fiber. In the taper region, a part of light is propagated from the fiber core into the cladding and a series of high-order modes are excited. The reflected light will be coupled back into the core to interfere with the residual core mode when passing through the taper again. Compared with the conventional single mode–multimode–single mode (SMS) fiber sensor, temperature sensitivity of the sensing probe, which consists of SMS and up-taper structure, is improved greatly. According to the experimental results, the temperature sensitivity of 89.42 pm/°C in the temperature range of 20 $^{\circ }\text{C}\sim 80~^{\circ }\text{C}$ can be achieved. The sensor has a series of advantages, such as high sensitivity, low cost, compact size, and reflective measurement.