Multimode Single-mode Fiber Research Articles

Investigation on a compact in-line multimode-single-mode-multimode fiber structure

We carried out a detailed investigation on a compact in-line multimode single-mode multimode (MSM) fiber structure. Both theoretical modal and experimental setup were established to demonstrate the transmission characteristics and the corresponding responses of the applied strain and temperature. The proposed structure simply involves a section of the single-mode fiber (SMF) spliced to two sections of multimode fiber (MMF) and lead-in and lead-out SMFs. The excited environment-sensitive cladding modes together with the fundamental mode in the central SMF form a typical Mach–Zehnder interferometer (MZI). We analyzed the transmission characteristics of the different length of the middle SMF and the MMF in detail. In the experiment, we obtained the extinction ratio of the MSM fiber structure based MZI comb spectrum which was up to 20dB, and sensitivities of 0.7096pm/με (0–2000με) and 44.12pm/°C (10–70°C), which proved the potential sensing applications of the proposed fiber structure.

Experimental Characterization of A Load Sensor Based on Singlemode – Multimode – Singlemode (SMS) Fiber Structure with A Variation of The Number of Bends

A load sensor based on singlemode-multimode-singlemode (SMS) fiber structure is presented in this paper. An SMS fiber structure with core diameter of 50 µm and length of 30 cm of the multimode fiber (MMF) section was chosen as a sensor. The measurement principle of the sensor is based on change of SMS fiber structure power response due to the applied load. A fiber bending and load transmitting (FBLT) device is designed to appropiately bend the SMS fiber structure under applied load in order to make the SMS fiber structure work as a load sensor. The applied load can bend the SMS fiber structure and hence affect the multimode interference (MMI) pattern that occured in SMS fiber structure. The number of bends that occured on SMS fiber structure is determined by the pin number of FBLT device. Five variations of pin number at 3, 5, 7, 9 and 11 pins were chosen. The applied load was varied at 0 – 100 kg. From the experiments, it is found that the relative outpout power response of the sensor gave a specific MMI pattern for each pin number variation. The configuration of 3 pins on FBLT device has the best response as a load sensor with the sensitivity at 1.32 μW/kg and R2 at 0.9783 with the longest measurement range from 1.82 – 91.32 kg.

Rotary propagation characteristics of light in multimode-single mode-multimode fiber structures using ray tracing method

The mode theory is the main way to study the propagation characteristics of light in fiber so far, but it is not suitable for analysis of light in duct. By using ray-tracing method, the rotary propagation characteristics of light in multimode-single mode-multimode (MSM) fiber structures are analyzed in this paper. Firstly, the light ray in fiber can propagate around an inscribed circle, and the central axis of this fiber is the propagation axis. Secondly, the radius of the inscribed circle is decided by both incident angle and incident position, and its variation is between 0 and RM, where RM is the radius of the multimode fiber. Lastly, the bigger the ratio of core and cladding diameter is, the higher the propagation efficiency is.

Multimode Interference in Single Mode–Multimode FBG for Simultaneous Measurement of Strain and Bending

In this paper, a discriminating measurement of strain and bending based on single mode–multimode fiber Bragg grating (FBG) structure is investigated experimentally. Experimental results show that the reflected modes of single mode–multimode FBG exhibit different responses toward the bending of transmitting multimode fiber because of the effect of multimode interference, which can be used for ratiometrical detection of bending, meanwhile the wavelength of the reflected modes is immune from bending but sensitive to strain. Using the different responses of the reflected modes of single mode–multimode FBG to bending, strain and bending can be discriminated by measuring simultaneously the wavelength shift of FBG resonance and the reflected intensity ratio between different modes.

A vibration-sensing system based on SMS fiber structure

A vibration-sensing system which is based on single-mode multimode single-mode (SMS) fiber structure was demonstrated in this paper. When the light is coupled from the lead-in single-mode fiber (SMF) into the multimode fiber (MMF), high-order Eigen modes of the MMF are excited and the interference between different modes occurs while the light propagates along the MMF. The interference results will be changed because the vibration signals are acted on the sensing MMF. Theoretical analysis was carried out, then preliminary experimental results verified the proposed idea. Vibration frequency measurement error of 2% as obtained within the range from 2Hz to 80Hz by using a lock-in amplifier as the signal detection method.

The use of a bend singlemode–multimode–singlemode (SMS) fibre structure for vibration sensing

A bend singlemode–multimode–singlemode (SMS) fibre structure based vibration sensor is proposed and developed. This sensor configuration is very simple and employs a bend SMS fibre structure and a broadband optical source. The vibration applied to the bend SMS fibre structure will change the bend radius and hence the intensity of the transmitted optical power will also vary. Experimental results show that the sensor can detect both vibration frequencies and amplitudes over a broad range with good sensitivity, from a hertz to 12 kHz.

Liquid concentration measurement based on SMS fiber sensor with temperature compensation using an FBG

A highly sensitive liquid concentration sensor based on a single-mode–multimode–single-mode (SMS) fiber structure cascaded with a FBG is utilized and experimentally demonstrated. The two kinds of dips that caused by the modal interference and FBG transmission are observed in the measured spectrum. Dip of SMS would shift while the liquid concentration is changed, but the dip of FBG is kept still. When the ambient temperature changed, the two kinds of dips would shift simultaneously with different sensitivities. The experimental results indicate that it is an effective method to compensate for the temperature perturbation via introducing a FBG. The approximate linear relationship between liquid concentration and spectrum dip wavelength shifts is obtained experimentally. Measurement sensitivity up to 0.52nm/% is showed with the concentration range of sucrose solution from 0 to 26%, which means the measurement resolution about 0.002% could be implemented if the wavelength shift measurement resolution of the optical spectrum analyzer is 1pm.

Estimation of LiBr-H2O Using Multimode Interference (MMI)

We use multimode interference (MMI) as an alternative optical technique to estimate lithium bromide (LiBr)concentration, of the work pair LiBr-H2O, in absorption heat pumps (AHP). The sensing element is a singlemodemultimode-singlemode (SMS) fiber optic structure. This is fabricated by splicing a precisely dimensioned multimodefiber (MMF) section between two singlemode fibers (SMFs). The operation principle is based on the multimodeinterference (MMI) effect occurring in the MMF section. For that purpose, different concentrations of the mixture wereprepared (from 44.30% to 60.69%) to study their optical response. The input field profile entering the sensing element,which is the naked (no cladding) MMF section of the SMS fiber structure, produced different transmitted intensityresponses for each of these concentrations. Thus the optical characterization of the mixture was used to establish amathematical relation to estimate the LiBr concentration. A linear fit for solutions with concentrations ranging from43.30% to 50.87% and refractive indices between 1.421 and 1.439 is demonstrated.

Investigation of the high sensitivity RI sensor based on SMS fiber structure

A high sensitive refractive index (RI) sensor based on a Single-mode Multimode Single-mode (SMS) fiber structure was used and experimentally demonstrated. The dip in the measured spectrum signal caused by the modal interference shifted obviously when the surrounding RI changed. The approximate linear relationship between surrounding RI and spectrum dip wavelength shift was obtained experimentally. The measurement sensitivity up to 286.2nm/RIU was showed with the surrounding RI range from 1.33 to 1.3775, which means the measurement resolution about 3.5×10−6 could be implemented if the wavelength shift measurement resolution of the Optical Spectrum Analyzer is 1pm.

A Static Axial Strain Fiber Ring Cavity Laser Sensor Based on Multi-Modal Interference

A novel and simple static strain fiber ring cavity laser sensor based on all-fiber single-mode-multimode-single-mode (SMS) structure filter is proposed and experimentally demonstrated. When an axial strain is applied on the SMS filter, the pass-band varies, resulting in shifts of the lasing central wavelength. Thus, the value of axial strain imposed on the structure can be calculated by the monitored central wavelength of the system via the function relationship between these two parameters. Compared with the conventional axial strain sensing system based on an SMS fiber structure, the proposal exhibits additional advantages in terms of high optical signal to noise ratio (~ 50 dB) and narrow 3 dB bandwidth apart from simple fabrication and low cost. A good linearity is obtained between the axial strain applied on the SMS filter and central wavelength of the proposed laser sensing system with R2 of 0.99954, and high sensitivity of 2.3 pm/μe in the range 0-2333 μe is successfully achieved.

Optimization of reversible LPFG for sensing application

We report here for the first time to our knowledge the characterization of mechanically induced long period fiber gratings in novel MSM fiber structure. Reversible grating of same period and length was induced in single mode fiber, multimode fiber and novel multimode-singlemode-multimode (MSM) fiber structure. The spectral response of reversible LPFG in SMF is verified experimentally as well as from simulation results and then compared with the experimental spectral response of reversible LPFG in multimode fiber and MSM fiber structure. Reversible LPFG in novel MSM fiber structure is the most optimized and suitable grating for sensing application. For this grating we have obtained single resonant wavelength over a wide wavelength range and maximum transmission loss peak of around 20dB.

Magnetic field sensing based on singlemode–multimode–singlemode fiber structures using magnetic fluids as cladding

Magnetic field sensing based on magnetic fluid (MF) and a singlemode-multimode-singlemode (SMS) fiber structure is proposed. The sensitivity of the proposed sensing system can be enhanced by corroding the cladding of the multimode fiber of the SMS fiber structure. The achieved maximum magnetic field sensitivity of our experimental structures is -16.86 pm/Oe as the fiber is corroded for 1680 s. The visibility of the interference dip for the MF-clad SMS fiber structure decreases with corrosion time. Considering the trade-off between sensitivity and visibility, the figure of merit of the sensing system is employed to evaluate the sensing performance comprehensively. In our experiments, the structure corroded for ~1620 s is found to have maximum sensing performance.

Sensitivity enhancement in a multimode interference-based SMS fibre structure coated with a thin-film: Theoretical and experimental study

Multimode interference in a single-mode–multimode–single-mode (SMS) fibre structure leads to the generation of transmission bands and attenuation bands in the optical spectrum. In this work, the progressive deposition, with layer-by-layer assembly technique, of a high refractive index thin-film onto the SMS structure permits to observe the evolution of the bands as a function of the coating thickness. The results show that it is possible to increase the sensitivity of the device to variations of the coating thickness, what indicates the adequateness of this structure as a sensing platform. The experimental results are corroborated with a numerical method based on coupled-mode theory. As a practical application, a sensitivity improvement has been obtained a pH sensor by increasing the coating thickness.

Designed and Fabricated Solution Concentration Sensor Based on the Single Mode-Etched Multimode-Single Mode Fiber Bragg Grating

A novel kind of solution concentration sensor is designed and fabricated based on fiber Bragg grating with the single mode-etched multimode-single mode fiber Bragg grating structure. Sensing characteristics of Bragg grating to solution concentration is investigated without considering the effects of temperature, sensing detection of four kinds of solution is experimentally realized with the wavelength differences between high modes and the fundamental mode, the higher order modes of the etched multimode fiber is more sensitive to the surrounding refractive index than the fundamental mode.

Relative humidity sensor based on SMS fiber structure with polyvinyl alcohol coating

A novel relative humidity (RH) sensor based on single-mode–multimode–single-mode (SMS) fiber structure is presented. The sensors are created through coating a thin layer of polyvinyl alcohol (PVA) on the multimode fiber deleted the cladding trough HF solution cauterization as the sensitive cladding film, whose refractive index varies as a function of humidity level. Due to the SMS fiber structure's sensitivity to ambient refractive index, the transmission spectra of SMS fiber structure coated PVA film are modified under exposure to different ambient humidity levels ranging from 30% to 80% RH. The related numerical simulations of transmission spectra of SMS fiber structure with different surrounding refractive index are also proposed. The sensitive of the RH measurement of 0.09nm/% RH in the range from 30% to 80% RH is experimentally achieved. Meanwhile the intensity of wavelength at 1543nm is decreasing as the humidity increasing. The experimental results obtained are consistent with the conclusion obtained by numerical simulating.

SMS fiber structure for temperature measurement using an OTDR

A singlemode-multimode-singlemode (SMS) fiber structure for temperature measurement using an optical time domain reflectometer (OTDR)-based interrogation system is proposed. A temperature measurement range of 40 °C–195 °C with a resolution of 0.12 °C and a linearity of 0.992 could be achieved for the multimode fiber (MMF) graded index with a length of 60 mm. It was also demonstrated that two-point temperature measurement with two SMS fiber structures as temperature sensors could be made. The proposed temperature measurement system offered a high resolution and also benefited from a simple configuration with a capability of multi-point temperature measurement.

Hetero-core fiber Bragg grating for simultaneous measurement of strain and bending based on cladding mode ratiometric detection

A simple but practical method using multimode-single mode fiber Bragg grating (FBG) for cladding mode based simultaneous measurement of strain and bending is proposed and investigated experimentally. Experimental results show that the intensity ratio between cladding mode and FBG resonance of hetero-core FBG show monotonous changes following with the increase of deflection, by which strain and bending discrimination can be achieved by measuring the wavelength shift of FBG resonance and ratiometrically detecting the intensity changes between cladding modes and FBG resonance.

Detection of liquid-level variation using a SMS fiber structure

A novel liquid-level sensor based on a single-mode-multimode-single-mode (SMS) fiber structure is proposed and demonstrated. The transmission spectra of the SMS fiber structure are dependent on the fraction of the length of the multimode fiber that is surrounded by the liquid. Both experimental and numerical results with different liquid levels are presented. The experiment results show that the resonance wavelength exhibits a total blue-shift 4nm with the variation of liquid-level 4mm. Meanwhile transmission optical power is obviously decreases in the wavelength range from 1523nm to 1534nm with the increase of the liquid-level. A measurement sensitivity of 1.02nm/mm and 1.03dB/mm is achieved, respectively.

Power-compensated displacement sensing based on single mode-multimode fiber Bragg grating structure

In this paper, power-compensated displacement sensing is proposed and investigated experimentally based on single mode-multimode fiber Bragg grating (FBG) structure, which is fabricated by a single mode fiber and an FBG written on 105/125μm graded-index multimode fiber (MMF). Experimental results verify that the reflected peak power of multiple wavelengths in single mode-multimode fiber Bragg grating structure shows different response to displacement induced bending of transmitting multimode fiber as the result of multimode interference (MMI). By employing different bending responses between multiple wavelengths of multimode FBG, ratiometric detection based high sensitive displacement measurement can be achieved, which provides a simple and practical method for displacement sensing and meanwhile a potential solution for multi-parameter measurement.

侧边抛磨SMS光纤传感器

A new type of optical fiber refractive index sensor was reported.The partial multimode fiber of Singlemode-Multimode Singlemode(SMS)fiber structures were side polished,and then the refractive index sensing properties were analyzed.The results show that multiple loss peaks can be observed in the spectrum of the SMS fiber structures because of the mutual interference by the high order modes of multimode fiber.The spectrum of the SMS fiber structures just drift a few nanometres when the SMS fiber structures are polished to close to the core;but when being side polished to the core of the fiber,the spectrum changes dramatically.After being side polished,the refractive index resolution of the SMS fiber structures will be higher.When the polishing depth is 10μm and the polishing length is 10mm,the refractive index sensitivity of the SMS fiber structure is 703.2nm/RIU.When the polishing depth is 15μm,the refractive index sensitivity of the SMS fiber structure is 798nm/RIU.When the polishing depth is 20μm,the refractive index sensitivity of the SMS fiber structure is up to 1 190nm/URI.A new type of optical fiber refractive index sensor can be realized with a single side polished SMS fiber structure,and the sensitivity of the sensor can be improved by increasing the depth of polishing area.