Single-mode-multimode-single-mode Structure Research Articles

Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing

A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed.

Highly Efficient Refractive Index Sensor Based on a Dual-Side Polished SMS Fiber Enabled by Femtosecond Laser Writing.

Fiber-optic refractive index (RI) sensors based on wavelength-shift-based interrogation continue to present a challenge in achieving high sensitivity for a wide detection range. In this paper, we propose a sensor for determining the RI of liquids based on femtosecond laser (fs-laser) writing of a dual-side polished singlemode-multimode-singlemode (SMS) fiber. The proposed sensor can determine the RI value of a surrounding liquid by detecting the dip wavelength in the transmission spectrum of the light propagating through the sensing area. The high RI sensitivity is attributed to the increased interaction area established by the fs-laser, which creates hydrophilic surfaces and maintains the wide detection range of the SMS structure. The results of the wavelength-shift-based interrogation reveal that the fabricated device exhibited a high sensitivity of 161.40 nm per refractive index unit (RIU) over a wide RI detection range of 0.062 RIU. The proposed device has high processing accuracy and a simple manufacturing process. Hence, it has the potential to be used as a lab-on-fiber sensing platform in chemical and biotechnological applications.

Temperature sensing based on multimode interference in polymer optical fibers: sensitivity enhancement by PC-APC connections

A simple, stable, and high-sensitivity temperature sensor based on multimode interference in a polymer optical fiber (POF) with higher-order mode excitation has been developed. In a single-mode–multimode–single-mode (SMS) structure, one end of the multimode POF with physical-contact (PC) connectors is connected to a silica single-mode fiber with an angled-PC (APC) connector. We compare the temperature sensing characteristics of the three configurations (no PC-APC, PC-APC at input, and PC-APC at output) and obtain the highest temperature sensitivity of 219.2 pm °C−1, which is more than double the value of the standard (no PC-APC) SMS structure.

Optical Fiber Magnetic Field Sensor Based on Silk Fibroin Hydrogel

In this work, an optical fiber magnetic field sensor is proposed and demonstrated based on silk fibroin hydrogel (SFH). The primary function of SFH is the carrier of material (Streptavidin magnetic nanoparticles, SMN) that is susceptible to the magnetic field. The multimode interference spectrum is obtained by configuring a single-mode -multimode -single-mode (SMS) structure. The SMS structure is consisted by sequentially connecting a single-mode fiber (SMF), a no-core fiber (NCF), and a SMF. In addition, the SMN responds to a magnetic field, and SMN will modify the effective refractive index (RI) of SFH. Hence, magnetic field sensing can be performed by coating a mixture of SMN and SFH on the SMS structure. Our experimental results show that the maximum sensitivity of the proposed sensor reaches 673.53pm/Oe, and the average sensitivity of 263pm/Oe in the magnetic field strength range from 0 to 115Oe. The proposed optical fiber sensor of the magnetic field, featuring the benefits of environmental friendliness, low cost, easy manufacturing, and simplicity of structure, can find potential application prospects in various areas such as the industry, environment, and military.

Yb-ASE Suppression in Single-Frequency Hybrid Double Cladding Erbium–Ytterbium Co-Doped Fiber Amplifier with SMS Structure

A hybrid double cladding erbium–ytterbium co-doped fiber (EYDF) amplifier with a single-mode-multimode-single-mode (SMS) active fiber is demonstrated in this study. The hybrid gain fiber with an SMS structure is composed of two kinds of EYDFs with 6 and 12 μm core diameters. The transmission spectra of the SMS fiber structure were theoretically analyzed and the simulation results indicated that the maximum loss in the 1~1.1 μm band where the Yb-band amplified spontaneous emission (Yb-ASE) located, was much larger than that of the 1.5-μm band because of the wavelength difference. The power performance and spectra properties of the hybrid fiber amplifier were theoretically and experimentally analyzed and compared with a typical uniform fiber amplifier under the same conditions. The experimental results demonstrated that this hybrid fiber amplifier can suppress the Yb-ASE by over 12 dB and increase the slope efficiency by more than 2%, but the ASE in the 1.5-μm band increases by 2~3 dB. This work provides a possible method to enable EYDF amplifiers to suppress the Yb-ASE and overcome the pump bottleneck effect.

Research on intelligent mattress based on improved SMS structure sensing fiber

The contact-type vital signs detection system affects the user’s comfort and causes inconvenience due to its monitoring through skin contact. And most of the non-contact equipment precision is not high, or the cost is too high. To solve the above problems, this article is based on the physiological activity of slight vibration in multimode optical fiber micro bending deformation which results in the change of light intensity principle, considering the singlemode-multimode- singlemode(SMS) fiber of the structure of the sensor has simple structure, low cost, high sensitivity, but get the signal to noise is bigger, the accuracy is not high, this paper designs a kind of intelligent mattress based on modified SMS sensing optical fiber. The mattress can obtain ballistocardiogram signal in real time by changing the small light intensity, and obtain the heart rate, respiration rate and other information through the algorithm processing signal. In this design, the sensing fiber of the mattress is embedded in a small core fiber to solve the shortcomings of signal noise interference and instability obtained by the SMS structure fiber, and then it is embedded in the mattress by means of serpentine routing, and uses the covering material of the mattress adjusts the micro-bending period of the optical fiber to reach the critical mechanical period, so that the mattress has high sensitivity. Through in the laboratory and SMS type sensor for heart and breath signal are compared, and contrast and by Fast Fourier Transform (FFT) in frequency domain signal is analyzed, the vital signs of signal more clearly this mattress stable noise smaller, replace the tester for many times after tests, get the signal of high accuracy, the error is very small. The experiment shows that the mattress can detect non-sensory vital signs, facilitate the user to monitor his/her physical condition in real time in daily work and life, and timely inform the family members and medical staff in case of emergency, so as to avoid the occurrence of accidents.

Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications—A Review

A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion- spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors.

Simultaneous Measurement of the Refractive Index and Temperature Based on a Hybrid Fiber Interferometer

A simple and compact sensor based on a hybrid interferometer, which is composed of a cascaded single-mode-multimode-single-mode (SMS) and bending single-mode fiber (BSF) structure, is proposed and investigated. By carefully designing the multimode fiber length of the SMS and the bending diameter of the BSF, the transmittance spectrum features two kinds of dips simultaneously, which show different responses to the surrounding refractive index (RI) and temperature. The SMS structure is only sensitive to the temperature, while the BSF structure is sensitive to both the RI and temperature. Therefore, the simultaneous measurement of the RI and temperature can be achieved by monitoring the two dips induced by the SMS and the BSF, respectively. According to this principle, a cascaded SMS-BSF hybrid sensor is designed, fabricated, and tested. Experimental results show that the sensor possesses an RI sensitivity of 331.71 nm/RIU (from 1.333 to 1.404) and a temperature sensitivity of -1.053 nm/°C (from 30 °C to 70 °C), which is outstanding among the reported results.

Optical Fiber Vacuum Sensor Based on Etched SMS Structure and PDMS Coating

In this work, an optical fiber vacuum sensor based on a single-mode multimode single-mode (SMS) structure coated with polydimethylsiloxane (PDMS) is studied. The SMS structure generates an interferometric pattern based on multimode interference. The structure is dip-coated with a layer of PDMS, whose optical properties change when it is subjected to varying vacuum pressure. Different strategies are applied in an attempt to improve the final performance of the sensor, such as decreasing the diameter of the fiber and modifying the properties of the coating by modifying the proportion of solvent. Decreasing the diameter of the optical fiber and using toluene as a solvent are both proved to be successful strategies for increasing the sensitivity of the sensor. The devices are studied in the $1\times 10^{-3}$ –10 mbar range with a maximum wavelength shift of 12 nm, leading to a maximum sensitivity of 35 nm/mbar. The simplicity of the fabrication process, which can be applied to more sensitive structures, suggests that PDMS may be a good choice for the development of optical fiber vacuum sensors.

Broadly tunable Tm/Ho-codoped fiber lasers based on temperature-sensitive single-mode-multimode-single-mode fiber structures.

We present all-fiber tunable lasers using single-mode-multimode-single-mode (SMS) structures involving a liquid cladding in the multimode section. We make use of large thermooptic coefficients of the refractive-index liquids to tune the laser oscillation wavelength. The oscillation wavelength is changed over a range of 100 nm from 1858 nm to 1958 nm by controlling the temperature of the SMS structures in the Tm/Ho-codoped fiber ring resonators.

Power Loss Classification on Shifts Based on SMS (Singlemode-Multimode-Singlemode) Structured Fiber Optic Using Gaussian Naïve Bayes Method

Singlemode-multimode-singlemode based optical fiber can be used as a good communication medium, but energy or power carried by light will be weakened (losses) due to leakage or due to lack of clarity or shift in optical fibers. In this study, power losses in fiber optics based on SMS will be classified based on changes in the values of power losses to shifts and divided them according to three classes, there are good, average, and bad. The shift will be used as a classification variable that is between 0 μm to 450 μm with an increment of 50 µm for every interval. The SMS optical fiber structure used is 5.5 with 25 attempts on different optical fibers. The classification method used is Naïve Bayes with a Gaussian distribution. Gaussian distribution is used in Naïve Bayes because the dataset will be processed in the form of continuous values. From the results of testing based on TP+TN = 6, FP = 6 FN = 6 on confusion matrix, the classification accuracy value was 42.86%. This indicates that this classification method is still less effective for classifying fiber optic power losses with an SMS structure. For further study, another classification methods can be used in the power loss classification to get better results.

Acoustic frequency vibration sensor based on tapered SMS fiber structure

The propagation of light from a singlemode fiber (SMF) to a multimode fiber (MMF) arouses a series of eigenmodes, which interference together along the MMF where the positions of the local maximum intensity are the self-imaging points (SIPs). When the light in the MMF propagates to a SMF, the maximum coupling efficiency is obtain when the SMF and MMF connect at a SIP. However the connection generally deviates the SIPs for the accurate connection is difficult to be achieved, which decreases the accuracy of the sensor severely. Herein a novel vibration sensor based on the singlemode multimode singlemode (SMS) fiber is developed, in which the MMF is etched, which turns the SIP to the connection of the MMF and the output SMF accurately. The etched SMS structure dispenses with the requirement of highly accurate connection at a SIP and provides a succedaneous scheme that is sensitive to vibration. The etched sensor responses to the vibration more easily comparing to the un-etched one. Experimental results demonstrate that the sensitivity of the SMS sensor with the both ends etched is respectively over 5 and 40 times as that of the middle etched and un-etched one. When the MMF length decreases from 8 to 2 cm, the vibrating sensitivity of the sensor increases 13.5 times. Experimental results demonstrate that the etched structure with MMF length of 2 cm responses well to the acoustic wave from 20 to 300 Hz, where a pressure sensitivity of 1.49 mV/mPa and linear correlation index of 0.9980 can be attained at 120 Hz.

High sensitivity optical fiber strain sensor using twisted multimode fiber based on SMS structure

A low-cost way of achieving a high sensitivity optical fiber strain sensor by introducing higher-order interference modes using a torsional multimode fiber (MMF) instead of normal MMF based on single-mode–multimode–single-mode (SMS) structure is proposed and the coupling mechanism of twist fiber is investigated theoretically. The sensor is fabricated by simple process of heating and twisting a small region of MMF. According to this method, the shift of multimode interference spectrum caused by an axial strain will be greatly magnified. Different strain sensitivities can be easily realized by controlling the torsional number of circles. The experimental results indicated a high strain sensitivity of 42.5 pm/με at most.

Fiber-Optic Immunosensor Based on an Etched SMS Structure

A novel and high sensitive IgG - anti-IgG biosensor based on multimode interference induced through a single-mode-multimode-single-mode (SMS) optical fiber structure is developed in this contribution. First, an improvement of the performance of a basic 125-μm-diameter SMS structure is carried out by etching the optical waveguide cladding to a diameter of 15 μm. This leads to sensitivity values eight times higher than the original SMS structure. Then, 3-glycidopropyl-trimethoxysilane is used to attach the IgGs to the optical substrate. Finally, a phosphate buffer saline solution containing an increasing concentration of antigoat IgGs is used to detect biological interactions up to 104 μg/ml. High-resolution mass spectrometry was used to corroborate the attachment of the biomolecules to the optic fiber. The analyzed bioprobe presents no cross sensitivity to other biomolecules immersed in the same detection solution.

Multiparameter POF Sensing Based on Multimode Interference and Fiber Bragg Grating

We report the fabrication of a multimode interferometer (MMI), based on a multimode polymer optical fiber (POF), sandwiched between two single-mode silica fibers. The POF used in this work is a step-index fiber with core and cladding composed of a cycloolefin polymer (COP) and PMMA, respectively. The low moisture absorption of the COP material and the benefit of having an exposed core for refractive index measurements in MMI devices lead us to perform an etching of the PMMA layer. Additionally, we report the inscription of a Bragg grating in the COP core material of an etched single-mode-multimode-single-mode (SMS) structure. The two fiber structures were simultaneously characterized to strain, temperature, humidity, and refractive index. The ability to detect refractive index by using a POF-based SMS structure is reported for the first time in this paper. Additionally, the capability for dual parameter measurement was discussed using both FBG and SMS structures.

Tunable multiwavelength Tm-doped fiber laser based on the multimode interference effect.

A simple multiwavelength Tm-doped fiber laser at the 2μm band based on multimode interference (MMI) is proposed and experimentally demonstrated. In this scheme, a 4m Tm-doped single-mode fiber is pumped by a 1568nm laser, and a single-mode-multimode-single-mode (SMS) fiber structure is used as an MMI filter in which the multimode fiber is used to tune the laser. Laser operation of up to three wavelengths is obtained based on the MMI filter. The wavelengths can be tuned by adjusting the polarization controller and rotating the multimode fiber in the SMS structure, and the tuning region is about 24nm, i.e., 1892-1916nm. The side-mode suppression ratio of the laser is about 54dB. The 3dB linewidth is less than 0.04nm. Peak fluctuation at each wavelength is analyzed, and the results show that the power fluctuation is less than 3dB around the average power.

Sensors Based on Thin-Film Coated Cladding Removed Multimode Optical Fiber and Single-Mode Multimode Single-Mode Fiber: A Comparative Study

Two simple optical fibre structures that do not require the inscription of a grating, a cladding removed multimode optical fibre (CRMOF) and a single-mode multimode single-mode structure (SMS), are compared in terms of their adequateness for sensing once they are coated with thin-films. The thin-film deposited (TiO2/PSS) permits increasing the sensitivity to surrounding medium refractive index. The results obtained can be extrapolated to other fields such as biological or chemical sensing just by replacing the thin-film by a specific material.

Temperature cross-sensitivity characteristics of singlemode-multimode-singlemode fiber structure.

The temperature cross-sensitivity characteristics of a singlemode-multimode-singlemode (SMS) fiber structure packaged by a shell are studied both theoretically and experimentally. By theoretical investigation, we found that the temperature sensitivity of a SMS structure is mainly determined by the thermo-optic effect (TOE) of the cladding of the multimode fiber (MMF). Meanwhile, the TOE of the MMF core, thermal expansion effects (TEEs) of the MMF core, and the packaging material also influence the ultimate sensitivity, and the magnitude of their effects depends on the refractive index of the MMF cladding. Among them, the TEE of the packaging material, inducing an axial strain, is considered to be the second main factor. A temperature sensor based on a packaged SMS structure is designed and investigated to experimentally verify the theoretical findings. The experimentally measured temperature sensitivity of the sensor is -453.4 pm/°C, which agrees well with the theoretical prediction.

High efficiency single-mode-multimode-single-mode fiber laser with diffraction-limited beam output.

We designed and tested an all-fiber, high efficiency Yb-doped laser operating at 1088nm with a single-mode-multimode-single-mode (SMS) structure. A larger-mode-area gain fiber of 1.5m length, with 20/130μm core/cladding diameters was used to increase the absorption, and a diffraction-limited Gaussian output beam was obtained from the single-mode output fiber. Using a 976nm laser diode as the pump source, the laser generated an output power up to 38.5W with a slope efficiency of 70%. The output beam qualities, with and without SMS structure, were compared and showed that the fiber laser with the SMS structure can achieve high gain, short fiber length, and excellent beam quality.

Multimode interference devices with single-mode-multimode-multimode fiber structure.

We present a single-mode-multimode-multimode (SMm) fiber structure with a few interesting device applications. Unlike the single-mode-multimode-single-mode (SMS) structure, SMm has the unique feature of more than one mode in the output fiber. A detailed physical understanding of the transmission properties, and the differences from the SMS structure, is presented. The device can be used to excite selected modes in the output multimode fiber (MMF). This can be used to reduce the number of modes in high-speed MMF applications using an all-fiber structure instead of bulk optics. In yet another possible application, we show a way of designing sensitive refractive index sensors for measurement in different RI ranges.