Microstructured fibers with geometric versatility, and exceptional guiding properties, show unique advantage in constructing high-performance sensors. In this paper, a sensitivity-enhanced fiber-optic sensor based on an exposedcore single-hole microstructured fiber (SHMF) is proposed and experimentally demonstrated for the refractive index (RI) measurement of liquid samples. Experimental results show that the RI sensitivity of the exposed-core microstructured fiber is 375.77 nm/RIU in the low-range of 1.3409~1.3522. With the advantages of enhanced lightmatter interaction ability, tight light confinement and mechanical robustness, the proposed sensor based on exposedcore single-hole microstructured fiber offers great potential in biological and chemical sensing applications.
A coreless-microfiber sensor operating near the turning around point is proposed and experimentally demonstrated for small-scale refractive index (RI) sensing. Numerical simulations verify the existence of turning around point in the coreless-microfiber as the group effective RI difference between the HE11 and the HE12 modes equals zero. Experimental results show the RI sensitivity of the coreless-microfiber sensor could reach 4037.5nm/RIU around 1.333, which is consistent with the theoretical analysis results. Moreover, with the high sensing sensitivity and flexible design, the coreless-microfiber sensor operating near the turning around point has great significance application on molecular trace detection, especially in the fields of photochemical analysis and bioengineering.
An optical fiber Febry-Perot (FP) temperature sensor based on virtual vernier effect is proposed. The parallel vernier effect is realized by superposing virtual reference interferometer to improve the sensitivity of temperature detection. This method can effectively avoid the measurement error caused by the external interference of the physical reference interferometer. The FP temperature sensor was made by sealed with thermosensitive polymer into a capillary. By measuring the wavelength shift of the interference valley, the temperature sensitivity of the sensor is 2.205 nm/°C. The generative approach of the virtual interference spectrum is proposed and the theoretical simulation is carried out. The physical and virtual reference interferometers were respectively used to carry out the comparative experiments of the vernier sensitization. The temperature sensitivities obtained by the physical and the virtual reference interferometers were 10.053 nm/°C and 9.911 nm/°C in the temperature range of 51-55°C, respectively. The sensitivity magnifications were 4.559 and 4.495 times respectively, which proved the feasibility of the FP sensor demodulation by using virtual vernier effect. This method can obtain an optical fiber temperature sensing system with high-sensitivity, low complexity and low cost.
A fiber optic refractive index sensor based on tapered capillary is proposed. A multimode interferometer generated by annular high-order mode is fabricated by fused thin-walled capillary between single-mode fibers, which has high refractive index sensitivity. The experimental results show that the sensor has a piecewise-linear refractive index response with gradually increasing sensitivity in the range of 1.3328~1.4553. It is found that the increase of capillary length is beneficial to the increase of refractive index sensitivity. For the sample with 10 mm capillary, the RI sensitivity is up to 7474.959 nm/RIU, the temperature sensitivity is only 9.48 pm/°C and the measurement error is ±2.12×10-3. The results show that the sensor has good accuracy and repeatability in refractive index measurement. The sensor is suitable for high refractive index range in biological process monitoring, where high refractive index sensitivity and low temperature cross sensitivity are required.
Bending vector sensing is demonstrated based on periodic core-offset ultra-long period fiber grating, which is mainly composed of splicing two fiber ends with a purposed lateral offset in a single mode fiber (SMF).Through the measured resonance wavelength and the peak attenuation of the core-offset ultra-long period fiber grating(CO-ULPFG), the curvature amplitude and orientation can be simultaneously determined in vertical directions, reaching up to -1.833 nm/m-1 and -1.674 nm/m-1 at the 0° and 180° orientations range and reaching up to -0.583 nm/m-1 and -0.479 nm/m-1 at the 90° and 270° orientations ranging from 0 m-1 to 7.041 m-1, respectively. The bending sensitivity in different directions is 6-10 times that of LPFG with similar structure. Because of the excellent linear response, the effect of temperature on bending sensing can be eliminated by using a measurement matrix. The advantages of bending direction recognition, high bending sensitivities and compact structure make the sensor attractive for structural deformation monitoring.
A fiber optic refractive index (RI) sensor that consists of cascaded single mode fiber-no core fiber-single mode fiber (SNS) structure followed by a fiber Bragg grating (FBG) was proposed and demonstrated. Because the SNS structure and FBG have similar temperature characteristics and the FBG is not sensitivity to the RI, temperature self-compensation during the RI measurement can be realized by calculating the difference between the characteristic wavelengths of the SNS and FBG. Three SNS-FBG-based sensors with different lengths of the NCF and different Bragg wavelengths were cascaded to achieve wavelength division multiplexing (WDM) in the range of 1510nm-1590nm. Experimental results show that three-point independent RI measurement can be accomplished with the maximum sensitivity of 211.088, 243.429 and 242.511nm/RIU (RI unit) in the RI range from 1.333 to 1.405, respectively. The RI errors caused by temperature are only 4.548×10-5RIU/°C, 3.779×10-5RIU/°C and 4.458×10-5RIU/°C respectively. These sensors feature the advantages of good reproducibility, small size, low cost, high sensitivity and very low temperature cross-sensitivity, which have a good practical prospect in the simultaneous multi-point RI measurement application area.
A fiber Bragg grating (FBG) textile-based pulse wave sensor is proposed and demonstrated based on multi-layers compound fabrics structure which can convert the fabric tension to the pressure of FBG sensing unit needed by pulse measurement and improve its sensitivity. On the basis of material mechanics theory, the sensing model is established and the theoretical formulas are deduced. A prototype of cuff-type FBG textile-based pulse sensor is achieved by the knitted way of compound fabrics. The test results show that the sensor can effectively obtain pulse wave which has higher signal-to-noise ratio and contains more detail features than the photoplethysmography (PPG) sensor. This sensor has potential applications in smart textile and a good practical prospect for the wearable human pulse measurement during the magnetic resonance imaging (MRI) scan.
We used a system that exploits the Mach–Zehnder interferometer structure and Young’s double-pinhole interference principle to measure the three-dimensional topographies of small objects at high precision. Next, we performed phase profilometry to measure small objects and achieve a height measurement within a 10 × 10-cm area. The accuracy of the measurement system improved by 44.1%, and the measurement time was reduced by 63.2%.
Polarization sensitivity is an important factor that affects the interrogation of ultrasmall arrayed waveguide grating (AWG) for fiber Bragg grating (FBG) sensor. An ultrasmall 1 × 8 silicon-on-insulator (SoI) AWG with a core size of less than 530 μm × 480 μm is proposed in this study. This ultrasmall SoI AWG exhibits good transmission spectra and high polarization sensitivity. The increased channel numbers and tight structure increase the polarization sensitivity of AWG. Temperature interrogation experiments show that the FBG sensor interrogation is drastically affected when the effect of polarization sensitivity on the ultrasmall AWG is sufficiently large.
A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718 nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22 pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73 pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61×10−4 RIU and 6.81×10−4 RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication.
Fiber optic sensor embedded in textiles has been a new direction of researching smart wearable technology. Pulse signal which is generated by heart beat contains vast amounts of physio-pathological information about the cardiovascular system. Therefore, the research for textile-based fiber optic sensor which can detect pulse wave has far-reaching effects on early discovery and timely treatment of cardiovascular diseases. A novel wavelength demodulation method based on photonic crystal fiber (PCF) modal interference filter is proposed for the purpose of developing FBG pulse wave sensing system embedded in smart clothing. The mechanism of the PCF modal interference and the principle of wavelength demodulation based on In-line Mach-Zehnder interferometer (In-line MZI) are analyzed in theory. The fabricated PCF modal interferometer has the advantages of good repeatability and low temperature sensitivity of 3.5pm/°C from 25°C to 60°C. The designed demodulation system can achieve linear demodulation in the range of 2nm, with the wavelength resolution of 2.2pm and the wavelength sensitivity of 0.055nm-1. The actual experiments’ result indicates that the pulse wave can be well detected by this demodulation method, which is in accordance with the commercial demodulation instrument (SM130) and more sensitive than the traditional piezoelectric pulse sensor. This demodulation method provides important references for the research of smart clothing based on fiber grating sensor embedded in textiles and accelerates the developments of wearable fiber optic sensors technology.
A sensor head consisting of an all single-mode fiber (SMF) in-line Mach–Zehnder interferometer (MZI) with an embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. It is fabricated by cascading two bulge-taper fusion structures in a section of SMF including an FBG. The MZI is sensitive to fiber bending and ambient temperature with a sensitivity of −16.59 nm/m−1 in the range of 1.05 to 4.05 m−1 and 58 pm/°C in the range of 30°C to 100°C, respectively. However, the FBG is only sensitive to the latter with a sensitivity of 13 pm/°C. Simultaneous measurement of curvature and temperature is obtained and the cross-sensitivity issue can be solved. The experimental results show that the average relative error of the curvature is 0.38%, which is about 18 times better than that without temperature compensating. The average error of temperature is only 0.21°C.
A highly efficient polarization-independent output grating coupler was optimized and designed based on silicon-on-insulator used for silica-based hybrid photodetector integration in an arrayed waveguide grating demodulation-integrated microsystem. The finite-difference time-domain (FDTD) method optimizes coupling efficiency by enabling the design of the grating period, duty cycle, etch depth, grating length, and polarization-dependent loss (PDL). The output coupling efficiencies of both the transverse electric (TE) and transverse magnetic (TM) modes are higher than 60% at 1517 to 1605 nm and ∼67% at around 1550 nm. The designed grating exhibits the desired property at the 3-dB bandwidth of 200 nm from 1450 to 1650 nm and a PDL <0.5 dB of 110 nm from 1513 to 1623 nm. The power absorption efficiency at 1550 nm for TE and TM modes reaches 78% and 70%, respectively. Both the power absorption efficiency of TE mode and that of TM mode are over 70% in a broad band of 1491 to 1550 nm.
A distributed optical fiber grating wavelength interrogator based on fiber Fabry Perot tunable filter(FFP-TF) was proposed, which could measure dynamic strain or vibration of multi-sensing fiber
gratings in one optical fiber by time division way. The wavelength demodulated mathematical model was built, the formulas of system output voltage and sensitivity were deduced and the method of
finding static operating point was determined. The wavelength drifting characteristic of FFP-TF was discussed when the center wavelength of FFP-TF was set on the static operating point. A wavelength locking method was proposed by introducing a high-frequency driving voltage signal. A demodulated system was established based on Labview and its demodulated wavelength dynamic range is 290pm in theory. In experiment, by digital filtering applied to the system output data, 100Hz and 250Hz vibration signals were measured. The experiment results proved the feasibility of the demodulated method.
Heart sound includes a lot of physiological and pathological information of heart and blood vessel. Heart sound detecting
is an important method to gain the heart status, and has important significance to early diagnoses of cardiopathy. In order
to improve sensitivity and reduce noise, a heart sound measurement method based on fiber Bragg grating was
researched. By the vibration principle of plane round diaphragm, a heart sound sensor structure of fiber Bragg grating
was designed and a heart sound sensing mathematical model was established. A formula of heart sound sensitivity was
deduced and the theoretical sensitivity of the designed sensor is 957.11pm/KPa. Based on matched grating method, the
experiment system was built, by which the excursion of reflected wavelength of the sensing grating was detected and the
information of heart sound was obtained. Experiments show that the designed sensor can detect the heart sound and the
reflected wavelength variety range is about 70pm. When the sampling frequency is 1 KHz, the extracted heart sound
waveform by using the db4 wavelet has the same characteristics with a standard heart sound sensor.
Magnetic resonance imaging (MRI) has become an indispensable aid to diagnosis and treatment. As the doctor cannot
accompany the patient, it is essential that the patient be monitored remotely to avoid the risk of respiration being
impaired by anesthetic drugs or upper airway obstruction. A smart wearable textile sensing system is described in this
paper. A fiber Bragg grating (FBG) with polymer encapsulation has been woven into an elastic bandage to detect the
respiration motion. According to the strain principle of FBG, the breathing rate and intensity can be obtained by
measuring the variety of FBG reflected wavelength. In order to eliminate the temperature cross-sensitivity, a FBG
temperature sensor has also been woven into the bandage to achieve the temperature compensation computing. Based on
the tunable Fabry-Perot filter wavelength demodulated theory, wavelength measuring method and data processing
arithmetic have been presented, and the system with ARM microprocessor has been designed to process and display the
breathing information. The experiments to the system have proved that the wavelength measuring range is about 40nm,
the resolution of wavelength can arrive at 2pm, and the sampling rate is 5Hz.
Based on optical fiber grating sensing technology, the signal processing system of smart clothes was designed. The
clothes embedded in fiber Bragg gratings (FBGs) sensor can measure the body physiological parameters, such as
temperature, and detect the healthy condition of wearer. The paper presented FBG wavelength detection and signal
processing methods with wearable characteristics, designed optical scheme and demodulated circuit in terms of tunable
F-P filer wavelength demodulation theory. The numerical arithmetic of processing wavelength data was researched and
realized in ARM. By testing, the measuring scope of wavelength is from 1520.5 to 1562nm, and the resolution of
wavelength can arrive at 2pm. By selecting appropriate center wavelength of sensing gratings and referenced gratings,
the relative error of wavelength can less than 0.001%. The signal processing system can adjust the excursion of F-P
cavity timely, and has characters of portability, wireless data transmission.
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