Dip Wavelength Research Articles

High sensitivity fiber temperature sensor based PDMS film on Mach-Zehnder interferometer

An optical fiber temperature sensor based on a Mach-Zehnder interferometer (MZI) coated with a film of polydimethylsiloxane (PDMS) was demonstrated. The MZI was fabricated by a fiber mismatch structure through core-offset fusion splicing method. The resonant dip wavelength of the MZI will shift with the changing of the environmental temperature own to the effects of expansion and thermo-optic of the PDMS. Experimental results showed that the proposed sensor shows the temperature sensitivity of 0.101 nm/°C under the temperature range of 20 °C to 100 °C, which is about 2 times of the existing similar temperature fiber sensors.

Humidity and temperature sensor based on a Mach-Zehnder interferometer with a pokal taper and peanut taper.

An optical fiber sensor for simultaneous measurement of humidity and temperature is proposed and investigated. The sensor is composed of a pokal taper and a peanut taper spliced at both ends of a single-mode fiber, respectively. In the experiment, the variations of the dip wavelength are used to measure the parameter sensing characteristics. The sensing area is coated with a concentration of 1 mg/ml graphene dispersion by using the optical driven deposition method. Experimental results show that the sensor has a good linear relationship with temperature and humidity. When the humidity measurement range is 30%RH∼70%RH, the sensitivities are 0.059 nm/%RH and 0.133 nm/%RH, respectively. When the temperature measurement range is 0∘C∼40∘C, the sensitivities are 0.254 nm/°C and 0.269 nm/°C, respectively. This sensor has the advantages of small size, low cost, and high response.

Temperature sensitivity investigation of tapered plastic optical fiber-Mach Zehnder interferometer for sensor application

Mach Zehnder interferometer (MZI) based on plastic optical fiber (POF) is interesting due to its robustness and simple fabrication. In this work, MZI-POF was designed and its sensitivity to temperature change was characterized to investigate its performance for sensor application. The MZI-POF was fabricated by forming two tapers at several distance by simple heat-pull method. The spectra was investigated by launching white light emitting diode (LED) and the output were observed using VIS-NIR spectrometer. It is shown that the wavelength of spectrum peak and spectrum dip were shifted compared to that of un-tapered POF which indicates that the structure serves well as MZI. Sensor sensitivity to temperature was determined by measuring the spectrum for various temperature which was varied from 40°C to 80°C with increment of 5°C. The result showed that the sensor sensitivity is 0.0693nm/°C in terms of peak measurement and 0.0487nm/°C for dip measurement. Therefore, MZI-POF is a potential candidate for temperature sensor.

Refractive index sensor based on concentric triple racetrack resonators side-coupled to metal–insulator–metal waveguide for glucose sensing

A refractive index sensor using two concentric triple racetrack resonators with plasmonic metal–insulator–metal (MIM) waveguides is suggested to provide more freedom for sensing applications. Due to momentum matching, intensive coupling happens between the equivalent modes of each section of the resonators. The sensing properties are numerically discussed in terms of the finite difference time domain method. According to the outcomes, when increasing the refractive index of the material in the outer sections of the racetrack resonators, the dip wavelengths show a notable red shift. The sensing performance can be enhanced by using two multiple concentric resonators that effectively increase the strength of the light–analyte interaction, which is useful for sensing applications. Our proposed nanosensor offers a high sensitivity value, sensing resolution, and figure of merit of 1618 nm/RIU, 6.18×10−4RIU, and 89RIU−1, respectively. Also, the racetrack resonators coupled to MIM waveguides can be simply integrated into chip circuits with other optical devices to perform monitoring and filtering tasks.

The Effect of Core Offset on the Mode Converting Characteristics in Twisted Single Mode Fibers

The effect of core offset on the mode converting characteristics in twisted single mode fiber (SMF) is investigated. If the core position of SMF is slightly shifted from its rotation center, multiple spectral dips appear in the transmission spectra due to the coupling between core and cladding modes. Guided mode solver and beam propagation method (BPM) based on finite-element method (FEM) specially formulated for helicoidal waveguides are used for the analysis. It is shown that a submicron offset of the core, which may not be seen in the microscope with 100-μm scale, causes strong modal coupling. Calculated positions of the dips for the submicron offset are in good agreement with the recently reported measured results, showing the slight core displacement may be one of the dominant mechanisms of the mode coupling in twisted SMF. Also, it is confirmed in the BPM analysis that the field in the twisted SMF carry an orbital angular momentum (OAM) at the spectral dip wavelength.

Long-period fiber gratings inscribed in few-mode fibers for discriminative determination.

We successfully fabricated the long-period fiber gratings in few-mode fibers (FMF-LPFGs) with micro-tapered method, which are different from the traditional LPFGs that only couple the fundamental mode to different cladding modes to obtain multiple resonant dips. There are two resonant dips on the transmission spectrum of the FMF-LPFGs, which are induced by the coupling between the fundamental mode and the low-order cladding mode LP03 (dip 1) and the coupling between the fundamental mode and the high-order core mode LP11 (dip 2). Due to the difference of the coupling mechanism involved in two dips, the shift of resonant wavelengths has different characteristics with the variation of the external environment parameter. The corresponding wavelength of dip 1 exhibits a red shift as the temperature increased. But for dip 2, the resonant wavelength has a blue shift. In addition, the two dips have different temperature and strain sensitivities. Therefore, discriminative determination of temperature and strain is realized by establishing the cross coefficient matrix, and the relative measurement error is less than 3%. What's more, we theoretically analyzed the reason why the two resonant wavelengths shift toward opposite direction with the increase of temperature and toward the same direction with the increase of strain.

Directional torsion and strain discrimination based on Mach-Zehnder interferometer with off-axis twisted deformations

An all-fiber sensor based on a Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated to measure directional torsion and strain simultaneously. The MZI employing a pair of off-axis twisted deformations (OATDs) on single mode fiber (SMF) is fabricated by continuous arc discharge method. It performs OATDs causing permanent twisted distortion to form light beam splitter and combiner of the MZI. The transmission spectrum of the OATDs-MZI has multiple resonance wavelength dips, which correspond to different cladding modes. The torsion and strain measurement can be simultaneously achieved by monitoring changes in the dips of wavelength. Due to the OATDs, the proposed sensor has the ability to distinguish torsional direction, and a strain sensitivity as high as −0.0285 nm/με was obtained. The proposed sensor exhibits excellent torsion and strain resolution of ±0.0611 rad/m and ±0.8232 με. The device has the advantages of low cost, small size, simple preparation steps, short preparation cycle, etc. Due to its excellent performance, it has the potential to become an excellent universal directional torsion and strain sensor.

Analysis of a Magnetic Field Sensor Based on a Sagnac Interferometer Using a Magnetic Fluid-Filled Birefringent Photonic Crystal Fiber

A magnetic field sensor based on a Sagnac interferometer using a magnetic fluid-filled birefringent photonic crystal fiber (MF-BPCF) was proposed and studied by finite element method. Two large air holes in the birefringent photonic crystal fiber were designed to fill with magnetic fluid. The MF-BPCF in the proposed Sagnac interferometer acted as the sensing head. As magnetic field perpendicular to the MF-BPCF, magnetic fluid was anisotropic. The characteristics of the output spectrum of the Sagnac interferometer was simulated. The results showed that when the phase difference $\phi$ satisfied integer multiples of $2\pi$ , several dip wavelengths appeared. Not only the intensity but also the direction of the magnetic field was measured. The measuring sensitivity could be further promoted by making the dip wavelength approach $\lambda _{B_g=0}$ .

Strain independent twist sensor based on uneven platinum coated hollow core fiber structure.

Optical fiber based twist sensors usually suffer from high cross sensitivity to strain. Here we report a strain independent twist sensor based on an uneven platinum coated hollow core fiber (HCF) structure. The sensor is fabricated by splicing a section of ~4.5-mm long HCF between two standard single mode fibers, followed by a sputter-coating of a very thin layer of platinum on both sides of the HCF surface. Experimental results demonstrate that twist angles can be measured by monitoring the strength change of transmission spectral dip. The sensor's cross sensitivity to strain is investigated before and after coating with platinum. It is found that by coating a platinum layer of ~9 nm on the HCF surface, the sensor's cross sensitivity to strain is significantly decreased with over two orders of magnitude less than that of the uncoated sensor sample. The lowest strain sensitivity of ~2.32×10-5 dB/𝜇𝛆 has been experimentally achieved, which is to the best of our knowledge, the lowest cross sensitivity to strain reported to date for optical fiber sensors based on intensity modulation. In addition, the proposed sensor is capable of simultaneous measurement of strain and twist angle by monitoring the wavelength shift and dip strength variation of a single spectral dip. In the experiment, strain and twist angle sensitivities of 0.61 pm/𝜇𝛆 and 0.10 dB/° have been achieved. Moreover, the proposed sensor offers advantages of ease of fabrication, miniature size, and a good repeatability of measurement.

Temperature- and strain- insensitive transverse load sensing based on optical fiber reflective Lyot filter

For the first time to our knowledge, we propose and experimentally demonstrate a transverse load sensor using an optical fiber reflective Lyot filter. Such an optical fiber reflective Lyot filter consist of a fiber polarizer, a segment of polarization-maintaining fiber and a single mode fiber based fiber reflector. By demodulating the wavelength of the interference dip, the proposed device exhibits a desirable high transverse load sensitivity of 64.72 nm/(N/mm) without temperature and strain confusion, which provides a new option for the transverse load measurement and further expands the application of the fiber Lyot filter.

Simultaneous measurement of temperature and RI based on an optical microfiber coupler assembled by a polarization maintaining fiber

A temperature and refractive index simultaneous measurement sensor based on an optical microfiber coupler (OMC) combined with a polarization maintaining fiber (PMF) is proposed and experimentally demonstrated. The OMC-PMF sensor is fabricated by inserting a section of PMF between two output ports of the OMC. The interference spectrum of the OMC-PMF sensor has different responses to temperature and refractive index variations. Temperature and the refractive index can be measured by monitoring the dip intensity variation and the dip wavelength shift of the interference spectrum, respectively. The experimental results show that the refractive index sensitivity is 12 020 nm/RIU, and the temperature sensitivity is 0.88 dB/°C upon increasing temperature from 35 °C to 41 °C.

Gelatin-Coated Michelson Interferometric Humidity Sensor Based on a Multicore Fiber With Helical Structure

A Michelson interferometer with gelatin coated for relative humidity (RH) measurement is proposed and demonstrated. It is fabricated by connecting the lead in/out single mode fiber, multimode fiber, triangular four core fiber (FCF) in a certain order. A gold film is deposited on the end face of the FCF as a reflector. A helical structure is fabricated by twisting the FCF under continuous arc discharging. Gelatin is coated onto the whole sensor to sense RH changes. When the RH of the external environment changes, both the refractive index (RI) and the size of the gelatin film changes, so the effective RI of the FCF cladding and the strain of the sensor changes accordingly, resulting in the resonance dip wavelength change in the interference signal. Such a probe is applied for RH measurement with a sensitivity of about -0.185 nm/%RH in the range from 45.0%RH to 81.7%RH with the response/recovery time of approximately 5.24/7.14 s, respectively.

Highly Sensitive Fabry–Perot Demodulation Based on Coarse Wavelength Sampling and Vernier Effect

A highly sensitive demodulation method for the low-finesse Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. To generate the Vernier effect, the sampling interval of a tunable laser source is set close to the free spectral range of the FPI sensor. In order to improve the demodulation precision, the spline fitting process was applied to the sampling data. The dip wavelength of the fitted curve was chosen to measure the spectral shift. In the strain test experiment, the sensitivity of 0.01836 nm/μϵ was obtained. A controllable sensitivity is achieved by changing the sampling interval. It is convenient to balance the sensitivity and the measuring time. The method can be used to enhance the sensitivity of the preexistent low-finesse FPI sensors by changing an optical source in the sensing system. There is no need to make any changes to the FPI sensors and sensing nodes. Furthermore, the method can also be applied for other two-beam interferometry sensors.

Hydrogen sulfide gas sensor based on copper/graphene oxide coated multi-node thin-core fiber interferometer.

A hydrogen sulfide gas sensor based on a copper/graphene oxide (Cu/GO) coated multi-point thin-core fiber Mach-Zehnder interferometer is proposed and experimentally demonstrated. The single-mode fiber (SMF) is sandwiched between the thin-core-fiber-1 (TCF-1) and thin-core-fiber-2 (TCF-2), and the SMF-TCF-1-SMF-TCF-2-SMF Mach-Zehnder interferometer is obtained. In order to detect the concentration of hydrogen sulfide, Cu/GO composite sensitive film was coated on the outside surface of two thin-core fibers. When the composite film absorbs the gases, it leads to a change of the effective refractive index of the cladding and causes the regular shift of dip wavelength. The result indicates that the thickness of the sensitive film is 1.6μm. With the increase of concentration of hydrogen sulfide, the transmission spectra appear blueshift in the range of 0-60ppm H2S. The linearity of 0.9884 and sensitivity of 4.83pm/ppm are achieved. In addition, the dynamic response time and recovery time of the hydrogen sulfide sensor are about 32s and 52s, respectively. This sensor has the advantages of the small size, simple structure, and easy manufacture, and it is suitable for the detection of trace hydrogen sulfide.

Construction of cascaded Fabry-Perot interferometers by four in-fiber mirrors for high-temperature sensing.

An optical fiber high-temperature sensor is proposed and demonstrated by use of cascaded Fabry-Perot interferometers based on four in-fiber mirrors fabricated by femtosecond laser inscription. The output fringe pattern of the device exhibits dominant/highly distinguishable dip wavelength, which helps in unambiguous measurement beyond the free spectral range. The device has excellent thermal stability in high temperature up to 1100°C, and the temperature sensitivity obtained is 9.91pm/°C within the temperature range of 100°C to 400°C, and 15.88pm/°C within the temperature range of 400°C to 1100°C, respectively. The device features compact size, simple fabrication, and convenient operation, which make it attractive for monitoring of extreme environment.

Refractive Index and Temperature Sensor Based on D-Shaped Fiber Combined With a Fiber Bragg Grating

A novel D-shaped fiber structure combined with a fiber Bragg grating for refractive index (RI) and temperature sensing is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding refractive index. By monitoring the dip wavelength of D-shaped fiber structure, the refractive index can be measured. Theoretical analysis and simulations were conducted to investigate the cladding mode propagating behavior influenced by the surrounding RI. Combined with the temperature sensing property of fiber Bragg grating (FBG), the possibility of simultaneously measuring the RI and temperature relies on monitoring the resonance dip of the D-shaped fiber modal interferometer and the Bragg wavelength of the FBG. Experimental results showed that the RI sensitivity of −31.79 nm/RIU within the range of 1.333 ~ 1.428, and a temperature sensitivity of 28.7 pm/°C can be obtained. The proposed sensor with simple configuration, low cost and easy fabrication is suitable for refractive index and temperature sensing applications.

Large-range refractive index measurement method for open cavity Fabry–Pérot interferometer

A large-range refractive index (RI) measurement method for a dual-beam open cavity Fabry–Pérot interferometer (FPI) is studied and demonstrated. Through theoretical derivation and simulation, the relationship between the target free spectral range (FSR) corresponding to a specified wavelength and the RI of the medium in the open cavity is revealed. Accordingly, a novel RI measurement method for the open cavity FPI is proposed, which uses the target FSR to track the shift of the specified interference dip. This method allows the shift distance of the tracked dip to be larger than the initial FSR, thus it can achieve a super-large RI measurement range compared to the conventional method. In addition, it allows a smaller dip bandwidth while measuring a wide range of RIs, which reduces the detecting error of the dip wavelength and improves the accuracy of RI measurement. By using a Fabry–Pérot sensor with a 68 µm-long open cavity, the change regularity of the value and order of the target FSR with the variation of the RI is verified. The experimental results show that, by using this method, the sensor can achieve a large measurement range from 1.333 to 1.4 RIU (RI unit), which is seven times higher than the conventional method and exhibits a good linear response with a sensitivity of 1260.282 nm/RIU. This method expands the application field of open cavity FPI and promotes its practical application.

Determination of physical location of fiber Bragg gratings using traversal heat‐induced phase shift

AbstractIn certain application, the exact location of the grating structure of fiber Bragg grating (FBG) is crucial for properly packaging, altering or exploiting the device. This article proposes a simple way to determine the location of FBG by applying heat induced phase shift. A traversing thin heating element is continuously scanned across the FBG while the local optical spectral profile is analyzed. The experiments are performed on an apodised uniform FBG and a chirped FBG. The results show that the symmetry of the local spectral profiles and prominence of the phase‐induced spectral dip can be used to determine the structure center and exact physical location of the apodised FBG. Meanwhile, the location of the linearly chirped FBG can be determined by simply observing the wavelength of the spectral dip.

Double-layer Fano resonance photonic-crystal-slab-based sensor for label-free detection of different size analytes

We propose a double-layer Fano resonance photonic-crystal-slab-based sensor (DFRPhCS) for label-free detection of different size analytes. One of the slabs is fully etched, while the other is partially etched, with three resonant dips in the transmission spectrum, which can detect and distinguish different sizes of analytes. As for bulk detection, all three resonant dips show red shifts, with the refractive index (RI) of the environment increasing, achieving sensitivity of 800 nm/RIU and detection limit of 1.71×10−7 RIU. For the detection of micrometer-level particles, due to the large analyte size, molecules are stuck on the upper slab, thereby changing the polarity and offset of the middle dip. Moreover, for nanoparticle detections, molecules can permeate the air holes. Therefore, the solution concentration variations of nanoparticles lead to middle dip wavelength shift with polarity remaining. Accordingly, the dimension and RI of the analyte can be distinguished, thereby indirectly presuming the category of the analyte. Furthermore, DFRPhCS is promising in preliminary detections and can be improved by adjusting the angle as well as polarization of the incidence appropriately.

Observation of multimodal interference in millimeter-long polymer optical fibers

We observe multimodal interference in single-mode-multimode-single-mode sensors comprising short polymer optical fibers (POFs) with lengths from 100 mm down to 7 mm. Characteristic spectral peaks/dips are observed not in the conventionally used telecom band but around 1000 nm. We find that the dip wavelength depends on temperature but that the sensitivity is much lower than those obtained in the longer wavelength range when longer POFs are used. We discuss the possibility that, even with the reduced sensitivity, our success in observing multimodal interference in millimeter-long optical fibers will be a basis toward the combined use of frequency and intensity information in conventional fiber-optic single-point sensors for discriminative measurement of multiple physical parameters.