Dispersion Turning Point Research Articles

Generating broadband cylindrical vector modes based on polarization-dependent acoustically induced fiber gratings using the dispersion turning point

Cylindrical vector beams (CVBs) with special polarization distribution have been extensively investigated due to the unique ways of interacting with matter. Although several configurations have been developed to generate CVBs, such as Q-plates and subwavelength gratings, the bandwidth of a single CVB is inherently narrow due to the phase geometry, which would limit its application for femtosecond lasers. Here, a broadband CVB mode converter based on an acoustically induced fiber grating (AIFG) and a tuning method of dispersion turning point (DTP) is demonstrated both theoretically and experimentally with the 3-dB bandwidth of 125 nm, which is more than 10 times that of conventional AIFGs. Not only can the DTP wavelength be tuned from the original 1500 nm to 1650 nm by thinning the fiber, but also the stable generation of a single broadband HE21odd/even mode can be controllably implemented by adjusting the polarization state of the incident light, owing to the larger beat length difference between HE21 and other CV modes. Additionally, the femtosecond CVBs and orbital angular momentum (OAM) modes are successfully generated and amplified by combining the broadband AIFG with a figure-9 mode-locked fiber laser. Meanwhile, it is verified by simulation that the choice of broadband CV mode and the tunability of DTP wavelength can be realized by designing ring-core fibers with different structures, which can furthermore improve the flexibility of generating high purity CVBs. This study provides a highly controllable technique for the generation of broadband CVBs and OAMs paving the way for high-capacity CVBs communication.

Generation of optical vortex beams with bandwidth exceeding 550 nm using a helical fiber needle exhibiting strong mode coupling.

A strong-coupling helical fiber needle (HFN) is proposed and demonstrated for the realization of bandwidth-enhanced broadband optical vortex beam (OVB) generation. The HFN is based on a single mode fiber and operates at the dispersion-turning-point (DTP) of the lowest radial order of the cladding mode (i.e., LP11) but with a remarkably high mode coupling efficiency. By utilizing this novel, to the best of our knowledge, HFN, successful generation of the first-order OVB with an impressive bandwidth up to 556 nm at -10 dB and a center wavelength of ∼1570 nm has been achieved. This represents the broadest bandwidth demonstrated among all fiber grating-based OVB generators to date. The proposed HFN-based OVB generator exhibits a relatively compact size, ultra-wide bandwidth, and customizable center wavelength, making it highly promising for applications in optical vortex-based endoscopic imaging as well as particle detection and manipulation.

Advances in dispersion turning point enhanced ultrasensitive optical fiber refractive index sensors

Optical fiber sensor operating at the dispersion turning point (DTP) has become a promising building block for future development in biosensing application due to its ultrahigh sensitivity. In this paper, we comprehensively review the fundamentals of various optical fiber sensors under the turning point condition, together with the basic definition, classification, design, fabrication and applications. Three main methods for DTP implementation are introduced and highlighted: i) phase matching condition in long period grating, ii) refractive index (RI) matching to the effective refractive index of the waveguide in modal interferometer, and iii) mode coupling via diameter regulation in modal interferometer, optical microfiber coupler and polarization interferometer. The aim here is to provide reliable and effective methods to achieve the superior performance on turning point-enhanced optical fiber RI sensors, accelerating the development of transduction technology for the research of novel biomedical sensors.

Ultrasensitive refractive index sensing of optical microfiber couplers coated with Ti3C2 MXene

Two-dimensional (2D) MXene has recently been found to play a significant role in constructing biosensing platforms due to its excellent biocompatibility, large specific surface area, and broad optical absorption. In this article, an ultra-sensitive refractive index (RI) sensor based on an optical microfiber coupler (OMC) coated with Ti3C2 MXene is proposed and experimentally demonstrated. Numerical results reveal that surface modification of MXene enhances the RI sensitivity of OMC and creates a new artificial dispersion turning point (ADTP). The OMC with a dispersion turning point (DTP) at 1300 nm is fabricated using a hybrid fusion elongation-wet etching method. It has a waist width of approximately 2.46 µm. We experimentally achieved the RI sensitivity of 9.1 × 104 ± 0.42 × 104 nm/RIU using the OMC. MXene improves the sensitivity of the OMC because Ti3C2 MXene can change the effective refractive index (ERI) for both the even and odd modes, leading to the interference dips shifting closer to the DTP. The experimental results demonstrate that the RI sensitivity of the OMC coated with Ti3C2 MXene can reach -1.7 × 105 ± 0.2 × 105 nm/RIU, which shows a ∼87% sensitivity boost compared with bare OMC. The proposed sensor is expected to be used for high-sensitivity biochemical molecule detection.

Ultrasensitive cancer cell sensing based on tapered optical fiber operating near the dispersion turning point

Cancer cell screening is very important for its treatment and prognosis. The screening techniques are required with high sensitivity and specific recognition. Here, an ultrasensitive tapered optical fiber (TOF) biosensor for cancer cell sensing is proposed. The unique property of the dispersion turning point (DTP) in the TOF is utilized to greatly improve the sensing sensitivity. Combined with numerical calculations, the TOF with the extremely high refractive index (RI) sensitivity (close to 18390 nm/RIU) near the DTP is achieved by precisely controlling the diameter of the TOF. In addition, biotin is used to functionalize the TOF for specific recognition of the biotin-overexpressing cancer cells. The sensitivity and selectivity of the TOF were demonstrated by the detection of the biotin receptor protein (streptavidin) and overexpressed cancer cells (lung epithelial adenocarcinoma cells, A549). The experiment results show that the TOF has a low limit of detection (LOD) for streptavidin and cancer cells, which are 28.73 pM and 49 cells/mL respectively. At the same time, the TOF showed the obvious response to another biotin-overexpressed cancer cell (breast cancer cells, 4T1), but is insensitive to normal cells (gastric mucosa cells, GES-1 and mouse epithelioid fibroblasts cells, L929). We believe that the proposed biosensor which exhibited extremely high sensitivity and good selectivity, will have potential application prospects in the field of chemical and biological detection.

Axial strain and temperature sensor based on tapered seven-core fiber operating at dispersion turning point

An axial strain and temperature sensor based on a tapered seven-core fiber (SCF) operating at the dispersion turning point (DTP) is proposed. The tapered SCF sensor with DTP was fabricated by the one-step tapering method, and its axial strain characteristics and temperature characteristics were simulated and experimentally investigated. The experimental results show that the tapered SCF sensor operates near DTP with the highest axial strain sensitivity and temperature sensitivity, consistent with the simulation results. The maximum axial strain sensitivity of the tapered SCF sensor is 391.2 pm/με in the range of 0–300 με, and the maximum temperature sensitivity is 6.44 nm/°C in the range of 30-70°C. The sensor is highly sensitive to axial strain and temperature. It is promising for measuring physical quantities such as weak strain and temperature.

Axial strain and temperature sensor based on tapered biased four-core fiber operating at dispersion turning point

An axial strain and temperature sensor based on tapered biased four-core fiber (BFCF) operating at the dispersion turning point (DTP) is proposed. The non-adiabatic tapered BFCF (Non-AT BFCF) and adiabatic tapered BFCF (AT BFCF) operating at the DTP were fabricated using the two-step tapering method and the one-step tapering method, respectively, and their axial strain and temperature characteristics were simulated and experimentally investigated. The experimental results show that the maximum sensing sensitivities of Non-AT BFCF are 365.6 pm/με and 2904 pm/°C in the ranges of 0–200 με and 30–70 °C, respectively. The maximum sensing sensitivities of AT-BFCF are 261.14 pm/με and 2280 pm/°C in the ranges of 0–300 με and 30–70 °C, respectively. The sensor is highly sensitive to axial strain and temperature. It has a promising future in measuring physical quantities such as weak strain and temperature.

Dispersion Turning Attenuation Microfiber for Flowrate Sensing.

We demonstrated a new optical fiber modal interferometer (MI) for airflow sensing; the novelty of the proposed structure is that an MI is fabricated based on a piece of HAF, which makes the sensitive MI itself also a hotwire. The interferometer is made by applying arc-discharge tapering and then flame tapering on a 10 mm length high attenuation fiber (HAF, 2 dB/cm) with both ends spliced to a normal single mode fiber. When the diameter of the fiber in the processing region is reduced to about 2 μm, the near-infrared dispersion turning point (DTP) can be observed in the interferometer's transmission spectrum. Due to the absorption of the HAF, the interferometer will have a large temperature increase under the action of a pump laser. At the same time, the spectrum of the interferometer with a DTP is very sensitive to the change in ambient temperature. Since airflow will significantly affect the temperature around the fiber, this thermosensitive interferometer with an integrated heat source is suitable for airflow sensing. Such an airflow sensor sample with a 31.2 mm length was made and pumped by a 980 nm laser with power up to 200 mW. In the comparative experiment with an electrical anemometer, this sensor exhibits a very high air-flow sensitivity of -2.69 nm/(m/s) at a flowrate of about 1.0 m/s. The sensitivity can be further improved by enlarging the waist length, increasing the pump power, etc. The optical anemometer with an extremely high sensitivity and a compact size has the potential to measure a low flowrate in constrained microfluidic channels.

CO2 laser-induced long-period fiber grating in the dispersion turning point

A CO2 laser-induced long-period fiber grating (LPFG) in the dispersion turning point (DTP) has been proposed in this work, which has demonstrated the possibility of fabricating LPFG with a shorter period of 115 μm in a thin single-mode fiber (SMF) without etching cladding. The LPFG working near DTP was also induced in a conventional SMF by CO2 laser after etching cladding, whose grating period was 196 μm. Then, the refractive index (RI) sensitivity of LPFG based on thin SMF was optimized by fully combining DTP and cladding reduction. Considering the sensor probe's mechanical strength comprehensively, the thin SMF cladding was etched to 51.03 μm to ensure cladding mode LP1,6 work near DTP, which can result in an average RI sensitivity of 2117.12 nm/RIU in the RI range of 1.33222 to 1.39018. Based on its excellent RI sensing performance, the proposed LPFG is a prospective sensor that can be applied to significant fields such as environmental, chemical, or biological parameters measurement. At the same time, this study provides a low-cost path to fabricate LPFG worked in DTP for most researchers.

A Seawater Salinity Sensor Based on Optimized Long Period Fiber Grating in the Dispersion Turning Point.

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‱ can be achieved when the salinity increases from 5.001‱ to 39.996‱, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing.

Ultrasensitive magnetic field sensor based on cladding-etched long-period grating.

We demonstrate a high-sensitivity fiber-optic magnetic field sensor, which consists of a cladding-etched long-period fiber grating (LPFG) near the dispersion turning point (DTP) integrated with a magnetic fluid (MF). By reducing the cladding diameter of the LPFG, the fundamental mode is coupled to the lowest order cladding mode (LP0,2) near the DTP, which has a much higher surrounding refractive index sensitivity. Thanks to the excellent magneto-optical characteristics of the MF, the proposed sensor can achieve a magnetic field intensity sensitivity of 44.69 nm/mT in the range of 3-7.4 mT. The minimum magnetic field intensity that can be detected is 0.45 µT due to the 0.02-nm wavelength resolution of the optical spectrum analyzer. The proposed etched DTP-LPFG-based sensor with ultrahigh magnetic field sensitivity could have potential applications in magnetic fields and electrical systems.

Fabrication of High-Sensitivity Optical Fiber Sensor by an Improved Arc-Discharge Heating System.

We proposed a high-sensitivity optical fiber sensor based on a dual-resonance helical long-period fiber grating (HLPG). The grating is fabricated in a single-mode fiber (SMF) by using an improved arc-discharge heating system. The transmission spectra and the dual-resonance characteristics of the SMF-HLPG near the dispersion turning point (DTP) were studied through simulation. In the experiment, a four-electrode arc-discharge heating system was developed. The system can keep the surface temperature of optical fiber relatively constant during the grating preparation process, which shows an advantage in preparing high-quality triple- and single-helix HLPGs. In particular, benefiting from this manufacturing system, the SMF-HLPG operating near the DTP was successfully prepared directly by arc-discharge technology, without secondary processing of the grating. As a typical application example of the proposed SMF-HLPG, physical parameters such as temperature, torsion, curvature and strain can be measured with high sensitivity by monitoring the variation of the wavelength separation in the transmission spectrum. Therefore, the proposed sensor and its fabrication technology have potential application prospects in practical sensing measurement.

Dispersion turning point-enhanced photothermal interferometry gas sensor with an optical microfiber interferometer

Photothermal interferometry (PTI) spectroscopy is a background-free and ultrasensitive gas or chemical sensing method. To date, most works have focused on applying different techniques to improve the sensitivity of PTI gas sensors via enhancing the photothermal (PT) phase modulation. Herein, we develop an all-fiber dispersion turning point (DTP)-enhanced PTI gas sensor with a taper-based mode interferometer in which the PT phase detection sensitivity of the interferometer is enhanced in a millimeter-long single fiber PTI configuration, which has not been studied yet. The design is demonstrated by conducting the measurement of acetylene with a 3-mm-long, 2.29-μm-diamter microfiber interferometer. A lower detection limit of 965 parts per billion (ppb) acetylene is achieved when the probe wavelength is operating near DTP, showing 16 times enhancement. Besides, theoretical analysis shows that the sensitivity of phase detection would be enormously enhanced when the dispersion factor approaching zero. By optimal design of the taper-based mode interferometer, together with thinner fiber diameter and longer fiber length, gas detection may be further improved by double enhancement of both PT phase generation and phase detection. With advantages of high sensitivity, compact size and low cost, the proposed all-fiber DTP-enhanced PTI gas sensor is potentially promising for ultra-sensitive trace chemical or biochemical sensing.

Ultra-Wideband OAM Mode Generator Based on a Helical Grating Written in a Graded-Index Few-Mode Fiber

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.

Ultrasensitive Optical Fiber Sensors Working at Dispersion Turning Point: Review.

Optical fiber sensors working at the dispersion turning point (DTP) have served as promising candidates for various sensing applications due to their ultrahigh sensitivity. In this review, recently developed ultrasensitive fiber sensors at the DTP, including fiber couplers, fiber gratings, and interferometers, are comprehensively analyzed. These three schemes are outlined in terms of operation principles, device structures, and sensing applications. We focus on sensitivity enhancement and optical transducers, we evaluate each sensing scheme based on the DTP principle, and we discuss relevant challenges, aiming to provide some clues for future research.

Temperature and strain self-compensated refractometer based on turning point of Mach–Zehnder interference in three-core fiber

We demonstrate an in-line Mach–Zehnder interference with a dispersion turning point (DTP) by sandwiching a section of three-core fiber (TCF) between two short no-core fibers (NCFs). The interference occurs mainly between the center core mode and a cladding mode of the TCF, which are excited and recoupled by the two NCFs. Two interference dips flanking the DTP are sensitive to ambient RI but shift oppositely. Moreover, the interval between these two dips responds linearly to ambient RI as well and has a higher sensitivity ( − 67.56 nm/RIU) within the RI range from 1.3518 to 1.4003. Meanwhile, the third dip near the DTP shows relatively low sensitivity to RI but is highly sensitive to temperature. By using the different responses of the third dip and the interval between flanking dips, this device is proposed to apply as a RI sensor with the compensation scheme of temperature. Finally, all three interference dips show very low sensitivity to axial strain, and the disturbance of strain can be further depressed by using the interval as the indicator. • We demonstrate an in-line fiber refractometer based on a Mach–Zehnder interference with a dispersion turning point, which is achieved by cascading a segment of three-core fiber with two short sections of no-core fibers. The proposed refractometer is featured with a higher sensitivity and the capability to eliminate the cross impact from temperature and axial strain without the assistance of an additional sensing unit. • Due to the special dispersion property, the wavelength interval of two interference dips flanking the dispersion turning point is proposed to be the indicator of the refractometer, since these two dips respond oppositely to the variation of ambient refractive index and their wavelength interval shows a higher linear sensitivity to ambient refractive index. Moreover, the interrogation of the wavelength interval helps to effectively depress the impact of axial strain, since both two dips shift to the same direction at a similar pace with varying axial strain. • By combining the third dip located at the dispersion turning point with the wavelength interval, we propose a temperature self-compensation solution for the fabricated device, in which all the indicators come from the interference itself. It used to be realized by using a second sensing unit, such as a fiber grating, another interference, and so on.

Wave-band-tunable optical fiber broadband orbital angular momentum mode converter based on dispersion turning point tuning technique.

A wave-band-tunable optical fiber broadband orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) and dispersion turning point (DTP) tuning technique is demonstrated both theoretically and experimentally. The DTP tuning is achieved by thinning the optical fiber during the HLPFG inscription. As a proof of concept, the DTP wavelength of the LP1,5 mode is successfully tuned from the original ∼2.4 µm to ∼2.0 µm and ∼1.7 µm. With the help of the HLPFG, broadband OAM mode conversion (LP0,1→LP1,5) is demonstrated near the 2.0 µm and 1.7 µm wave bands. This work addresses a longstanding problem that the broadband mode conversion is limited by the intrinsic DTP wavelength of the modes and provides a new, to the best of our knowledge, alternative for broadband OAM mode conversion at the desired wave bands.

Highly sensitive based on a Mach-Zehnder interferometer with double-slot hybrid plasmonic waveguide

A refractive index sensor based on the Mach-Zehnder interferometer (MZI) with double-slot hybrid plasmonic waveguide is proposed. Numerical simulation shows that the dispersion turning point (DTP) will appear when the group effective refractive index equals zero and ultra-high refractive index sensitivities can be achieved around the DTP. By changing the surrounding refractive index (SRI), the sensitivity can reach 120,000 nm/RIU with the SRI ranging from 1.0000 to1.0004 and 40,000 nm/RIU with the SRI ranges from 1.3330 to 1.3334. More importantly, the unique dual interference dips around the DTP double the sensitivity. Finally, by taking advantages of the exceptional refractive index sensitivity, we demonstrate a pressure sensor with sensitivity of 800 nm/MPa.

High-Sensitivity Magnetic Field Sensor Based on Long-Period Grating Near the Dispersion Turning Point

We demonstrate the fabrication of a high-sensitivity magnetic field sensor based on the packaged long-period fiber grating (LPFG) with magnetic fluid. The transmission spectra and the surrounding refractive index (SRI) characteristics of the LPFGs near the dispersion turning point (DTP) were investigated theoretically and experimentally. We have studied the influence of mode order and wavelength separation at DTP on the SRI sensitivity of the DTP-LPFG. The experimental results are consistent with the simulation. The LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,10</sub> mode has the highest SRI sensitivity of 1956.4 nm/RIU in the range of 1.333-1.435 when the wavelength separation at DTP is 89.1 nm. The sensor achieves a magnetic field sensitivity of 1.9 nm/mT in the magnetic field intensity range of 0-20.5 mT, which indicates that the sensor can be used as a high-sensitivity magnetic field sensor.

Broadband Acoustic Sensing with Optical Nanofiber Couplers Working at the Dispersion Turning Point.

Herein, a broadband ultrasensitive acoustic sensor based on an optical nanofiber coupler (ONC) attached to a diaphragm is designed and experimentally demonstrated. The ONC is sensitive to axial strain and works as the core transducing element to monitor the deformation of the diaphragm driven by acoustic waves. We first theoretically studied the sensing property of the ONC to axial strain and the deformation of the diaphragm. The results reveal that ONC working at the dispersion turning point (DTP) shows improved ultra-sensitivity towards axial strain, and the largest deformation of the circular diaphragm occurs at the center. Guided by the theoretical results, we fabricated an ONC with a DPT at 1550 nm, and we fixed one end of the ONC to the center of the diaphragm and the other end to the edge to construct the acoustic sensor. Finally, the experimental results show that the sensor can achieve accurate measurement in the broadband acoustic wave range of 30~20,000 Hz with good linearity. Specifically, when the input acoustic wave frequency is 120 Hz, the sensitivity reaches 1923 mV/Pa, the signal-to-noise ratio is 42.45 dB, and the minimum detectable sound pressure is 330 μPa/Hz1/2. The sensor has the merits of simple structure, low cost, and high performance, and it provides a new method for acoustic wave detection.