Helical Long-period Fiber Gratings Research Articles

Full C- and L-band covered second-order OAM mode generator based on a thinned helical long-period fiber grating

A full C- and L-band covered second-order orbital-angular-momentum (OAM) mode generator has been proposed and experimentally demonstrated, which is realized by using a helical long-period fiber grating (HLPG) but inscribed in a thinned four-mode fiber. By optimizing the design of grating period and fiber diameter of the proposed HLPG, an ultra-broadband rejection filter with a depth of ∼23 dB, a bandwidth of ∼156 nm @-10 dB (ranging from 1522 nm to 1678 nm) and a bandwidth of ∼58 nm @-20 dB (ranging from 1574 nm to 1632 nm), has been successfully obtained as a typical sample. To the best of our knowledge, this is the first demonstration of such ultra-broadband second-order OAM mode generator by using only one fiber component, i.e., the thinned HLPG. In addition, the proposed generator is less polarization-dependent and less temperature-sensitive than those of the conventional HLPGs, which is believed to be considerably helpful to find potential applications of the device itself in wavelength division multiplexing (WDM) and OAM mode division multiplexing (MDM) optical fiber communication systems.

High-Sensitivity Salinity Sensor based on Helical Long-Period Fiber Grating in Thin-cladding Fiber

High-Sensitivity Salinity Sensor based on Helical Long-Period Fiber Grating in Thin-cladding Fiber

On-demand flat-top wideband OAM mode converter based on a cladding-etched helical fiber grating.

A new method enabling to provide an on-demand flat-top wideband orbital angular momentum (OAM) mode converter is proposed and experimentally demonstrated, which is based on utilization of a cladding-etched helical long-period fiber grating (CEHLPG). By appropriately selecting the grating period and precisely controlling the diameter of the CEHLPG in-situ, both the radial order and central wavelength of the flat-top band for the generated OAM mode can be flexibly tailored according to specific requirements. As typical examples, the first azimuthal order OAM modes with a flat-top bandwidth of 95 nm at -20 dB, a central operating wavelength of ∼1500 nm, and the radial-orders of 9, 8, 5, and 2, respectively, have been demonstrated consecutively. The proposed method provides an excellent flexibility and robustness in controlling both the radial order and the central wavelength of the resulting flat-top wideband OAM mode conversion, which may support a variety of practical optical vortex applications.

A Novel Method for Measuring the Concentration of Liquids using Helical Long-Period Fiber Gratings

A Novel Method for Measuring the Concentration of Liquids using Helical Long-Period Fiber Gratings

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings.

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

Power-Interrogated Torsion and Strain Sensor Based on a Helical Long-Period Fiber Grating Written in a Thinned Four-Mode Fiber

A helical long-period fiber grating (HLPG)-based temperature-insensitive torsion and strain sensor is proposed and experimentally demonstrated, which was realized by using a second-order HLPG written in a thinned four-mode fiber. Simultaneous measurement of the torsion and strain was accomplished by using the power-interrogated scheme, i.e., the torsion- and strain-induced changes in transmission spectrum of two linear regions were especially used as the testing objectives. The maximum torsion sensitivity is 665 pm/(rad/m) under the torsion ranging from -22.4 to 22.4 rad/m and the maximum strain sensitivity is -5.42 pm/με under the strain ranging from 0 to 1588 με, which are about 2-fold and 10-fold higher than those of the conventional long-period fiber grating (LPG)-based fiber sensors, respectively. In addition, the temperature sensitivity of the proposed sensor is ~13.7 pm/°C at a temperature ranging from 20 to 70 °C, which is almost 4-fold lower than that of conventional LPGs.

Helical Long-Period Fiber Grating-Based OAM Interferometer and Its Application to Fiber Sensing

Orbital-angular-momentum (OAM) beams-based interferometers have recently been attracted significant interest and found various applications in the field of optical precision measurement. Whereas all-fiber OAM interferometers featured with the superior characteristics of the compact size, low cost, extremely-low insertion-loss, and the inherent compatibility with other fiber devices, have less been studied and especially demonstrated in experiment. In this study, we proposed and experimentally demonstrated an all-fiber OAM beams-based interferometer, where the two conjugated OAM beams generated by helical long-period fiber gratings (HLPGs) but with opposite helicities, were particularly utilized as the tested and referenced beams, respectively. The resulted interferograms are of the polarization independence, robust, and particularly suitable for phase-demodulation in azimuthal angle domain. As application examples of the proposed all-fiber OAM beams-based interferometer, changes in temperature ranging from 30 °C to 60 °C and changes in the strain ranging from 0 to 100 μϵ were measured, respectively. The obtained maximum sensitivity in temperature and strain are 26.585 rad/°C and 2.130 rad/μϵ, respectively. The accuracies for temperature and strain measurements are estimated to be 0.0016 °C and 0.0195 μϵ, respectively, which are about 2–3 order higher than those obtained from the conventional optical fiber-based sensors. It is believed that the proposed all-fiber OAM beams-based interferometer would find potential applications to ultra-high precision and sensing measurements.

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.

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.

Properties of off-axis helical long-period fiber gratings

In this paper, a new four-electrode arc discharge device with large constant temperature region is designed, which is used to prepared high-quality off-axis helical long-period fiber grating. The larger constant temperature heating area is more conducive to releasing the stress of optical fiber, so that the prepared device is less off-axis. In order to show that low off-axis is a key parameter of high-quality off-axis helical long-period fiber grating, the effects of single mode fiber on transmission spectrum of off-axis helical long-period fiber grating under different coupling lengths, pitches, core refractive indexes, cladding refractive indexes, core diameters, cladding diameters and off-axis quantity are simulated by using beam propagation method. Since traditional methods are difficult to measure the off-axis helical long-period fiber grating with small off-axis quantity, the off-axis quantity of the prepared device is estimated by using the method of spectral comparison and back-thrust off-axis quantity in this work. The off-axis helical long-period fiber grating is prepared by using the established processing device. The off-axis quantities of the prepared devices are about 0.12, 0.13 and 0.16 µm, respectively, according to the comparison between the simulated transmission spectrum and the actual spectrum. Finally, experiments on the torsional resistance and repeatability of the off-axis helical long-period fiber grating prepared by the device are carried out. The experimental results show that the prepared grating has certain torsional resistance and good spectral repeatability.

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.

Stable orbital angular momentum mode generator based on helical long-period fiber grating

We developed a helical long-period fiber grating (HLPG), which was formed by twisting step-index double-clad fiber (SDF) under the irradiation of a CO2 laser, and can generate stable orbital angular momentum (OAM) mode. The mode coupling and transmission characteristics of the SDF-HLPG were investigated both theoretically and experimentally. The results prove that this grating can couple the light wave into the inner cladding mode of the SDF, and directly generate OAM mode. Subsequently, the tolerance of the SDF-HLPG to environmental disturbance was also evaluated. The OAM mode has low sensitivities to mechanical torsion and bending, and has almost no responses to strain and surrounding refractive index. This indicates that the SDF provides a channel for the OAM mode to avoid high external interference. Therefore, this kind of SDF-HLPG has the potential for communication applications.

A Compact Sensor Capable of Temperature, Strain, Torsion and Curvature Measuring

We experimentally proposed a helical long-period fiber grating (HLPG), which was produced by twisting a tri-hole fiber (THF) with arc discharge technology. The optical field distribution in THF-HLPG was analyzed and its transmission spectrum was studied. The simulation results prove that the light wave propagating along the fiber core can be strongly coupled into cladding modes within a short transmission distance, thus making the THF-HLPG more compact. In the experiment, a sensor with a length of 4.2 mm was manufactured by using an arc heating system. With this processing device, the air holes of the THF-HLPG can effectively avoid shrinkage during heating. Temperature, strain, torsion and curvature properties were investigated experimentally. Owing to the peculiarity of THF-HLPG, this sensor shows a high mechanical response and is suitable for identifying curvature changes with a large bending radius. The demonstrated THF-HLPG offers a promising application for precisely detecting mechanical deformation of micro-structure.

Torsion-tunable OAM mode generator based on oxyhydrogen-flame fabricated helical long-period fiber grating.

We demonstrate a class of all-fiber torsion-tunable orbital angular momentum (OAM) mode generators based on oxyhydrogen-flame fabricated helical long-period fiber gratings (HLPFGs). The 1-order and 3-order OAM modes are excited based on the HLPFGs inscribed in the single-mode fiber (SMF) and six-mode fiber (6MF), respectively. Theoretical analysis reveals that the twisting can result a resonant wavelength shift of the HLPFG, which means that the OAM modes can also be excited at various wavelength by simply applying a twist rate on the HLPFG. Experiments are carried out to characterize the torsional tunability of the OAM modes, and the results show that the 1-order and 3-order OAM modes can be excited at various wavelength of ∼1564 - 1585 nm and ∼1552 - 1574 nm, respectively, when the torsion angle varied from -360° to 360°, which is consistent with the theoretical analysis. Therefore, the HLPFG can be a candidate for all-fiber wavelength tunable OAM mode generator.

Ultrasensitive refractometer based on helical long-period fiber grating near the dispersion turning point.

Precise and accurate measurements of the optical refractive index (RI) for liquids are increasingly finding applications in biochemistry and biomedicine. Here, we demonstrate a dual-resonance helical long-period fiber grating (HLPFG) near the dispersion turning point (DTP), which exhibits an ultrahigh RI sensitivity (∼25546 nm/RIU at ∼1.440). The achieved RI sensitivity is, to the best of our knowledge, more than one order of magnitude higher than a conventional HLPFG. The ultrahigh RI sensitivity can improve the RI measurement precision and accuracy significantly. Furthermore, ultralow wavelength shifts (nearly zero) with temperature and strain ranging from 20 to 100°C and 0 to 2226 µε, respectively, are also demonstrated for the proposed HLPFG, which may be a good candidate for developing new low-cross-talk sensors.

Broadband edge filter based on a helical long-period fiber grating and its application to a power-interrogated torsion sensor

A broadband edge filter with linear dynamic ranges of 40 n m in wavelength and 93 % in transmission loss has been demonstrated both theoretically and experimentally, which is achieved by using a helical long-period fiber grating (HLPG) operating near the dispersion turning point (DTP) of the L P 1 , 10 mode. To take full advantage of the wide dynamic ranges, the fabricated edge filter has been successfully used as a power-interrogated torsion sensor, and the torsion responsivity presented is about 0.501 nm/(rad/m), which is higher than most conventional HLPG-based torsion sensors. The results indicate that the proposed edge filter may find applications in any other kinds of power-interrogated sensors as well.

Femtosecond laser inscribed helical long period fiber grating for exciting orbital angular momentum.

A method employing femtosecond lasers to inscribe helical long period fiber grating (HLPFG) for exciting orbital angular momentum (OAM) of light is experimentally demonstrated. In this method, the refractive index modulation (RIM) of HLPFG is realized by three-dimensional translation of a fiber without rotation, indicating better stability, repeatability and flexibility. The coupling efficiency can be customized by varying the radius of the helical RIM, except laser energy. The characteristics of phase and polarization purity of the coupled modes in HLPFGs are studied. Results show that HLPFGs can directly excite OAM modes, the polarization state and helical phase of the mode can be adjusted independently, and the purity is the highest at resonant wavelength, over 91%.

Reflective-Type Multiparameter Sensor Based on a Paired Helical Fiber Gratings and a Trapezoid-Like Microcavity

A reflective-type fiber sensor allowing the simultaneous measurements of directional torsion, strain, and temperature is proposed and experimentally demonstrated, which is realized by using a paired helical long-period fiber gratings and a trapezoid-like microcavity (PHLPGs-TLMC). With aid of the proposed sensor, the crosstalk effects among torsion, strain, and temperature can be well distinguished. The measurement resolutions for such three parameters are 0.017 rad/m, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.1~\mu \varepsilon $ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.452~^{\circ }\text{C}$ </tex-math></inline-formula> , respectively, which are competitive to those of the fiber-based sensors. In comparison with the transmission-type sensors reported to date, the proposed sensor is particularly used in reflection, which provides more flexibilities for sensor’s installation and is suitable for real-time and in-site measurements also. The proposed sensor could be found potential applications in structural health monitoring of the buildings, the bridges, the ships, the aircrafts, and so on.

High-order OAM mode generation in a helical long-period fiber grating inscribed by an oxyhydrogen-flame

In this study, an all-fiber ±3-order orbital angular momentum (OAM) mode generator with ∼90% conversion efficiency is proposed and experimentally demonstrated. By using a helical long-period fiber grating (HLPFG) inscribed by an oxyhydrogen-flame, the fundamental mode (LP01) of the six-mode fiber (6MF) can be effectively converted into the OAM±3 mode without the need for polarization control. This is the first demonstration of the all-fiber ±3-order OAM mode generator by an oxyhydrogen-flame-induced HLPFG approach as far as we know. In addition, the broadband tunability of the ±3-order OAM mode generator within 1520–1630 nm is demonstrated just by changing the pitch of the helical grating. We believe that the proposed tunable ±3-order OAM mode generator can offer a new alternative for high-order OAM mode generation.

A combined fiber sensor for simultaneous measurement of directional torsion and displacement

A combined sensor for simultaneous measurement of directional torsion and displacement is proposed and experimentally demonstrated. The sensor is constructed with a helical long-period fiber grating (H-LPG) and an air-cavity fiber-optic Fabry–Perot interferometer (FPI) in series. By investigating the resonant wavelength shift of the H-LPG and the FSR variation of the FPI in the transmission spectra of the sensor, the applied torsion and displacement can be unambiguously discriminated and determined simultaneously. The experimental results show that the sensor achieves to the directional torsion responsivity of −2.693 nm/rad/m, and the displacement responsivity of 0.0275 nm/μm in ranges of −240 to 240 degrees and 0 to 200μm, respectively.