Long-period Gratings Research Articles

Helical Long-Period Grating Through a Commercial CO2 Laser Fiber Processing Machine for Torsion Sensor Applications

Helical Long-Period Grating Through a Commercial CO₂ Laser Fiber Processing Machine for Torsion Sensor Applications

Real-time optical fibre near-infrared chromatic dispersion analyser using collocated optical fibre gratings

We present what we believe to be a novel optical fibre device for real-time measurements of near-infrared chromatic dispersion of a liquid medium from 1100 nm to 1700 nm. This inline optical fibre chromatic dispersion analyser is based upon collocated fibre long-period gratings written adjacently in the core by femtosecond laser inscription yielding 8 attenuation bands associated with different cladding modes, yielding 8 independent measurements. This fibre device is tested on a series of chemical compounds associated with wines. The results for each compound yielded distinctive chromatic responses. A machine learning pipeline was developed that successfully clustered and classified spectral responses of the different chemicals to illustrate the distinctive responses. The limits of detection for these fibre devices ranged from 1×10−3 to 6×10−5 mol/L for the compounds associated with wines, demonstrating the potential usefulness of an optical fibre chromatic dispersion analyser is capable of monitoring in real-time effective chromatic dispersion that is not offered by any current technology.

Design and analysis of an integrated optical coupler based on three-core fiber with long-period gratings

Design and analysis of an integrated optical coupler based on three-core fiber with long-period gratings

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

High-sensitive refractive index sensor based on the long-period gratings inscribed in the tapered fiber at dispersion turning point

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the tapered single-mode fiber by CO2 laser. The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters have been studied theoretically and experimentally. When the waist diameter of the tapered fiber decreased from 90 μm to 22 μm, the slope of the phase-matching curve changed dramatically. The refractive index sensitivity can be increased by inscribing the LPFGs in the tapered fiber around the dispersion turning point. A maximum refractive index sensitivity of 21702.78 nm/RIU has been achieved in the range of 1.445–1.457 for the LPFG inscribed in the tapered fiber with a waist diameter of 50 μm. This kind of LPFGs inscribing in the tapered fiber may be a good candidate for the development of refractive index based chemical and biological sensors.

Refractive index sensing characteristics of long-period gratings in linearly arranged three-core fiber

We demonstrate the refractive index (RI) sensing characteristics of long-period gratings (LPGs) in linearly arranged three-core fiber (TCF), which are fabricated using arc discharge technique. Due to the asymmetric index modulation during fabrication, the RI sensitivity of TCF-LPGs in the 0° grating inscription direction is greater than that in the 90° grating inscription direction. The spectral and RI sensing characteristics of TCF-LPGs in the 0° grating inscription direction with different grating periods have been investigated experimentally. In the RI range from 1.4538 to 1.4589, the resonance at 1533.3 nm for TCF-LPGs with grating period of 465 μm could achieve the RI sensitivity of 15943.3 nm/RIU, which could be optimized to 18160.7 nm/RIU at 1676.3 nm as the TCF-LPGs with grating period of 600 μm. Moreover, the proposed TCF-LPGs exhibit notably greater RI sensitivity compared to arc-discharged LPGs in conventional single-mode fibre, presenting substantial advantage in RI sensing. And, changing the polarization state of the incident light enhances the RI sensitivity of TCF-LPGs.Additionally, the influence of bending or temperature interference on the RI sensitivity of TCF-LPGs is minimal, ensuring that interference with RI measurements can be neglected. The proposed sensor boasts cost-effectiveness, simplicity in structure, and high sensitivity to RI, showing substantial potential for applications in chemical and biological sensing.

Characteristics of Helical Long-Period Gratings Inscribed in Seven-Core Fibers by CO2 Laser

Characteristics of Helical Long-Period Gratings Inscribed in Seven-Core Fibers by CO₂ Laser

Torsion Sensor Based on Helical Long-Period Grating Inscribed in the Etched Double Cladding Fiber

Torsion Sensor Based on Helical Long-Period Grating Inscribed in the Etched Double Cladding Fiber

Optical properties of thin films monitored in real-time at high gamma radiation doses using long period fiber gratings

In this work, we report an all-optical-fiber-based method to monitor in real-time the changes in thin film optical properties when exposed to harsh environments such as high doses of gamma radiation. The method is based on a long period grating (LPG) inscribed in a radiation resistant optical fiber, which is coated with a thin film to be analyzed. Due to impact of the environment on the film properties, the transmission spectrum of the LPG changes. To validate this methodology, we deposited oxides of various metals, i.e. aluminum (Al2O3), titanium (TiO2), and tantalum (Ta2O5), as well as a polymeric film, namely polystyrene (PS), on the LPG surface and measured the resonant wavelength shift induced by high doses of gamma exposure (dose rate of 2.3 kGy/h, up to a total dose of 46 kGy). The changes could be observed in real-time up to maximum reached dose and during the recovery period. The results are significantly dependent on thin film material. Among the oxides, TiO2 thin film demonstrated the highest susceptibility to irradiation, whereas Al2O3 exhibited the least impact. Additionally, a good recovery potential for Ta2O5 thin film was observed, hinting at its promising application in dosimetry. Conversely, the PS film exhibited a permanent effect induced by irradiation. This is the first real-time experimental study of the impact of high dose gamma radiation on thin film optical properties using a universal optical-fiber-based device. The findings pave the way for assessing radiation hardness of thin film materials or utilizing such a method to optimize material treatment involving ionizing radiation.

Design of a Broadband Integrated Mode Converter Based on Non-Uniform Binary Long-Period Grating by Direct Multiparameter Optimization

Design of a Broadband Integrated Mode Converter Based on Non-Uniform Binary Long-Period Grating by Direct Multiparameter Optimization

Long period grating based molecularly imprinted fiber optic sensor for the label-free detection of bisphenol A

The molecularly imprinted fiber optic (MIFO) sensors are recognized for its reliability, sensitivity and selective analyte identification. In this study, we report a Long-Period Grating (LPG) based MIFO sensor developed for the label-free detection of Bisphenol A (BPA) in water samples. The sensor is fabricated by polymerizing hydrogel precursor containing BPA on the LPG, followed by etching BPA molecules. The rebinding of the BPA molecule to the molecularly imprinted long period grating Bisphenol A (MILPG-BPA) sensor causes the hydrogel to swell, leading to a peak shift in the LP07 resonant mode of the sensor. This shift is linear across BPA concentrations ranging from femto molar to nano molar range with a low limit of detection of 0.146 fM. The non-imprinted long period grating (NILPG) sensor showed no selective binding of the BPA molecule, underscoring the significance of the microenvironment established through molecular imprinting for effective sensing. The MILPG-BPA sensor exhibits high selectivity in detecting the BPA molecules among the potential interferents. Furthermore, it has a rapid response time and reusability, thus holding promise for a wide range of applications.

Multiparameter sensor based on long-period grating in few-mode ring-core fiber for vector curvature and torsion measurement

With the rapid development of optical fiber sensor in physical measurement, it is of practical significant to realize multi-parameter measurement. In this work, we proposed and demonstrated a multiparameter sensor for curvature, torsion and temperature. The sensor is formed from CO2 laser-inscribed long-period grating (LPG) in few-mode ring-core fiber (RCF), which can convert fundamental mode to LP11 mode. One-side inscription enhances asymmetric index modulation in fiber, resulting in directionally sensitive to bend by intensity interrogation. The curvature sensitivities in various bending directions perform linearity in curvature range of 5–9.466 m−1. Curvature sensitivities are related to the bending directions. The intensity sensitivity achieves − 0.584 dB/m−1, 0.879 dB/m−1, and − 0.735 dB/m−1, respectively. And torsion sensitivity from -6.98 rad/m to 6.98 rad/m displays linearity with −0.554 dB/(rad/m). The measured influence of temperature can be negligible. The proposed RCF-LPG could not only be used as high-efficiency mode converter in mode division multiplexing systems but also be potential in few-mode fiber based multiparameter sensing fields.

Magneto-optical fiber-based orbital angular momentum mode converters.

Orbital angular momentum (OAM) mode division multiplexing (MDM) systems can support large-capacity and high-speed rate information transmission, in which the OAM mode conversion devices play an important role. In this paper, the mode conversion principle of magneto-optical fiber-based long-period grating (MOF-LPG) is analyzed for further developing new magneto-optical (MO) OAM mode converters, including three types of C P 01 to O A M ±1,1, O A M ±1,1 to O A M ±2,1, and O A M ±1,1 to C P 02. It is shown that the magnetic tunability of the mode converters through the propagation constants of the eigenmodes is useful for compensating for process errors and increasing the operating wavelength range. The implementation of MOF-LPGs is also discussed from the aspect of the prospective experiments.

Switchable dual-wavelength mode-locked cylindrical vector beam fiber laser based on unpumped EDF saturable absorber and nonlinear polarization rotation

A switchable single- and dual-wavelength mode-locked pulse laser with cylindrical vector beam (CVB) based on an unpumped EDF saturable absorber and a nonlinear polarization rotation (NPR) hybrid mode-locked mechanism has been proposed and experimentally demonstrated. An unpumped EDF saturable absorber is utilized to generate a mode-locked operation that generates high-order laser pulses. Utilizing a hybrid mode-locked mechanism, the signal-to-noise ratio (SNR) and operating stability of the laser are improved compared to the mode-locked operation with a single unpumped EDF saturable absorber. Through adjusting the polarization controller, single- and dual-wavelength stable mode-locked pulses can be easily realized. The single-wavelength mode-locked pulse operates with central wavelengths of 1550.16 nm and 1549.80 nm, repetition frequencies of 6.33 MHz and 6.35 MHz, corresponding to pulse intervals of 158 ns and 157 ns. Further, the long-period grating (LPFG) is used as a mode converter to achieve pulse output with a purity exceeding 95 % in the TM01 and TE01 modes at different operating wavelengths. The proposed fiber laser with a simple structure has provided a flexible pulsed CVB laser source for laser processing, time division, and mode division multiplexing applications.

Optically tunable mode-locked fiber laser using long-period grating coated with multiwall carbon nanotubes

We demonstrate an optically tunable mode-locked fiber laser using long-period fiber grating (LPFG) coated with multi-walled carbon nanotubes (CNTs). The multi-walled CNTs can absorb light to convert it into thermal energy, and the resonance wavelength of the grating can be easily turned by varying the external modulated light power. This multi-walled CNT coated LPFG-based all-optical fast and efficient spectrum tunable filter enables continuous tuning of the central wavelength of the laser by manipulating the loss of the mode-locked laser, ensuring the stability of the mode-locking state. In the absence of modulated light on multi-walled CNTs, the soliton laser could generate 890 fs pulses at 1546.7 nm with a spectrum bandwidth of 3.26 nm and a signal-to-noise ratio of 73.1 dB. Through adjustment of the pump power of the modulation light on multi-walled CNTs, the mode-locked fiber laser can be continuously tuned from 1546.71 to 1563.15 nm. The response time of the optically tunable system was measured to be in the order of hundreds of milliseconds. The presented optical tuning filter shows great potential in the fiber laser system, offering a repeatable, straightforward, and rapidly responsive laser tuning technique.

Long-Period Grating with Asymmetrical Modulation for Curvature Sensing

We propose and demonstrate a curvature sensor based on long-period fiber grating (LPFG) with asymmetric index modulation. The LPFG is fabricated in single-mode fiber with femtosecond laser micromachining. The grating structure is not introduced in the central fiber core, but is located off-axis with a distance of a few micrometers. Experimental results indicate that the offset distance has direct influence on the grating spectra. By utilizing such an asymmetric structure, two-dimensional vector curvature sensing can be realized. For an LPFG with an offset distance of 6 μm, the curvature sensitivity is around 29 nm/m−1 in the 0° and 180° direction and about 20 nm/m−1 in the 90° and 270° direction. The difference in curvature sensitivity in different bending directions makes the sensor capable of distinguishing the curvature orientation. The temperature response of the sensor is also experimentally investigated, and results indicate that the sensor has a very low temperature cross-sensitivity of 0.003 m−1/°C. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, and compact structure make the device an ideal candidate to be applied in the field of power grid health monitoring and intelligent robotics.

Ultra-broadband multi-channel orbital angular momentum mode generator using a helical long-period grating pair

We demonstrate an ultra-broadband multi-channel orbital angular momentum (OAM) mode generator using a helical long-period grating (HLPG) pair in a few-mode fiber. The numbers of the channel of the OAM mode generator can be flexibly adjusted by designing the cascaded length of the HLPG pair. When the cascaded length is 50 cm, the maximum channel number of OAM mode generator reaches 46, and the mode convert efficiencies of the channels are more than 90 %, which is, to the best of our knowledge, the highest number of channels among OAM mode converters. By reducing grating length and enhancing the refractive index modulation depth, the 3 dB bandwidth of the HLPG is significantly increased, and thus the wavelength range of the HLPG pair can be expanded. The maximum wavelength ranges of the channels with coupling efficiency greater than 90 % are measured to be 379 nm experimentally. By adjusting the spiral direction of the helical refractive index modulation, we can achieve multi-channel ± 1 order OAM mode conversion for each channel. The multi-channel OAM mode generator holds substantial potential for applications in mode division multiplexing systems, wavelength filters, and fiber sensing.

Few-Mode Fiber-Based Long-Period Fiber Gratings: A Review

Long-period fiber gratings (LPFGs) are efficient ways to achieve high-order core mode conversion and vortex mode conversion in few-mode fibers (FMFs), which have the benefits of flexible structure, high integration, low insertion loss, and strong wavelength selectivity. With the rapid development of mode division multiplexing (MDM) optical communications, the FMF-LPFGs could have promising applications in the field of MDM optical communications and optical sensors. In this paper, we briefly introduce the principle of optical coupling, theoretical analysis, fabrication techniques, and applications of the FMF-LPFGs in optical communications and optical sensors. The CO2-laser writing technique and arc discharge technique are widely used for the fabrication of FMF-LPFGs and FMF helical long-period gratings (HLPGs). The hydrogen-oxygen flame technique has the advantage on the fabrication of FMF-HLPGs. The mechanical micro-bending and acoustic induction techniques are flexible and effective methods in fundamental researches. The femtosecond laser technique has the advantages for the fabrication of parallel FMF-LPFGs and micro-structure FMF-LPFGs. The mode converters based on the FMF-LPFGs have been demonstrated using different inscription techniques. The FMF-LPFG converters have the advantages such as temperature insensitivity and wavelength tunability. The broadband converters have been demonstrated based on the phase-shift FMF-LPFG, cascaded FMF-LPFGs, and FMF-LPFG operating at dispersion turning point. The FMF-LPFGs and FMF-HLPGs are an excellent option to generate all-fiber orbital angular momentum (OAM) modes in different specialty FMFs, especially the FMF-HLPGs can excite different-order OAM modes directly. As the novel fiber component, the sensing application of FMF-LPFGs and FMF-HLPGs is also briefly introduced.

Broadband OAM Mode Generator Based on Helical Long-Period Grating Inscribed by Automated CO-Laser

Broadband OAM Mode Generator Based on Helical Long-Period Grating Inscribed by Automated CO-Laser<sub />

All-fiber broadband spectral acousto-optic modulation of a tubular-lattice hollow-core optical fiber.

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time to our knowledge. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60% are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices.