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25 Years of Long-Period Fiber Gratings
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25 Years of Long-Period Fiber Gratings

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 10067

Special Issue Editor

Prof. Dr. Gaspar Rego

E-Mail Website1 Website2
Guest Editor
1. proMetheus-Research Unit in Materials, Energy and Environment for Sustainability, Polytechnic Institute of Viana do Castelo, Portugal
2. Institute for Systems and Computer Engineering, Technology and Science, Porto, Portugal
Interests: optical fiber sensors; arc-induced long-period fiber gratings; fiber gratings; biosensors; environmental sensors
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

In 2021, we will be celebrating the 25 years of the introducion of the concept of long-period fiber grating. Several techniques were proposed in order to fabricate these gratings in all kind of fibers. The mechanisms of grating formation were investigated, and the gratings’ responses to changes in physical parameters were analyzed. Based on this knowledge, several applications in optical communications and sensing domains were implemented. Nevertheless, despite their identified enormous potential, long-period gratings did not share in the success of fiber Bragg gratings with respect to the availability of commercial products. In this context, it is pertinent to create an up-to-date record of the achievements of each fabrication technique and to envision real application fields for long-period fiber gratings.

Prof. Dr. Gaspar Rego
Guest Editor

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Keywords

  • Long-period fiber gratings
  • Theory, fabrication techniques, characterization, and properties
  • Optical fiber sensors
  • Speciality optical fiber sensors
  • Mechanical and physical sensors
  • Bio, chemical, and environmental refratometric sensors
  • Extreme environment sensors
  • Optical communication devices/sensors interrogation schemes

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Published Papers (4 papers)

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15 pages, 3302 KiB  
Article
Simulation of the Transmission Spectrum of Long-Period Fiber Gratings Structures with a Propagating Acoustic Shock Front
by Oleg V. Ivanov, Paulo Caldas and Gaspar Rego
Sensors 2021, 21(21), 7212; https://doi.org/10.3390/s21217212 - 29 Oct 2021
Cited by 1 | Viewed by 2028
Abstract
In this paper, we investigate modification of transmission spectra of long-period fiber grating structures with an acoustic shock front propagating along the fiber. We simulate transmission through inhomogeneous long-period fiber gratings, π-shift and reflective π-shift gratings deformed by an acoustic shock front. Coupled [...] Read more.
In this paper, we investigate modification of transmission spectra of long-period fiber grating structures with an acoustic shock front propagating along the fiber. We simulate transmission through inhomogeneous long-period fiber gratings, π-shift and reflective π-shift gratings deformed by an acoustic shock front. Coupled mode equations describing interaction of co-propagating modes in a long-period fiber grating structures with inhomogeneous deformation are used for the simulation. Two types of apodization are considered for the grating modulation amplitude, such as uniform and raised-cosine. We demonstrate how the transmission spectrum is produced by interference between the core and cladding modes coupled at several parts of the gratings having different periods. For the π-shift long-period fiber grating having split spectral notch, the gap between the two dips becomes several times wider in the grating with the acoustic wave front than the gap in the unstrained grating. The behavior of reflective long-period fiber gratings depends on the magnitude of the phase shift near the reflective surface: an additional dip is formed in the 0-shift grating and the short-wavelength dip disappears in the π-shift grating. Full article
(This article belongs to the Special Issue 25 Years of Long-Period Fiber Gratings)
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11 pages, 2365 KiB  
Communication
Wavelength-Tunable, Ultra-Broadband, Biconical, Long-Period Fiber Grating Mode Converter Based on the Dual-Resonance Effect
by Yu Zheng, Huiyi Guo, Mao Feng, Zhi Wang and Yange Liu
Sensors 2021, 21(17), 5970; https://doi.org/10.3390/s21175970 - 6 Sep 2021
Cited by 7 | Viewed by 2631
Abstract
We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO2 laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing [...] Read more.
We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO2 laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP01 to LP11, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength. Full article
(This article belongs to the Special Issue 25 Years of Long-Period Fiber Gratings)
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14 pages, 4937 KiB  
Communication
UV Inscription and Pressure Induced Long-Period Gratings through 3D Printed Amplitude Masks
by Ricardo Oliveira, Liliana M. Sousa, Ana M. Rocha, Rogério Nogueira and Lúcia Bilro
Sensors 2021, 21(6), 1977; https://doi.org/10.3390/s21061977 - 11 Mar 2021
Cited by 12 | Viewed by 2586
Abstract
In this work, we demonstrate for the first time the capability to inscribe long-period gratings (LPGs) with UV radiation using simple and low cost amplitude masks fabricated with a consumer grade 3D printer. The spectrum obtained for a grating with 690 µm period [...] Read more.
In this work, we demonstrate for the first time the capability to inscribe long-period gratings (LPGs) with UV radiation using simple and low cost amplitude masks fabricated with a consumer grade 3D printer. The spectrum obtained for a grating with 690 µm period and 38 mm length presented good quality, showing sharp resonances (i.e., 3 dB bandwidth < 3 nm), low out-of-band loss (~0.2 dB), and dip losses up to 18 dB. Furthermore, the capability to select the resonance wavelength has been demonstrated using different amplitude mask periods. The customization of the masks makes it possible to fabricate gratings with complex structures. Additionally, the simplicity in 3D printing an amplitude mask solves the problem of the lack of amplitude masks on the market and avoids the use of high resolution motorized stages, as is the case of the point-by-point technique. Finally, the 3D printed masks were also used to induce LPGs using the mechanical pressing method. Due to the better resolution of these masks compared to ones described on the state of the art, we were able to induce gratings with higher quality, such as low out-of-band loss (0.6 dB), reduced spectral ripples, and narrow bandwidths (~3 nm). Full article
(This article belongs to the Special Issue 25 Years of Long-Period Fiber Gratings)
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11 pages, 2101 KiB  
Letter
Analysis of Long Period Gratings Inscribed by CO2 Laser Irradiation and Estimation of the Refractive Index Modulation
by Ana M. Rocha, Ana I. Machado, Telmo Almeida, Joana Vieira and Margarida Facão
Sensors 2020, 20(22), 6409; https://doi.org/10.3390/s20226409 - 10 Nov 2020
Cited by 2 | Viewed by 1722
Abstract
Long period gratings (LPGs) inscribed in single mode fibers (SMFs) using CO2 laser irradiation were modelled numerically using the coupled mode method. The model considers the specifications of the inscription technique, such as the shape of the refractive index modulation that mimics [...] Read more.
Long period gratings (LPGs) inscribed in single mode fibers (SMFs) using CO2 laser irradiation were modelled numerically using the coupled mode method. The model considers the specifications of the inscription technique, such as the shape of the refractive index modulation that mimics the circularly symmetric point-to-point laser irradiation profile. A simple expression for predicting the resonant wavelength was obtained assuming a two-mode coupling model. However, to explain the spectra of the experimental LPGs, it was necessary to assume a reasonably high refractive index change and a multimode coupling model. Furthermore, using the developed model and a genetic algorithm to fit experimental resonances to simulated ones, we were able to estimate the maximum refractive index change, obtaining a value of 2.2 × 10−3, confirming the high refractive index change. The proposed model also predicts a second order resonance for this high value of refractive index change that was confirmed experimentally. Hence, with this model, we found some significant differences in the LPGs behavior when compared with conventional ones, namely, the emergence of coupling between different cladding modes and the competition of first and second order resonances which change the LPG transmission spectrum. Full article
(This article belongs to the Special Issue 25 Years of Long-Period Fiber Gratings)
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