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Optics, Volume 2, Issue 2 (June 2021) – 6 articles

Cover Story (view full-size image): The first part of the article is focused on the characterization of the refractometric measurement performance (stability, repeatability, resolution, cross-strain sensitivity for double parameter measurements) of a TFBG sensor demodulated with the Delaunay triangulation technique to obtain the best measurement configuration. Then, the last part is focused on the investigation of the TFBG spectrum evolution during partial immersion of the sensor in a refractive index liquid. The results are considerably important for monitoring the manufacturing of composite materials during RTM processes, as this allows one to detect and inspect the interlaminate speed of the resin during the infusion, the permeability of the reinforcement fiber layers and the eventual voids, as well as to evaluate the infusion state of composites. View this paper

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21 pages, 9517 KiB

Open AccessArticle

Refractometric Properties of a TFBG Sensor Demodulated Using α-Shape Modified Delaunay Triangulation

by Luigi Fazzi and Roger M. Groves

Optics 2021, 2(2), 113-133; https://doi.org/10.3390/opt2020012 - 9 Jun 2021

Cited by 6 | Viewed by 3484

Abstract

In our previous research, a novel demodulation technique based on α-shape Delaunay triangulation (D-T) was developed to obtain the refractive index of the medium surrounding the optical fibre using the envelope of the cladding peaks from the spectrum of the tilted fibre Bragg grating (TFBG) sensor. This technique was demonstrated to be efficient, easy to implement, powerful, faster than the previous ones and applicable for real-time measurements. In this paper, a deep parametric analysis of the resolution, repeatability and accuracy of the D-T demodulation technique for a TFBG refractometer sensor is performed and presented. The spectral properties of the TFBG sensor as a refractometer are explored using the same demodulation technique. Specifically, supposing the use of the TFBG as a two-parameter optical sensor, the influence of the strain on the envelope area is analysed, and the measurement stability regarding the external RI is reported. Then, the cladding resonance peaks in the spectrum are observed experimentally as the TFBG undergoes partial immersion in a defined refractive index liquid. This last experiment allowed a better understanding the evolution of the TFBG transmission spectrum when the Bragg gratings were partially surrounded by a medium with a different RI. Full article

(This article belongs to the Special Issue Feature Papers in Optics)

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10 pages, 3148 KiB

Open AccessArticle

Design of Refractive/Diffractive Hybrid Projection Lens for DMD-Based Maskless Lithography

by Zhuohui Xu, Jinyun Zhou, Bo Wang and Ziming Meng

Optics 2021, 2(2), 103-112; https://doi.org/10.3390/opt2020011 - 8 Jun 2021

Viewed by 4031

Abstract

The projection lens is the core component of DMD-based maskless lithography and its imaging quality directly affects the transferal of exposure pattern. Based on the traditional projection lens system, we have designed diffractive optical element (DOE) and aspheric surfaces to optimize the refractive/diffractive hybrid projection lens system to improve its imaging quality. We found that the best effect is obtained when DOE is very close to the front lens group before the diaphragm of the hybrid system. Compared with the traditional projection lens system, this hybrid projection lens system has lower wave aberration with the help of DOE, and higher image quality owing to the modulation transfer function (MTF) value being improved. Finally, a hybrid projection lens system with working distance of 29.07 mm, image Space NA of 0.45, and total length of 196.97 mm is designed. We found that the maximum distortion and field curvature are 1.36 × 10⁻⁵% and 0.91 μm, respectively. Full article

(This article belongs to the Special Issue Fabrication and Applications of Photonic Micro-Devices)

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7 pages, 595 KiB

Open AccessArticle

Modelling Dispersion Compensation in a Cascaded-Fiber-Feedback Optical Parametric Oscillator

by Ewan Allan, Craig Ballantine, Sebastian C. Robarts, David Bajek and Richard A. McCracken

Optics 2021, 2(2), 96-102; https://doi.org/10.3390/opt2020010 - 28 May 2021

Cited by 2 | Viewed by 4224

Abstract

Fiber-feedback optical parametric oscillators (OPOs) incorporate intracavity fibers to provide a compact high-energy wavelength-tunable laser platform; however, dispersive effects can limit operation to the sub-picosecond regime. In this research article, we modeled pulse propagation through systems of cascaded fibers, incorporating SMF-28 and ultra-high numerical aperture (UHNA) fibers with complementary second-order dispersion coefficients. We found that the pulse duration upon exiting the fiber system is dominated by uncompensated third-order effects, with UHNA7 presenting the best opportunity to realise a cascaded-fiber-feedback OPO. Full article

(This article belongs to the Special Issue Recent Developments in Novel Solid State Lasers)

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9 pages, 3307 KiB

Open AccessArticle

Dual-Output Mode Analysis of Multimode Laguerre-Gaussian Beams via Deep Learning

by Xudong Yuan, Yaguang Xu, Ruizhi Zhao, Xuhao Hong, Ronger Lu, Xia Feng, Yongchuang Chen, Jincheng Zou, Chao Zhang, Yiqiang Qin and Yongyuan Zhu

Optics 2021, 2(2), 87-95; https://doi.org/10.3390/opt2020009 - 24 May 2021

Cited by 6 | Viewed by 3052

Abstract

The Laguerre-Gaussian (LG) beam demonstrates great potential for optical communication due to its orthogonality between different eigenstates, and has gained increased research interest in recent years. Here, we propose a dual-output mode analysis method based on deep learning that can accurately obtain both the mode weight and phase information of multimode LG beams. We reconstruct the LG beams based on the result predicted by the convolutional neural network. It shows that the correlation coefficient values after reconstruction are above 0.9999, and the mean absolute error (MAE) of the mode weights and phases are about 1.4

{\times 10}^{- 3}

and 2.9

{\times 10}^{- 3}

, respectively. The model still maintains relatively accurate prediction for the associated unknown data set and the noise-disturbed samples. In addition, the computation time of the model for a single test sample takes only 0.975 ms on average. These results show that our method has good abilities of generalization and robustness and allows for nearly real-time modal analysis. Full article

(This article belongs to the Special Issue Recent Developments in Optical Communications)

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17 pages, 5375 KiB

Open AccessReview

A 100 Gbps OFDM-Based 28 GHz Millimeter-Wave Radio over Fiber Fronthaul System for 5G

by James Dzisi Gadze, Reynah Akwafo, Kwame Agyeman-Prempeh Agyekum and Kwasi Adu-Boahen Opare

Optics 2021, 2(2), 70-86; https://doi.org/10.3390/opt2020008 - 30 Apr 2021

Cited by 6 | Viewed by 4954

Abstract

Due to the unprecedented growth in mobile data traffic, emerging mobile access networks such as fifth-generation (5G) would require huge bandwidth and a mobile fronthaul architecture as an essential solution in providing a high capacity for support in the future. To increase capacity, utilizing millimeter waves (mm-waves) in an analog radio over fiber (RoF) fronthaul link is the major advancement and solution in achieving higher bandwidth and high data rate to cater for 5G mobile communication. In this paper, we demonstrate the feasibility of transmission and reception of a 100 Gbits/s data rate link at 28 GHz. The performance of three modulation formats (16-PSK, 16-QAM and 64-QAM) have been compared for an optical fiber length from 5 km up to 35 km for two detection systems; coherent and direct detection. Also, in this paper, the transmission impairments inherent to transmission systems are realized through the implementation of a digital signal processing (DSP) compensation scheme in the receiver system to enhance system performance. Quality factor (QF) and bit error rate (BER) are used as metrics to evaluate the system performance. The proposed system model is designed and simulated using Optisystem 16. Full article

(This article belongs to the Special Issue Recent Developments in Optical Communications)

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7 pages, 26181 KiB

Open AccessArticle

Research of Scattering Properties in Solid-Core Polarization-Maintaining Photonic Crystal Fibers

by Xueyan Hu, Shengheng Zheng, Xingyuan Song, Xiaoxia Huang, Huaiwen Guo, Bowang Zhao and Wei Zhou

Optics 2021, 2(2), 63-69; https://doi.org/10.3390/opt2020007 - 26 Mar 2021

Cited by 2 | Viewed by 2136

Abstract

The scattering from air–glass interfaces within solid-core polarization-maintaining photonic crystal fiber (PM–PCF) will increase the fiber attenuation coefficient, which may lead to high transmission loss. Therefore, it is necessary to describe scattering properties to guide research into reducing fiber loss. In this paper, the loss resulting from roughness scattering at multi-hole interfaces within PM–PCF was theoretically and experimentally analyzed. A PM–PCF scattering model was established to explore the scattering distribution. On the other hand, a fully automatic testing device was built to enable the measurement of a three-dimensional scattering sphere. Simulations were in good agreement with experimental measurements. Moreover, this new proposed measurement method could apply to other PCFs and it will be a useful tool for further scattering research. Full article

(This article belongs to the Special Issue Fabrication and Applications of Photonic Micro-Devices)

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