Photonics

Editorial

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3 pages, 186 KiB

Open AccessEditorial

Special Issue “Novel Specialty Optical Fibers and Applications”: An Overview

by Dora Juan Juan Hu and Georges Humbert

Photonics 2022, 9(7), 497; https://doi.org/10.3390/photonics9070497 - 15 Jul 2022

Viewed by 1613

Abstract

Novel specialty optical fibers refer to optical fibers that have been engineered in terms of design, material and structure, and have been post-processed for novel functionalities and applications [...] Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

Research

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12 pages, 61382 KiB

Open AccessArticle

A Broadband Gold-Coated Photonic Crystal Fiber Polarization Filter with a High Loss Ratio of Both Polarizations at 1550 and 1310 nm

by Chao Wang, Guoxu Zhang, Zheng Wu, Yajing Zhang, Yiyang Zhang, Linghong Jiang and Weihong Bi

Photonics 2021, 8(11), 488; https://doi.org/10.3390/photonics8110488 - 1 Nov 2021

Cited by 7 | Viewed by 2006

Abstract

A new kind of gold-coated hexagonal photonic crystal fiber polarization filter is designed in this paper. The filtering properties can be adjusted through varying the structural parameters. With the 25.60 nm gold film thickness, the losses of the respective modes of Y and [...] Read more.

A new kind of gold-coated hexagonal photonic crystal fiber polarization filter is designed in this paper. The filtering properties can be adjusted through varying the structural parameters. With the 25.60 nm gold film thickness, the losses of the respective modes of Y and X-polarized core mode at 1550 nm are 1024.84 and 0.12 dB/cm with the loss ratio of 8540.33 between two polarizations. However, the losses of Y and X-polarized core mode at 1310 nm are 682.14 and 0.03 dB/cm, and the loss ratio is 22,738 with the gold film thickness of 55.30 nm. That indicates that the proposed filter has a higher loss ratio. Moreover, the crosstalk value with the fiber length of 200 μm at 1550 and 1310 nm are 178.01 and 118.49 dB, respectively. The bandwidths with crosstalk value greater than 20 dB are 640 and 180 nm. The designed polarization filter represents good filtering characteristics and allows great fabrication tolerances. Therefore, the designed hexagonal filter can be well applied in the domain of optical fiber communication. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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17 pages, 6772 KiB

Open AccessArticle

A Novel Liquid Crystal-Filled, Dual-Core Photonic Crystal Fiber Polarization Beam Splitter Covering the E + S + C + L + U Communication Band

by Yuwei Qu, Ying Han, Jinhui Yuan, Xian Zhou, Binbin Yan, Kuiru Wang, Xinzhu Sang and Chongxiu Yu

Photonics 2021, 8(11), 461; https://doi.org/10.3390/photonics8110461 - 21 Oct 2021

Cited by 12 | Viewed by 2129

Abstract

This paper proposes a novel liquid crystal-filled, dual core photonic crystal fiber polarization beam splitter (LC-DC-PCF PBS) based on the coupled mode theory of DC-PCF. The mode birefringence of odd and even modes, coupling lengths (CLs) of the X-polarization (X-pol) and [...] Read more.

This paper proposes a novel liquid crystal-filled, dual core photonic crystal fiber polarization beam splitter (LC-DC-PCF PBS) based on the coupled mode theory of DC-PCF. The mode birefringence of odd and even modes, coupling lengths (CLs) of the X-polarization (X-pol) and Y-polarization (Y-pol), and the corresponding coupling length ratio (CLR) of the proposed LC-DC-PCF PBS filled without LC E7 and with LC E7 are compared. The change rules of the CLs of the X-pol and Y-pol and CLR of the proposed LC-DC-PCF with wavelengths for different cladding microstructure parameters were investigated. The relationships between the X-pol and Y-pol normalized output powers in core A of the proposed LC-DC-PCF PBS and the propagation length at the wavelength of 1.604 μm are discussed. Finally, by studying the change of extinction ratio (ER) with wavelength, the LC-DC-PCF PBS ER of 60.3 and 72.2 dB at wavelengths 1.386 and 1.619 μm are achieved, respectively. The final splitting length (L_S) is 94 μm, and the splitting bandwidth is 349 nm (1.352~1.701 μm), covering the whole of the E + S + C + L + U communication bands. The proposed LC-DC-PCF PBS has good beam-splitting performance, such as ultra-short L_S and ultra-wide splitting bandwidth, with potential applications in laser, sensing, and communication systems. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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11 pages, 2263 KiB

Open AccessCommunication

All-Fiber Gas Raman Laser by D₂-Filled Hollow-Core Photonic Crystal Fibers

by Wenxi Pei, Hao Li, Wei Huang, Meng Wang and Zefeng Wang

Photonics 2021, 8(9), 382; https://doi.org/10.3390/photonics8090382 - 9 Sep 2021

Cited by 5 | Viewed by 2405

Abstract

We report here an all-fiber structure tunable gas Raman laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs). An all-fiber gas cavity is fabricated by fusion splicing a 49 m high-pressure deuterium-filled HC-PCF with two solid-core single-mode fibers at both ends. When pumped [...] Read more.

We report here an all-fiber structure tunable gas Raman laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs). An all-fiber gas cavity is fabricated by fusion splicing a 49 m high-pressure deuterium-filled HC-PCF with two solid-core single-mode fibers at both ends. When pumped with a pulsed fiber amplifier seeded by a tunable laser diode at 1.5 μm, Raman lasers ranging from 1643 nm to 1656 nm are generated. The maximum output power is ~1.2 W with a Raman conversion efficiency of ~45.6% inside the cavity. This work offers an alternative choice for all-fiber lasers operating at 1.6–1.7 μm band. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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13 pages, 1958 KiB

Open AccessArticle

Sensitivity Enhanced Refractive Index Fiber Sensor Based on Long-Range Surface Plasmon Resonance in SiO₂-Au-TiO₂ Heterostructure

by Wenyi Bu, Zhifang Wu, Perry Ping Shum, Xuguang Shao and Jixiong Pu

Photonics 2021, 8(9), 379; https://doi.org/10.3390/photonics8090379 - 9 Sep 2021

Cited by 9 | Viewed by 2781

Abstract

Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. [...] Read more.

Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. Many useful applications related to light–medium interaction have been demonstrated based on the LRSPR effect, especially in the sensing area. Here, we propose and demonstrate an LRSPR-based refractive index sensor by using a SiO

_{2}

-Au-TiO

_{2}

heterostructure, in which a D-shaped honeycomb-microstructure optical fiber (MOF) is designed as the silica substrate and then deposited with a gold film and thin-layer titanium dioxide (TiO

_{2}

). By using the full-vector finite-element method (FEM), this heterostructure is numerically investigated and demonstrated to excite LRSPR without a buffer layer, which is usually necessary in previous LRSPR devices. Through comprehensive discussion about the influence of structural parameters on the resonant wavelength, the excitation of the LRSPR in the proposed heterostructure is revealed to be highly related to the effective refractive index of MOF’s fundamental core mode, which is mainly determined by the MOF’s pitch, the thicknesses of the silica web and the planar-layer silica. Moreover, the thin-layer TiO

_{2}

plays an important role in significantly enhancing the resonance and the sensitivity to analyte’s refractive index as well, when it is coated on the top of the Au film rather than between the metal and waveguide. Finally, the proposed LRSPR sensor based on SiO

_{2}

-Au-TiO

_{2}

heterostructure shows an ultra-high wavelength sensitivity of 20,100 nm/RIU and the corresponding minimum resolution is as low as

4.98 \times 10^{- 7}

RIU. Thus, the proposed LRSPR device offers considerable potential for sensing applications in biomedical and biochemical areas. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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7 pages, 1872 KiB

Open AccessCommunication

All-Fiber Gas Cavity Based on Anti-Resonant Hollow-Core Fibers Fabricated by Splicing with End Caps

by Jing Shi, Xinyu Ye, Yulong Cui, Wei Huang, Hao Li, Zhiyue Zhou, Meng Wang, Zilun Chen and Zefeng Wang

Photonics 2021, 8(9), 371; https://doi.org/10.3390/photonics8090371 - 3 Sep 2021

Cited by 8 | Viewed by 2424

Abstract

In recent years, fiber gas lasers have obtained a rapid development, however, efficient and stable pump coupling is a key limitation for their applications in the future. Here, we report an all-fiber gas cavity based on anti-resonant hollow-core fibers which have the beneficial [...] Read more.

In recent years, fiber gas lasers have obtained a rapid development, however, efficient and stable pump coupling is a key limitation for their applications in the future. Here, we report an all-fiber gas cavity based on anti-resonant hollow-core fibers which have the beneficial properties of adjustable broad transmission bands and potential low transmission attenuation, especially in the mid-infrared. This kind of all-fiber gas cavity is fabricated by directly splicing with end caps at both ends for the first time. The high-power laser transmission characteristics were studied, and the experimental results show that the all-fiber gas cavities have a very stable performance. The maximum input laser power at 1080 nm is about 260 W, and the output power is 203 W, giving a total transmission efficiency of 78.1%. This work opens a new opportunity for the development of high-power all-fiber structured fiber gas lasers. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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7 pages, 2684 KiB

Open AccessCommunication

Nanograting-Enhanced Optical Fibers for Visible and Infrared Light Collection at Large Input Angles

by Ning Wang, Matthias Zeisberger, Uwe Hübner and Markus A. Schmidt

Photonics 2021, 8(8), 295; https://doi.org/10.3390/photonics8080295 - 24 Jul 2021

Cited by 6 | Viewed by 2253

Abstract

The efficient incoupling of light into particular fibers at large angles is essential for a multitude of applications; however, this is difficult to achieve with commonly used fibers due to low numerical aperture. Here, we demonstrate that commonly used optical fibers functionalized with [...] Read more.

The efficient incoupling of light into particular fibers at large angles is essential for a multitude of applications; however, this is difficult to achieve with commonly used fibers due to low numerical aperture. Here, we demonstrate that commonly used optical fibers functionalized with arrays of metallic nanodots show substantially improved large-angle light-collection performances at multiple wavelengths. In particular, we show that at visible wavelengths, higher diffraction orders contribute significantly to the light-coupling efficiency, independent of the incident polarization, with a dominant excitation of the fundamental mode. The experimental observation is confirmed by an analytical model, which directly suggests further improvement in incoupling efficiency through the use of powerful nanostructures such as metasurface or dielectric gratings. Therefore, our concept paves the way for high-performance fiber-based optical devices and is particularly relevant within the context of endoscopic-type applications in life science and light collection within quantum technology. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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Review

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24 pages, 9308 KiB

Open AccessReview

Advances in Multicore Fiber Grating Sensors

by Zhiyong Zhao, Yunli Dang and Ming Tang

Photonics 2022, 9(6), 381; https://doi.org/10.3390/photonics9060381 - 26 May 2022

Cited by 23 | Viewed by 4768

Abstract

In recent years, multicore fiber (MCF) has attracted increasing interest for sensing applications, due to its unique fiber structure of multiple parallel cores in a single fiber cladding, which offers a flexible configurable platform to establish diverse functional fiber devices for sensing applications. [...] Read more.

In recent years, multicore fiber (MCF) has attracted increasing interest for sensing applications, due to its unique fiber structure of multiple parallel cores in a single fiber cladding, which offers a flexible configurable platform to establish diverse functional fiber devices for sensing applications. So far, a variety of discrete fiber sensors using MCF have been developed, among which one of the major categories is the MCF grating sensors. The most distinct characteristic of MCF that differs from the normal single mode fibers is that the off-center cores of a MCF are sensitive to bending, which is caused by the bending induced tangential strain in off-center waveguides through either compression or stretching. The bending sensitivity has been widely developed for bending/curvature sensing or measuring physical parameters that are associated with bending. In this paper, we review the research progress on MCF-based fiber grating sensors. MCF-based diverse fiber grating sensors will be introduced, whose working principles will be discussed, and various types of applications of the MCF grating sensors will be summarized. Finally, the challenges and prospects of MCF grating for sensing applications will be presented. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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15 pages, 5313 KiB

Open AccessReview

Characteristics of Critical-Wavelength-Existed Fiber-Optic Mach–Zehnder Interferometers and Their Sensing Applications

by Chenxu Lu, Xiaopeng Dong and Chi Wu

Photonics 2022, 9(6), 378; https://doi.org/10.3390/photonics9060378 - 26 May 2022

Cited by 5 | Viewed by 2382

Abstract

In this paper, we review the characteristics of critical wavelength (CWL)-existed fiber-optic Mach–Zehnder interferometers (MZIs), including special few-mode fibers and microfibers, and their sensing applications in physical, chemical, and marine fields. Owing to the existence of CWL in the transmission spectra, the in-line [...] Read more.

In this paper, we review the characteristics of critical wavelength (CWL)-existed fiber-optic Mach–Zehnder interferometers (MZIs), including special few-mode fibers and microfibers, and their sensing applications in physical, chemical, and marine fields. Owing to the existence of CWL in the transmission spectra, the in-line MZIs show some specific characteristics. The closer the peak/dip wavelength to the CWL, the larger the wavelength shift or the related sensitivity when the interferometer is under testing. Meanwhile, CWL shifts monotonically with the variations in measurands, such as temperature (in the air or seawater), axial strain, water pressure, surrounding refractive index, etc., when they are applied to the sensing fibers. These characteristics of the CWL-existed in-line MZIs make them appealing solutions for fabricating various interferometric sensors, with the advantages of large measurement range, high sensitivity, multiparameter sensing, etc. Theoretical and experimental studies on the properties of the CWL-existed in-line MZIs are reviewed and discussed in this paper. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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23 pages, 13558 KiB

Open AccessReview

Review of a Specialty Fiber for Distributed Acoustic Sensing Technology

by Yixiang Sun, Hao Li, Cunzheng Fan, Baoqiang Yan, Junfeng Chen, Zhijun Yan and Qizhen Sun

Photonics 2022, 9(5), 277; https://doi.org/10.3390/photonics9050277 - 20 Apr 2022

Cited by 31 | Viewed by 5604

Abstract

Specialty fibers have introduced new levels of flexibility and variability in distributed fiber sensing applications. In particular, distributed acoustic sensing (DAS) systems utilized] the unique functions of specialty fibers to achieve performance enhancements in various distributed sensing applications. This paper provides an overview [...] Read more.

Specialty fibers have introduced new levels of flexibility and variability in distributed fiber sensing applications. In particular, distributed acoustic sensing (DAS) systems utilized] the unique functions of specialty fibers to achieve performance enhancements in various distributed sensing applications. This paper provides an overview of recent preparations and developments of specialty-fiber-based DAS systems and their sensing applications. The specialty-fiber-based DAS systems are categorized and reviewed based on the differences in scattering enhancement and methods of preparation. The prospects of using specialty fibers for DAS systems are also discussed. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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26 pages, 5035 KiB

Open AccessReview

Review on All-Fiber Online Raman Sensor with Hollow Core Microstructured Optical Fiber

by Haonan Ding, Dora Juan Juan Hu, Xingtao Yu, Xiaoxian Liu, Yifan Zhu and Guanghui Wang

Photonics 2022, 9(3), 134; https://doi.org/10.3390/photonics9030134 - 25 Feb 2022

Cited by 16 | Viewed by 5047

Abstract

Raman spectroscopy is widely used for qualitative and quantitative analysis of trace components in scientific fields such as food safety monitoring, drug testing, environmental monitoring, etc. In addition to its demonstrated advantages of fast response, non-destructive, and non-polluting characteristics, fast online Raman detection [...] Read more.

Raman spectroscopy is widely used for qualitative and quantitative analysis of trace components in scientific fields such as food safety monitoring, drug testing, environmental monitoring, etc. In addition to its demonstrated advantages of fast response, non-destructive, and non-polluting characteristics, fast online Raman detection is drawing growing attention for development. To achieve this desirable capability, hollow core optical fibers are employed as a common transmission channel for light and fluid in the Raman sensor. By enhancing the interaction process between light and matter, the detection sensitivity is improved. At the same time, the Raman spectroscopy signal light collection efficiency is significantly improved. This article summarizes enhancement techniques reported for Raman sensors, followed by a detailed review on fiber-based Raman sensor techniques including theoretical analyses, fabrication, and application based on hollow core photonic crystal fibers and capillary-based hollow core fibers. The prospects of using these fibers for Raman spectroscopy are discussed. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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18 pages, 5259 KiB

Open AccessReview

Chalcogenide Glass Microfibers for Mid-Infrared Optics

by Dawei Cai, Yu Xie, Xin Guo, Pan Wang and Limin Tong

Photonics 2021, 8(11), 497; https://doi.org/10.3390/photonics8110497 - 5 Nov 2021

Cited by 11 | Viewed by 3374

Abstract

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to [...] Read more.

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As₂S₃ and As₂Se₃. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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14 pages, 2417 KiB

Open AccessFeature PaperReview

Review of Specialty Fiber Based Brillouin Optical Time Domain Analysis Technology

by Dora Juan Juan Hu, Georges Humbert, Hui Dong, Hailiang Zhang, Jianzhong Hao and Qizhen Sun

Photonics 2021, 8(10), 421; https://doi.org/10.3390/photonics8100421 - 30 Sep 2021

Cited by 19 | Viewed by 3730

Abstract

Specialty fibers have introduced new functionalities and opportunities in distributed fiber sensing applications. Particularly, Brillouin optical time domain analysis (BOTDA) systems have leveraged the unique features of specialty fibers to achieve performance enhancement in various sensing applications. This paper provides an overview of [...] Read more.

Specialty fibers have introduced new functionalities and opportunities in distributed fiber sensing applications. Particularly, Brillouin optical time domain analysis (BOTDA) systems have leveraged the unique features of specialty fibers to achieve performance enhancement in various sensing applications. This paper provides an overview of recent developments of the specialty fibers based BOTDA technologies and their sensing applications. The specialty fibers based BOTDA systems are categorized and reviewed based on the new features or performance enhancements. The prospects of using specialty fibers for BOTDA systems are discussed. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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24 pages, 7209 KiB

Open AccessReview

Transmission and Generation of Orbital ANGULAR Momentum Modes in Optical Fibers

by Zhuo Wang, Jiajing Tu, Shecheng Gao, Zhaohui Li, Changyuan Yu and Chao Lu

Photonics 2021, 8(7), 246; https://doi.org/10.3390/photonics8070246 - 30 Jun 2021

Cited by 13 | Viewed by 5709

Abstract

The orbital angular momentum (OAM) of light provides a new degree of freedom for carrying information. The stable propagation and generation of OAM modes are necessary for the fields of OAM-based optical communications and microscopies. In this review, we focus on discussing the [...] Read more.

The orbital angular momentum (OAM) of light provides a new degree of freedom for carrying information. The stable propagation and generation of OAM modes are necessary for the fields of OAM-based optical communications and microscopies. In this review, we focus on discussing the novel fibers that are suitable for stable OAM mode transmission and conversion. The fundamental theory of fiber modes is introduced first. Then, recent progress on a multitude of fiber designs that can stably guide or generate OAM modes is reviewed. Currently, the mode crosstalk is regarded as the main issue that damages OAM mode stability. Therefore, the coupled-mode theory and coupled-power power theory are introduced to analyze OAM modes crosstalk. Finally, the challenges and prospects of the applications of OAM fibers are discussed. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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13 pages, 11010 KiB

Open AccessFeature PaperReview

Recent Advancement of Anti-Resonant Hollow-Core Fibers for Sensing Applications

by Wenjun Ni, Chunyong Yang, Yiyang Luo, Ran Xia, Ping Lu, Dora Juan Juan Hu, Sylvain Danto, Perry Ping Shum and Lei Wei

Photonics 2021, 8(4), 128; https://doi.org/10.3390/photonics8040128 - 19 Apr 2021

Cited by 37 | Viewed by 8190

Abstract

Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review [...] Read more.

Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review presents an overview of recent progress in anti-resonant hollow-core fibers for sensing applications. Both regular and irregular-shaped fibers and their performance in various sensing scenarios are summarized. Finally, the challenges and possible solutions are briefly presented with some perspectives toward the future development of anti-resonant hollow-core fibers for advanced sensing. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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Other

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11 pages, 4059 KiB

Open AccessLetter

Operation of a Single-Frequency Bismuth-Doped Fiber Power Amplifier near 1.65 µm

by Grzegorz Gomółka, Monika Krajewska, Małgorzata Kaleta, Aleksandr M. Khegai, Sergey V. Alyshev, Aleksey S. Lobanov, Sergei V. Firstov and Michał Nikodem

Photonics 2020, 7(4), 128; https://doi.org/10.3390/photonics7040128 - 9 Dec 2020

Cited by 6 | Viewed by 3292

Abstract

The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral [...] Read more.

The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral region is still limited. In this paper, a single-frequency continuous-wave bismuth-doped fiber amplifier (BDFA) operating at 1651 and 1687 nm is presented. With the improved signal/pump coupling and modified pump source design, the output powers of 163 mW (at 1651 nm) and 197 mW (at 1687 nm) were obtained. Application of the BDFA to the optical spectroscopy of methane near 1651 nm is also described. We demonstrate that the BDFA can be effectively used for signal amplitude enhancement in photothermal interferometry. Full article

(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)

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