Optical fibers provide much more than a means to transport light between different locations. This article reviews how integration of functional fluid, solid, and gaseous materials in photonic crystal fibers enables control of their linear and nonlinear properties with applications in optoelectronics, sensing, and laser science.

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Hybrid photonic-crystal fiber

Symmetrical dual D-shape photonic crystal fibers (PCFs) for surface plasmon resonance (SPR) sensing are designed and analyzed by the finite element method (FEM). The performance of the sensor is remarkably enhanced by the directional power coupling between the two fibers. We study the influence of the structural parameters on the performance of the sensor as well as the relationship between the resonance wavelengths and analyze refractive indexes between 1.36 and 1.41. An average spectral sensitivity of 14660 nm/RIU can be achieved in this sensing range and the corresponding refractive index resolution is 6.82 × 10-6 RIU. The characteristics of a single D-shape PCF-SPR sensor with the same structural parameters are compared with those of the dual PCFs sensor and the latter has distinct advantages concerning the spectral sensitivity, resolution, amplitude sensitivity, and figure of merits (FOM).

Symmetrical dual D-shape photonic crystal fibers for surface plasmon resonance sensing.

High-quality optical fibers can be produced now at a low cost and large quantity, and this has further promoted the development of fiber optic (chemical) sensors. After over 30 years of innovation, fiber optic sensing technology has become mature because of acceptable costs, compact instrumentation, high accuracy and the capability of performing measurements at inaccessible sites, over large distances, in strong (electro)magnetic fields and in harsh environment. The technology is still proceeding quickly in terms of innovation, and respective applications have been found in highly diversified fields. This review covers work published in the time period between October 2015 and October 2019. It is written in continuation of previous reviews.

Fiber-Optic Chemical Sensors and Biosensors (2015-2019).

A Review of advancements (2007–2017) in plasmonics-based optical fiber sensors

Surface plasmon resonance (SPR) infrared sensor based on D-shape photonic crystal fibers with ITO coatings

In this paper, an isolator-free unidirectional multiwavelength Tm-doped fiber laser is experimentally investigated. The isolator-free unidirectional operation of the laser is realized by the construction of a type of theta cavity. A single-mode-multimode-single-mode fiber structure served as a multimode interference filter is employed for implementing multiwavelength operation. Wavelength numbers from one to three can be obtained by increasing the pump power and properly adjusting the polarization controller. Besides the single wavelength emission at ∼2002.6 nm, dual-wavelength operations (∼1952.6 nm, ∼1954.7 and ∼1954.8 nm, ∼2002.7 nm) with signal-to-noise ratios larger than ∼46 dB and three-wavelength operation (∼1952.8, ∼1954.5, and ∼2002.9 nm) with signal-to-noise ratios larger than ∼50 dB have also been obtained, respectively. The maximum output power of ∼545 mW was obtained at pump power of ∼7.3 W, when the laser operates at three-wavelength oscillation and the slope efficiency is ∼10%. Our demonstrated fiber laser source represents a type of all-fiberized solution for the realization of high-power, unidirectional and controllable multiwavelength operations, simultaneously.

Isolator-Free Unidirectional Multiwavelength Tm-Doped Double-Clad Fiber Laser Based on Multimode Interference Effect

A high sensitivity fiber-optic sensor based on self-imaging effect in a hollow-core capillary waveguide (HCCW) is presented for sensing applications. The sensor is composed of a section of HCCW fusion spliced between single mode fibers (SMFs). The self-imaging effect in the HCCW is investigated with different fiber lengths and arc-fusion parameters. By infiltrating the hollow core with index matching liquids, the peak wavelength of the proposed device shifts towards longer wavelengths. The temperature and refractive index (RI) responses of the sensor are studied systematically. When temperature is increased from 25 °C to 75 °C, the temperature sensitivity of the device can be improved significantly with the infiltrated structure, and reaches −0.49 nm/°C, compared with that of the un-filled device, which is 9.8 pm/°C. For the RI response, the liquid-filled structure achieves sensitivity of 12,005 nm/RIU in the range between 1.448 and 1.450, slightly higher than the 11,920 nm/RIU achieved by the un-filled one. The proposed sensor exhibits the advantages of simple structure, high sensitivity and low cost, which may find potential applications in physical and chemical sensing.

https://www.mdpi.com/1424-8220/20/1/135/pdf

Self-Imaging Effect in Liquid-Filled Hollow-Core Capillary Waveguide for Sensing Applications.

A highly sensitive torsion sensor can be constructed by combining a twisted photonic crystal fiber with a liquid-filled waveguide in its air-hole cladding. The torsion sensitivity of this type of sensor is determined directly by the phase-matching conditions between the fiber core mode and the liquid waveguide mode, which can be improved by tuning the helicity (denoted by the initial twist rate, α0) of the twisted photonic crystal fiber. The enhancement mechanism of α0 on the sensitivity of the proposed torsion sensor is investigated theoretically, followed by experimental verifications, and a torsion sensitivity as high as 446 nm∙mm∙rad−1 can be obtained by tailoring these parameters. Experimental results show that the torsion sensitivity increases with α0 decreasing from 3.142 to 3.925 rad/mm, which are in consistence with that of the numerical predictions. The demonstrated torsion sensor is expected to contribute to the development of highly sensitive torsion-related photonic crystal fiber devices.

https://www.mdpi.com/1424-8220/20/5/1490/pdf

Helicity Enhanced Torsion Sensor Based on Liquid Filled Twisted Photonic Crystal Fibers.

We propose a simple deep ultraviolet (UV) sensor consisting of a conventional single-mode optical fiber capped with a ZnO microwire. The ZnO microwire positioned on the fiber-tip acts as a Fabry-Perot interferometer, of which the reflection spectrum can be employed for UV light monitoring. When ZnO microwire is exposed to UV irradiation, variations in the concentration of photogenerated carriers will result in the change of the refractive index (RI) of the ZnO microwire, and thus interference wavelengths of the proposed device exhibits red-shift with a sensitivity of 0.288 nm/(W·cm−2) under 266-nm deep UV laser irradiation. Meanwhile, a fast response time of 0.56 ms is experimentally obtained. The results may pave a new way for fast responsive, highly spatial-resolved optical fiber UV sensors.

ZnO Microwire-Based Fiber-Tip Fabry-Pérot Interferometer for Deep Ultraviolet Sensing

We report on mode-locked thulium-doped fiber lasers with high-energy nanosecond pulses, relying on the transmission in a semiconductor saturable absorber (SESA) and a carbon nanotube (CNTs-PVA) film separately. A section of an SMF–MMF–SMF structure multimode interferometer with a transmission peak wavelength of ∼2003 nm was used as a wavelength selector to fix the laser wavelength. When the SESA acted as a saturable absorber (SA), the mode-locked fiber laser had a maximum output power of ∼461 mW with a pulse energy of ∼0.14 μJ and a pulse duration of ∼9.14 ns. In a CNT-film-based mode-locked fiber laser, stable mode-locked pulses with the maximum output power of ∼46 mW, pulse energy of ∼26.8 nJ and pulse duration of ∼9.3 ns were obtained. To the best of our knowledge, our experiments demonstrated the first 2 μm region ‘real’ SA-based dissipative soliton resonance with the highest mode-locked pulse energy from a ‘real’ SA-based all-fiberized resonator.

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Yijian Huang

Papers

Isolator-Free Unidirectional Multiwavelength Tm-Doped Double-Clad Fiber Laser Based on Multimode Interference Effect

Self-Imaging Effect in Liquid-Filled Hollow-Core Capillary Waveguide for Sensing Applications.

Helicity Enhanced Torsion Sensor Based on Liquid Filled Twisted Photonic Crystal Fibers.

ZnO Microwire-Based Fiber-Tip Fabry-Pérot Interferometer for Deep Ultraviolet Sensing

Two-micron all-fiberized passively mode-locked fiber lasers with high-energy nanosecond pulse