Considering the problem that the optical fibers prepared via the modified chemical vapor deposition method suffer from the restriction of limited variation range of fiber core components, the rapidly developed melt-in-tube (MIT) fiber drawing method is introduced. The recent progress in the preparation of special optical fibers based on the MIT method is summarized from the perspective of glass core fiber, semiconductor core fiber, and crystal core fiber. Further, the performances and applications of these optical fibers are also analyzed. Finally, the existing problems and development trend of this method are discussed from a practical perspective.

In recent years, ytterbium (Yb)-doped large mode field photonic crystal fibers have attracted significant attention owing to their applications in high peak-power picosecond ultrafast laser amplifiers. Herein, the difficulties in the development of Yb-doped large mode field photonic crystal fibers are briefly analyzed, and the research progress of these novel fibers both at home and abroad is examined. The preparation methods and optical/spectral properties of Yb-doped silica glass mandrels used for Yb-doped large mode field photonic crystal fibers are summarized. The preparation of large diameter, low NA ytterbium-doped silica glass mandrels, large mode area Yb doped photonic crystal fibers and their applications in picosecond pulse laser amplification at the Shanghai Institute of Optics and Fine Mechanics are then discussed. Finally, the development and application of Yb-doped large mode field photonic crystal fibers are summarized, and their future prospects are detailed.

The structure and performance advantages of microstructured optical fibers (MOFs) have aroused an immense interest from researchers both locally and internationally, and the MOF emerges as a hot spot in the field of optoelectronics. Currently, according to the structure, MOF can either be solid-core or hollow-core. With respect to the transmission mechanism, they can be categorized as total internal reflection MOF, photonic bandgap MOF, and anti-resonant MOF. The important applications of MOF can be found in laser technology, optical sensing technology, optical communication technology, optoelectronic integration, and optical fiber devices. This paper conducts a review on the development history of MOF, presents a comprehensive analysis and summary of their various types, transmission mechanism, design, and fabrication, and provides a reference for exploring new research directions and applications of MOF in the future.

Mid-infrared fiber lasers have important applications in fundamental researches, optical communications, biomedicine, environmental monitoring, and national defense security. Currently, researchers are mainly focused on two main mid-infrared fiber lasers, including supercontinuum (SC) lasers and rare-earth-ions-doped fiber lasers. For the application requirements, the authors developed a fluorotellurite glass fiber with relatively good stability and high damage threshold. By using the fluorotellurite glass fibers as the nonlinear media, broadband SC laser source from 0.6 to 5.4 μm was obtained in the experiments. Moreover, SC light source with an average power of about 20 W was also obtained, and the spectral range covered 1-4 μm wavelength. The authors fabricated Ho 3+-doped AlF3 based glass fibers with relatively good water resistance. By using the Ho3+-doped AlF3 based glass fibers as gain media, the authors obtained about 2868 nm lasers. The authors also fabricated Ho3+-doped InF3 based glass fibers with relatively low phonon energies. By using the Ho3+-doped InF3 based glass fibers as gain media, the authors obtained about 2875 nm lasers. This paper mainly focuses on introducing the current progress on mid-infrared glass fibers, including the material characteristics of fluorotellurite glass fibers, AlF3 based glass fibers, InF3 based glass fibers, and relative lasers.

Herein, fiber hosts and rare earth gain ions commonly used in 3-μm fiber lasers are introduced, and the working principle of 3-μm rare earth ion-doped fiber lasers is briefly analyzed. Further, research progress on various 3-μm rare earth ion-doped fiber lasers is reviewed, revealing that mode-locked mid-infrared fiber lasers, miniaturized mid-infrared all-fiber lasers, and mid-infrared fiber lasers with long wavelengths of 3-4 μm are currently hot research topics. With the rapid development of 3-μm mid-infrared fiber lasers in recent years, 3-μm fiber lasers with a compact structure and excellent performance are emerging, which will greatly promote their commercialization and practical application in addition to fulfilling the requirements of different fields.

Due to the characteristics of wide mid-infrared transmission range, high non-linear coefficients, and easily fiberized performance, chalcogenide glass has very important applications in mid-infrared photonic integration, optical fiber source, sensing, and other optical fields. In recent years, with further development of the infrared optics, some progresses have been made in the research of chalcogenide glasses and their optical fibers. From this point of view, the research progress of the fabrication of low-loss chalcogenide fiber, and its application fields including fiber devices (couplers, combiners, bundles,and fiber gratings), fiber-based light sources, and optical fiber sensing at home and abroad are summarized in this review. Besides, some existing problems are analyzed and the future development direction is prospected.

High-power erbium-ytterbium codoped fiber lasers have been widely applied to optical fiber communication, lidar, satellite remote sensing, and precision measurement owing to its “eye safety” and low loss characteristics in both optical fibers and atmosphere. This study briefly introduces the developments of the erbium-ytterbium codoped fiber and associated laser system and emphatically expounds the latest progress in research on erbium-ytterbium codoped fiber lasers with high power and narrow linewidth. Additionally, we analyzes factors which restrict the increase of power of the erbium-ytterbium codoped fiber laser. In view of ytterbium-band amplified spontaneous emissions and optical fiber thermal effects, optical fiber and laser system structures are optimized to realize an erbium-ytterbium codoped fiber laser with higher output power.

Fiber lasers with 2-3 μm center wavelength have broad application prospects in the military, sensing, communication, biomedical, and environmental monitoring fields. The use of the rare earth doped fluorophosphate glass to prepare optical fiber is an effective method for directly fabricating 2-3 μm fiber lasers. The current research progress of rare earth doped fluorophosphate glass fibers for 2-3 μm fiber laser application is reviewed. In addition, the development and current challenges associated with rare earth doped fluorophosphate glass fiber are explored.

Fiber lasers have been widely used in industry, medicine, military defense,etc. The investigations of gain fibers play an important role for accelerating the development of fiber lasers. Glass ceramic fiber, possessing strong crystal field environments, is a promising material for application in high-efficiency and tunable fiber lasers. Moreover, the glass ceramic fibers containing nonlinear crystals can be used for the frequency conversion of lasers and further broaden the application of fibers. This paper introduces the concepts and optical properties of glass ceramic fibers, emphatically describes the research progresses of rare-earth ions doping, transition metal ions doping, quantum dots doping and second harmonic generation glass ceramic fibers, and discusses the fabrication techniques of glass ceramic fibers. Finally, the applications of glass ceramic fibers in the future are envisioned.

Metal nanocrystals hybridized optical fibers (MNCs-OFs) combine the unique localized surface plasmon resonance (LSPR) property afforded by metal nanocrystals and the advantages of small size, simple structure, stable performance, and high resilience to disturbance of optical fibers. On the one hand, high optical nonlinearity, metal enhanced fluorescence,and surface enhanced Raman scattering, which are caused by LSPR of metal nanocrystals, can impart optical fiber with new functionalities and better performance. On the other hand, evanescent wave transmitting property of optical fiber can tremendously enhance the excitation efficiency of LSPR of metal nanocrystals. Therefore, MNCs-OFs are very useful in many applications such as optical tuning, fiber laser, physical/biochemical sensing and detection, which have drawn extensive research interests. Here, we make a brief review to the LSPR mechanism of metal nanocrystals and the fabrication and applications of MNCs-OFs,and provide a prospect to the future development of this type of optical fibers.

Single crystal fiber (SCF) is a combination of bulk crystal and conventional fiber, it has gradually become a research hotspot in the field of solid-state lasers because of the excellent physical and chemical properties accompanying with good thermal management. Laser heated pedestal growth (LHPG) and micro-pulling-down (μ-PD) growth methods are introduced in detail. Meanwhile, the research status about the fabrication of SCF and SCF lasers at home and abroad are summarized. Finally, combined with the current research foundation, the research prospects and development trends of SCF are analyzed.

As the remaining frontier for increasing the communication capacity of optical fibers, multi-core fibers with space division multiplexing have attracted much attention in recent years, with many solutions proposed for expanding the capacity and improving the transmission speed. This paper comprehensively reviews the basic properties of multi-core optical fibers, especially in non-optical-fiber communications and sensing applications. We focus on the main issues related to multi-core fibers, namely, inter-core coupling and crosstalk problems, the transmission and bending characteristics of the fibers, fan-in/fan-out and connection technologies, the taped fiber coupling technique for multicores, and other dominant problems in optical fiber communication technology. A new hollow elliptic multi-core polarization-maintaining fiber is discussed as well. In a series of examples, we finally show how multi-core fibers are applied in high-power fiber lasers and fiber interference, emphasizing their applications in various sensing fields.

The functionalized graphene materials link optoelectronics, micro- and nano-technology, and material physics; further, these materials exhibit good integration capability with fibers and waveguides because of their atomically thin nature. The recent development of optoelectronics has been driven by the graphene-based fiber functional devices. This study focuses on the extensively used sensors, lasers, and nonlinear devices and briefly introduces the fabrication of graphene and related materials along with the implementations of the fiber/waveguide structures. Furthermore, we demonstrate the advantages of graphene-based fiber devices based on their performances and applications by reviewing the relevant research results.

Optical micro-nanofibers (MNFs) have diameters close to or smaller than the wavelength of the guided light. Recently, MNFs have attracted wide attention due to their important applications in optofluidic sensors. This paper begins with a brief introduction of the fabrication methods and waveguiding properties of MNFs. Based on intensity and phase modulation, we introduce the research progress of optofluidic MNF sensors in chemical and biological sensing with high sensitivity, fast response, and small footprint. Finally, the development and application of optofluidic MNF sensors are summarized and prospected.

Recently, fiber optics have become indispensable infrastructural components for the transmission of optical signals and energy. We provide an overview of functional fibers. The light guiding mechanism and preparation scheme of microstructure optical fibers (MOFs) are introduced. The usage of MOF has emerged in optoelectronic sensing and laser applications because of its flexible preform preparation, hollow core transmission ability, and ultralow theoretical attenuation. Because the development of optical fibers is progressing toward the integration of multi-functional units into single fibers, the preparation and potential applications of a nanomechanical optical fiber are introduced in detail. This new type of optical fiber represents a crucial research direction for the development of all-optical devices and optical integration technologies.

Glass microsphere laser device is a kind of whispering-gallery mode micro-nano laser device. In recent years, it has attracted a lot of attention as its low threshold, narrow line width and integration advantages. Various types of glass microsphere laser device are reported continually. In this paper, the preparation methods of the whispering-gallery mode glass microsphere cavity and the latest research progress of silica and compound glass microsphere laser are briefly introduced.

Herein, using a fast Brillouin optical time-domain reflectometry system with a wideband receiver and distributed Raman amplification, a long-distance distributed Brillouin sensing is realized. We study the performance difference and nonlinear phenomena observed in the system before and after Raman amplification, and also jointly optimize the Raman pump and launched pulse powers. Our experiments achieve a sensing distance of around 100 km with a spatial resolution of 50 m and accuracy of 1.2 ℃ at a sensing position of 50 km. In comparison to the system without Raman amplification, the sensing distance is increased by approximately 50 km and measurement can be completed in approximately 10 s only.

In order to fabricate an arsenic-free infrared chalcogenide optical fiber,two kinds of high purity glass samples, i.e., Ge20Se79Te1 and Ge20Se80, are prepared by the melt-quenching method and dynamic vacuum distillation purification process. Then a Ge-Se fiber preform with ideal core-clad structure is prepared by an optimized peel-off extrusion method. The drawn Ge-Se fiber has an average loss of 4.8 dB/m in the range from 7.5 to 8.7 μm,and has a minimum loss of 3.2 dB/m at 7.7 μm. A flat supercontinuum in the range of 1.5-11.2 μm is generated by using a 17-cm long fiber which is pumped by an optical parametric amplifier, and the relationship among supercontinuum wide, pumping wavelength, and pumping power is obtained.

A temperature compensated fiber optic microfluidic sensor by using the femtosecond laser drilling technique is realized and studied. A laser-induced water breakdown method is used to fabricate a uniform microfluidic channel perpendicular with the fiber core, between a fiber Bragg grating (FBG) and the gold-coated end face of the fiber. Then a single end reflective sensor is made. By performing the fast Fourier transformation to the reflection spectrum of the fabricated device, both the FBG and Fabry-Perot (F-P) cavity wavelength information can be restored. Experiment shows that the FBG and F-P cavity can have different responses to the external temperature and the microfluidic refractive index. As a result, the temperature information can be sensed by measuring the FBG spectral shift, while the refractive index information can be extracted from the FBG spectral shift with the temperature compensated. In our experiment, the measured refractive index sensitivity of the sensor at a center wavelength of 1550 nm is about 1.2038 nm·RIU-1 (RIU is the unit refractive index). Analysis shows that the sensor performance can be further improved by optimizing the fiber structure. Our device is featured with structural simplification, easy operation, and real-time monitoring, and thus it can have good potential in both biochemical and medical areas.

This paper reviews a specially designed few-mode fiber and a single mode fiber-few mode fiber-single mode fiber (SFS) sensing structure. Only the fundamental core mode LP01 and the first symmetric high order core mode LP02 are excited in the few-mode fiber under the axial to axial splicing condition, and there is a critical wavelength existing in the transmission spectrum of the SFS structure at the operational wavelength. Near the critical wavelength, the wavelength spacing between adjacent interference peaks reaches maximum, and the output intensity of the interferometer is fixed while the wavelength changes. Therefore, the critical wavelength is exclusive and easy to identify from the transmission spectrum. The sensing characteristics of the critical wavelength and the interference fringes in the transmission spectrum under strain, temperature, bending, and liquid refractive index are studied theoretically and experimentally, and the SFS sensor structure is applied to measure a wide range of sensing parameters including strain, temperature, curvature, displacement, surrounding refractive index, and relative humidity, with the advantages of large measurement ranges, high sensitivity, and multi-parameter measurement. Thus, the current problems of limited measurement range and multi-value outputs existing in the traditional interferometers can be solved.

Herein, two sets of interference spectra corresponding to the ordinary and extraordinary waves are obtained by combining the nematic E7 liquid crystal (LC) with a U-shaped optical fiber cavity and applying an external electric field to the LC. In particular, their differing temperature characteristics are investigated. Experimental results show that when the temperature is below the LC’s clearing point, the interference spectrum of the extraordinary wave blue-shifts, whereas the spectrum of the ordinary wave generally red-shifts with the increasing temperature. As the temperature approaches the LC’s clearing point, these shifts become increasingly faster. However, when the temperature is above the clearing point, the polarization dependence of the structure vanishes; further, the interference spectrum blue-shifts as the temperature increases. Then, the temperature coefficients of the refractive index in case of the LC are theoretically analyzed, which is consistent with the experimental results. The proposed system is compact, highly integrated, and stable, denoting its considerable potential in fields such as LC temperature characterization, polarization optics, and optical sensing.

In order to improve the sensitivity and accuracy of fiber Bragg grating (FBG) monitoring micro-displacement, a substrate-based FBG displacement sensor is proposed and implemented. The sensing is realized based on beat-frequency signals of different modes in the resonator. The Sagnac ring cavity effectively improves the utilization of the pump light source, and has smaller phase noise and higher signal-to-noise ratio than the straight cavity. The substrate-based strain sensing structure can effectively protect the optical fiber and accurately measure the frequency variation caused by the displacement. In the experiment, the drift of the sensing signal is observed every 5 mm. The results of repeated experiments are recorded. The results show that the frequency shift varies linearly with the displacement, the degree of linear fit is up to 0.9995, and the sensitivity is -45.4 kHz/mm. According to the highest precision of the spectrometer, we obtain that the measurement accuracy of the sensor is 0.88×10-3 mm, which is basically consistent with the theoretical derivation.

An optofluidic refractive index sensor based on large air-hole polarization-maintaining microstructured optical fiber polarization rocking filter (PM-MOF-PRF) is proposed. Resonant coupling between orthogonal polarization modes occurs when a periodic back-and-forth twist is induced by PM-MOF along the axial. Via polarized detection, a transmission spectrum similar to a long-period fiber grating can be observed, and thus a polarization rocking filter (PRF) is obtained. The transmission spectrum of this device is simulated based on mode-coupling theory. When splicing a short piece of C-shaped fiber between the PM-MOF and single-mode fibers (SMF) at both end of the PRF, it allows fluids running into and out of the air hole channels without affecting light coupling between the MOF and SMF. Therefore, an all-fiber optofluidic refractive index sensor can be constructed. Then the phase modal birefringence dispersion curves of the PM-MOF for microfluidics with refractive index around 1.333 is simulated by finite element method, and the transmission spectra of the PRF for different microfluidic refractive index values are obtained. By tracking the wavelength shift of the spectra, a refractive index sensitivity of 7196.4 nm/RIU is achieved, and the value can be improved to 16754.0 nm/RIU when the MOF is tapered to half of the initial diameter.

Herein, we design and implement a laser frequency stability measurement system based on coherent detection and Rayleigh scattering to measure the instantaneous small frequency deviation of the laser. Based on the proposed system, the study examines performance degradation due to laser frequency fluctuation in distributed Brillouin sensors. Results show that large frequency fluctuation of the laser can result in the broadening of the Brillouin gain spectrum and deviation from the Lorentz shape. This produces a large error in the measurement of Brillouin frequency and limits the sensing range to 15 km for the laser studied. The sensing range can be extended to 50 km by a laser with good frequency stability.

In this paper, a high-fineness optical fiber Fabry-Perot (F-P) pressure sensor based on micro-electro-mechanical system (MEMS) is proposed and experimentally demonstrated. The sensor is a high-fineness F-P interferometer formed by a silicon diaphragm and a Pyrex#7740 glass sheet both coated with high-reflection film. The change of pressure causes the length change of F-P cavity. Thus, based on the high-sensitive optical fiber white light interferometry, the pressure can be obtained by measuring the cavity length of the F-P. Experimental results show that the pressure sensor has properties of good measurement resolution, high linearity and low temperature drift characteristics.

This study proposes a Brillouin refractive index sensor based on the evanescent field effect of a micro-nano fiber. Multipeak Brillouin spectrum caused by hybrid acoustic field of the fiber is used for liquid refractive index sensing. Experimental results show that, for a 2 μm-micro-nano fiber, Brillouin peaks at approximately 10.4 GHz have a relatively high signal-to-noise ratio in NaCl solution, and their frequency shifts have a linear dependence on the liquid refractive index. A refractive index sensing sensitivity of 369.6 MHz/RIU (RIU is refractivity unit) is obtained within the range of 1.3333-1.3612. The proposed sensor will provide a new alternative for refractive index sensing.

A novel continuous-wave fiber optics cavity ring-down refractive index sensing method based on the frequency-shifted interferometry is researched. We made a sensor by using a section of fused fiber optics taper in the fiber ring-down cavity to experimentally study the refractive index of liquids at different concentrations. The experimental results show that the detection sensitivities of NaCl and glucose solutions are 3.779 dB·RIU-1 and 6.413 dB·RIU-1 (RIU is refractivity unit), respectively, in the range of solution mass fraction less than 9%, and the minimum detection limit of the system is 10-4. The relative deviation of measured cavity loss is less than 0.4% over 40 min continuous observation, showing good stability. In contrast to conventional cavity ring-down schemes, the frequency-shifted interferometry does not require fast detection electronics and reduces the need for hardware devices. The sensing system has the advantages of simple structure, low cost and high sensitivity, and has some application prospects of chemical detection and biosensing.

An in-fiber Michelson interferometer based on multimode interference is proposed and fabricated. The proposed interferometer comprises a section of multimode fiber (MMF) fusion-spliced with single mode fiber (SMF). When light enters the MMF from the SMF, multiple high-order modes are excited by core mismatching. These high-order modes and core mode propagate in the MMF and couple with each other to form intermodal interference. The interference fringes of the proposed sensor samples are clear with high contrast. Liquid refractive index (RI) sensing and temperature measurement experiments demonstrate that a liquid refractive index sensitivity of -92.43 dB/RIU is achieved for the liquid RI range from 1.3333 to 1.3796 RIU, and a temperature sensitivity of 0.01 dB/℃ is obtained for a water temperature range of 25-75 ℃. The proposed sensor is easy to fabricate and cost-effective. The probe structure of the proposed sensor has promising applications in biomedical and petrochemical fields.

In order to reduce the thickness of fiber cladding and improve the refractive index sensitivity of the interferometer, a simple photonic crystal fiber (PCF) Mach-Zehnder interferometer which is easy to fabricate is developed. First, a PCF is fused between two single-mode fibers; then, in a self-made etching tank, hydrofluoric acid is used to reduce the thickness of the cladding. The effects of the length, corrosion time, and ambient temperature on the sensitivity of PCFs are studied by controlling the above variable factors. The results show that the sensitivity increases with the increasing length of the optical fiber, and increases about three times after 40 min corrosion in hydrofluoric acid solution with mass fraction of 40% for 3 cm PCF. The ambient temperature has little influence on the sensitivity of the PCF Mach-Zehnder interferometer.

A highly sensitive fiber-optic Fabry-Perot microcavity strain sensor is fabricated using single-mode fibers and a glass capillary tube; the fabricated strain sensor is demodulated using the non-scanning correlation demodulation method. The sensor is fabricated by penetrating two vertically cut single-mode fused silica fibers into a hollow core fused silica fiber and fixing the fibers to both ends of the hollow core fused silica fiber. Thus, the enhanced cavity length-strain sensitivity of the fiber-optic Fabry-Perot microcavity strain sensor is realized. The sensor has a cavity length with micron dimension. The non-scanning correlation demodulation method is used for demodulation based on varying cavity lengths. For a fiber-optic Fabry-Perot microcavity strain sensor with an initial cavity length of 30.129 μm and a hollow core capillary length of 40 mm, the cavity length-strain sensitivity reaches 14.08 nm/με and its linearity is up to 99.7%.

The distributed optical fiber sensing system based on Brillouin optical time domain analysis (BOTDA) can be used to measure temperature, stress, and other information in the external environment. In order to further improve the measurement accuracy and speed of distributed optical fiber sensing system, a Brillouin frequency shift extraction method based on adaptive gradient descent algorithm is proposed, and a 24.4 km distributed temperature sensing system based on BOTDA is setup. The results show that, compared with the traditional Levenberg-Marquardt Lorentz fitting algorithm, the proposed method can extract Brillouin frequency shift spectrum quickly and accurately, which is of great significance to improve the measurement accuracy of BOTDA distributed temperature sensing system.

We propose an approach based on the four-wave mixing (FWM) effect for improving the sensitivity of a dual-wavelength fiber laser sensor with polarization-maintaining fiber Bragg grating (PM-FBG). The wavelength interval of the dual wavelength output is proportional to external measures such as temperature and strain. Experimental results show that the wavelength interval can be incresaed using the FWM effect. In particular, we observe that the temperature sensitivity of the conventional laser sensor with PM-FBG is improved from -0.62 pm/℃ to -4.32 pm/℃ while using third-order FWM-based scheme, and a sensitivity enhancement of nearly 7 times is achieved.

With two ends of the multi-mode fiber and a single-mode fiber spliced together, a complex Mach-Zehnder interferometer is constructed by splicing fiber micropores with fibers of different core diameters. A new fiber optic sensor with high temperature and refractive index is constructed via the interferometer. Through experiment, it is found that the wavelength of the resonant peak valley in the transmission spectrum of the sensor linearly drifts only with the ambient temperature, while the peak intensity of the wave valley dip 1 linearly drifts only with the refractive index in the environment. A measurement matrix for temperature and refractive index of wave valley dip 1 can be obtained. The experimental results show that the high temperature sensitivity is 18.55 pm/℃ and the refractive index sensitivity is -155.2 dB/RIU (RIU is refractivity unit). Simultaneous measurement of the high temperature and refractive index is realized, and there is no cross sensitivity. The sensor has the advantages of compact and simple structure, stable performance and high sensitivity.