We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of ${\mathrm{CO}}_2$-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (${\mathrm{LP}}_{01}$) to the high-order LP core mode (${\mathrm{LP}}_{21}$) with efficiency of 99.7% and then transform the ${\mathrm{LP}}_{21}$ mode into a high-order OV mode ($\pm 2$ order). This is the first time, to the best of our knowledge, that $\pm 2$-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

A miniaturized tip Fabry-Perot interferometer (tip-FPI) is proposed for high-temperature sensing. It is simply fabricated for the first time by splicing a short length of microfiber (MF) to the cleaved end of a standard single mode fiber (SMF) with precise control of the relative cross section position. Such a MF acts as a Fabry-Perot (FP) cavity and serves as a tip sensor. A change in temperature modifies the length and refractive index of the FP cavity, and then a corresponding change in the reflected interference spectrum can be observed. High temperatures of up to 1000 °C are measured in the experiments, and a high sensitivity of 13.6 pm/°C is achieved. This compact sensor, with tip diameter and length both of tens of microns, is suitable for localized detection, especially in harsh environments.

Erbium (Er)-doped fiber amplifier (EDFA) has achieved great success in modern optical communications for decades due to its outstanding gain performance in C-band. Nowadays, people are making efforts to make it monolithic integration due to the imperious demands from the advancement of high-performance and multi-function optoelectronic devices and chips. Except for the Er-doped waveguide amplifier, the other ion-doped on-chip waveguide amplifiers have also attracted much attention for multi-band optical communications, since they have own advantages in gain, bandwidth, selectivity of host materials, and integrability. Herein, we review the transmission characteristics of ion-doped on-chip waveguide amplifiers including gain and bandwidth, combining with the emission mechanisms and fabrication processes in applications of long-haul and short-reach optical communications. To facilitate the practical progress of the integrated waveguide amplifiers, from the perspective of gain medium, we also discuss the challenges and potential solutions for future technology roadmap. We expect that the bismuth (Bi)-doped waveguide amplifier would present impressive advantages in multi-band optical communications.

In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8 km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8 km submarine optical cable causes four spectral nulls on the 36 GHz bandwidth of 72Gbit/s OOK signal, which is the main obstacle for achieving an acceptable bit-error-rate (BER) performance. Decision feedback equalizer (DFE) is effective to compensate for the spectral nulls. However, DFE has a serious defect of burst-error propagation when the burst errors emerge due to the unstable submarine environment. Weighted DFE (WDFE) can be used to mitigate the burst-error propagation, but it cannot fully compensate for the spectral nulls because only a part of feedback symbols is directly decided. Fortunately, maximum likelihood sequence estimation (MLSE) can be added after the WDFE to simultaneously eliminate the residual spectral distortions and implement optimal detection. Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm can effectively suppress the burst-error propagation to obtain a maximum 2.9 dB improvement of Q factor and eliminate the phenomenon of BER floor. In conclusion, the joint WDFE and MLSE algorithm can solve the burst-error propagation for the field-trial fiber-optic communications.

A liquid-filled D-shaped fiber (DF) cavity serving as an in-fiber Mach–Zehnder interferometer (MZI) has been proposed and experimentally demonstrated for temperature sensing with ultrahigh sensitivity. The miniature MZI is constructed by splicing a segment of DF between two single-mode fibers (SMFs) to form a microcavity (MC) for filling and replacement of various refractive index (RI) liquids. By adjusting the effective RI difference between the DF and MC (the two interference arms), experimental and calculated results indicate that the interference spectra show different degrees of temperature dependence. As the effective RI of the liquid-filled MC approaches that of the DF, temperature sensitivity up to −84.72 nm/°C with a linear correlation coefficient of 0.9953 has been experimentally achieved for a device with the MC length of 456 μm, filled with liquid RI of 1.482. Apart from ultrahigh sensitivity, the proposed MCMZI device possesses additional advantages of its miniature size and simple configuration; these features make it promising and competitive in various temperature sensing applications, such as consumer electronics, biological treatments, and medical diagnosis.