We report on the design and characterization of ring-assisted (RA) few-mode fibers with improved minimum mode effective index difference MinΔn _eff . By introducing a high-index RA structure whose position coincides with the trough of mode power distribution of LP ₀₂ mode, the effective index of the LP ₂₁ mode can be largely elevated while that of LP ₀₂ mode remains almost unaffected. The dependences of both Δn _eff and effective mode area A _eff on different ring parameters have been investigated. We fabricated RA 4-mode fibers with a minimum Δ _neff (1.8 × 10 ^-3 ) which is 2.2 times larger than that of step-index (SI) 4-mode fiber (0.8 × 10 ^-3 ) without obvious degradation on other characteristics. The fabricated RA 4-mode fiber has a low mode-average loss and a low mode dependent loss of 0.23 and 0.02 dB/km at 1550-nm wavelength, respectively, which are comparable with that of an SI 4-mode fiber fabricated for reference purposes. The distributed inter-mode crosstalk of the fibers has been measured using swept-wavelength interferometry with a winding tension of 0.5 N. We observed similar distributed inter-mode crosstalk between the RA-FMF and the SI-FMF with low tension and the results show that no additional deformation was introduced by the high-index ring. As a result, the fabricated RA-FMF with a larger MinΔn _eff is expected to be more robust than SI-FMF in real applications. Moreover, we adopted this method for the design of 7-mode fibers and successfully obtained a minimum Δn _eff as high as 1.7 × 10 ^-3 and an effective area larger than 100 μm ² .

We designed and fabricated a graded-index few-mode fiber (GI-FMF) with large effective mode area and low intermodal dispersion for Raman distributed temperature sensor (RDTS) to simultaneously achieve high spatial and temperature resolution over long distance. In experiment, we measured the spatial and temperature resolution of the RDTS using different types of fibers under different launch conditions based on a commercially available RDTS system. By using the GI-FMF under the overfilled launch condition, we achieved a 1 °C temperature resolution with a spatial resolution of 1.13 m at the distance of 25 km. The spatial resolution using the standard MMF degraded to 2.58 m with only a 0.3 °C higher temperature resolution in comparison. As a result, the GI-FMF under the few-mode operation condition can provide a desirable temperature resolution comparable with that of the MMF with a negligible degradation on spatial resolution. Moreover, the RDTS using the GI-FMF under the quasi-single mode operation condition achieved a temperature resolution of 4.7 °C at the distance of 25 km with a 2.2 °C improvement and no degradation on spatial resolution compared with that using the standard SMF.

We proposed and experimentally demonstrated paralleled Mach-Zehnder interferometers (MZIs) in few-mode multicore fiber (FM-MCF) for temperature and strain discriminative sensing. A section of FM-MCF is sandwich-spliced between two single-mode multicore fiber (SM-MCF) with a rotational offset. The arbitrarily controlled angular misalignment generates intentional intermodal interferences in outer cores of the FM-MCF thus multiple MZI structures are implemented. Experimental results show that the temperature sensitivities are 105.8 pm/°C and 223.6 pm/°C for two outer cores, strain sensitivity is 13.96 pm/με for the outer core 1 and 11.7 pm/με for the outer core 2, respectively. Due to the low condition number of the cross coefficient matrix dependent on the temperature and strain response indexes, the temperature-strain cross sensitivity can be efficiently eliminated. In addition, the structure’s fabrication process is simple, cost effective, and repeatable. The sensing structure can be applied to a wide range of measurements and is expected to develop potentials by building a higher dimensional matrix with more cores.

We demonstrate, for the first time, an ultra-low- noise mode-division multiplexed transmission with 8 wavelength- division multiplexed channels over 100-km few-mode fiber based on second-order few-mode Raman amplifier. To minimize the mode dependent gain, two backward depolarized pumps at the wavelengths of 1455 nm (75 mW) and 1360 nm (1890 mW) are equally launched both into the LP11a and LP11b modes. The Raman on-off gains of the LP01 and LP11 modes are both above 8 dB and the mode dependent gain is less than 0.7 dB. Amplified spontaneous emission noise improvement of 1.8 dB is achieved in the second-order few-mode Raman amplifier, with respect to the first-order Raman amplification. The effective noise figure of the second-order few-mode Raman amplifier is about -3 dB, which is ~1.4 dB lower than that in the first-order pumping case. In the transmission experiments, single-mode transmission over LP01 mode and mode-division multiplexed transmission over LP11a and LP11b modes are performed separately. The bit-error-rates for all the transmission channels are significantly improved due to the low noise figure amplification of the second-order few-mode Raman amplifier.

Ultra-low-loss all-fiber mode selective couplers (MSCs) have been designed and fabricated using fused biconical taper technique with precise control on phase matching condition. An LP01 MSC with minimum insertion loss about 0.1 dB is fabricated. It is promising for single-mode excitation in large-mode-area few-mode fibers. The insertion loss of the demonstrated LP11 MSC is 0.3 dB at 1540 nm with mode extinction ratio higher than 28 dB. It has a broad operation wavelength from 1480 nm to 1640 nm with wavelength dependent loss about 0.3 dB. To the best of our knowledge, it shows the lowest loss in the reported broadband LP11 mode converters with high mode purity. The fabrication process is reliable and repeatable with smooth power curves, which is of great importance to the practical fabrication of these components. MIMO-less 2×25-Gb/s OOK signal and 2×20-Gb/s PAM-4 signal transmission over 15-km few-mode fiber have been demonstrated using the fabricated components. The low loss, large bandwidth and the high fabrication repeatability of the mode selective couplers make them promising for low-cost MIMO-less WDM-MDM transmission.

An advanced fiber optic current sensor (FOCS) is proposed based on recirculating fiber loop architecture for significantly enhancing the current sensitivity. The recirculating loop is constructed by a 2X2 optical switch and the standard single mode fiber (SSMF) is used as the sensing head. The proposed FOCS is coupler-free with low insertion loss which results in a significantly improved current sensitivity. We experimentally obtained a sensitivity of 11.5 degrees/A for 1-Km SSMF FOCS and a sensitivity of 21.2 degrees/A for 500-m SSMF FOCS, both of which have been enhanced by more than ten times. The flexible switch control of recirculating can support the FOCS to work for different current scenarios with the same system and thus reconfigurable operation of the FOCS has been achieved. The significantly enhanced high sensitivity with reconfigurable operation capability makes the proposed FOCS a promising method for practical applications.