On-chip integrated mode-division multiplexing (MDM) is an emerging technique for large-capacity data communications. In the past few years, while several configurations have been developed to realize on-chip MDM circuits, their practical applications are significantly hindered by the large footprint and inter-mode cross talk. Most importantly, the high-speed MDM signal transmission in an arbitrarily routed circuit is still absent. Herein, we demonstrate the MDM circuits based on digitized meta-structures which have extremely compact footprints. 112 Gbit/s signals encoded on each mode are arbitrarily routed through the circuits consisting of many sharp bends and compact crossings with a bit error rate under forward error correction limit. This will significantly improve the integration density and benefit various on-chip multimode optical systems.

A high-sensitivity fiber-optic strain sensor, based on the Vernier effect and separated Fabry-Perot interferometers (FPIs), is proposed and experimentally demonstrated. One aircavity FPI is used as a sensing FPI (SFPI) and another is used as a matched FPI (MFPI) to generate the Vernier effect. The two FPIs are connected by a fiber link but separated by a long section of single-mode fiber (SMF). The SFPI is fabricated by splicing a section of microfiber between two SMFs with a large lateral offset, and the MFPI is formed by a section of hollow-core fiber sandwiched between two SMFs. By using the Vernier effect, the strain sensitivity of the proposed sensor reaches 1.15 nm/με, which is the highest strain sensitivity of an FPI-based sensor reported so far. Owing to the separated structure of the proposed sensor, the MFPI can be isolated from the SFPI and the detection environment. Therefore, the MFPI is not affected by external physical quantities (such as strain and temperature) and thus has a very low temperature cross-sensitivity. The experimental results show that a low-temperature cross-sensitivity of 0.056 με/°C can be obtained with the proposed sensor. With its advantages of simple fabrication, high strain sensitivity, and low-temperature cross-sensitivity, the proposed sensor has great application prospects in several fields.

We reported on the generation of the dual-wavelength rectangular pulse in an erbium-doped fiber laser (EDFL) with a topological insulator saturable absorber. The rectangular pulse could be stably initiated with pulse width from 13.62 to 25.16 ns and fundamental repetition rate of 3.54 MHz by properly adjusting the pump power and the polarization state. In addition, we verified that the pulse shape of the dual-wavelength rectangular pulse can be affected by the total net cavity dispersion in the fiber laser. Furthermore, by properly rotating the polarization controllers, the harmonic mode-locking operation of the dual-wavelength rectangular pulse was also obtained. The dual-wavelength rectangular pulse EDFL would benefit some potential applications, such as spectroscopy, biomedicine, and sensing research.

A binary polarization shift-keying scheme with two circle polarization states, circle polarization shift keying (CPolSK), is proposed and discussed. The propagation of the circle-polarized Gaussian Schell model beam is studied based on the cross-spectral density matrix. The results show that the state of polarization remains unchanged and the degree of polarization slightly increases upon propagation in a turbulent atmosphere. The analysis of the bit-error-rate performance shows that the CPolSK system has about a 3 dB lower requirement in signal-to-noise ratio than the on–off keying system. The modulation scheme will be helpful for practical free-space optical communication systems in the future.

A high-sensitivity Fabry-Perot interferometer (FPI) strain sensor based on a concave-core photonic crystal fiber (CPCF) was proposed and demonstrated. The FPI was formed by directly splicing a CPCF to a standard single-mode fiber. The CPCF was fabricated by cleaving a high-numerical-aperture solid-core photonic crystal fiber under axial tension using a traditional fiber cleaver. An FPI strain sensor with a cavity length of 4.85 μ m and a strain sensitivity of 31.58 pm/με was successfully produced. The sensor also demonstrated a low temperature sensitivity of 0.653 pm/°C, reducing the cross-sensitivity between tensile strain and temperature. The benefits of the proposed sensor include simple fabrication, high strain sensitivity, and low temperature cross-sensitivity, making it attractive for practical applications.