A few-mode erbium-doped fiber amplifier (FM-EDFA) is a key device for realizing long-haul and high-capacity mode-division multiplexing (MDM) transmission. However, different mode channels across a wide wavelength band generate both mode-dependent and wavelength-dependent gains, leading to differential mode gain (DMG) over the wavelength span. This severely constrains the capacity enhancement of MDM transmission systems. In this study, we provide a comprehensive review of recent progress in the gain regulation of FM-EDFA. We systematically discuss the generation mechanism of DMG and its optimization strategies. The performance of DMG under various regulation schemes is then presented. Finally, we explore the research challenges related to DMG equalization under conditions of higher mode counts and wavelength-division multiplexing (WDM) transmission.

Stable time and frequency transfer is one of the two core technologies in time and frequency infrastructure systems. Optical fiber enables high precision and long-distance transfer, making it the best choice for terrestrial time and frequency transfer. In this study, we introduce the principles of fiber-optic time and frequency transfer and elaborate on the phase noise compensation methods using optical frequency transfer as an example. Addressing the limitations of current time and frequency transfer equipment that uses discrete optoelectronic devices, we explore and study the design approaches for optoelectronic integrated chips and demonstrate the advantages of photonic integration. Finally, we review recent progress in fiber-optic time and frequency transfer globally and present prospects.

We introduce a unique type of partially coherent light (PCL) that simultaneously carries a vortex phase and exhibits a special spatial correlation structure, known as radially polarized multi-Gaussian Schell-model fractional vortex (RP-MGSM-FV) beams. We outline the fundamental requirements for generating such light beams and derive the analytical expression for their cross-spectral density matrix after transmission through ABCD optical systems. We further examine the influence of the topological charge magnitude, sign, and coherence width of its vortex phase component on the intensity distribution at the focal plane. The results indicate that as the coherence width increases, the intensity distribution at the focal plane translates from a flat-top to a Gaussian-like shape, then to a flat-top, and ultimately to a ring pattern. An increase in the topological charge numbers leads to a distinct separation in the spatial distribution of the beam at the focal plane. Additionally, changes in the sign of the topological charge cause an inversion in the spatial distribution pattern, allowing for the detection of both the magnitude and sign of the topological charge in RP-MGSM-FV beams. These findings are significantly valuable in applications such as free-space optical communications and particle trapping in micro domains.

The squeezing direction of the quadrature squeezed state is highly compatible with the quadrature amplitude modulation in classical communication, making the quantum communication protocol based on the quadrature squeezed state easier to implement and commercialize. Here, we experimentally prepare a 200 MHz broadband quadrature squeezed state corresponding to the optical fiber communication window. First, we construct an optical parametric amplifier with a cavity length of 8 mm and a full width at a half maximum of 210 MHz. Then, we develop a 1‒300 MHz low-noise balanced homodyne detector with a common-mode rejection ratio of up to 50 dB. Finally, we obtain a quadrature amplitude squeezed state with a bandwidth of 200 MHz at a wavelength of 1.3 μm, providing the necessary quantum light source for high-speed quantum communication.