Advancements in Mode-Add/Drop Technology Using Few-Mode Fiber Bragg Gratings

The impressive growth of the Internet industry has led to a substantial increase in the demand for information transmission across various sectors of society. As data volumes continue to surge, optical fiber communication networks are faced with the challenge of meeting ever-increasing transmission capacity requirements. The current transmission capacity of single-mode fiber, however, is bounded by the Shannon limit, and it is projected that bandwidth exhaustion will soon become a reality. To address this challenge, researchers have turned to few-mode fibers, which offer multiple orthogonal modes, each capable of serving as an independent channel for information transmission. Leveraging mode-division multiplexing (MDM) technology, these fibers have the potential to exponentially increase information transmission capacity, a development that has gained significant momentum in recent years.

In MDM systems, multiple modes traverse a few-mode optical fiber, necessitating the ability to add or drop specific modes at various nodes within a long-distance transmission network. Existing add-drop technologies for MDM systems can be broadly categorized into spatial optical path types and on-chip waveguide types. Spatial optical path solutions are relatively straightforward to implement but tend to exhibit complex structures and significant insertion losses. Conversely, on-chip waveguides offer greater controllability and integration capabilities but pose challenges in terms of realization. Another study introduced a mode-selective coupler based on long-period gratings to achieve mode add-drop functionality. While this approach demonstrated promising performance and effectively mitigated coupling interference between modes, it involved the intricate fabrication of the core component—the mode-selective coupler. This fabrication process demanded precise control of parameters in both the few-mode fiber and the single-mode fiber, as well as the creation of long-period gratings, necessitating a high level of accuracy and specific conditions.

In a new study published in the Journal Optics Express by Xiuquan Li, Dr. Haiyan Wang, Hongjun Zhu, and led by Professor Guijun Hu from the College of Communication Engineering at Jilin University developed a new mode add-drop technology based on few-mode fiber Bragg gratings (FM-FBGs). Unlike other methods, this technology requires only the fabrication of Bragg gratings in the few-mode fibers without any additional processing. FM-FBGs exhibit unique reflection characteristics, allowing specific modes to be reflected at particular wavelengths while leaving other modes unaffected. By utilizing multiple Bragg gratings at specific wavelengths, this approach effectively separates different modes, enabling mode add-drop functionality. This technology boasts several advantages, including ease of component fabrication, low cost, and excellent performance. Furthermore, the fabrication process employs femtosecond laser techniques to write the gratings in parallel. By adjusting the writing position of the gratings to match the energy distribution of the mode field of high-order modes, the self-coupling reflectivity of LP₀₁, LP₁₁, and LP₂₁ modes is significantly enhanced, ultimately improving the performance of the mode add-drop technology.

In practice, the Bragg grating fabrication process involves the use of femtosecond lasers, specifically the Pharos series femtosecond laser with a wavelength of 1030 nm. The laser’s flexibility in pulse width (ranging from 290 fs to 10 ps) and its maximum single pulse energy of 200 μJ make it well-suited for high-precision machining. The 515 nm femtosecond laser is generated by frequency doubling the 1030 nm laser through a β-BaB₂O₄ (BBO) crystal. The laser beam is focused onto the fiber core using an Olympus oil-immersed objective lens, and a specific fiber fixture securely holds the fiber in place. Additionally, the system incorporates a 3D displacement platform for precise positioning, including ABL1000 series air-floating displacement platforms for X and Y axes and a QF46Z piezoelectric displacement table for the Z-axis. These platforms provide exceptional accuracy, enabling the creation of fine Bragg gratings. The system also incorporates vibration isolation measures to mitigate external interference. Finally, the system’s performance is evaluated through optical spectrum analyzers.

The authors validated thoroughly the effectiveness of the proposed FM-FBG-based mode add-drop technology through experimental testing in a mode-division multiplexing system. The experimental setup involved the use of 8 Gbit/s quadrature phase shift keying (QPSK) signals, tunable lasers, coherent receivers, and digital signal processing (DSP) algorithms for equalization, clock recovery, re-sampling, channel equalization, frequency offset recovery, and phase noise compensation.

The researchers’ findings demonstrated that the FM-FBG-based mode add-drop technology effectively allowed for the extraction and insertion of specific modes within the MDM system. It was observed that the quality of signal transmission significantly improved when equalization algorithms were applied. The bit error rate (BER) curves confirmed the robustness of the technology, with error rates below 10^-3 achieved at sufficient received power levels for LP₀₁, LP₁₁, and LP₂₁ modes. The optimal step size parameters for the Constant-Modulus Algorithm (CMA) algorithm were carefully selected to balance convergence speed and equalization effectiveness, further enhancing the performance of the technology.

In conclusion, Professor Guijun Hu and his team new the mode add-drop technology based on FM-FBGs represents a significant advancement in optical communication systems. This innovative approach addresses the ever-growing demand for higher data transmission capacity by leveraging the unique reflection characteristics of FM-FBGs. By fabricating Bragg gratings within few-mode fibers and utilizing femtosecond laser techniques, this technology enables the efficient separation and addition of different modes within MDM systems. As data demands continue to escalate, innovations like this will be pivotal in meeting the evolving needs of the Internet industry and beyond.