MDL Equalization Technology Using Long-Period Fiber Grating

Mode division multiplexing (MDM) is attracting attention as one of the optical fiber communication technologies to address the explosive growth in data traffic. This technology enables simultaneous transmission of multiple optical signals through a single optical fiber by utilizing different propagation modes. However, higher-order modes are more susceptible to signal attenuation, and the variation in signal intensity between modes (Mode Dependent Loss: MDL) has been a limiting factor in communication performance by complicating signal processing.

To solve this challenge, our laboratory is developing MDL equalization devices using Long Period Fiber Gratings (LPFG). By forming periodic refractive index changes (gratings) in the fiber core, LPFG enables mode coupling between the information-carrying “core modes” and the high-loss “cladding modes,” allowing for loss control. The figure below illustrates an example of MDL equalization: ① Two core modes with different signal intensities are input (LP₀₁: 100%, LP₁₁: 50%). ② In the grating section, part of the LP₀₁ core mode couples with the cladding mode, reducing the LP₀₁ core mode signal intensity. ③ As a result, the signal intensities of the two core modes (LP₀₁ and LP₁₁) become equal (50%) at the output.

This technology is expected to contribute to the realization of future high-speed, large-capacity communication networks by enabling flexible and high-quality mode division multiplexed transmission.