Large-bandwidth, low-loss, efficient mode mixing using long-period mechanical gratings

Area of Science:

• Optical Communications
• Photonics
• Fiber Optics

Background:

• Space-division multiplexing (SDM) is crucial for increasing optical network capacity.
• Efficient mode mixing in few-mode fibers is challenging due to high losses.
• Current methods struggle to control mode coupling effectively in SDM systems.

Purpose of the Study:

• To propose and experimentally validate a novel architecture for inducing strong mode mixing with low loss in few-mode fibers.
• To enable efficient spatial mode manipulation for advanced space-division multiplexing applications.
• To characterize the performance of the proposed long-period fiber grating (LPG) based system.

Main Methods:

• Implementing long-period fiber gratings (LPGs) within step-index (SI) few-mode fibers.
• Utilizing a mechanical LPG on an eight-spatial-mode SI fiber to induce mode mixing.
• Experimentally measuring insertion loss, coupling matrix, and mode-dependent loss (MDL).

Main Results:

• Achieved efficient mixing of three spatial modes over a broad bandwidth (C band).
• Demonstrated strong mode mixing between LP01 and LP11 modes.
• Recorded insertion loss below 0.5 dB, including splice losses.
• Characterized the coupling matrix and MDL for the first time, showing negligible MDL.

Conclusions:

• The proposed LPG architecture in SI few-mode fibers effectively induces strong mode mixing with low loss.
• This approach significantly reduces losses associated with coupling to cladding modes.
• The demonstrated performance, including negligible MDL, is promising for practical SDM systems.