Modal decomposition in helical side-core fibers through controlled excitation

Area of Science:

• Optical Fiber Communications
• Photonics
• Wave Propagation

Background:

• Helical side-core fibers (HSCFs) present unique modal decomposition challenges due to their complex geometry and core coupling.
• The helical structure supports a rich spectrum of supermodes, complicating analysis beyond individual core modes.

Purpose of the Study:

• To introduce a controlled excitation methodology for separating and characterizing modal interference mechanisms in HSCFs.
• To differentiate between intra-core and inter-core interference phenomena based on excitation methods.

Main Methods:

• Utilized spatially and spectrally resolved (S² ) imaging combined with principal component analysis (PCA) for modal analysis.
• Employed controlled excitation techniques, including single-mode fiber splicing and free-space coupling.
• Conducted control experiments on phase-mismatched fibers to validate findings.

Main Results:

• Identified two distinct, launch-dependent mode pair interferences: intra-core guided-mode interference (fiber splicing) and inter-core supermode interference (free-space coupling).
• Demonstrated that supermode excitation depends critically on the inter-core phase-matching condition.
• Reported mode-resolved propagation losses for left-circularly polarized (LCP) modes: 0.079 ± 0.005 dB/m (HE₁₁⁺) and 0.267 ± 0.015 dB/m (HE₂₁⁺).

Conclusions:

• Established a systematic framework for characterizing complex helical fibers.
• Provided essential design parameters for optimizing high-power fiber lasers and sensing applications using HSCFs.