Femtosecond laser micro-machining enabled all-fiber mode selective converter

Area of Science:

• Photonics and Optical Engineering
• Fiber Optics Technology
• Nanofabrication

Background:

• Few-mode fibers (FMFs) are crucial for advanced optical communication systems, but controlling light propagation within them remains challenging.
• Mode selective converters are essential for routing and manipulating different light modes in FMFs.
• Existing methods for mode conversion often lack compactness, efficiency, or precise control.

Purpose of the Study:

• To demonstrate a compact, all-fiber mode selective converter.
• To achieve efficient and selective conversion between optical modes in few-mode fibers.
• To investigate the use of femtosecond laser micro-machining for creating precise micro-structures on fiber facets.

Main Methods:

• Femtosecond laser micro-machining was employed to create specific micro-structures on the facet of few-mode fibers.
• Theoretical optimization was performed to determine the optimal micro-structure designs for selective mode conversion.
• A proof-of-concept experiment was conducted using a two-mode fiber with an inscribed micro-slot.

Main Results:

• Micro-structures on the FMF facet were shown to induce a π spatial phase difference for mode conversion.
• Selective conversion of higher-order modes (LP11, LP02, LP21) was achieved with >25 dB extinction ratio and >45% coupling efficiency over the C-band.
• A proof-of-concept LP01 to LP11 mode conversion demonstrated an average insertion loss of 2.7 dB and >65% mode intensity profile correlation.

Conclusions:

• Femtosecond laser micro-machining offers a precise and effective method for fabricating compact all-fiber mode selective converters.
• The developed technique enables efficient and selective conversion of various higher-order modes in few-mode fibers.
• This technology holds significant potential for enhancing the capacity and functionality of optical communication systems.