[Paper Review] Laser Nano-Filament Explosion for Enabling Open-Grating Sensing in Optical Fibre

Keivan Mahmoud Aghdami, Abdullah Rahnama+2 more|arXiv (Cornell University)|Nov 29, 2020

Advanced Fiber Optic Sensors65 references38 citations

TL;DR

This paper presents a laser nano-filament explosion technique to create high-aspect-ratio, sub-wavelength nano-holes through silica optical fibre cladding and core, enabling robust, open-grating fibre Bragg gratings (FBGs) with strong photonic stopbands. By combining femtosecond laser filamentation with post-chemical etching, the method achieves sharply resolved p-shifted FBGs with high refractive index (RI) sensitivity up to 600 nm/RIU, enabling real-time, high-resolution sensing of liquids and gases through capillary filling of the nano-holes without fibre thinning or damage.

ABSTRACT

Embedding strong photonic stopbands into traditional optical fibre that can directly access and sense the outside environment is challenging, relying on tedious nanoprocessing steps that result in fragile thinned fibre. Ultrashort pulsed laser filaments have recently provided a non contact means of opening high aspect ratio nanoholes inside of bulk transparent glasses. This method has been extended here to optical fibre, resulting in high density arrays of laser filamented holes penetrating transversely through the silica cladding and guiding core to provide high refractive index contrast Bragg gratings in the telecommunication band. The point by point fabrication was combined with post-chemical etching to engineer strong photonic stopbands directly inside of the compact and flexible fibre. Fibre Bragg gratings with sharply resolved pi-shifts are presented for high resolution refractive index sensing from n = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding.

Motivation & Objective

• To develop a non-contact, robust method for embedding high-contrast photonic stopbands directly into solid-core optical fibre for environmental sensing.
• To overcome the fragility and complexity of traditional nano-processed fibre sensors by eliminating the need for mechanical thinning or cladding removal.
• To enable strong, tunable, and highly sensitive refractive index sensing by creating open, capillary-accessible nano-hole arrays in the fibre core and cladding.
• To achieve high-resolution, p-shifted FBG resonances with narrow linewidths (<200 pm) for enhanced sensing performance.

Proposed method

• Femtosecond laser filamentation with an aberration-correcting plate enables precise, non-contact drilling of high-aspect-ratio nano-holes (diameter ~200 nm) through the fibre cross-section.
• Laser-induced nano-explosions create isolated, through-holes with minimal heat-affected zone, preserving fibre mechanical integrity.
• Post-chemical etching (HF acid) is used to controllably increase nano-hole diameter from 200 nm to 700 nm, enabling photonic bandgap engineering.
• A periodic array of nano-holes (period Λ = 1.072 μm) is patterned to form a second-order Bragg grating with strong stopband responses in the 1550 nm telecommunication band.
• p-shifted defects are introduced into the nano-hole array to generate narrowband, high-Q resonances (Q ~ 10^4) for enhanced sensing resolution.
• Capillary forces enable spontaneous filling of nano-holes with liquids of varying refractive index (n = 1.002 to 1.67), enabling real-time optofluidic sensing.

Experimental results

Research questions

• RQ1 Can femtosecond laser filamentation be used to create high-aspect-ratio, through-holes in optical fibre without damaging the cladding or requiring fibre thinning?
• RQ2 What is the maximum refractive index sensitivity achievable with nano-hole FBGs formed via laser filamentation and chemical etching?
• RQ3 How does the inclusion of a p-shifted defect in the nano-hole array affect the spectral response and sensing resolution of the FBG?
• RQ4 To what extent can capillary action enable spontaneous and reversible filling of nano-holes with diverse liquids for optofluidic sensing?
• RQ5 Can the combination of laser nano-filamentation and chemical etching produce robust, mechanically stable FBGs with strong photonic stopbands suitable for practical sensing applications?

Key findings

• The laser nano-filament explosion technique successfully created 200 nm diameter nano-holes with an aspect ratio of ~500, penetrating both the cladding and core of standard SMF-28 fibre without inducing structural damage.
• Chemical etching increased nano-hole diameter at a rate of 100 nm/min after an initial 2-minute delay, enabling tunable photonic bandgap engineering.
• The fabricated FBGs exhibited a refractive index sensitivity of up to 600 nm/RIU at high RI (n = 1.66) and large hole diameter (700 nm), significantly exceeding conventional FBGs.
• p-shifted FBGs achieved a linewidth of 100 pm (3 dB), indicating a high-quality factor (Q ~ 10^4) suitable for high-resolution sensing.
• Capillary flow enabled complete wetting of nano-holes with liquids ranging from air (n = 1.002) to oils (n = 1.670) in sub-seconds to tens of seconds, enabling real-time sensing.
• Simulations and experiments confirmed strong, spectrally narrow stopbands that shifted rapidly with refractive index, validating the high sensitivity and robustness of the open-grating design.