**1. Introduction**

174 Photonic Crystals – Introduction, Applications and Theory

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Arc fusion splicing is an established method for joining optical fibres in communication networks, ensuring splice loss down to 0.05 dB and excellent reliability. Telecom fibres are covered by IEC 60793 and ITU-T G.651.1-G.657 standards, with common material (fused silica) and cladding size (125 μm). Splicing equipment for these fibres is widely available.

Fusion splicing of specialty fibres, like dispersion compensating fibres (DCF), polarizationmaintaining fibres (PMF), rare-earth doped active fibres and photonic crystal fibres (PCF) having varying, not standardized designs, dimensions and materials is considerably more difficult. Some, like PMF and many PCFs lack axial symmetry, requiring rotational alignment before fusion. However, this functionality is not available in common splicing machines. A length of specialty fiber enclosed inside a device like optical amplifier, sensor or dispersion compensator is usually spliced at both ends to telecom single mode fibres (SMF) for connections to other components and external interfaces.

Splicing procedure must be tailored to particular fibre, often in a time-consuming trial-anderror way. Special solutions, like fiber pre-forming or insertion of intermediate fiber are sometimes needed. Dedicated splicing machines for such fibres employing both arc fusion (OFS, 2008) and hot filament methods (Vytran, 2009) exist, but are expensive.

Fusion splicing of photonic crystal or other "holey" fibres with numerous tiny, ca. 0.5-4 μm gas-filled holes is particularly hard because holes collapse quickly once glass is heated to melt; this disturbs radiation guiding, introduces loss and causes fiber shrinkage.

There is a need to splice "holey" or "microstructured" fibres for characterization and experiments using typical tools and equipment. PCF-SMF splices are most common, as PCFs need to be connected to test instruments, optical devices and circuits incorporating or designed for SMFs. Splicing PCF to a SMF requires special fiber handling and machine settings different from splicing SMF to SMF, like reduced arc power and fusion time shortened to 0.2-0.5 s. More difficult splicing of two lengths of PCF is much less common.

Fusion splicing has the advantage of gas-tight sealing a length of PCF, which is of importance in making gas-filled absorption cells or protecting the fibre against penetration of humidity, dust or vapours in hostile operating environment.

Arc Fusion Splicing of Photonic Crystal Fibres 177

Fig. 1. Two identical 125 μm silica fibres before (top) and after (bottom) arc fusion. Electrode

Fig. 2. Two 125 μm single-mode fibres spliced with 10 μm perpendicular offset. Top to bottom: fibres before fusion and after 0.5 s, 1 s and 2 s long fusion with 17 mA current.

insertion loss are possible. The heat for fusion is provided by either:





Glass evaporating from the hot zone is partly deposited on fibres in the vicinity - degrading surface quality and strength, and on parts of splicing machine - contaminating them. Evaporation is compensated for by fibre overlap: a reduced volume of glass is accommodated in shorter length of fibre without change in diameter. As no air, gel, glue etc. separates fibres after fusion, strength close to one of pristine fibre, no reflection and low

tip is visible as a black spot at the bottom of upper image.

This chapter focuses on procedures and experiences with fusion splicing using equipment and tools intended for standard single mode and multimode telecom fibres, which gave acceptable results for most, but not all PCFs the authors encountered. Before this matter is presented in section 4, overview of "holey" fibres and their properties is made in section 2, and arc fusion physics and technology are summarized in section 3.
