**4. Fusion splicing of PCFs**

This section presents some issues specific to fusion splicing of silica "holey" fibres, primarily of single mode PCF to SMF. Due to large variety of designs, actual procedure, power, time and geometry settings must be individually tailored.

Finished splice must be protected to have adequate mechanical strength, as holes and flaws on their surfaces make PCFs inherently weaker than solid fibres. In all experiments at NIT, fusion splices were protected with commercial 60 mm heat shrinkable sleeves reinforced with stainless steel rod. Protected splices performed well during temperature cycling between -40°C and +80C, with loss stability better than 0.05 dB, and as grips for application of twist and tensile forces in mechanical tests (Figure 11).

Fusion splices are hermetic, keeping external contaminants out, but trapping whatever entered earlier. Exceptions include helium and hydrogen, diffusing through 60 μm thick fused silica cladding in few hours. Short suspended core PCF infiltrated with acetylene (C2H2) or hydrogen cyanide (HCN) and fusion spliced to SMF pigtails is used as optical frequency reference, e.g. for calibration of optical spectrum analyzers (Thapa et al., 2006).

Out-gassing of cleaved PCF is fast, but removal of liquid or dust is essentially impossible due to high pneumatic resistance of thin holes and adsorption to the surface of their walls.

Infiltration of holes with gas or liquid allows to make fibre sensors for chemical analysis, detection of pollutants or poison gas, medical diagnostics, etc. through spectral absorption

Arc Fusion Splicing of Photonic Crystal Fibres 187

Unless PCF end is sealed by fusion, low-viscosity liquid applied to it penetrates holes. Filling with colourless acetone, isopropyl alcohol, ethyl alcohol or water is invisible through



For the same reason, PCFs cannot be connectorized in conventional way because water and small (0.5-3 μm) particles of polishing materials enter holes. To avoid infiltration, fibre ends shall be protected against liquids, dust or vapours during handling and storage. In particular, water vapour degrades fibre strength by producing flaws on walls of holes. PCF is best stripped mechanically and dry wiped to remove remains of coating. When use of solvent, acid, etc. is required, fibre end must be first sealed by fusion. PCF contamination in storage or shipping can be prevented by fusing both ends. To fit a connector, PCF can be stripped, cleaved and fused to collapse holes over a 100-200 μm length and fixed in the connector ferrule for polishing. This procedure works best for fibres with doped core, whose

Cleaving of glass fibres uses perpendicular propagation of indentation-initiated break at the speed of sound, approx. 5950 m/s for fused silica. Structures made of differing materials, like arrays of holes in PCF or inserts of B2O3-SiO2 glass in PANDA fibre distort this propagation; these fibres are reportedly more difficult to cleave than conventional ones.

PCFs tested at NIT, with 80-200 μm cladding diameter were cleaved using a typical, simple cleaver for telecom fibres with tungsten carbide blade. Proportion of bad cleaves was around 20%, a little higher than experienced with most SMFs. It rose to some 50% for the 80 µm IPHT 252b5, presumably because tensile load was excessive for this fibre with equivalent diameter of solid glass of just 72 μm. This is consistent with literature data that best tensile load is proportional to cladding diameter raised to power

Cleaved PCFs shall be carefully inspected for perpendicular cut before further work. For non-standard fibre sizes, use of cleaver with adjustable tensile force is recommended.

In absence of differential pressure, surface tension in molten glass causes the holes to reduce

(2)

*collapse v*

their radius at constant linear speed set by the following formula (Yablon, 2005):

coating. Removal is practically impossible, with the following consequences:

changes with movement of solvent, interfering with alignment of fibres.

liquid deeper into PCF. Resulting loss observed at NIT was up to 50 dB.

light guiding properties are retained without photonic structure.

**4.1.1 Fibre infiltration** 

**4.1.2 Fibre cleaving** 

of 2/3 (Yablon, 2005).

**4.1.3 Collapse of holes and thermal issues** 

measurements. Filling with liquids, including liquid crystals or suspensions of solid particles in oils allows to build tuneable, nonlinear or electrically controlled optical devices.

Fig. 11. 80 μm PCF (IPHT 252b5) in acrylate coating broken by 180 rev/m twist applied Damaged PCF PCF break Sleeve (3 mm diameter) PCF-SMF fusion splice

using heat shrinkable sleeve as grip. 4 mm section of fibre inside sleeve was long enough to transfer a destructive force. Splice was illuminated with 650 nm laser through SMF (right).

PCFs are sometimes fused for purposes other than splicing, like:


Fusion power and duration must ensure robust collapse of all holes.
