**4.1.1 Fibre infiltration**

186 Photonic Crystals – Introduction, Applications and Theory

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 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).

Damaged PCF PCF break Sleeve (3 mm diameter) PCF-SMF fusion splice

Besides issues presented in section 3, work with PCF brings several new challenges:

4. PCF has lower fusion temperature and thermal conductivity than solid fibre. 5. Mismatch in fiber cladding diameters results in sharp edges, splice strength suffers.

designed strictly for telecom fibres with 125 μm cladding and 250-900 μm coating.

available for trials are limited and less-than-perfect solutions need to be accepted.

7. Photonic structure supports undesirable propagation of light outside of fibre core. 8. High attenuation and strong backscattering in PCF affect splice loss measurements.

It is often needed to splice PCFs having unusual cladding diameter, core design and MFD, as there are no standards for this category of fibres. Most commercially available splicing and test equipment, tools and accessories like fibre adaptors or protective sleeves are

A compromise between achieving different goals is often required, particularly between splice loss and its mechanical strength. Also, unless optimization of manufacturing process, device performance, etc. justifies labour and equipment costs involved, time and funds

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

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

**4.1 Issues and solutions specific to fusion splicing of PCFs** 

2. Holes distort propagation of crack and hamper fibre cleaving. 3. Surface tension of molten silica causes collapse of holes.

6. Mode fields of PCFs do not exhibit full radial symmetry.

1. Solvents used for fibre cleaning infiltrate PCF holes.



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 light guiding properties are retained without photonic structure.

## **4.1.2 Fibre cleaving**

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 of 2/3 (Yablon, 2005).

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.

#### **4.1.3 Collapse of holes and thermal issues**

In absence of differential pressure, surface tension in molten glass causes the holes to reduce their radius at constant linear speed set by the following formula (Yablon, 2005):

$$
\sigma\_{collopse} = \frac{\mathcal{Y}}{\eta} \tag{2}
$$

Arc Fusion Splicing of Photonic Crystal Fibres 189

Collapse is minimized by shortening fusion time to 0.2-0.5 s from 1-2 s for solid 125 μm fibres and reducing power, fusion time being more important. However, too short fusion time and too low temperature prevent full fusion of fibre-fibre boundary and proper rounding of edges if fiber diameters don't match, as the glass is too viscous and/or doesn't have enough time to flow. There is a trade-off between achieving low splice loss with little heat or good strength with more, and splice with excellent optical transmission may not be

Fig. 14. Splices between 204 μm PCF (IPHT 212b1) and SMF, fused with 150 μm axial offset. Splice with intact photonic structure and lowest loss, which broke during handling (top),

In splicing dissimilar fibres, axial offset of fibre contact point from the axis of electrodes is useful. The more heat-sensitive fibre - PCF in splice to SMF, or smaller of two PCFs, is kept away from centre of discharge column and its temperature is lower. In experiments at NIT, maximum axial offset was 1.2-1.5x fibre cladding diameter, otherwise unacceptable fibre

The power required to achieve given fibre temperature is approximately proportional to cladding diameter, and when fibres of different diameters are fused, the thinner fibre must receive a smaller share of arc power to obtain symmetrical temperature distribution. This is ensured by axial offset of arc centre in direction of thicker fibre. When splicing PCF to solid fibre, PCF shall be colder to prevent collapse of holes, adding second component of axial offset. In effect, even when PCF is moderately thicker than solid fibre, there is usually no

During fusion of fibres of different diameters, poor smoothing of corners at fibre-fibre transition and fragility of splice are common, as seen in Figure 14. Therefore, fusion power and duration are often selected to obtain the minimum splice strength allowing handling

Typical photonic structure, e.g. of "honeycomb" type, lacks full radial symmetry and mode field distribution reflects shape of it. In PCF with circular doped core, mode field can be still

without break, even if collapse of holes and increased splice loss are to be accepted.

and splice that survived (bottom). Fusion current: 18-19 mA, fusion time: 0.5 s.

deformation occurred in the hottest zone. Reduced fibre overlap can help.

**4.1.4 Mismatch in cladding diameter** 

offset towards PCF.

**4.1.5 Non-circular mode** 

strong enough even for removal from splicing machine, as shown in Figure 14.

where is surface tension, almost constant, and is glass viscosity falling with temperature (Figure 3). If this continues long enough, holes collapse and solid fibre of reduced diameter is created. Collapse of holes can be prevented by internal gas pressure ("inflation"); equilibrium pressure Pcritical for capillary is a function of its inner (ri) and outer (ro) radius:

$$P\_{critical} = \mathcal{V}\left(\frac{\mathbf{1}}{r\_i} + \frac{\mathbf{1}}{r\_o}\right) \tag{3}$$

In PCF with holes of differing sizes, the largest holes disappear last and over the shortest length. Due to longitudinal temperature gradient, only some length of PCF is subjected to collapse of holes, with gradual "thinning" in the intermediate zone - see Figures 12 and 13.

For internal fibre temperature independent of depth, all holes of given size shall collapse simultaneously, but in experiments (Xiao et al., 2007, Bourliaguet et al., 2003) holes located deeper are less affected. Example from our work is shown in Figure 13. Transfer of heat from fibre surface to its interior, predominantly of radiative type, is apparently delayed.

Fig. 12. Structure of PCF (UMCS 070119p2) with 3.5 μm and 1.3 μm holes and views of fusion splice to SMF. Fusion time: 0.3 s, fusion current from 13 mA (top) to 15 mA (bottom).

Fig. 13. Depth-dependent and diameter-dependent collapse of holes (UMCS 070119p2).

In the solidified length of fibre, light beam expands freely and proportion of power coupled to core of other fibre drops with increase of collapsed zone. Collapse of holes shall be avoided as much as possible, and if it cannot be avoided, fibre length affected must be reduced to absolute minimum. PCFs with doped core are partial exception.

where is surface tension, almost constant, and is glass viscosity falling with temperature (Figure 3). If this continues long enough, holes collapse and solid fibre of reduced diameter is created. Collapse of holes can be prevented by internal gas pressure ("inflation"); equilibrium pressure Pcritical for capillary is a function of its inner (ri) and outer (ro) radius:

In PCF with holes of differing sizes, the largest holes disappear last and over the shortest length. Due to longitudinal temperature gradient, only some length of PCF is subjected to collapse of holes, with gradual "thinning" in the intermediate zone - see Figures 12 and 13. For internal fibre temperature independent of depth, all holes of given size shall collapse simultaneously, but in experiments (Xiao et al., 2007, Bourliaguet et al., 2003) holes located deeper are less affected. Example from our work is shown in Figure 13. Transfer of heat from fibre surface to its interior, predominantly of radiative type, is apparently

Fig. 12. Structure of PCF (UMCS 070119p2) with 3.5 μm and 1.3 μm holes and views of fusion splice to SMF. Fusion time: 0.3 s, fusion current from 13 mA (top) to 15 mA (bottom).

Fig. 13. Depth-dependent and diameter-dependent collapse of holes (UMCS 070119p2).

reduced to absolute minimum. PCFs with doped core are partial exception.

In the solidified length of fibre, light beam expands freely and proportion of power coupled to core of other fibre drops with increase of collapsed zone. Collapse of holes shall be avoided as much as possible, and if it cannot be avoided, fibre length affected must be

*critical*

*P*

delayed.

1 1

 

*i o*

(3)

*r r*

Collapse is minimized by shortening fusion time to 0.2-0.5 s from 1-2 s for solid 125 μm fibres and reducing power, fusion time being more important. However, too short fusion time and too low temperature prevent full fusion of fibre-fibre boundary and proper rounding of edges if fiber diameters don't match, as the glass is too viscous and/or doesn't have enough time to flow. There is a trade-off between achieving low splice loss with little heat or good strength with more, and splice with excellent optical transmission may not be strong enough even for removal from splicing machine, as shown in Figure 14.

Fig. 14. Splices between 204 μm PCF (IPHT 212b1) and SMF, fused with 150 μm axial offset. Splice with intact photonic structure and lowest loss, which broke during handling (top), and splice that survived (bottom). Fusion current: 18-19 mA, fusion time: 0.5 s.

In splicing dissimilar fibres, axial offset of fibre contact point from the axis of electrodes is useful. The more heat-sensitive fibre - PCF in splice to SMF, or smaller of two PCFs, is kept away from centre of discharge column and its temperature is lower. In experiments at NIT, maximum axial offset was 1.2-1.5x fibre cladding diameter, otherwise unacceptable fibre deformation occurred in the hottest zone. Reduced fibre overlap can help.
