**4.1.4 Mismatch in cladding diameter**

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 offset towards PCF.

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