**4.1.5 Non-circular mode**

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

Arc Fusion Splicing of Photonic Crystal Fibres 191





While effective, these techniques are sensitive to deviations in process parameters. Specific advice for different cases can be found in literature, but several techniques require splicing

In single-mode propagation regime, insertion loss of splice is independent of transmission

Excitation of higher order modes, e.g. at splice with lateral offset is known in telecom systems, but mostly limited to short fibres, as higher order modes are strongly attenuated. In PCFs, photonic structure can support persistent propagation of own modes, especially as short lengths of such fibres, 1 m are common. This produces interference in measurements, sensing or operation of optical devices, as detectors in active instruments respond to total optical power of all modes. Examples of related measurement problems are:


SMF PCF

**(b) Propagation from SMF to PCF** 

PCF SMF

**(a) Propagation from PCF to SMF** 

al., 2008, Borzycki et al., 2010a). Fibres with lenses are than fused – see 4.2.2.

type and length must be carefully chosen to ensure proper focusing of light.

machine with precise control of fibre movements, in particular for fibre tapering.

**4.1.7 Propagation of light in photonic structure and splice loss measurements** 

direction. Non-reciprocity indicates multimode propagation in one or both fibres.

arc power somewhat lower than required for pre-fusion.

light beam towards interface to fibre with larger core.


Fig. 16. Mechanism creating non-reciprocal loss in PCF-SMF splices.

shows average loss value.

more or less distorted, depending on interactions with surrounding holes. Many PCFs for signal processing, sensing, optical switching etc. have cores and/or photonic structure made deliberately non-symmetrical; mode field distribution is non-circular, but usually with lateral symmetry, and propagation is polarization-dependent.

Additionally, current PCF manufacturing technology cannot ensure perfect fibre geometry. Holes are often distorted during fibre drawing (Figure 15), affecting mode field shape.

Fig. 15. Central part of doped-core PCF (IPHT 282b4) with deformed holes.

If the PCF is spliced to radially symmetrical fibre, like single-mode or multimode telecom fibre, relative rotation has no effect on splice parameters, and simpler fusion splicing machine without fibre rotation is sufficient. Rotational alignment is necessary for splicing PCF to another PCF or specialty fibre of non-circular design like Bow-Tie or PANDA. Alignment is based either on observation of fibre structure through microscope of splicing machine or monitoring of transmission through butt-coupled fibres. In the latter case, a source of linearly polarized light properly coupled to one fibre is often required.
