Guidelines for Visual, Rotational Alignment of
FIBERCORELIMITED HB-T (‘Bow-Tie’) to PANDA-type
Polarisation Maintaining Fibres

The Fibres
The first thing to consider when contemplating the rotational alignment of Bow-Tie and PANDA fibres is the fundamental similarity of the two designs:

Bow-Tie

PANDA

Both fibres are stress birefringent and achieve their birefringence by means of two, diagonally-opposed stress members. In addition, these stress members are all boron-doped and therefore have highly compatible fusion properties.

In short, the practical difference between the two fibres is merely the shape of their stress members.

Rotational Alignment
Most fusion splicers view the fibre from the side, focusing on the core, with a fairly narrow depth of field. Since the stress members extend in both directions, well beyond the plane of the core, they remain substantially out of focus. For this reason, neither fibre provides the distinct pattern of two dark lines, transitioning to a single dark line, during rotation through 90°, that many people expect. In practice, the internal structures of both fibres appear as the series of lighter and darker lines that may be seen in Figs. I to V (magnification X200, Sumitomo 35SE Rotary Fusion Splicer).

In these images, the main structures that relate to the stress members are the pair of distinct, parallel, ‘shadowed’ lines that run longitudinally. As you rotate the fibre, you can see one line grow stronger and the other fade as they move in and out of the focal plane, during their orbit of the core.

In essence, the images you will see for Bow-Tie and PANDA will be similar, but with the following key differences:

1. The image of the internal structure of a PANDA Fibre will appear somewhat broader because the stress members typically occupy around 70% of the total width of the fibre, compared with only 45% for the more efficient, Bow-Tie structure.
2. More structure can be seen in the Bow-Tie Fibre when viewed parallel to the stress members because of the close proximity of the inner ‘corners’ of the bow-ties to the focal plane (Fig III).

When aligning the fibres, the key feature to look out for is symmetry. During rotation, both Bow-Tie and PANDA Fibres will only exhibit two perfectly symmetrical images. These images correspond to the two birefringent axes of the fibre and will be found 90° apart (see Figs. I/V and III).

When viewed perpendicular to the stress members (along the fast axis), you will be able to see a pale band in the centre of the fibre (the core) flanked by two lines with dark ‘shadows’ (the stress members). This image is illustrated in Fig.I and in most cases, this will be the simplest and clearest image to use for alignment.

Rotating through 90° to view the fibres parallel to their stress members (along their slow axes) will reveal a similar pattern of shadowed lines, but located closer together. In addition, a broad, pale band will be seen between these lines in the PANDA fibre, compared with a series of three fine, dark parallel lines in the Bow-Tie fibre. This image is illustrated in Fig.III and may be used to confirm that alignment has been achieved by using the 90° view rotation facility found on most fusion splicers.

C.Emslie May 17th 2000

Fig I Fibres viewed perpendicular to stress members (along fast axis)
Fig II Fibre axes rotated by 45°
Fig III Fibres viewed parallel to stress members (along slow axis)
Fig IV Fibre axes rotated by 45°
Fig V Fibres viewed perpendicular to stress members (along fast axis)

Insertion Loss Results

Telcordia (Bellcore) TR-NWT-000765

‘Webfacts’ Factnote 4.3.11