**4.1.6 MFD mismatch**

This problem is not limited to PCFs - see section 3.4. Besides use of intermediate fibre(s), one can locally change MFD of one fibre to make it more compatible with another or modify propagation in the zone between fibres. Several methods were reported, including:


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

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

lateral symmetry, and propagation is polarization-dependent.

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

required.

**4.1.6 MFD mismatch** 

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

This problem is not limited to PCFs - see section 3.4. Besides use of intermediate fibre(s), one can locally change MFD of one fibre to make it more compatible with another or modify


Edvold & Gruner-Nielsen, 1996). Transition zone shall be at least 300 μm long.

propagation in the zone between fibres. Several methods were reported, including:


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 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**

In single-mode propagation regime, insertion loss of splice is independent of transmission direction. Non-reciprocity indicates multimode propagation in one or both fibres.

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:


Non-reciprocity of splice loss measured with optical source and optical power meter is due to different propagation of higher-order modes. In SMF, this part of radiation escapes fibre core and is lost, leaving only the fundamental mode. In PCF, part of it reaches the end of fibre and detector of power meter (Figure 16); loss indicated by instrument connected to PCF is lower than "true" value for fundamental mode. OTDR test is less affected, as optical pulse travels through splice in both directions and the instrument shows average loss value.

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

Arc Fusion Splicing of Photonic Crystal Fibres 193

PCF

PCF

Splice 1 Splice 2

Bare fibre adaptor

Optical power meter

SMF pigtail 2

> Optical power meter

> > SMF pigtail

Fig. 18. Setups for loss measurements. Measurement of splice No. 1 after separate PCF

Splice 1

be cleaned by gentle contact with suitable sticky tape, like Scotch Magic (Figure 19).

Fig. 19. Cleaved IPHT 282b4 in bare fibre adapter after cleaning of dust with sticky tape.

Fig. 20. Arrangement for characterization of PCF and PCF-SMF splices with OTDR.

SMF pigtail

OTDR

Fusion splices

PCF

SMF 1 SMF 2

OTDR measurement of SMF-PCF splice(s) and PCF itself requires certain length of fibre before and after the splice, at least 50 m for instrument with 10 ns pulse width (Figure 20).

When bare fiber adapters are used, debris on PCF endface can produce errors. PCF tip could

attenuation measurement (top), direct measurement of splice No. 2 (bottom).

SMF pigtail 1

FC/PC connector

SMF patchcord

FP laser source

FP laser source

Fig. 17. DGD and PDL of 20.2 m and 18.7 m long samples of PCF prone to multimode propagation (IPHT 252b3) with poor (top) and optimized (bottom) fusion splices to SMFs.

Loss measurements are needed to align fibres, monitor fusion, and evaluate finished splice. It is often necessary to measure total loss of circuit or device incorporating PCF rather than splice(s) alone. There are two basic test methods:


The first technique allows fast measurements (0.1-1 s) with high resolution (0.001 dB). As typical laser source emits linearly polarized light, apparent PCF-SMF splice loss during alignment varies with fiber rotation due to PCF non-circular structure. In experiments presented in section 4, loss was monitored during fibre alignment and after each fusion step. Test setup shown in Figure 18 was used, including HP8153A optical multimeter with HP81553SM laser source (1558 nm) and HP81532A power meter modules. For splice No. 1 it was necessary to subtract PCF loss, measured separately with OTDR and connector loss; loss of splice No. 2 was measured directly. Loss calculations must take into account high attenuation of PCF, usually 20-200 dB/km. Data in section 4.2 refer to splice No. 2.

Fig. 17. DGD and PDL of 20.2 m and 18.7 m long samples of PCF prone to multimode propagation (IPHT 252b3) with poor (top) and optimized (bottom) fusion splices to SMFs.


splice(s) alone. There are two basic test methods:

in section 4.2 refer to splice No. 2.


Loss measurements are needed to align fibres, monitor fusion, and evaluate finished splice. It is often necessary to measure total loss of circuit or device incorporating PCF rather than

The first technique allows fast measurements (0.1-1 s) with high resolution (0.001 dB). As typical laser source emits linearly polarized light, apparent PCF-SMF splice loss during alignment varies with fiber rotation due to PCF non-circular structure. In experiments presented in section 4, loss was monitored during fibre alignment and after each fusion step. Test setup shown in Figure 18 was used, including HP8153A optical multimeter with HP81553SM laser source (1558 nm) and HP81532A power meter modules. For splice No. 1 it was necessary to subtract PCF loss, measured separately with OTDR and connector loss; loss of splice No. 2 was measured directly. Loss calculations must take into account high attenuation of PCF, usually 20-200 dB/km. Data

Fig. 18. Setups for loss measurements. Measurement of splice No. 1 after separate PCF attenuation measurement (top), direct measurement of splice No. 2 (bottom).

When bare fiber adapters are used, debris on PCF endface can produce errors. PCF tip could be cleaned by gentle contact with suitable sticky tape, like Scotch Magic (Figure 19).

Fig. 19. Cleaved IPHT 282b4 in bare fibre adapter after cleaning of dust with sticky tape.

OTDR measurement of SMF-PCF splice(s) and PCF itself requires certain length of fibre before and after the splice, at least 50 m for instrument with 10 ns pulse width (Figure 20).

Fig. 20. Arrangement for characterization of PCF and PCF-SMF splices with OTDR.

Arc Fusion Splicing of Photonic Crystal Fibres 195

Fig. 22. OTDR trace of PCF (IPHT 282b4) spliced to SMFs as in Figure 20. Wavelength:

Cladding diameter m 82.7 127.5 125.9 124.4 126 Hole diameter (d) m 3.6 5.8 0.7 0.7 3.5/1.3 Hole spacing (Λ) m 4.2 6.5 4.4 4.2 3.5 Diameter of holey package m 42.8 61.5 44.6 43.0 55

IPHT 252b3

1.4/3.3/6.6\*

IPHT 282b3

0.8/2.8/7.1\*

IPHT 282b4

1.2/3.9/7.3\*

UMCS 070119p2

N/A \*\*

252b5

\*) Central high GeO2 doped core / GeO2 doped socket / total core diameter.

Fig. 23. Cross-sections of fibres: IPHT 282b4 (left) and IPHT 252b5 (right).

1550 nm, pulse width: 100 ns, PCF length: 104 m.

Parameter Unit IPHT

Diameter of doped core m 0.5/2.0/4.1\*

Table 2. Data of photonic crystal fibres

\*\*) No doped core.

Large differences of backscattering intensity in PCF and SMF are common, and true loss Γ of SMF-PCF splice can be established only with bi-directional measurement and averaging:

$$
\Gamma = \frac{\Gamma\_A + \Gamma\_B}{2} \tag{4}
$$

where ΓA and ΓB are apparent splice losses measured in A and B directions.

Many PCFs produce strong backscattering due to entrapment of scattered light by photonic structure and intense scattering in doped core, if present (Borzycki et al., 2010b, 2011a). This brings noise-free OTDR trace, but one-way OTDR measurement are misleading (Figures 21 and 22). In Figure 22, trace of highly GeO2-doped PCF (IPHT 282b4) was shifted by 9.5 dB vs. traces of SMFs, producing "gain" in splice before PCF and exaggerated loss of splice after PCF (Borzycki et al, 2011b). Testing such samples requires high-performance OTDR.

Fig. 21. OTDR trace of SMF, PCF-SMF splice and PCF (IPHT 282b3) acquired with setup shown in Figure 20. Wavelength: 1550 nm, pulse width: 10 ns, PCF length: 91 m. Instead of 8.1 dB "gain", the splice had actual loss of 2.2 dB.

In PCF characterization, improvements to quality of splices pay off with improved accuracy and fewer measurement artifacts, especially in measurements of polarization properties.

### **4.2 Examples**

Descriptions below apply to splicing of SMF to PCFs designed and made at IPHT Jena, Germany as highly nonlinear single-mode fibres for signal processing, like wavelength conversion. For this, fibre cores were strongly doped with GeO2, up to 36% mol. The reader is referred to separate papers on manufacturing of these fibres (Schuster et al, 2007) and their characterization (Borzycki et al., 2010b). Data of all PCFs referred to in this chapter are presented in Table 2. UMCS 070119p2 mentioned in section 4.1.3 was a "PANDA-like" birefringent fibre developed at UMCS Lublin, Poland for use in polarimetric sensors.

Large differences of backscattering intensity in PCF and SMF are common, and true loss Γ of SMF-PCF splice can be established only with bi-directional measurement and averaging:

Many PCFs produce strong backscattering due to entrapment of scattered light by photonic structure and intense scattering in doped core, if present (Borzycki et al., 2010b, 2011a). This brings noise-free OTDR trace, but one-way OTDR measurement are misleading (Figures 21 and 22). In Figure 22, trace of highly GeO2-doped PCF (IPHT 282b4) was shifted by 9.5 dB vs. traces of SMFs, producing "gain" in splice before PCF and exaggerated loss of splice after PCF (Borzycki et al, 2011b). Testing such samples requires high-performance OTDR.

Fig. 21. OTDR trace of SMF, PCF-SMF splice and PCF (IPHT 282b3) acquired with setup shown in Figure 20. Wavelength: 1550 nm, pulse width: 10 ns, PCF length: 91 m. Instead of

In PCF characterization, improvements to quality of splices pay off with improved accuracy and fewer measurement artifacts, especially in measurements of polarization properties.

Descriptions below apply to splicing of SMF to PCFs designed and made at IPHT Jena, Germany as highly nonlinear single-mode fibres for signal processing, like wavelength conversion. For this, fibre cores were strongly doped with GeO2, up to 36% mol. The reader is referred to separate papers on manufacturing of these fibres (Schuster et al, 2007) and their characterization (Borzycki et al., 2010b). Data of all PCFs referred to in this chapter are presented in Table 2. UMCS 070119p2 mentioned in section 4.1.3 was a "PANDA-like"

birefringent fibre developed at UMCS Lublin, Poland for use in polarimetric sensors.

8.1 dB "gain", the splice had actual loss of 2.2 dB.

**4.2 Examples** 

where ΓA and ΓB are apparent splice losses measured in A and B directions.

2

*<sup>A</sup> <sup>B</sup>* (4)

Fig. 22. OTDR trace of PCF (IPHT 282b4) spliced to SMFs as in Figure 20. Wavelength: 1550 nm, pulse width: 100 ns, PCF length: 104 m.


\*) Central high GeO2 doped core / GeO2 doped socket / total core diameter.

\*\*) No doped core.

Table 2. Data of photonic crystal fibres

Fig. 23. Cross-sections of fibres: IPHT 282b4 (left) and IPHT 252b5 (right).

Arc Fusion Splicing of Photonic Crystal Fibres 197

After measuring loss with butt coupling (Figure 26), fibres were melted to form ball lenses (Figures 26-28). Melting of SMF tip was repeated to obtain the required shape. After fusion (Figure 29), the splice was repeatedly heated at the same settings to reduce loss, but without further movement (Figures 29-30). Light transmission was monitored and work terminated

Fig. 26. Left: PCF (left) and SMF (right) aligned. Electrode tip is visible as dark triangle at the

Fig. 27. Left: SMF tip melted – Phase 2. Right: PCF positioned for melting.

Fig. 28. Left: PCF tip melted. Right: fibres aligned for fusion with axial offset.

Fig. 29. Left: fibres fused. Right: splice after additional heating No. 1.

bottom of picture. Right: SMF tip melted – Phase 1.

after splice loss stopped to significantly decrease any further – see data in Table 4.
