**3.1 Process basics and physics**

Fusion splicing involves localized melting of two fibre butts pressed together, with fibre coating removed. Surface tension forces cause glass to flow when viscosity is low enough, forming a joint with continuous structure and smooth, round external surface (Figure 1).

While smoothing of edges improves splice strength, self-centring of fibres shown in Figure 2 is undesirable when lateral shift is needed to align non-concentric fibre cores.

This chapter focuses on procedures and experiences with fusion splicing using equipment and tools intended for standard single mode and multimode telecom fibres, which gave acceptable results for most, but not all PCFs the authors encountered. Before this matter is presented in section 4, overview of "holey" fibres and their properties is made in section 2,

Their common feature is substantial modification of optical characteristics by presence of multitude of longitudinal holes or concentric layer(s) of solid micro- or nano-particles

b. PCFs with doped core surrounded by layers of holes, like HAF and nonlinear fibres.

d. Fibres with single central hole surrounded by dielectric mirror (hollow fibers and

Bending-tolerant Hole-Assisted Fibres (HAF) and fibres with solid nanostructured barrier found use in optical access networks (FTTH) and are covered by ITU-T G.657 recommendation, specifying their properties - but not designs. Corning ClearCurve fibre with layer of embedded solid particles around core is fusion spliced as SMF, while HAF is converted to SMF by collapse of holes on fusion splicer (Nakajima et al., 2003); both are not covered here. Properties differ: while fibres (b) and (c) are "splicing friendly", work with suspended-core PCFs and other fibres from group (a) is more difficult. Large-core "dielectric mirror" fibres for delivery of high-power laser radiation (d) are used in short lengths and not spliced. Experience of authors applies to first two groups, but recommendations are applicable to most other microstructured fibres. Key properties for

This section will introduce reader to arc fusion splicing of conventional, solid fibres and

Fusion splicing involves localized melting of two fibre butts pressed together, with fibre coating removed. Surface tension forces cause glass to flow when viscosity is low enough, forming a joint with continuous structure and smooth, round external surface

While smoothing of edges improves splice strength, self-centring of fibres shown in Figure 2

is undesirable when lateral shift is needed to align non-concentric fibre cores.

a. PCFs without doped core, e.g. suspended-core and highly birefringent fibres.

and arc fusion physics and technology are summarized in section 3.

around the light guiding core. There are several variants, including:

c. Fibres with solid nanostructured barrier around doped core.

The core is capable of guiding light on its own.


**3. Physics and technology of arc fusion splicing** 

techniques adopted for splicing fibres in communication networks.


**3.1 Process basics and physics** 

(Figure 1).


**2. Microstructured optical fibres** 

Photonic Bandgap Fibres).

fusion splicing include:

Fig. 1. Two identical 125 μm silica fibres before (top) and after (bottom) arc fusion. Electrode tip is visible as a black spot at the bottom of upper image.

Fig. 2. Two 125 μm single-mode fibres spliced with 10 μm perpendicular offset. Top to bottom: fibres before fusion and after 0.5 s, 1 s and 2 s long fusion with 17 mA current.

Glass evaporating from the hot zone is partly deposited on fibres in the vicinity - degrading surface quality and strength, and on parts of splicing machine - contaminating them. Evaporation is compensated for by fibre overlap: a reduced volume of glass is accommodated in shorter length of fibre without change in diameter. As no air, gel, glue etc. separates fibres after fusion, strength close to one of pristine fibre, no reflection and low insertion loss are possible. The heat for fusion is provided by either:


Arc Fusion Splicing of Photonic Crystal Fibres 179

Material CTE (10-6/K-1) Heat conductivity (W/m•K)

Table 1. Comparison of thermal properties of fused silica and selected other materials.

which the glass is transparent, and conduction along the fibre (Yablon, 2005), and:




required.

**3.3 Fusion procedure** 

Heat transfer during fusion is mostly through bulk radiation at wavelengths of 0.6-2 μm, to

Doping with germania (GeO2) or titania (TiO2) considerably reduces silica viscosity, and fusing multimode fibres with 50-62.5 μm core taking 20-25% of cross-section requires lower temperature than single-mode fibres, where core is small (4-10 μm) and contains less GeO2. In a hot fibre, diffusion of dopant(s) is observed, changing refractive profile and core size. The problem is acute during splicing of "depressed cladding" or "pure silica core" singlemode fibres, where core is surrounded by a layer doped with fluorine (F), a light element easily diffusing at high temperatures; careful control of arc power and short fusion time are

Out-gassing of boron, phosphorus, germanium or fluorine compounds from melted glass may cause problems as well, in particular during splicing of highly-doped specialty fibres. Silica does not burn, decompose or oxidise when heated in the air. However, exposure to humidity, alkalis or sharp objects produces surface flaws; damaged fibre breaks easily. All lengths of fibre stripped of coating must be promptly protected against humidity, abrasion, etc. Only splices intended for short-term use in laboratory conditions may be exempted.

Fusion of fibres with different dopants or doping levels can produce significant internal stress due to uneven thermal contraction after fusion, potentially affecting splice strength.

Arc fusion splicing of two single, polymer-coated, multimode or single-mode silica fibres of

3. fibre cleaving by scoring with a blade and applying controlled strain till it breaks

125 μm cladding diameter usually includes steps listed below:

("scribe and load"); cleave angle shall be less than 1,

4. clamping of fibres in supports with V-grooves,

1. removal of coating from fibres, usually by mechanical stripping,

2. fibre cleaning with solvent: isopropyl alcohol, acetone, etc. or dry wiping,

Fused silica 0.55 1.38 Tungsten 4.3 173 Steel 13-18 12-45 Copper 16.6 401 Aluminium 22.2 237 PVC (hard) 50-80 0.13-0.29 PET 60-70 0.17-0.24 PMMA 50-90 0.17-0.25

The first method is preferred due to compact equipment, fast operation and flexible control. Filament splicing is used for specialty fibres and when high splice strength is required. Other techniques are rarely used. Descriptions below cover arc fusion splicing only.

### **3.2 Fused silica properties and fusion splicing**

Fused silica is a glassy form of silicon dioxide (SiO2). In comparison to most multicomponent glasses, fused silica exhibits relatively slow decrease of viscosity with temperature (Figure 3). For fusion splicing, this property is desirable, as larger variations in temperature can be tolerated.

Fig. 3. Viscosity of pure fused silica as function of temperature. Approximate data compiled from sources not in full agreement (Shand, 1968, Yablon, 2005, Schott, 2007).

To ensure fusion in 0.5-2 s, glass viscosity must be reduced to about 104 Pa-s. Temperatures during different stages of fusion splicing range approximately from 1500°C (softening of fibres) to 2100°C (fusion), and electric power required to fuse two 125 μm solid silica fibres is about 8-10 W. As voltage drop between electrodes is almost constant, 500-600 V for 1 mm gap, discharge power is essentially proportional to arc current. The range for work with 125 μm fibres is approximately 7-20 mA.

Glass melting during fusion of 125 μm fibres takes place in zone 0.5-1 mm long, including effects of distribution of energy inside arc column, which is approx. 0.2 mm wide for 1 mm electrode gap. Because fused silica has exceptionally low linear thermal expansion coefficient and heat conductivity, as shown in Table 1, fibre strain during post-fusion cooling from the annealing temperature of approx. 1100C to room temperature is negligible: only 0.0099% for a 1 mm hot zone and 6 mm fibre length between clamps.

The first method is preferred due to compact equipment, fast operation and flexible control. Filament splicing is used for specialty fibres and when high splice strength is required.

Fused silica is a glassy form of silicon dioxide (SiO2). In comparison to most multicomponent glasses, fused silica exhibits relatively slow decrease of viscosity with temperature (Figure 3). For fusion splicing, this property is desirable, as larger variations in

Fig. 3. Viscosity of pure fused silica as function of temperature. Approximate data compiled

To ensure fusion in 0.5-2 s, glass viscosity must be reduced to about 104 Pa-s. Temperatures during different stages of fusion splicing range approximately from 1500°C (softening of fibres) to 2100°C (fusion), and electric power required to fuse two 125 μm solid silica fibres is about 8-10 W. As voltage drop between electrodes is almost constant, 500-600 V for 1 mm gap, discharge power is essentially proportional to arc current. The range for work with

Glass melting during fusion of 125 μm fibres takes place in zone 0.5-1 mm long, including effects of distribution of energy inside arc column, which is approx. 0.2 mm wide for 1 mm electrode gap. Because fused silica has exceptionally low linear thermal expansion coefficient and heat conductivity, as shown in Table 1, fibre strain during post-fusion cooling from the annealing temperature of approx. 1100C to room temperature is

negligible: only 0.0099% for a 1 mm hot zone and 6 mm fibre length between clamps.

from sources not in full agreement (Shand, 1968, Yablon, 2005, Schott, 2007).

125 μm fibres is approximately 7-20 mA.

Other techniques are rarely used. Descriptions below cover arc fusion splicing only.

**3.2 Fused silica properties and fusion splicing** 

temperature can be tolerated.


Table 1. Comparison of thermal properties of fused silica and selected other materials.

Heat transfer during fusion is mostly through bulk radiation at wavelengths of 0.6-2 μm, to which the glass is transparent, and conduction along the fibre (Yablon, 2005), and:


Doping with germania (GeO2) or titania (TiO2) considerably reduces silica viscosity, and fusing multimode fibres with 50-62.5 μm core taking 20-25% of cross-section requires lower temperature than single-mode fibres, where core is small (4-10 μm) and contains less GeO2.

In a hot fibre, diffusion of dopant(s) is observed, changing refractive profile and core size. The problem is acute during splicing of "depressed cladding" or "pure silica core" singlemode fibres, where core is surrounded by a layer doped with fluorine (F), a light element easily diffusing at high temperatures; careful control of arc power and short fusion time are required.

Out-gassing of boron, phosphorus, germanium or fluorine compounds from melted glass may cause problems as well, in particular during splicing of highly-doped specialty fibres.

Silica does not burn, decompose or oxidise when heated in the air. However, exposure to humidity, alkalis or sharp objects produces surface flaws; damaged fibre breaks easily. All lengths of fibre stripped of coating must be promptly protected against humidity, abrasion, etc. Only splices intended for short-term use in laboratory conditions may be exempted.

Fusion of fibres with different dopants or doping levels can produce significant internal stress due to uneven thermal contraction after fusion, potentially affecting splice strength.

## **3.3 Fusion procedure**

Arc fusion splicing of two single, polymer-coated, multimode or single-mode silica fibres of 125 μm cladding diameter usually includes steps listed below:


Arc Fusion Splicing of Photonic Crystal Fibres 181

Fibre core guiding radiation is small, with effective diameter usually in the 2-15 μm range in single-mode fibres, with extreme values often encountered in PCFs, and accurate alignment of fibre cores during splicing is essential. Main factors introducing splice loss include:


For splice between single mode fibres, insertion loss Γ resulting from core offset, MFD difference and angular misalignment is given by the following formula (Yablon, 2005),

<sup>2</sup> 2 2 2 2 1 2 1 2

*g g g g*

*w w w w*

where wg1 and wg2 are Gaussian radii of spliced fibres, δ is lateral offset between fibre cores, θ is the angular misalignment, n is refractive index of fibre material and the operating wavelength. Characteristics of loss due to each factor are shown in Figures 5, 6 and 7. Loss


The mode field diameter (MFD) of fibre included in technical specifications, usually measured in accordance with Petermann II definition is roughly twice its Gaussian mode

For fibres with smaller MFD, splice loss rises faster with lateral offset, but slower with angular misalignment; the resultant loss is approximately proportional to square of given misalignment. While lateral offset between fibre claddings is easy to detect visually through splicer microscope, even large angular misalignment, most often due to debris in V-grooves holding fibres may be overlooked when field of view is small or the optical system produces image distortion. This is critical in splicing large-MFD fibres for high power applications. Accuracy of fibre alignment depends on core size and accepted loss. For splicing singlemode fibres (wg = 2.5-5 μm) with loss below 0.2 dB, lateral core offset must be reduced to 0.5-1 μm (Figure 6). If the alignment requires cladding offset, increase of it is needed to compensate for self-centring of fibres during fusion. Splice loss can be estimated from MFD mismatch, core misalignments and deformations measured by automated analysis of splice image. True value is obtained from bi-directional measurements with optical time domain reflectometer (OTDR), as differences between backscattering intensity in fibres can produce

 

2 2 2 2 22 2 2 1 2 1 2

4 4 2/ sin

*g g g g*

*w w nw w*

(1)

mode fibres and physical core diameter in multimode fibres,



values calculated this way are only approximate because:


radius, but definitions of both parameters are not directly comparable.

aligned with lateral offset (Figure 2),

10log exp <sup>2</sup>


assuming a Gaussian approximation of mode fields:

**3.4 Fibre alignment and splice loss** 



Figure 4 shows typical sequence of arc current and fibre movement.

Fig. 4. Example of fibre-fibre gap and arc current variations during fusion splicing of SMF. Negative value of gap means an overlap of fibres pressed into each other.

Arc current and duration of heating in steps 9-11 and fibre overlap in step 9 depend on fibres spliced. 10% deviation from optimum arc current is usually enough to significantly rise splice loss or reduce splice strength, if current and temperature are too low. To reduce self-centring, fusion time is shorter for single mode than multimode fibres: 0.5-1 s vs. 1-2 s.

Some coatings require thermal stripping, softening with chemicals or burning for removal. When high splice strength is required, acrylate, epoxy-acrylate or polyimide coatings can be dissolved during 20-30 s immersion in hot (180-200C) sulphuric acid (H2SO4) - either pure (95%) or with addition of approx. 5% nitric acid (HNO3). Residual acid is removed by rinsing the fibre with water and later acetone (Matthewson et al., 1997).

### **3.4 Fibre alignment and splice loss**

180 Photonic Crystals – Introduction, Applications and Theory

8. alignment of fibres for lowest transmission loss; this may involve application of perpendicular offset with monitoring of loss or observation of cladding or cores, 9. softening of fibres by low power discharge: 6-9 mA current, 0.5-3 s duration, than

Fig. 4. Example of fibre-fibre gap and arc current variations during fusion splicing of SMF.

0 2 4 6 8 10

**Time [s]** 

Arc current and duration of heating in steps 9-11 and fibre overlap in step 9 depend on fibres spliced. 10% deviation from optimum arc current is usually enough to significantly rise splice loss or reduce splice strength, if current and temperature are too low. To reduce self-centring, fusion time is shorter for single mode than multimode

Some coatings require thermal stripping, softening with chemicals or burning for removal. When high splice strength is required, acrylate, epoxy-acrylate or polyimide coatings can be dissolved during 20-30 s immersion in hot (180-200C) sulphuric acid (H2SO4) - either pure (95%) or with addition of approx. 5% nitric acid (HNO3). Residual acid is removed by

Negative value of gap means an overlap of fibres pressed into each other.

rinsing the fibre with water and later acetone (Matthewson et al., 1997).

5. cleaning of fibres by short (0.2-0.5 s), low power electric arc, 6. visual inspection of fibre tips for proper cleave and cleanliness,

15. splice protection by heat-shrinkable sleeve, re-coating, etc.

Figure 4 shows typical sequence of arc current and fibre movement.

pressing together with 6-15 μm overlap,

13. insertion loss measurement (optional),

14. tensile strength test (optional),

fibres: 0.5-1 s vs. 1-2 s.

0

10

**Arc Current [mA]** 

20

7. placing fibre tips between electrodes with 10-20 μm gap (butt coupling),

10. fusion of fibres by high power electric arc: 12-20 mA current, 0.5-2 s duration,

Prefusion

Cleaning Annealing

Fusion

**Gap [μm]** 

0

10

20


11. annealing / polishing of fibres with low power electric arc (optional), 12. visual inspection of splice: no distortions, slits or bubbles allowed,

Fibre core guiding radiation is small, with effective diameter usually in the 2-15 μm range in single-mode fibres, with extreme values often encountered in PCFs, and accurate alignment of fibre cores during splicing is essential. Main factors introducing splice loss include:


For splice between single mode fibres, insertion loss Γ resulting from core offset, MFD difference and angular misalignment is given by the following formula (Yablon, 2005), assuming a Gaussian approximation of mode fields:

$$\Gamma = -10\log\left[\frac{4w\_{\text{g1}}^2 w\_{\text{g2}}^2}{\left(w\_{\text{g1}}^2 + w\_{\text{g2}}^2\right)^2} \exp\left(-\frac{4\delta^2 + \left(2\pi/\lambda\right)^2 n^2 w\_{\text{g1}}^2 w\_{\text{g2}}^2 \sin^2\theta}{2\left(w\_{\text{g1}}^2 + w\_{\text{g2}}^2\right)}\right)\right] \tag{1}$$

where wg1 and wg2 are Gaussian radii of spliced fibres, δ is lateral offset between fibre cores, θ is the angular misalignment, n is refractive index of fibre material and the operating wavelength. Characteristics of loss due to each factor are shown in Figures 5, 6 and 7. Loss values calculated this way are only approximate because:


The mode field diameter (MFD) of fibre included in technical specifications, usually measured in accordance with Petermann II definition is roughly twice its Gaussian mode radius, but definitions of both parameters are not directly comparable.

For fibres with smaller MFD, splice loss rises faster with lateral offset, but slower with angular misalignment; the resultant loss is approximately proportional to square of given misalignment. While lateral offset between fibre claddings is easy to detect visually through splicer microscope, even large angular misalignment, most often due to debris in V-grooves holding fibres may be overlooked when field of view is small or the optical system produces image distortion. This is critical in splicing large-MFD fibres for high power applications.

Accuracy of fibre alignment depends on core size and accepted loss. For splicing singlemode fibres (wg = 2.5-5 μm) with loss below 0.2 dB, lateral core offset must be reduced to 0.5-1 μm (Figure 6). If the alignment requires cladding offset, increase of it is needed to compensate for self-centring of fibres during fusion. Splice loss can be estimated from MFD mismatch, core misalignments and deformations measured by automated analysis of splice image. True value is obtained from bi-directional measurements with optical time domain reflectometer (OTDR), as differences between backscattering intensity in fibres can produce

Arc Fusion Splicing of Photonic Crystal Fibres 183

As all single-mode and multimode telecom fibres exhibit radial symmetry of core and

Properly made splice of identical fibres has insertion loss of around 0.05 dB and return loss in excess of 70 dB. Butt-coupling with 10-15 μm gap, e.g. before fusion (Fig. 1) introduces insertion loss of approx. 0.40 dB and 15 dB return loss due to Fresnel reflection from a pair of glass/air interfaces. If a transmission loss is monitored during splicing, this difference helps to estimate splice quality. Because loss measured with laser source varies periodically with gap width due to interferometric effects (Yablon, 2005), incoherent source like LED is best. Hot fibre is a strong source of broadband thermal radiation, with emission peak close

Fig. 7. Calculated loss caused by angular misalignment of identical single mode fibres.

Loss of splice between single-mode fibres usually shows weak wavelength dependence, because MFD increases with wavelength, typically by 10-20% between 1310 and 1550 nm (Corning, 2008). If the main source of loss is core offset, it falls with wavelength, while angular misalignment produce loss rising with wavelength, in both cases in proportion to square of MFD. For multimode fibres, splice loss is essentially wavelength-independent.

Strong increase of splice loss with wavelength shown in Figure 8 indicates excessive fibre

For example, while single splice between fibres having 1:2 MFD ratio has best-case loss of 1.94 dB, two splices with 1 : 1.41 : 2 MFD ratio have combined loss 2 x 0.51 = 1.02 dB. This approach was adopted for splicing a small-core DCF with MFD of 2-4 μm to typical SMF having 8-11 μm MFD (Edvold & Gruner-Nielsen, 1996), and later to splice dissimilar PCFs (Xiao et al., 2007). Extra intermediate fibres can reduce loss further, e.g. to 0.56 dB for 4 splices and 1:2 MFD ratio, but difficulty with finding necessary fibres, losses due to other

bending due to improper handling, tight coiling or squeeze in the vicinity of splice.

factors and additional labour usually make such efforts impractical.

= 1.55 μm, n = 1.5. Blue: wg = 7.5 μm, green: wg = 5 μm, red: wg = 2.5 μm.

cladding, rotational alignment of fibres is not required.

to 780 nm at 2000C, preventing loss measurements during discharge.

relative shift of fibre traces and error in OTDR measurement of splice loss in one direction, often exceeding 1 dB.

Fig. 5. Calculated loss caused by mismatch in mode field diameters of single mode fibres.

Fig. 6. Calculated loss caused by lateral offset between cores of identical single mode fibres.

relative shift of fibre traces and error in OTDR measurement of splice loss in one direction,

Fig. 5. Calculated loss caused by mismatch in mode field diameters of single mode fibres.

Fig. 6. Calculated loss caused by lateral offset between cores of identical single mode fibres.

often exceeding 1 dB.

As all single-mode and multimode telecom fibres exhibit radial symmetry of core and cladding, rotational alignment of fibres is not required.

Properly made splice of identical fibres has insertion loss of around 0.05 dB and return loss in excess of 70 dB. Butt-coupling with 10-15 μm gap, e.g. before fusion (Fig. 1) introduces insertion loss of approx. 0.40 dB and 15 dB return loss due to Fresnel reflection from a pair of glass/air interfaces. If a transmission loss is monitored during splicing, this difference helps to estimate splice quality. Because loss measured with laser source varies periodically with gap width due to interferometric effects (Yablon, 2005), incoherent source like LED is best. Hot fibre is a strong source of broadband thermal radiation, with emission peak close to 780 nm at 2000C, preventing loss measurements during discharge.

Fig. 7. Calculated loss caused by angular misalignment of identical single mode fibres. = 1.55 μm, n = 1.5. Blue: wg = 7.5 μm, green: wg = 5 μm, red: wg = 2.5 μm.

Loss of splice between single-mode fibres usually shows weak wavelength dependence, because MFD increases with wavelength, typically by 10-20% between 1310 and 1550 nm (Corning, 2008). If the main source of loss is core offset, it falls with wavelength, while angular misalignment produce loss rising with wavelength, in both cases in proportion to square of MFD. For multimode fibres, splice loss is essentially wavelength-independent.

Strong increase of splice loss with wavelength shown in Figure 8 indicates excessive fibre bending due to improper handling, tight coiling or squeeze in the vicinity of splice.

For example, while single splice between fibres having 1:2 MFD ratio has best-case loss of 1.94 dB, two splices with 1 : 1.41 : 2 MFD ratio have combined loss 2 x 0.51 = 1.02 dB. This approach was adopted for splicing a small-core DCF with MFD of 2-4 μm to typical SMF having 8-11 μm MFD (Edvold & Gruner-Nielsen, 1996), and later to splice dissimilar PCFs (Xiao et al., 2007). Extra intermediate fibres can reduce loss further, e.g. to 0.56 dB for 4 splices and 1:2 MFD ratio, but difficulty with finding necessary fibres, losses due to other factors and additional labour usually make such efforts impractical.

Arc Fusion Splicing of Photonic Crystal Fibres 185

Fig. 10. Deformation of fibre tips by heat of electric arc: DTU MIK125/0.5 made of PMMA (left) and Corning SMF-28 made of fused silica (right). Top: cleaved fibres, middle: after 1st

Handling and splicing of thin mPOF with standard equipment is difficult due to softness of polymers in comparison to fused silica. In experiments at NIT, mPOF of 125 μm diameter exhibited unacceptable sag and curl when clamped 4 mm away from tip, while standard

This section presents some issues specific to fusion splicing of silica "holey" fibres, primarily of single mode PCF to SMF. Due to large variety of designs, actual procedure, power, time

Finished splice must be protected to have adequate mechanical strength, as holes and flaws on their surfaces make PCFs inherently weaker than solid fibres. In all experiments at NIT, fusion splices were protected with commercial 60 mm heat shrinkable sleeves reinforced with stainless steel rod. Protected splices performed well during temperature cycling between -40°C and +80C, with loss stability better than 0.05 dB, and as grips for

Fusion splices are hermetic, keeping external contaminants out, but trapping whatever entered earlier. Exceptions include helium and hydrogen, diffusing through 60 μm thick fused silica cladding in few hours. Short suspended core PCF infiltrated with acetylene (C2H2) or hydrogen cyanide (HCN) and fusion spliced to SMF pigtails is used as optical frequency reference, e.g. for calibration of optical spectrum analyzers (Thapa et al., 2006).

Out-gassing of cleaved PCF is fast, but removal of liquid or dust is essentially impossible due

Infiltration of holes with gas or liquid allows to make fibre sensors for chemical analysis, detection of pollutants or poison gas, medical diagnostics, etc. through spectral absorption

to high pneumatic resistance of thin holes and adsorption to the surface of their walls.

heating, bottom: after 2nd heating.

**4. Fusion splicing of PCFs** 

V-groove clamps usually damaged the fibre.

and geometry settings must be individually tailored.

application of twist and tensile forces in mechanical tests (Figure 11).

Fig. 8. Sharp bend of SMF observed with OTDR at 1310 nm (left) and 1550 nm (right).

If fibres having substantially different MFD must be spliced, loss can be reduced by introduction of short fibre with intermediate MFD, as shown in Figure 9.

Fig. 9. Principle of splicing through intermediate fibre.
