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Fusion splicing single-mode G.655, G.656 or G.657 onto G.652D

Fusion splicing single-mode G.655, G.656 or G.657 onto G.652D 
It appears as if an OTDR knows not its A from its E, when testing G.652D Non-Dispersion-Shifted Fibre (NDSF), connected to the following fibre types: (a) G.655D or G.656, variants of non-zero dispersion-shifted fibre (NZDSF) and (b) G.657A bend insensitive fibre. It is a difference in backscatter just before and just after the splice that confuses an OTDR:

a) The issue here is mode field diameters (MFDs). When splicing G.652D (smaller MFD) onto G.655/6 (larger MFD) a negative contribution (gainer) is incorrectly reported and G.655/6 onto G.652D reports an exaggerated loss… e.g. a real splice of 0.05 dB could bi-directionally measure -0.10 dB and +0.20 dB. These phenomena are well known, and operating procedures calling for bi-directional OTDR measurements and averaging the results has been a time-honoured tradition. More importantly though, using either Fujikura SM or NZ splicing modes, averaged dB splice losses as low as 0.04 to 0.02 are accomplishable without any obvious effort.

I recently subjected my field (who all happened to be novices) to splicing G.652D onto G.657A, using Auto, SM and NZ splice modes and below, the outcome:

Auto, SM and NZ modes all deliver decent-looking splices, with the true loss being the average of bi-directional measurements. NZ proved to be superior by a whisker. Experienced splice techs are predisposed not to like Auto-mode because for them, it is intolerably slow. Note that while BI G.657.A-compliant fibres are required to be backward compatible with G.652.D - G.657.B-compliant fibres (called bend-tolerant), are not.

OPTICAL FIBER NETWORK: Example cable tension calculation method

OPTICAL FIBER NETWORK: Example cable tension calculation method: Example cable tension calculation method The following are example formulae used in tension calculation.

OPTICAL FIBER NETWORK: TOPOLOGY AND POWER LOSS POINT ANALYSIS

OPTICAL FIBER NETWORK: TOPOLOGY AND POWER LOSS POINT ANALYSIS: TOPOLOGY AND POWER LOSS POINT ANALYSIS

TOPOLOGY AND POWER LOSS POINT ANALYSIS

TOPOLOGY AND POWER LOSS POINT ANALYSIS

Example cable tension calculation method

Example cable tension calculation method

The following are example formulae used in tension calculation.

Example overhead trunk installation method

Example overhead trunk installation method

• Use an anti-twist fitting or the like to avoid twisting in the optical cable during installation.
• Hanger rollers tend to cause twisting in the cable for structural reasons. If you are using hanger rollers, use them with the utmost care during the installation of a long cable since hanger rollers are likely to affect the cable in such installation.
• Place the cable drum 2H or more apart from the utility pole (H: cable roller mounting height), as shown above, to avoid sharp bends in the optical cable. It is recommended to use an 11-wheel cable roller with a 300 mm corner radius to avoid squashing the cable during installation.
• To lay an optical cable over a long distance, pull the tension member instead of the cable sheath and monitor the tension in the cable to avoid over-tension. Over-tension can result in detaching the pulling eye or squashing the cable. The maximum allowable tension differs according to the cable type, and specifications are given for individual cables. For more detailed information, refer to relevant specifications.
• If it is unavoidable to use a ribbon slotted-core cable (helical), always secure the fibers in order to prevent them from moving due to vibration after installation.
• Figure 8 cables need to be twisted once every 10 m or so to reduce vibration caused by winds.

Selecting a basic optical cable structure (recommended structure)

Selecting a basic optical cable structure (recommended structure)

Example underground trunk installation method

Example underground trunk installation method

• Attach a pulling eye or other similar fitting on one end of the optical cable.
• Use an anti-twist fitting or the like, as shown above right, to avoid twisting in the optical cable during installation.
• Place the cable drum right above the opening of the conduit, as shown above left, in order to unwind the cable in a smooth curve from the drum. When unwinding, take care so as not to twist or form a kink in the cable. Moreover, use a corrugated flexible pipe and a bellmouth fitting to protect the cable.
• To lay an optical cable over a long distance, pull the tension member instead of the cable sheath and monitor the tension in the cable to avoid over-tension. Over-tension can result in detaching the pulling eye or squashing the cable. The maximum allowable tension differs according to the cable type, and specifications are given for individual cables. For more detailed information, refer to the relevant specifications.
• For installation along railroad tracks or in other places where strong vibrations can cause fibers to move, use a ribbon slotted-core cable (helical) with fibers secured or an SZ slotted-core ribbon cable.

Optical Fiber Terms

Optical Fiber Terms

Core diameter

A parameter of multimode optical fibers. It represents the diameter of the circle that best approximates the circumference of the core region. The smaller the value of the core diameter, the broader the services band. Fibers commonly used today have a core diameter of 50μm.

Mode field diameter (MFD)

A parameter of single-mode optical fibers, MFD corresponds to the diameter of the spread of electric field distribution in propagation mode (light path). Light usually passes through the core region. However, in the case of a single-mode optical fiber, the light leaks into the cladding region. Therefore, single-mode optical fibers are specified by MFD rather than core diameter. MFD is slightly greater than the core diameter. The smaller the MFD, the higher the required accuracy of alignment for connection/splicing. Furthermore, the larger the MFD difference of two joined fibers, the greater the connector/splice loss.

Cladding diameter

The diameter of the circle best approximating the cladding surfaces. The larger the cladding diameter difference of two joined fibers, the greater the connector/splice loss.

Cable cutoff wavelength

A parameter of single-mode optical fibers. An optical fiber cannot be a single-mode fiber if it is used at a wavelength shorter than the cable cutoff wavelength, which is determined by optical fiber structure, involving refraction index distribution and core diameter.

Proof test

Screening is a technique intended to remove the glass defects of a fiber and improve its structural reliability. A given level of elongation strain is applied to the overall length of an optical fiber to break the fiber at its low-strength section. The screening level is the value of the elongation strain. The higher the value of screening level, the higher the reliability of the optical fiber.

Transmission loss

Transmission loss is a value that indicates the decrease of optical power of light propagating between two points of optical fiber. It is expressed

The transmission distance becomes short when transmission loss grows.

Transmission band

A parameter of multi-mode optical fibers. The transmission band is the frequency at which the magnitude of the baseband transfer function decreases to a specified value (6 dB). In other words, the value indicates to what frequency the signal is transmitted without distortion. The higher the transmission band, the higher the usable transmission frequency, hence larger-capacity transmission.

Zero-dispersion wavelength

A parameter of single-mode optical fibers. At the zero-dispersion wavelength, the wavelength dispersion decreases to zero. Transmission at a wavelength of a large absolute value of wavelength dispersion results in greater dispersion and therefore higher optical pulse distortion. Optical fibers designed so that the zero-dispersion wavelength is about 1310 nm or 1550 nm are known as the general-purpose SM or the dispersion-shifted optical fiber, respectively.

Zero-dispersion slope

A parameter of single-mode optical fibers. The zero-dispersion slope represents the gradient of dispersion at the zero-dispersion wavelength. In general, the greater the zero-dispersion slope, the higher the absolute value of dispersion at any wavelength.

Optical Fiber Notes on Fusion Splicing

Notes on Fusion Splicing

Fusion splicing procedures comprise (1) the fitting of a fiber protection sleeve, (2) removal of cover layers, (3) fiber cleaning, (4) fiber cleaving, (5) fusion splicing, and (6) reinforcing the splice.

(1) Fitting of Fiber Protection Sleeve

The fiber protection sleeve is used to protect optical fibers exposed at the splice. Make sure that one of the optical fibers is passed through the protection sleeve before fusion splicing.

(2) Removal of Cover Layers

Using a jacket remover, remove the cover layers to expose the fiber glass.

Notes:

• After cover layer removal, off-cuts are present in the jacket remover. Remove off-cuts from the jacket remover and clean the blade.
• To remove cover layers from a fiber ribbon, use a heated jacket remover. For successful removal, warm the cover layers for about 5 seconds before removal.

(3)Fiber Cleaning

After cover layer removal, clean the fiber glass with alcohol.

Notes:

• Debris of cover layers if remaining on the fiber glass can cause poor concentricity in fusion splicing or increased splice loss. Clean the glass fiber thoroughly.
• In the case of a multi-fiber cable, fiber ends may stick together due to alcohol, causing defective cleaving of fibers. Flip lightly with a finger to spread out the fibers.

(4) Fiber Cleaving

Follow the optical fiber cleaver operating procedure to cut the fiber.

Notes:

• The loss characteristic of a fusion splice depends on the cleaving. To reduce cleaving defects, clean the fiber holder and blade of optical fiber cleaver on a regular basis
• Keep the cleaved end of an optical fiber away from an object including your fingers to eliminate the causes of defective splices.
• Avoid scattering fiber off-cuts.

(5) Fusion Splicing

Fusion-splice optical fibers following the operation manual of the fusion splicer.

Notes:

• Dirt in the V-grooves or clamp of a fusion splicer can cause an unusual light loss due to poor concentricity. Clean the fusion splicer thoroughly.
• It is possible to detect faulty conditions of cleaved end if pre-splicing inspection capability with dual-axis observation is available.
• If the fiber has a curl, lightly squeeze the fiber with fingers to remove the curl. The placed fiber should bend downward.

(6) Splice Reinforcing

Cover the optical fiber splice with the fiber protection sleeve. Reinforce the fiber with the sleeve on the heater.

Notes:

• Avoid bending or twisting the fiber when moving it so as not to break the fiber.
• Position the fiber protection sleeve so that its midpoint is close to the center of the splice.
• When placing the reinforcement, make sure that the glass fiber is straight.

Classification and Principles of Fusion Splices

Classification and Principles of Fusion Splices

Fusion splicing involves the melting and joining of optical fibers using heat generated by an electric arc between electrodes. Fusion splicing is classified into the two methods, as follows.

(1) Core alignment method(core alignment)

Optical fiber cores observed with a microscope are positioned with the help of image processing so that they are concentrically aligned. Then, an electric arc is applied to the fiber cores. The fusion splicer used has cameras for observation and positioning in two directions.

(2) Stationary V-groove alignment method (cladding alignment)

This fusion splicing method uses V-grooves produced with high precision to position and orient optical fibers and utilizes the surface tension of melted optical fibers for alignment effects (cladding alignment). Splices made by this method achieve low loss thanks to the recent advancement of optical fiber production technology, which has improved the dimensional accuracy regarding the placement of core. This method is primarily used for splicing a multi-fiber cable in a single action.

Mechanisms of Light Loss at Optical Fiber Joint

Classification of Techniques Used for Optical Fiber Connection/Splicing
Optical fibers are joined either by fusion/mechanical splice, which is a permanent joint, or by connectors, which can be disengaged repeatedly. Optical connectors are used mostly at joints that need to be switched for optical service operation and maintenance reasons, while permanent joints are in use mostly in other applications.

When joining optical fibers, the opposed cores must be properly aligned. Optical fiber connector/splice loss occurs mostly in the following manner.

Mechanisms of Light Loss at Optical Fiber Joint

(1) Poor concentricity

Poor concentricity of joined optical fibers causes a connector/splice loss. In the case of general purpose single-mode fibers, the value of connector/ splice loss is calculated roughly as the square of the amount of misalignment multiplied by 0.2. (For example, if the light source wavelength is 1310 nm, misalignment by 1 µm results in approximately 0.2 dB of loss.)

(2) Axial run-out

A connector/splice loss occurs due to an axial run-out between the light axes of optical fibers to be joined. For example, it is necessary to avoid an increased angle at fiber cut end when using an optical fiber cleaver before fusion splicing, since such an angle can result in splicing of optical fibers with run-out.

(3) Gap

An end gap between optical fibers causes a connector/splice loss. For example, if optical fiber end faces are not correctly butt-joined in mechanical splicing, a splice loss.

(4) Reflection

An end gap between optical fibers results in 0.6 dB of return loss at the maximum due to the change in refractive index from the optical fiber to the air. Cleaning optical fiber ends is important for optical connectors. In addition, the whole optical connector ends should be cleaned because loss can also occur due to dirt between optical connector ends.

Optical Fiber Categories

Optical Fiber Categories

Here’s the most common description of the varieties of telecommunication fibers.

MMF (multimode fiber)
- OM1 or MMF(62.5/125)
- OM2/OM3 (G.651 or MMF(50/125))
SMF (single-mode fiber)
- G.652 (dispersion non-shifted SMF)
- G.653 (dispersion shifted SMF)
- G.654 (cut-off shifted SMF)
- G.655 (NZDSF)
- G.656 (low dispersion slope NZDSF)
- G.657 (bending insensitive SMF)

Technically you can use any fibers for FTTx as far as the optical budget allows, but the most common application for FTTx shall be by G.652 and G.657.

G.651 (multi mode fiber)

Multi mode fiber (MMF) is used for communication over short distance, such as LAN and datacenter. MMF classified M1 to M4 according to ISO/ IEC. Each bandwidth and distance is defined as follows.

ITU-T G.651 is another name for OM2/OM3 or MMF(50/125). ITU-T recommendation does not have OM1 or MMF(62.5/125) which is still popular in US. The core of MMF(50/125) has a refractive index profile gradually changing from the center of the core to the cladding, which enables multiple of transmission light (mode) travel with nearly the same velocity.

G.652 (dispersion non-shifted SMF)

It is the most common SMF in the world. It is tuned to minimize the dispersion (which gives the deformation to the signal) around the wavelength at 1310nm. You can use 1550 nm wavelength window for the shorter distance or with the dispersion compensating fiber or module. G.652A/B is the basic SMF and G.652C/D is the category for Low-waterpeak SMF.

G.653 (dispersion shifted fiber)

It is designed to minimize the dispersion at around 1550nm where the optical loss is the smallest.

G.654 (cut-off shifted fiber)

Official name for G.654 is “cut-off shifted fiber”, but it is better known as low attenuation fiber. Sumitomo’s Z Fiber™ has the world record of 0.154 dB/km. Thanks to this low attenuation the major application for G.654 is in the submarine and terrestrial long-haul application such as 400km reach without repeater.

G.655 (NZDSF)

NZDSF is short for NZDSF for wide band transport fiber. G.653 has designed to have zero dispersion at 1550nm, but G.655 has positive or negative dispersion intently. The reason for that is to reduce the undesirable effect of the non-linear phenomenon which interfere with the adjacent wavelength in DWDM system. The first generation NZDSF such as PureMetroTM has smaller dispersion of around or less than 5ps/nm/km to make the dispersion compensation easier. On the other hand the second generation NZDSF such as Pure- Guide® has larger dispersion of around 10ps/nm/km to enhance the DWDM capacity to double.

G.656 (low-slope dispersion NZDSF)

It is a kind of NZDSF which has stricter requirements on the dispersion slope which enables to guarantee the DWDM performance in wider wavelength range.

G.657 (bending insensitive fiber)

This category is introduced to specify macrobending performance, which sports implementation in FTTH and access network. G.657.A is fully compliant with ITU.T-G.652 specification, on the other hand, G.657.B is required higher macrebending performance but not necessary to comply with G.652.

Construction of Optical Fiber

Construction of Optical Fiber

An optical fiber for the telecommunication is made of glass designed to guide light along its length by total internal reflection. The glass fiber has nominal diameter of 125μm (0.125mm) and covered with plastic jacket for protection to form 250μm or 900μm in diameter. The central part of the glass fiber which guides light is called “core” and the “cladding” around it has lower refractive index than the “core” to confine the guided light. Silica glass is fragile; therefore, it is covered with a protective jacket. There are three typical coatings for the optical fiber.

Primary coated fiber

This is covered with a UV cured-Acrylate to a diameter of nominal 0.25mm. Since it has an extraordinary small diameter, it has a superior capacity to fit a large number of fibers into a cable and is used widely.

Secondary jacketed fiber

Or tight or semi-tight buffered fiber. This is optical fiber covered with thermoplastic to a diameter of 0.9mm. Compared to 0.25mm fiber, it is stronger, easier to handle and is widely used in LAN wiring and other small fiber-count cable.

Ribbon fiber

Ribbon fiber provides an excellent way to boost the productivity of connector assemblies and facilitates mass fusion splicing for greater productivity. The ribbon is composed of 4,8 or 12 colored fibers for fiber counts as great as 1000. The fibers are encapsulated by a UV-acrylate material which can be easily removed with standard strippers for mass splicing or easily peeled apart for single fiber access. Ribbon can be spliced at once with mass –fusion splicer and easy for identification in high fibercount cable.

Mode Field Diameter (MFD)

The mode field diameter(MFD) is the width of the fundamental (most strongly guided) mode in a single mode fiber above its cutoff wavelength. 

Any significant difference in MFD at the splice/joint will cause unacceptably high losses.

Whilst the MFD characteristics are frozen at the time of manufacture and as such are relatively fixed for the life of the fiber, it is a parameter that is influenced by various properties within the fiber that do vary along its length. It is therefore is useful and considered good practice to measure the MFD even within a cable manufacturing environment to ensure the consistency of the product.

The 'Production/Lab/R&D' range is suitable for manufacturers of fiber and cables. It is considered unlikely that this measurement would be required in a field environment. 

Multi-fiber cables

Multi-fiber cables

Individual fibers in a multi-fiber cable are often distinguished from one another by color-coded jackets or buffers on each fiber. The identification scheme used by Corning Cable Systems is based on EIA/TIA-598, "Optical Fiber Cable Color Coding." EIA/TIA-598 defines identification schemes for fibers, buffered fibers, fiber units, and groups of fiber units within outside plant and premises optical fiber cables. This standard allows for fiber units to be identified by means of a printed legend. This method can be used for identification of fiber ribbons and fiber subunits. The legend will contain a corresponding printed numerical position number and/or color for use in identification.

Optical Fiber Patch Cords Color Coding

Optical Fiber Patch Cords Color Coding
The buffer or jacket on patchcords is often color-coded to indicate the type of fiber used. The strain relief "boot" that protects the fiber from bending at a connector is color-coded to indicate the type of connection. Connectors with a plastic shell (such as SC connectors) typically use a color-coded shell. Standard color codings for jackets and boots (or connector shells) are shown below:

Buffer/jacket color	Meaning
Orange	multi-mode optical fiber
Aqua	OM3 or OM4 10 gig laser-optimized 50/125 micrometer multi-mode optical fiber
Violet	OM4 multi-mode optical fiber (some vendors)[15]
Grey	outdated color code for multi-mode optical fiber
Yellow	single-mode optical fiber
Blue	Sometimes used to designate polarization-maintaining optical fiber

Connector Boot		Meaning	Comment
Blue		Physical Contact (PC), 0°	mostly used for single mode fibers; some manufacturers use this for polarization-maintaining optical fiber.
Green		Angle Polished (APC), 8°
Black		Physical Contact (PC), 0°
Grey,	Beige	Physical Contact (PC), 0°	multimode fiber connectors
White		Physical Contact (PC), 0°
Red			High optical power. Sometimes used to connect external pump lasers or Raman pumps.

Remark: It is also possible that a small part of a connector is additionally colour-coded, e.g. the leaver of an E-2000 connector or a frame of an adapter. This additional colour coding indicates the correct port for a patchcord, if many patchcords are installed at one point.