OPTICAL FIBER TS & CONNECTIONS

Dr. BC Choudhary

NITTTR, Sector-26, Chandigarh.

Basic Fiber Optic Link Transmitter

Splice

Cable

Connector

Cable

Receiver

OPTICAL FIBER TS  Technical

requirement for both ting & termination of transmission media



Number of ts or Connections  Link length between repeaters  Continuous length of fiber  Length of fiber cable practically or conveniently installed as continuous length

 Repeaters Spacing (A continuously increasing parameter) Ranges from

 40-60 km at 400 Mbits/s

 100 km at 2.4 Gb/s - SMFs  300 km at 1.7-10 Gb/s using SMDSFs

FIBER TS • Source- Fiber • Fiber- Fiber • Fiber- Detector  Manufacturers supply Electro-optical devices (Sources and Detectors) with fiber optic pigtail or connector to facilitate direct fiber-fiber connection  IMPORTANT ASPECT IS FIBER-TO - FIBER CONNECTION WITH LOW LOSS AND MINIMUM DISTORTION

Two major categories of fiber ts  FIBER SPLICES: Permanent or Semi-permanent ts

Soldering  FIBER CONNECTORS: Demountable or Removable ts

Plugs or Sockets FIBER COUPLERS: Branching devices  Splitters or Combiners  Importance in Networks

 Crucial

aspect of fiber ts concerning Optical Losses associated with the connection • Fiber Alignment

Loss Mechanisms at ts 1. FRESNEL REFLECTION 

Optical Loss encountered at the interfaces (Even when two fiber ends are smooth, perpendicular to fiber axes and perfectly aligned)



A small proportion of light may be reflected back into transmitting fiber causing attenuation at the t.  Fresnel Reflection

 Occurs due to step change in refractive index at ted interface Glass – Air - Glass

Reflection Loss

Fraction of light reflected at a single interface

 n1  n   r    n1  n 

2

n1 : R.I. of core, n : R.I. of interfacing medium (= 1 for air)

Loss in decibel due to FR at single interface

LossFres = -10 log10(1-r)  Can be reduced to a very low level using index matching fluid in the gap between ted fibers.

2. Deviation in Geometrical & Optical Parameters  All light from one fiber is not transmitted to another fiber ;

Because of mismatch of mechanical dimension

Three major cases : a) Core mismatch b) NA mismatch c) Index Profile

Intrinsic Losses Losses due to: • Fresnel Reflection • Deviation in Geometrical & Optical parameters  Minimized using fibers manufactured with lowest tolerance or same fiber

Extrinsic Losses 

Losses due to some imperfection in splicing 

Caused by Misalignment

Three possible types of misalignment at t (a) Longitudinal misalignment (b) Lateral misalignment; (c) Angular misalignment

(a) Loss due to lateral and longitudinal misalignment for a 50 m core diameter GI fiber; (b) insertion loss due to angular misalignment for ts in two MMSI fibers with NA of 0.22 and 0.3.

FIBER SPLICES A

permanent t formed between two fibers

TWO BROAD CATEGORIES

• Fusion Splicing or Welding Accomplished by applying localized heating (a flame or an electric arc) at the interface between two butted, prealigned fiber ends causing them to soften and fuse.

• Mechanical Splicing Fibers are held in alignment by some mechanical means  Achieved by various methods; o Tube Splices o Groove Splices

 MUST HAVE SMOOTH AND SQUARE END FACES  End preparation achieved using suitable tools - “ Cleavers”

“ Scribe and Break”

or “Score and Break”

 Scoring of fiber surface under tension with cutting tool (Sapphire, Diamond or Tungsten Carbide blade)

Optical fiber end preparation: principle of scribe and break cutting.

Fiber Cleavers

Two Action Cleaver: Fiber cleaving & Fusion splicing tool

One Action Cleaver

Handheld Cleaver

Cable Preparation Equipment Multipack; • Enhanced quality to prevent cracks and fiber strength degradation. • Allow skill-free operation of factory fiber prep and field splicing applications. • Equipped with a high precision tensile strip and automatic ultrasonic cleaning action.

Fusion Splicing of Optical Fibers  Manual Processing • Require Fiber end surfaces to be prepared for t

• Heating of prepared fiber ends to fusion point with application of axial pressure between two fibers. • Positioning & alignment using microscopes Electric Arc Fusion splicing

Prefusion Method (Automated)  No need for smooth end preparation

Prefusion method for accurate splicing 

Smaller Fresnel Reflection loss  Typical Losses : 0.1 to 0.2 dB for MMF

Fusion Splicers

 Drawback: Fiber get weakened near splice (30%) 



Fiber fracture occurs near the heat-affected zone adjacent to the fused t. Splice be packaged to reduce tensile loading

Protection of ts

Protection Sleeves for spliced fibers

Fiber t enclosures Underground fiber splice tray

Mechanical Splicing  Uses accurately produced rigid alignment tubes into which the prepared fiber ends are permanently bonded.

 Techniques for tube splicing of optical fibers: (a) Snug Tube Splice (b) Loose Tube Splice; Square Cross section Capillary

Ultra Splice

Ultra Splice: Reusable mechanical splice. Average Loss  0.2 dB

Groove Splices  Use of grooves to secure the fibers to be ted  better alignment to the prepared fiber ends.

V-groove splices 

Insertion losses  0.1 dB using jigs for producing V-groove splice.

Elastic Tube or Elastomeric Splice  Comprises of two elastic parts (inner with V-groove) in compression to ensure alignment of fibers.

Elastomeric Splice: (a) Cross section (b) Assembly



Fibers of different diameters tend to be centred and hence successfully spliced.



General loss ~ 0.25 dB for commercial product

Spring Groove Splice  Utilizes a bracket containing two cylindrical pins, which serve as an alignment guide for two prepared fibers.

 An elastic element (a spring) used to press the fibers into groove and maintain alignment of fiber ends.

Mean Losses  0.05 dB with MMGI Fibers.  Practically used in Italy.

Springroove Splice : (a) Expanded overview (b) Cross-section Schematic

Secondary Alignment Techniques  Alignment of secondary elements around the bare fibers • Increased ruggedness • Easy ground and polish of fiber end • Better termination Drawbacks:  Time consuming for termination  Increased losses due to tolerances on secondary elements  Fiber misalignment.

Glass capillary tubes (Ferrules)

 Fixing of glass ferrules Alignment sleeve of metal or plastic in which glass tube fibers are aligned

Average loss  0.2 dB

Mechanical splice using glass capillary tubes.

Rotary Splice  Use glass capillary tubes for fiber termination with small eccentricity. • Built-in offset and rotation, for excellent alignment • Alignment accuracy of 0.05 m using three glass rod alignment sleeve. (necessary for SMFs; 8-10 m MFD)

• Mean Losses  0.03 dB using Index matching gels (Not affected by skill levels of the splicer). Rotary Splice for SMF: (a) Alignment using glass ferrules (b) Glass rod alignment sleeve

 Used in large installations in USA

MULTIPLE SPLICES 

Commercially available for splicing number of fibers simultaneously • Simultaneous Splicing of Five fibers in 5 minutes;

• 15 minutes for five single fusion splicing.  Splice Losses: • Ranging 0.04 to 0.12 dB- MM GI fibers

• 0.13 to 0.4 dB – SM fibers.

A. Silicon Chip Array  Utilize trapezoidal grooves of a silicon chip using a comb structure for fiber laying and top silicon chip

 End faces ground & polished after curing.

Average Splice loss  0.12 dB.

Multiple fibers splicing using a Silicon chip array

B. V-groove flat Chip  Moulded from glass filled polymer resin  Direct mass splicing of 12 fiber ribbons with simultaneous end preparation using ribbon grinding and polishing procedures.

 Fibers positioned in grooves in glass filled plastic substrate.  Vacuum technique to hold fibers at position whilst cover plate is applied.



Spring clips to hold assembly and hole in cover plate for index matching gel.

 Average Splice Losses 0.18 dB with MM fiber.

V-groove polymer resin ribbon fiber splice.

FIBER CONNECTORS  Demountable fiber connectors  More difficult to achieve than fiber splices

 Must maintain similar tolerance , but in a removable fashion.  Must allow for repeated connection and disconnection without problems for fiber alignment - without degradation in performance.

 Must protect the fiber ends from damage – due to handling  Must be insensitive to environmental factors (moisture & dust)  Must cope with tensile load on the cable and can be fitted with relative ease.  Should ideally be a low cost component,

Three Major Parts:  Fiber Termination : protects and locates the fiber ends

 Fiber end Alignment : provide optimum optical coupling  Outer shell : maintains the connection and fiber alignment, protects the fiber ends from the environment and provides adequate strength at the t.

 Losses

in the range 0.2 to 0.3 dB

A. Butt ted Connectors  Alignment of two prepared fiber ends in close proximity (butted) to each other so that the fiber axes coincide.

Cylindrical Ferrule Connector  Glass Ferrules with central drilled hole  Concentric alignment sleeve

• Preparation of fiber ends before fixing the ferrules • Insertion Losses  1 to 2 dB with MMSIF

• Watch jewel for close end approach and tolerance requirement

Ferrule Connectors: (a) structure of a basic ferrule connector; (b) structure of a watch jewel connector ferrule.

Ceramic Capillary Ferrules  Ferrules made from ceramic material  End preparation after fixing ceramic ferrules 

Outstanding

• Thermal, • Mechanical

• Chemical Resistance  Average Losses

 0.2 dB with MMGI ST series multimode fiber connector using ceramic capillary ferrules.

 0.3 dB with SMF

Commonly Used Connectors

FC Connectors

ST Connectors

SC Connector

DIN Connectors

MTRJ Connector

(Spring loaded free-floating Zirconia ceramic ferrule)

SMA Connector

Biconic Connectors

D4 Connectors

Double Eccentric Connector  Does not rely on a concentric sleeve approach  Consists of two eccentric cylinders within outer plug.  An

active assembly adjustable, allowing close alignment of fiber ends

 Operation performed

under inspection microscope or peak optical adjustment.

Connector Structure

 Mean insertion loss  0.48 dB with MMGIFs reduces to 0.2 dB with index matching gel.  Also used with SMFs giving losses 0.46 dB.

Multiple Fiber Connectors  Utilizes V grooved Silicon chips for mounting



Metal guiding rods and coil springs for precise alignment

 Average Losses •  0.8 dB with MMFs • Reduced to 0.4 dB using index matching fluids

(a) Fiber ribbon connector (b) SM Ten fiber connector.

B. Expanded-Beam Connectors  Utilize interposed optics at the t to expand the beam from the transmitting fiber end before reducing it again to a size compatible with the receiving fiber end. 

Collimating and refocusing the light from one fiber into other.

Principle of Operation

 Very attractive for multi-fiber connections and edge connections for PCBs

Lens Coupled Expanded beam connectors  Utilize spherical micro-lenses ( 50 m ) for beam expansion and reduction

(a) Two Micro lenses connector (b) Moulded plastic lens connector

 Average Loss  1 dB, reduced to 0.7 dB with AR coating

GRIN-rod Lenses 

An alternative lens geometry to facilitate efficient beam expansion and collimation

 Arose from development of GI fiber waveguides  A cylindrical glass rod 0.5 to 2 mm in diameter with parabolic refractive index profile.  Light propagation is determined by the lens dimension and wavelength of the light.

 Produce a collimated output beam with divergent angle of 1o to 5o from light source.onto the opposite face of lens

 Traversing of one sinusoidal period : one full pitch

Various fractional pitch GRIN-rod lenses  0.25, 0.23, 0.29 etc.  SELFOC from Nippon Sheet Glass Co. Ltd.

 Losses  1 dB Average Losses

 0.2 dB with MMGI  0.3 dB with SMF

Fiber Reels, Connectors & Patch cords Adapters

Connectors

Patch cords

Fiber Splicing and Connectorization kits

Bibliography: The excerpts of this lecture are based on the information drawn from following references. 1. John M. Senior “Optical Fiber Communications: Principles and Practice,” 2nd edn., PHI, 2001. 2. Gerd Keiser, “Optical Fiber Communica tion” , 3rd edn., Mc Graw Hill, 2000. 3. Djafar K. Mynbaev, Lowell L. Scheiner, “Fiber-Optic Communication Technology”, LPE, Pearson Education Asia, 2002. 4. www.google.co.in

THANK YOU