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1. UNIT II Fiber Couplers and Connectors

2. An optical fiber connector is a flexible device that connects fiber cables requiring a quick connection and disconnection . Optical fibers terminate fiber-optic connections to fiber equipment or join two fiber connections without splicing . Optical fiber connector

3. Basic Fiber Optic Link Transmitter Connector Cable Cable Receiver Splice

4. OPTICAL FIBER JOINTS Ranges from  40-60 km at 400 Mbits/s  100 km at 2.4 Gb/s  300 km at 1.7-10 Gb/s using SMDSFs Technical requirement for both jointing & termination of transmission media . Number of Joints 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)

5. E l e c tr o -opti c al devi c es (S o u r c e s and D e tecto r s) with fi b er o p tic pi g t ail to facilitate direct fiber-fiber connection IMPORTANT ASPECT IS FIBER-TO- FIBER CONNECTION WITH LOW LOSS AND MINIMUM DISTORTION Source- Fiber Fiber- Fiber Fiber- Detector Manufacturers supply FIBER JOINTS

6. Two major categories of fiber joints FIBER SPLICES : Permanent or Semi-permanent joints Soldering FIBER CONNECTORS : Demountable or Removable joints Plugs or Sockets FIBER COUPLERS: Branching devices Splitters or Combiners Importance in Networks

7. Fiber Alignment In any fiber optic communication system, in order to increase fiber length there is need to joint the length of fiber. The interconnection of fiber causes some loss of optical power. Different techniques are used to interconnect fibers. A permanent joint of cable is referred to as splice and a temporary joint can be done with the connector.

8.   The fraction of energy coupled from one fiber to other proportional to common mode volume M common . The fiber-to-fiber coupling efficiency is given as where, M E is number of modes in fiber which launches power into next fiber. The fiber-to-fiber coupling loss L F is given as L F = -10log η F

9. Fiber Ali g nment LOSS MECHANISMS AT JOINTS : 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 joint. Fresnel Reflection

10. Return Loss or reflection loss The return loss RL is a measure of the portion of light that is reflected back to the source at the junction. In optics (particularly in fiberoptics) a loss that takes place at discontinuities of refractive index, especially at an air-glass interface such as a fiber end face. At those interfaces, a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss," or simply "Fresnel loss."

11. Occurs du e to st e p c h anges i n re f racti v e index at jointed interface Glass – Air - Glass Reflection Loss

12. Fraction of light reflected at a single interface : Where, n 1 : R.I. of core, n : R.I. of interfacing medium ( = 1 for air) Loss in decibel due to FR at single interface Loss Fres = -10 log 10 (1-r) Can be reduced to a very low level using index matching fluid in the gap between jointed fibers.  

13. 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 : Core mismatch NA m i sm atc h Index Profile

14. Minimized using fibers manufactured with lowest tolerance i.e.(same fiber) Intrinsic Losses : Losses due to: Fresnel Reflection Deviation in Geometrical & Optical parameters

15. Extrinsic Losses : Losses due to some imperfection in splicing Caused by Misalignment The diameter of fiber is few micrometer hence the microscopic alignment is required. If the radiation cone of emitting fiber does not match the acceptance cone of receiving fiber, radiation loss takes place. The magnitude of radiation loss depends on the degree of misalignment. Mechanical Misalignment

16. Different types of mechanical misalignments are Lateral misalignment 2. Longitudinal misalignment 3. Angular misalignment

17. Lateral misalignment : Lateral or axial misalignment occurs when the axes of two fibers are separated by distance ‘d’. 2. Longitudinal misalignment : Longitudinal misalignment occurs when fibers have same axes but their end faces are separated by distance ‘S’. 3. Angular misalignment : Angular misalignment occurs when fiber axes and fiber end faces are no longer parallel. There is an angle ‘θ’ between fiber end faces.

18. The axial or lateral misalignment is most common in practice causing considerable power loss. The optical power coupled is proportional to common area of two fiber cores. The axial offset reduces the common core area of two fiber end faces as shown in Fig.

19. (a) Insertion loss due to lateral and longitudinal misalignment for a 50 µ m core diameter graded index ( GI ) fiber . (b) I nsertion loss due to angular misalignment for joints in two Multi mode step index fiber ( MMSI ) fibers with NA of 0.22 and 0.3.

20. Fiber Splices A permanent or semi permanent connection between two individual optical fibers is known as fiber splice . And the process of joining two fibers is called as splicing . Typically, a splice is used outside the buildings and connectors are used to join the cables within the buildings. Splices offer lower attenuation and lower back reflection than connectors and are less expensive.

21. Types of Splicing There are two main types of splicing Fusion splicing Mechanical splicing / V groove

22. Fusion Splicing or Welding Fusion splicing involves butting two cleaned fiber end faces and heating them until they melt together or fuse. Fusion splicing is normally done with a fusion splicer that controls the alignment of the two fibers to keep losses as low as 0.05 dB. Fiber ends are first pre aligned and butted together under a microscope with micromanipulators. The butted joint is heated with electric arc or laser pulse to melt the fiber ends so can be bonded together.

23. Fusion Splicers

24. Drawback: Fiber get weakened near splice (  30%) Fiber fracture occurs near the heat-affected zone adjacent to the fused joint. Splice be packaged to reduce tensile loading

25. Protection Sleeves for spliced fibers Protection of Joints Fiber joint enclosures Underground fiber splice tray

26. Mechanical Splicing / V Groove Mechanical splices join two fibers together by clamping them with a structure or by epoxying the fibers together. Mechanical splices may have a slightly higher loss and back reflection . These can be reduced by inserting index matching gel. V groove mechanical splicing provides a temporary joint i.e., fibers can be disassembled if required.

27. Mechanical Splicing Uses accurately produced rigid alignment tubes into which the prepared fiber ends are permanently bonded. Techniques for tube splicing of optical fibers : Snug Tube Splice Loose Tube Splice; Square Cross section Capillary

28. Comparison of Two Approaches Snug Tube Splices Exhibits problems with capillary tolerance requirements Losses  up to 0.5 dB with Snug tube splice (ceramic capillaries) using MMGI and SM fibers. Loose Tube Splices Avoids the critical tolerance requirements. Losses  0.1 dB with loose tube splice using MMGI fibers.

29. Groove Splices V-groove splices Insertion losses  0.1 dB using jigs for producing V-groove splice. Use of grooves to secure the fibers to be jointed better alignment to the prepared fiber ends.

30. Spring Groove Splice Mean Losses  0.05 dB with MMGI Fibers. Practically used in Italy. Springroove Splice : (a) Expanded overview (b) Cross-section Schematic 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.

31. Connectors Connectors are mechanisms or techniques used to join an optical fiber to another fiber or to a fiber optic component. Three different types of connectors are used for connecting fiber optic cables. Subscriber Channel (SC) connector Straight Tip (ST) connector MT-RJ connector

32. Subscriber Channel (SC) connector: SC connectors are general purpose connections. It has push-pull type locking system.

33. Straight Tip (ST) connector ST connectors are most suited for networking devices. It is more reliable than SC connector. ST connector has bayonet type locking system.

34. MT-RJ connector is similar to RJ45 connector.

35. Principles of Good Connector Design Low coupling loss. Inter-changeability – No variation is loss whenever a connector is applied to a fiber. Ease of assembly. Low environmental sensitivity. Low cost – The connector should be in expensive also the tooling required for fitting. Reliable operation. Ease of connection. Repeatability – Connection and reconnection many times without an increase in loss.

36. Connectors use variety of techniques for coupling such as screw on, bayonet-mount, push-pull configurations, butt joint and expanded beam fiber connectors. Fiber is epoxied into precision hole and ferrules are used for each fiber. The fibers are secured in a precision alignment sleeve. Butt joints are used for single mode as well as for multimode fiber systems.

37. Two commonly used butt-joint alignment designs are: Straight-Sleeve. Tapered-Sleeve/Biconical. Straight-Sleeve In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of two fibers.

38. Tapered-Sleeve/Biconical In tapered sleeve or biconical connector mechanism, a tapered sleeve is used to accommodate tapered ferrules. The fiber end separations are determined by sleeve length and guide rings.

39. Connector Return Loss At the connection point of optical link low reflectance levels are desired since the optical reflections can be source of unwanted feed back into the laser cavity. Due to this unwanted feedback the optical frequency response may degrade, also it generates internal noise within the source affecting overall system performance.

40. The return loss for the index-matched gap region is given by,   Where, d is the separation between fiber ends. n 1 is index-matching material. R is reflectivity constant.

41. FIBER CONNECTORS Demountable fiber connectors More difficult to achieve than fiber splices Must maintain similar tolerance requirements, 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 ( e.g. moisture & dust) Must cope with tensile load on the cable and can be fitted with relative ease. Should ideally be a low cost component ,

42. 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 joint. Three Major Parts : Losses in the range 0.2 to 0.3 dB

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

44. B. Expanded-Beam Connectors Utilize interposed optics at the joint in order to expand the beam from the transmitting fiber end before reducing it again to a size compatible with the receiving fiber end.

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

46. Ceramic Capillary Ferrules ST series multimode fiber connector using ceramic capillary ferrules. Ferrules made from ceramic material End preparation after fixing ceramic ferrules Ou t st a n d ing The r m a l , Mechanical Chemical Resistance Average Losses  0.2 dB with MMGI  0.3 dB with SMF

47. Commonly Used Connectors FC Connectors ST Connectors SC Connector

48. DIN Connectors (Spring loaded free-floating Zirconia ceramic ferrule) MTRJ Connector SMA Co n nector Biconic Connectors D4 Connectors

49. Biconical Connectors Mean insertion losses  0.21 dB with connectors of 50  m diameter GI fibers. Cross-section of biconical connector Widely used as part of jumper cable Fiber end faces polished after plug attachment

50. Double Eccentric Connector Mean insertion loss  0.48 dB with MMGIFs reduces to dB with index matching gel. Also used with SMFs giving losses 0.46 dB. 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

51. Duplex Fiber Connector Developed to provide two way communications Uses ferrules of different types Mostly used in LANs Comme r c i a lly av a il a ble for use in FDDI  loss of 0.6 dB. Media interface plug with DFC

52. Multiple Fiber Connectors 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. Utilizes V grooved Silicon chips for mounting

53. EXPANDED BEAM CONNECTORS Collimating and refocusing the light from one fiber into the other. Principle of Operation Very attractive for multi-fiber connections and edge connections for PCBs

54. Lens Coupled Expanded beam connectors (a) Two Micro lenses connector (b) Moulded plastic lens connector Average Loss  1 dB, reduced to 0.7 dB with AR coating Utilize spherical micro-lenses ( 50  m  ) for beam expansion and reduction

55. 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 1 o to 5 o from light source.onto the opposite face of lens

56. Ray propagation determined by paraxial ray equation d 2 r  1 dn dz 2 n dr Solution is r = k 1 cos A 1 / 2 r + k 2 sin A 1 / 2 r : A sinusoidal path Traversing of one sinusoidal period : one full pitch GRIN-rod Lenses

57. Various fractional pitch GRIN-rod lenses 0.25, 0.23, 0.29 etc. S E LFOC f r om Nippon Sheet Glass Co. Ltd. Losses  1 dB Average Losses  0.2 dB with MMGI  0.3 dB with SMF

58. Fiber Reels, Connectors & Patch cords Connectors Ada p ters Patch cords

59. Fiber Splicing and Connectorization kits