Fiber Joints and Couplers; Cable Design. Dr. Mohammad Faisal Dept. of EEE, BUET

Transcription

1 Fiber Joints and Couplers; Cable Design Dr. Mohammad Faisal Dept. of EEE, BUET

2 Fiber Joints and Couplers For fiber-fiber connection we need joints which are of two major types Fiber splices: these are semipermanent or permanent joint (like electrical soldered joints) Demountable fiber connector or simple connector: these are removable joints, easy and fast coupling and uncoupling of fibers (like electrical plugs and sockets) Purpose: fiber to fiber joints are designed to couple ideally all the light in one fiber to adjoining fiber

3 Why Fiber Joints and Couplers are necessary? Optical fiber links require connection in order to make long fiber or for fiber coupling to sources or amplifier or termination to receiver The number of intermediate fiber connections or joints depends on link length between repeaters

4 The Main Concern of Joints Optical power loss at the joint is the main concern The major categories of loss: Intrinsic joint loss: 1) Loss due to Fresnel reflection Fresnel reflection loss is related to step changes in refractive index at the jointed interface. even when two jointed fiber ends are smooth and perpendicular to fiber axis and two fiber axes are perfectly aligned, A small portion of light reflects back into the transmitting fiber. The amount of this partial reflection can be estimated by using classical Fresnel formula: r n n 1 1 n n 2 r is the fraction of light reflected at a single interface n 1 is the core refractive index n is refractive index of the medium between two Jointed fibers, say for air n=1

5 The loss due to Fresnel reflection at a single interface LossFres 10log 1 r db The effect of Fresnel reflection at a fiber-fiber connection can be reduced by using an index-matching fluid between the gaps. 2. Loss due to deviations in the geometrical and optical parameters of the two jointed fibers. The problems may occur for (i) Different core and/or cladding diameters; (ii) Different NA and/or relative refractive index differences; (iii) Different refractive index profiles; (iv) Fiber faults: core ellipticity, core concentricity etc Use same fibers to keep this loss minimum Loss due to misalignment A major source of loss at a fiber-fiber connection is caused by misalignment of two jointed fibers 10

6 a) Longitudinal misalignment b) Lateral misalignment c) Angular misalignment

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8 Fiber Splices Permanent and semi permanent joint between two optical fibers: (1) Fusion Splicing (Permanent) (2) Mechanical splicing (semi permanent) (1) Fusion splicing is accomplished by applying localized heat (by flame or electric arc) at the interface between two butted, prealigned fiber end causing them to soften and fuse. Fibers are positioned and clamped with the help of inspection microscope. (2) In mechanical splicing fibers are held aligned by some mechanical means, like, using tubes around fiber ends or V- grooves into which butted fibers are placed.

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10 (1) splice loss: 0.1 db-0.2db weak in the vicinity of splice, tensile strength reduces Mechanical Splices: (i) tube splice and (ii) groove splices (i) tube splice: Snug tube splice; Transparent adhesive is

11 Injected through a transverse bore in the capillary to give mechanical sealing and index matching of splice Insertion loss: 0.1 db 0.5 db (ii) groove splices In V-groove two fibers are jointed V-groove splices are formed by sandwiching the butted fiber ends. Insertion loss: 0.1 db

12 (i) tube splice and

13 Fiber Connectors Demountable fiber connectors are more difficult to achieve than fiber splices: Because: In order to maintain similar tolerance requirements to splices and it is difficult to maintain in removable fashion Require repeated connection and disconnection without problem of alignment To maintain optimum performance, fiber ends require to be protected from damage due to frequent handling To make insensitive to environmental factors (e.g. moisture and dust) (i) Therefore THREE MAJOR CONSIDERATIONS: The fiber termination which protects and locates the fiber ends (ii) End alignment to provide optimum optical coupling (iii) Outer shell maintains the connection and fiber alignment, protects the ends from environments and stress

14 Cylindrical Ferrule Connector: Basic Ferrule Connector (The simplest fiber connector)

15 Two fibers are permanently bonded (with epoxy resin) in metal plugs known as ferrules (two ferrules) which have an accurately drilled central hole in their end faces where stripped fiber is located The two ferrules are placed in alignment sleeve which allows fiber ends to be butt jointed The ferrules are held in place via a retaining spring Some ferrule connectors have incorporated a watch jewel in the ferrule end face in order to have fiber alignment accuracy In this case the fiber is centered with respect to the ferrule through the watch jewel hole. The use of watch jewel allows close diameter and tolerance requirements of the ferrule end face hole to be obtained more easily than simply through drilling of metallic ferrule end face alone. Insertion Loss: 1 ~ 2 db

16 List of common fiber connector types used for both multimode and single-mode fiber systems Multimode connectors are generally used for data communication (LANs), transport (automobiles and aircraft), test instrument Single-mode connectors in optical fiber telecommunication systems

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18 Fiber Couplers Fiber Couplers are branching devices that split all the light from a main fiber into two or more fibers Or Alternately couple a proportion of light propagating in the main fiber into a branch fiber they can combine light from two or more branch fibers into a main fiber Optical fiber couplers are passive devices in which power transfer takes place in two ways: (i) Core-interaction type: through fiber core cross-section by butt jointing the fibers (ii) Surface-interaction type: through fiber surface and normal to its axis by converting the guided core modes to both cladding and refracted modes

19 Power Transfer in Couplers

20 Optical Fiber Coupler Types

21 Functions of Multiport Couplers (1) Three and four-port couplers: For signal splitting, distribution and combining (2) Star Couplers: for distributing a single input signal to multiple outputs (3) WDM devices: are specialized couplers designed to permit a number of different wavelength channels to be transmitted in parallel in a single fiber WDM MUX: combine/multiplex different wavelength channels onto a fiber WDM DeMux: separate different wavelength channels output from a fiber

22 Three and Four-Port Couplers (a) Lateral Offset Method: -Fiber end faces are overlapped, Light from input fiber is coupled to output fibers according to degree of overlap. -Input power can be distributed in a welldefined proportion by appropriate control of lateral offset between fibers. -Suitable for multimode fibers but with high excess loss. (b) Semitransparent Mirror Method: -A beam splitter element between the fibers; A partially reflecting surface is placed to the end face cut at an angle 45 o to form a thin beam splitter -The input power may be split in any desired ratio between the reflected and transmitted beams - Typical excess loss: 1~2 db, suitable for both MMF and SMF

23 3-Port Coupler Based on Micro-optic Components These couplers utilize the beam expansion and collimation properties of graded index (GRIN) rod lens combined with spherical retro-reflecting mirrors (which reflects light back to its source) It consists of two quarter pitch lenses with a semitransparent mirror in between Light rays from input fiber F 1, collimate in the first lens before incident on mirror. A portion of incident beam is reflected back and is coupled to fiber F 2. The transmitted light is focused in 2 nd lens and then coupled to F 3. Parallel surface type is more attractive because of ease of fabrication, compactness, simplicity and relatively low insertion loss For both, insertion loss <1dB

24 Fused Biconical Taper (FBT) Method Most common method of manufacturing couplers Fibers are twisted together and then spot fused under tension such that the fused section is elongated to form a biconical taper structure A three port coupler is formed by removing one of the input fibers

25 Optical power launched into the input fiber propagates in the form of guided core modes. The higher order modes leave the fiber core because of its reduced size in the tapered-down region and therefore guided as cladding modes. These modes transfer back to guided core modes in the tapered-up region of the output fiber with approximately even distribution the two fibers Various Loss Parameters Associated with Four-Port Coupler Excess Loss (Scattering loss): ratio of power input to power output P1 Excess Loss 10log10 db P3 P4 Insertion Loss (optical loss through connection): loss obtained for a particular port-to-port optical path P1 Insertion Loss (port 1 to 4) 10log10 db P4 P1 Insertion Loss (port 1 to 3) 10log10 db P 3

26 Cross talk: the ratio of back scattered power received at the 2 nd input port to input power P2 Crosstalk 10log10 db P Splitting or Coupling ratio: it indicates the percentage division of optical power between the output ports Star Coupler It distributes an optical signal from a single input fiber to multiple output fibers Two main techniques: Mixer rod method FBT method 3 Split Ratio 100% P P3 P4 1 P P3 P %

27 A thin platelet of glass (mixer rod) is used which efficiently mixes the light from one fiber, dividing it among outgoing fibers

28 FBT method: The fibers which constitute the star coupler are bundled, twisted, heated and pulled to form the coupler. Highly mode dependent which results in a relatively wide port-to-port output variation In an ideal star coupler, the optical power from any input fiber is evenly distributed among output fibers

29 Ladder Coupler: -This is an alternative technique to construct a star coupler. - The ladder coupler comprises a number of cascaded stages, each incorporating three or four-port FBT couplers coupler consists of three stages which gives 8 output ports - If three port FBT coupler, then 1 N coupler - If four port FBT coupler, then N N coupler - It has relatively low insertion loss - Widely used for single-mode fiber star coupler

30 Wavelength Division Multiplexing Couplers WDM Couplers: are specialized devices which enable light from two or more optical sources of differing nominal peak optical wavelength to be launched into a single optical fiber The spectral performance characteristic for a typical five-channel WDM device is shown below

31 The important parameters associated with WDM coupler: Attenuation of light over a particular wavelength band: should be low The interband isolation to minimize crosstalk: should be high but channel separation be as small as may be permitted Wavelength band (channel BW) Diffraction grating type WDM Coupler Diffraction grating is an angularly dispersive element which reflects light in a particular direction according to spacing of the grating The angle at which light is incident on the grating optical wavelength In Littrow type grating, the blaze angle is such that incident and reflected light beams follow virtually the same path.

32 A blazed grating is a special type of diffraction grating. Blazed gratings produce maximum efficiency at a specified wavelength; that is, a diffraction grating that is "blazed at 250nm" will operate most efficiently when light with a wavelength of 250 nm passes through the grating. Like standard diffraction gratings, blazed gratings diffract incoming light using a series of grooves. However, in blazed gratings the grooves have been manufactured such that they form right angles with a specified "blaze angle," which is the angular distance from the surface normal of the diffraction plate. The magnitude of the blaze angle determines the wavelength at which the grating will be most efficient.

33 A diffraction grating consists of a series of equally spaced parallel grooves formed in a reflective coating deposited on a suitable substrate. The distance between adjacent grooves and the angle the grooves form with respect to the substrate influence both the dispersion and efficiency of a grating. If the wavelength of the incident radiation is much larger than the groove spacing, diffraction will not occur. If the wavelength is much smaller than the groove spacing, the facets of the groove will act as mirrors and, again, no diffraction will take place. d is the grating constant (the distance between successive grooves), i is the angle of incidence measured from the normal and i' is the angle of diffraction measured from the normal.

34 Two main structural type: (i) Littrow device employing lens and a separate plane grating (ii) Concave grating Littrow type grating demux (a) using a conventional lens (b) using a GRIN-rod lens Single input fiber and multiple output fibers are arranged on the focal plane of the lens Lens is used to collimate the optical beam. The input light is collimated by lens and hence transmitted to the diffraction grating which is offset at the blaze angle so that the incoming light is incident virtually normal to the groove faces. On reflection from the grating the diffraction process causes the light o be angularly dispersed according to wavelength Finally the diffracted λs pass through the lens and are focused onto different output collecting fibers.

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36 Arrayed Waveguide Grating (AWG): passive optical MUX, DeMUX coupler using diffraction grating mechanism. Can perform MUXing and DeMUXing operation in DWDM network with narrow channel spacing An AWG comprises of a number of waveguides with different lengths converging at the same point(s) WDM Devices

37 optical signal passing through each of these waveguides interfere with the signals passing through their neighboring waveguides at the convergence points depending upon the phase difference of interfering signals (constructive or destructive) an optical signal at a desired wavelength can be obtained at device out put AWG: Five elements: Input/output waveguides (WG) Arrayed waveguides of different lengths Two focusing slab waveguides The basic operation is carried out in the two focusing slabs: each of them acts as a multimode interference coupler or a free space propagation region When a WDM signal is coupled to input WG, this signal propagates through input WG slab region where it illuminates the grating by splitting the optical signal into each arrayed WG with Gaussian distribution.

38 Curved arrayed waveguides are preferred to produce waveguide channels over a suitable distance Optical signals travel down the WG array to the other WG slab Since each arrayed WG exhibits a different path length then the optical wavefronts reach the input ports of 2 nd slab out-ofphase with one another 2 nd slab (output slab) performs as a combiner the overall AWG becomes a WMD demultiplexer Each input signal from arrayed WG interferes with all others within the output slab WG. As a consequence of constructive interference each single wavelength signal present in the original WDM signal will be coupled into exactly one of the output waveguides. AWG is a passive device which can be operated as a multiplexer when operating in opposite direction

39 Optical Isolator It is essentially a passive device which allows the flow of optical signal power (for a particular wavelength or a wavelength band) in only one direction preventing reflection in backward direction. Optical isolators can be implemented by using FBGs (FBGs are wavelength dependent which can be designed to allow or block the optical signal at a particular or a range of wavelength/s) or by using Magneto optic devices

40 FBG: Fiber Bragg Grating is an optical fiber component having a periodic variation in the refractive index of its core along the fiber length. An FBG acts as a highly wavelength selective reflector, with a high reflectivity at a given central wavelength. The central wavelength, the peak value of reflectivity and the bandwidth of reflection spectrum depends on the period of refractive index modulation, on strength of index modulation grating and length of grating

41 Circulator An optical circulator is a multiport device with nonreciprocal transmission characteristics. When light enters port 1 of circulator, it exits through port 2. If light enters port 2, instead of emerging from port 1, it emerges from port 3. Isolators can be connected to form circulator P P1 2 Insertion loss, IL 10 log ;(db) P P2 3 Cross talk, CT 10log ;(db)

42 Isolators are interconnected to form a 3-port device which does not discard backward reflections but directs them to another isolator. No connection is usually permitted between port 3 and port 1. Commercially available circulators have 1dB insertion loss and high isolation in the range of 40 to 50 db centered at 1.3 and 1.5 μm

43 Optical Add/Drop Multiplexer (OADM) OADM is used for adding and dropping of optical channels in a fiber link while preserving the integrity of other channels An FBG is placed with a central wavelength λ 1 at port 2, if light at λ 1 and λ 2 are incident on port 1 of circulator, then out of 2 wavelengths exiting from port 2, FBG reflects back λ 1. this wavelength propagates back towards port 2 of circulator and exits from port 3 while the wavelength λ 2 continue to propagate along port 2. Thus it acts as a drop filter. In the right side, it is adding λ 1 to the network.

44 λ 1 is dropped in circulator 1, then another optical signal at this wavelength can be added at circulator 2. Multiple channels can be dropped and/or added by using a combination of an FBG and optical circulators.

45 Cable Design Optical fibers are required to be safely installed and maintained in all environmental conditions. Unprotected optical fibers (only core and cladding) have many disadvantages with regard to strength and durability Bare glass fibers are brittle and very susceptible to damage due to environment or mechanical stress and strain. So it is necessary to cover them to improve tensile strength, to protect them against external influences

46 The Function of Fiber Cable and Related Concerns Fiber protection: to protect against damage, and breakage during installation and maintenance Stability of fiber characteristics: Cabled fiber must maintain good stable characteristics compared with uncabled fiber Cable strength: Must maintain similar mechanical strength as electrical power cable. Mechanical properties like tension, torsion, compression, bending, squeezing, and vibration. Cable strength may be improved by using extra strength member These can be achieved by surrounding the fiber with a series of protective layers which is referred to as coating and cabling.

47 Cable Design Considerations A number of major considerations: (i) Fiber Buffering: primary coating during production (typically 5 to 10 μm Teflon) in order to prevent scratch of glass surface and subsequent flaws in material. Primary coated fiber is given secondary or buffer coating (jacket) to provide protection against external mechanical and environmental influences, to reduce microbending losses

48 Tight buffer jacket: consists of hard plastic (nylon, hytrel, tefzel) and is direct contact with primary coated fiber with a typical diameter of 250 μm. Loose tube buffer jacket: it provides an oversized cavity in which fiber is fiber is placed which mechanically isolates the fiber from external forces. It is usually achieved by using a hard, smooth, flexible materials (polyester or polyamide) in the form of an extruded tube with an outer diameter of typically 1.4 mm. Filled loose tube buffer: the oversized cavity is filled with a moisture resistant compound. The filling material must be soft, self-healing and stable over a wide temperature range (-30 to +70 degree Celsius), e.g., specially blended petroleum or silicon-based compounds. Buffer size could be larger like 1.8 to 3.5 mm to accommodate multiple fibers

49 (ii) Cable Structural Member: one or more structural members are provided to serve as a cable core foundation around which buffered fibers are wrapped or slotted. Structural me may be nonmetallic with plastics, fiber glass or Kevlar (iii) Cable Strength Members: have high tensile strength. The structural member may be strength member if it consists of suitable material such as solid or stranded steel wire, dielectric yarns or Kevlar (polyester). In this case the central structural member is the primary load-bearing element. (iv) Cable Sheath: cable is covered with Plastic sheath in order to reduce abrasion and to provide the cable with extra protection against external mechanical effects. Polyethylene (PE) sheath is commonly used. (v) Water Barrier: The ingress of water may be prevented by filling the spaces in the cable with moisture-resistant compounds. Specially formulated silicon rubber or petroleum-based compounds are often used.

50 Typical Structure of SI SMF and MMF Core dia: 5 to 10 μm Cladding dia: ~ 125μm Buffer Jacket dia: 250 to 1000 μm NA: around 0.1 α: 2 to μm, μm and 0.35 μm Suited for high BW (>500 MHz-km), medium and long-haul applications

51 Core dia: 50 to 400 μm Cladding dia: 125 to 500 μm Buffer Jacket dia: 250 to 1000 μm NA: around 0.16 to 0.5 α: 2 to μm, μm and 0.4 μm Best suited for short-haul, limited BW (50 MHz-km) and relatively low cost applications

52 Some Typical Cable Structure

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54 Multifiber cable with central steel wire structural and strength member Description/Characteristics: Suitable for SMF or MMF. High strength loose tube made of a high modulus plastic Tubes are filled with waterresistant filling compound Steel wire located at the center as structural and strength member Tubes are stranded around strength member Polyethylene Steel Polyethylene (PSP) is longitudinally applied over the cable core and then the core is filled with filled with filing compound to protect it from water ingress. The cable is then completed with a PE sheath.

55 Description/Characteristics: Suitable for SMF or MMF. High strength loose tube made of a high modulus plastic Tubes are filled with waterresistant filling compound Steel wire (sometimes sheathed with PE) located at the center as structural and strength member Tubes are stranded around strength member An Aluminum Polyethylene Laminate (APL) is applied around cable core which is filled with filling compound to protect water ingress. APL as moisture barrier. PSP is enhancing moisture proof, also enhances crush resistance Stranded Loose Tube Cable with Aluminum Tape and Steel Tape (Double Sheath) Application: Duct/Aerial

56 Stranded loose tube cable with aluminum and steel tape plus non-metallic central strength member (Double sheaths) A piece of Fiber Reinforced Plastic (FRP) locates in the center of core as a non-metallic strength member Application: Duct/Aerial/Direct Buried

57 Loose Tube Corrugate Steel Armored Fiber Optic Cable It has high tensile strength for long spans Stranded wires (7 1.6mm) are load bearing and gives high tensile strength Cable can carry up to 144 fibers High stregth loose tube with special tube filling compound for water resistance

58 A steel wire or a piece of Fiber Reinforced Plastic (FRP) locates in the center of core as a non-metallic central strength member HDPE jacket for weather and UV protection Rip cord, of an optical cable, a parallel cord of strong yarn that is situated under the jacket(s) of the cable for jacket removal Application: Best for Aerial and outdoor application. Telecom data link, long-haul networking, or campus/office building interconnections Tube filling compound: Tubes are filled with thixotropic Jelly and necessary plastic fillers to have circular cable core. As a lubricant and water resistant. Cable core filling compound: Petroleum jelly, Paraffin wax, Polyolefin wax etc.