94639

RELEASABLE AND PERMANENT CONNECTIONS OF FIBERS

Лабораторная работа

Коммуникация, связь, радиоэлектроника и цифровые приборы

Standard quartz multimode fibers have core diameter of 50 microns singlemode fibers - 4.1 Permanent connections Permanent optical connections splices serves for permanent junction of optical fibers. Releasable optical connections connectors are used for multiple junction-disjunction of fibers.

Английский

2015-09-15

357 KB

0 чел.

Laboratory work № 2

RELEASABLE AND PERMANENT CONNECTIONS OF FIBERS

1 PURPOSE OF WORK

To study methods of carrying out releasable and permanent connections of optical fibers, to measure insertion loss|what| and signal power|capacity| in| permanent connections.

2 KEY STATUTES

When building cable consturctions of fiber-optic transmission systems (FOTS), optical connectors are necessary  in order to link up all the equipment of terminal and intermediate stations with optical cable as well as join cable factory lengths. As a rule, in the first case releasable optical connectors are utilized, whereas in the second one permanent connections find a use.

There are following basic requirements for|main||claiming||by|| optical junctions|downstream|: |mini|low attenuation and reflection of radiated power, high mechanical strength, insensibility to effect of environmental factors, simpleness in carrying out, small price, possibility to carry out junctions in field conditions.

Difficulties in joining OF are caused by their small size. Standard quartz multimode fibers have core diameter of 50 microns (singlemode fibers - 4...12 microns), when the diameter of light-reflecting cladding is 125 microns.

 2.1 Permanent connections

Permanent optical connections (splices) serves for permanent junction of optical fibers. Releasable optical connections (connectors) are used for multiple junction-disjunction of fibers. The basic methods of splicing include welding (fusion) of ends of fibers or fixating them coaxially in a calibrator. Fixation of fibers being joined can be provided by adhesive material, mechanical pressure or both of them.

2.1.1 Fiber joining by welding

Splices are carried out by welding two glass fibers (Figuer 2.1). To fuse the butts of fibers electric arc is used in devices for fiber’s welding. Fiber ends are prepared for welding according to method of incision and subsequent cleavage. Then it is necessary to attempt to obtain coaxiality of fibers by regulating fiber position by use of three-dimensional micromanipulators. Positioning is controlled visually by microscope or any other mean of zoom.

Accuracy of alignment can be also checked up by controlling power which passes through the joint before the fibers are welded. In case transmitter and receiver are located far from the place of joint (for example a few haundreds metres and more), than such a measurement might be difficult and take a lot of time. The problem’s solution is method of light injecting and detection (LID). When using LID method, light is injected into one of the fibers being joining at the small distance away from joint (10...20 cm) and brought out to be detected from other fiber (at the same distance). Light injection into a fiber and outlet it from another one is carried out at fiber bend around a cylinder with small radius. The bend is rather strong (the radius is usually  about a few millimeters) so that the energy can be injected into the fiber when an illuminant is placed at the point of bend of source fiber and a photoreceiver nearby the bend of the outlet fiber. In most singlemode fibers buffer covering is transparent so that there isn’t any necessary to remove it when LID method is used. In some cases fiber coverings are coloured (labeled) by dye for identification, which can be non-transparent. The dye at the bends should be removed. To attain it dissolvent of acetone type is used.

In welding surface tension forces provide alignment of fiber axes and in this way they minimize transverse displacement. Splices which are carried out by using  conventional welding devices provide loss less than 0.25 dB. A skilled welder using a modern welding device can make a joint with loss less than 0.1 dB. On carrying out a splice the phase of recovering of protective (polymeric) coating arises. Bare section of welded fibers is enveloped (protected) by either epoxy resin or thermocontracting tube. Joining fibers by welding them is used for quartz fibers (both multimode and singlemode ones).

2.1.2 Joining fibers by using adhesive

A lot of calibrators in which adhesive material is employed to join fibers was proposed and taken in use.  The constructions of 4 of them are shown in Figure 2.2. Each of the structures mechanically adjust fibers and then provide them to be joined reliably. Fibers are fixed in set position by epoxy resin. So far as certain time is needed for epoxy resin to solidify enough, these connectors can’t be used instantly. Setting time of resin can be reduced by high-temperature influence or, for some epoxy resins, by ultraviolet radiation.

V- slot (Figure 2.2, a) is the basis of the simplest mechanical connector. Bared fibers which are being joined are placed in the notch. In this case high-quality angular alignment is attained. Fibers can slide along the notch to meet each other until touch. Then they are fixed in the position by epoxy resin. On solidifying adhesive error though the clearance between fiber butts is minimal. If reflective index of epoxy resin is matched with  reflective index of joined fiber cores, then even small clearance between the ends doesn’t cause high loss. There will be slight in-plane shear in the notch in case either fiber has the same diameters of core and cladding and cores are placed concentricly against fiber claddings. Core shift against cladding (eccentricity) can be revealed by rotating a fiber around the axis when control of passing power is performing.

Fibers with identical cross sizes and without core eccentricity would produce the same output power in any position of the input fiber during its rotation. None of the connectors shown in Figure 2.2 can provide core eccentricity compensation. A protective layer might be applied on an V slot’s surface to provide additional protection of a joint.

A precision capillary (Figure 2.2, b) has central aperture for lead-in of fibers in buffer covering. Capillary’s ends are slightly expanded to simplify inputting fibers. Epoxy resin with matched load can be spread on the fiber butts before inputting to the capillary. Some capillaries have a side aperture intended for observing the butts of contacting fibers and injecting drop of epoxy resin or special liquid with matched index of refraction (immersion liquid). There are metallic, plastic or glass capillaries. There are splicing devices which use plastic as material for capillaries. In case fibers are inserted to an aperture with slightly reduced diameter, elastic material force both of fibers align with respect to central general axis. Even fibers with different diameters of claddings can be coaxial in such an elastic capillary.

In connectors with free tube (Figure 2.2, c) fibers are freely placed in a rectangular tube. Bend of fibers make theirs ends move inside the tube and place alongside one of the tube’s inner arrises. Being aligned in such a way fibers are fixed by epoxy resin.

Three precision rods made of glass of metal are also applied to align fibers (Figure 2.2, d ). Rod’s diameters are selected to make the aperture between cylinders a bit larger than diameters of joined fibers (with claddings). On inserting fibers to the aperture and joining them to ends’ touch epoxy resin with matched index of refraction is used. Then the thermocontracting tube is to be laid over the adjuster. On performing warming and cooling-down of the tube it provides hold of the rods and presses fibers against them.

2.2 Releasable connections of fibers (cables)

To perform effective releasable connection it is necessary to meet strict mechanical limits. It conditions optical connectors to be complicated in designing and expensive in producing. There are basic requirements to high-quality connector given.

2.2.1 Low loss. The construction of such connectors must guarantee error of positioning to be minimized automatically when joining respective parts of connector. Some possibility exists that position of joint is inaccessible for observing and correcting errors, in this case positioning can’t be performed. If there are a few connectors along the line, then each of them need to be high-quality one. For example if 5 connectors are used and each of them has attenuation of 2 dB, then loss totals 10 dB. In this case received signal power is diminished by a factor of 10 dB.

2.2.2 Insertion loss repeatability.  Effectiveness of connection shouldn’t vary widely when connection-disconnection is performed.

2.2.3 Predictability. Identical effectiveness of power transmission should be obtained in case the same combinations of fibers and connectors are used, i.e. power loss  shouldn’t depend on qualification of an erector.

2.2.4 Long operability life. Connecting-disconnecting fibers shouldn’t impair effectiveness of power transmission or strength of connection. Attenuation of light in connectors shouldn’t alter after with the lapse of time.

2.2.5 High strength. Effectiveness of connection shouldn’t impair in consequence of increment of load on connector’s shell or tension of fibers.

 2.2.6 Compatibility with environmental condidtions. Connectors need to withstand variation of temperature and humidity, chemical influence, smear, pressure drop, vibrations.

2.2.7 Simplicity of performing connections. Preparing fiber and mounting it into plug should not be difficult and take a lot of time.

2.2.8 Easiness of usage. Process of connection-disconnection of connector’s respective parts should be easy.

2.2.9 Efficiency. High-precision connectors are expensive. Cheaper connectors are usually made of plastic, they have worse parameters though.

Most of connectors implement method of tight joint. Fibers’ butts  are placed as close to each other as it is possible. The main options of tight joint are: 1) straight bushing and 2) conic bushing. Lens connector [1] is an alternative of tight joint. Structures of connectors given underneath explain general approaches which are used in conventional releasable connections. The descriptions don’t give complete insight of all the details of concrete connectors but include theirs basic elements.

Tight connector consists of two plugs (one for each fiber) and one precision adjusting bushing where the plugs are joined. Figure 2.3 explains structure of connector with straight (cylindrical) bushing. Some plugs of optial connectors with straight bushing are designed alike SMA-connectors for coaxial cables (see underneath). Axial (crosscut) and angular alignment is obtained by coupling smoothly butts of plugs inside a cylindrical bushing. Obviously strict tolerance limits are necessary. Size of clearance between fiber’s butts is defined by length of the part from the butt to the projecting end, which sets against the butt of the adjusting bushing and by length of the latter. Connector is fixed bodily by cover nuts. According to Figure 2.3 cable outer shell is fixed inside a plug's tube by epoxy resin, which provides durability of the whole connection. In alternative construction (Figure 2.4) aramid fibers  are mounted by crimping tube, which provides additional breaking strength. Tension of cable passes to aramid fibers, not to fragile optical fibers and thus integrity of the latter is provided.

Connector with conical (biconical) bushing which longitudinal section is shown in Figure 2.5 consists of plastic or ceramic biconical bushing which takes and turns two conical plugs. Slight abrasive wear takes place in such a construction when connecting-disconnecting them. Cable shells are joined to the plugs by the use of adhesive material as it is implemented in connector with straight bushing. Clearance between ends of fibers being joined is defined by elements of mechanical construction (Figure 2.5). Limiting ring inhibits plugs from approaching at the distance when the butts can be damaged. If the ring doesn’t restrict plugs penetration into the bushing (for example in case length of the adjusting bushing in Figure 2.5 is too short), then the clearance between fiber ends will depend on compression force, which appears in screwing cover nut when connector is being assembled.

2.3 Classes of releasable connectors

Appearance of male part of the most widespread optical connectors is shown in Figure 2.6. Let us consider them.

 Fiber connector (FC)

This class of connectors was designed in Japan in the 80s for multimode and singlemode fibers. To fix connector captive nut is used. Ceramic plugs are 2.5 mm over 4 mm long.

Such connectors are designed to connect-disconnect up to 1000 times. Flat finish of plug’s butts was performed in first modifications, which led to high insertion and return losses. A new method of “physical contact” is employed in modern models ( FC-РС), which consists in spherical finishing plug’s butts. It provides insertion loss of 0.3 dB and return loss no less than 40 dB for singlemode fibers.

D4 connector

These connectors have analogy with FC ones with exception of length of plugs fixing singlemode or multimode fibers is 2 mm. They are designed for 1000 actions of connection-disconnection.

 SТ connector (Straight tip ).

It was designed in 1985 for multimode and singlemode fibers. These connectors are the most widespread. Bayonet fixing mechanism and ceramic, metallic or plastic plugs 2.5 mm over are used in the connectors. Insertion loss is 0.3 dB for ceramic plugs (for plastic ones it is 0.7 dB). Return loss is no less than 40 dB for singlemode fibers.

Subscripter connector ( SC)

These connectors are easily joined-disjoined. To disjoin them pushing special lever or button is enough. They are used both for multimode and singlemode fibers and designed for 1000 actions of connection-disconnection. Insertion loss is about 0.3 dB, return loss is no less than 40 dB. Nowadays subscriber connectors are the most popular and exclude FC and D4 connectors as from telecommunications so from computer networks. Meanwhile they are not so commonly used as ST connectors.

 SMA connector (Sub-miniature type A – subminiature microwave coaxial connector of type A. Later it was modified for the purpose of joining optical fibers, sometimes therefore letter ‘F’ (fiber) is added to its name: FSMA. Connectors were developed in the 70s for multimode fibers and later for singlemode ones. To fix fibers threaded screw mechanism is used. The plug is cylindrical 3.2 mm over for SMA-905 and cylindrical stepped with the thickest part 3.0 mm over for SMA-906. Insertion loss is about 1.5 dB (30%). ( One reason why these connectors are so popular is that they meet severe standards of U.S. Army).

Biconical connector (BIC)

There is used an adjusting barrel in the connector, which intakes two conical plugs and directs them. Such a construction conditions extra abrasive wear when joining-disjoining it.

MIC connector (Medium interface). It is also known as FDDI-connector (it isn’t shown in Figure 2.6). This is a double (duplex) connector designed specially for FDDI networks (Fiber distributed data interface). It provides simultaneous connection of two singlemode or multimode fibers, which are used to create basic and reserve rings of FDDI network. Jointing mechanism is used in MIC connectors, which is fixed similarly to SC connectors. Such connectors are designed for approximately 500 connections-disconnections. Insertion loss is about 0.5 dB and 0.3 dB respectively for multimode and singlemode fibers. Return loss is 35 dB.

Enterprise system connector (ESCON). It is not shown in Figure 2.6. It is similar to MIC-connectors and meant for using in FDDI networks, however it has a movable case which simplifies operation of connection-disconnection of transceivers. It disadvantage is lower reliability. Such connectors are designed for approximately 500 connections-disconnections and provide insertion loss about 0.5 dB and return loss of 35 dB.

Basic parameters of releasable optical connectors are given in the Table 2.1

Table 2.1 – Parameters of optical connections

Class

OF type, material of plug

Insertion loss, dB

Return loss, dB

Number of connections-disconnections

FC/PC

sm, mm

0,3

40

1000

ST

ОМ

0,3

40

1000

mm

Ceramic

0,3

-

1000

noncorrosive steel

0,6

0,7

-

1000

Plastic

-

250

SC

sm/mm

0,3

40

1000

SMA

mm

1,5

-

200

ВIC

sm, mm

1,0

40

500

MIC (FDDI)

sm

0,3

35

500

mm

0,5

-

500

ESCON

mm

0,5

35

500

DNP

mm OF p/p type

2,0

-

-

splice

0,2

40

-

Note. sm, mm are abbreviations for respectively singlemode and multimode optical fibers, p/p – fiber of polymer-polymer type. Ceramic plugs are used in singlemode connectors of all types.

Figure 2.6 – Appearance of plug couplers of optical connectors

2.4 Causes of loss in fiber joints

Causes of extrinsic-joint loss are shown in Figure 2.7. An accurate joint must be performed without in-plane shear and angular difference, clearance between touching fiber ends.

In joined fibers internal loss occurs. Power transfer efficiency declines in case: 1) there is difference in numerical apertures or diameters of fibers being joined 2) fiber cores are oval (not round) and fibers are joined in a way when theirs major and minor axes are mutually perpendicular 3) centers of cores doesn’t coincide with claddings’ ones (eccentricity). By carrying out positioning accurately  the mentioned losses can be minimized what enables performing splices with loss of 0.1dB and multiuse releasable connectors with loss <1 dB.

 

For multimode step-index fibers with core diameter 2a and numerical aperture NA power transfer efficiency  (dB) can be calculated according formulae [1]:

а) in case of in-plane shear, d (microns):

, dB;     (2.1)

б) in case of angular difference,  (rad):

, dB;    (2.2)

б) in case of clearance between fiber butts, х (microns)

,dB     (2.3)

where  is refractive index of medium between fiber ends. Computing formulae and diagrams for other types of optical fibers  are given in [1].

3 key questions

3.1 Tell basic option for joining active and passive components of fiber-optic lines and explain difficulties which arise at performing it.

3.2 Tell causes of loss in joining identical fibers. Which alignment errors insert the highest loss?

3.3 Which kinds of losses occur in joining fibers with technological aberrations?

3.4 When loss in joints depends on propagation direction of light?

3.5 Which methods of preparing fiber ends are used in practice and in which cases?

3.6 Tell methods of performing permanent connections  and compare them according theirs loss, strength and cost.

3.7 Point out the way of enhancement loss in adhesive joints.

3.8 Point out the way of enhancement loss in mechanical loints.

3.9 Tell basic requirements for various releasable junctions.

3.10 Which variants of matching butts of plugs with fiber ones are used in releasable junctions?

3.11  Which kinds of materials are used for plugs in optical connectors and why?

3.12 Which variant of preparing (finishing) plug’s butts (physical contact, angular physical contact) provides minimal insertion loss and minimal reflected power and why?

4 Home task

4.1 Learn methods of joining optical fibers by the use of the literature [1], [2] and [3].

4.2 Prepare recitation for key questions.

4.3 For multimode step-index OF with diameter of 2а = 50 microns and numerical aperture of NA = 0,24 calculate power transfer efficiency а (dB) for in-plane shear, angular difference and longitudinal displacement according  the formulae (2.1)...(2.3) in case n0 = 1 (air). Basic data for variants 0…9 which correspond with the last number of student’s mark book are given in the Table 4.1.

Table 4.1 – Basic data for doing home work

variant 

0

1

2

3

4

5

6

7

8

9

d, micron

6

8

10

12

14

16

18

20

22

25

, deg.

9

8

7

6

5

4

3

2

1

10

Х, micron

50

45

40

35

30

25

20

15

10

5

4.4 Prepare report, which must include: title of the work, its purpose, results of performing home work, structure chart of laboratory model.

5. Laboratory model

5.1 Structure chart of laboratory model is shown in the picture 5.1, where 1 is emission source (converts electric signal into optical one, based on light-emitting diode (LED)) from the nest of Optical-loss Test Sets (OLTS); 2 is mode mixer; 3 is optical radiation input unit; 4 is optical multimode fiber fixed inside welding equipment; 5 is radiation detector (converts optical signal into electric one, based on photodiode) from the nest of OLTS; 6 is indicator unit of the OLTS; 7 is electric discharge forming unit of welding equipment; 8 is positioner with a microscope; 9 is power supply unit 220 V/ = 12 V of welding equipment; 10 is the nest of toolware for cable working and preparing OF for welding.

5.2 Operation of the laboratory model.

5.2.1. Electric discharge forming unit of the welding equipment converts electric energy of dc voltage of 12 V into energy of alternating voltage of approximately 3 kV (controlled). Such voltage is necessary for producing electric arc between the electrodes of the welding equipment. Temperature in the arc zone can reach 1800С. Positioner has two carriages, which fix fibers, prepared for welding. One of the carriages is fixed and other can be moved by special compression nuts

in three directions - xyz. One can visually control positioning by the use of a microscope. It is possible to observe the fibers from above through the microscope’s eyepiece and from one side through the special mirror, i.e. in two planes (remember that the microscope reverses image). The tool kit serves for optical cable processing, stripping fibers and shearing fiber ends.

5.2.2. To measure attenuation of permanent junction ОМКЗ-76 optical-loss test sets is used. The optical emission source is LED. Its radiation is inputted into mode mixer, which provides permanent composition of radiation. Input ubit provides inputting radiation into fiber being researched. Radiation of the output fiber end is inutted into optical receiver (photodiode) where it is converted electric current which strength is proportional to received optical power. Then the electric signal passes the indication unit to be amplified and indicated. Its numeric display indicates value of optical power in absolute units (W) and relative ones (dB).

6. PROCEDURE OF WELDING FIBERS

Attention! The special toolkit is used to work fiber cable and prepare fibers for welding.

6.1 Pull off cable shielding.

6.2 Strip fiber on the length of 3…4 cm.

6.3 Sponge off the lacquer film which covers the fiber’s surface with spirit.

6.4 Shear fiber end by the use of shearing device.

6.5 Fix prepared fibers’ ends in movable and fixed carriages of adjusting device of welding equipment.

6.6 Visually inspect quality of the work.

6.7 By using the adjusting compression nuts attain register of fibers in two perpendicular planes.

6.8 Set the mode of fusing ends (there should be some clearance between them). Current strength (mA) and discharge time (s) give the lecturer.

6.9 Turn on the voltage to create electric arc in order to fuse fiber butts.

6.10 Visually estimate quality of fusion and shift the fibers closely to each other.

6.11 Set the mode of welding. Current strength (mA) and discharge time (s) give the lecturer.

6.12 Turn on the voltage to create electric arc and perform weld simultaneously pressing fiber ends to one another.

6.13 Recover strength of protective coating by using thermocontracting tube (according to lecturer’s task).

7 accident prevations when welding

7.1. Safety when using electrical equipment. Power supply unit is under tension  of 220 V while electrodes are under tension of 3 kV!!!

7.2. It is prohibited to look in fiber butts where infrared light (invisible for your eyes) can emit. It is dangerous for your organs of sight.

7.3. When shearing fibers removed parts of them must be piled into a special bucket which is on the laboratory table in order to prevent them from getting on the unprotected parts of your body.

ATTENTION!

WELDING EQUPMENT OPERATES AT VOLTAGE OF 3000 V.

SWITCH ON AND SWITCH OFF WITH LECTUERER’S PERMISSION ONLY!

8 lABORATORY TASK

8.1 Look over the technical specification of the equipment and safety rules, which are on-site.

8.2 Prepare fiber (1-2 metres long) lecturer has given to you. One end of the fiber join to mode mixer and another to radiation detector of optical tester.

8.3 Adjust the input end of fiber in order to attain maximum power at the output end. Write down the reading Р1 W or dB of the numerical display of tester’s indication unit.

8.4 Cut the fiber into two sections by using an instrument and prepare fiber ends for being welded.

8.5 Fix the ends in adjusting carriages of welding equipment and attain them to be coaxial controlling the process with microscope.

8.6 Fuse the butts.

8.7 Weld fibers, but don’t change conditions of inputting optical signal into the fiber being measured. Measure and write down power Р2 (W or dB) at the output end having performed welding.

8.8 Calculate attenuation being inserted by the splice according following expressions:

а = lg(Р1(W)/Р2(W)), dB or а = Р1(dB) – Р2(dB), dB.

In case the determined attenuation is more then 0.3 dB, then the fiber is to be cut again at the point of welding, then repeat the procedure.

8.9 Having performed it three or four times, average the results and compare the attenuation of splice with standard value.

9 conetnt of the Report

9.1 Title and purpose of the laboratory work.

9.2 Block diagram of the laboratory model, configuration, purpose of the equipment.

9.3 Results of calculation of home task according to individual variant.

9.4 Measuring results of attenuation at the splice.

9.5 Conclusion (comparison of determined results with required ones).

10 LITERATURE

10.1 Корнейчук В. И., Шевчук О. Б., Панфилов И. П. Волоконно-Оптические системы передачи: Учебник: Одесса: Изд-Во “Печать”, С. 2001. - 426.

10.2 Корнейчук В. И. Измерение параметров компонентов и устройств ВОСП: Учебное пособие / Одесса: УГАС,1999. - С. 65-82.

10.3 Корнейчук В. И., Макаров Т. В., Панфилов И. П. Оптические системы передачи: Учебник: К.: Изд-Во “Техника”, С. 1994. - 387.


Bare OF 1

Tungsten electrode

Fixed carriage

Figure 2.1 – Principle of electroarc welding  optical fibers

Bare OF 2

y

х

z

Mobile carriage

Рисунок 2.1 – Принцип электродугового сваривания оптических волокон

Рисунок 2.1 – Принцип электродугового сваривания оптических волокон

                     

Неподвижная каретка

Штекер ОВ2

Кабель

ОВ

Кольцо для

колпачковой  гайки

Юстировочная втулка

с внешней резьбой

Зазор между юстировочным фланцем и кольцом

Рисунок 2.3 – Разрез конструкции соединителя с прямой втулкой

 

Штекер ОВ1

Figure 2.7 – Causes of loss: а) in-plane shear; b) angular one; c) clearance between fiber ends; d) roughness of the butts

х

d

c

b

a

d

2а

Figure 5.1 – Laboratory model’s structure chart

1

2

3

4

5

6

8

7

9

10

=12 V

3 kV

LED

PD

Размер поперечного сечения трубки намного больше диаметра оболочки соединяемых волокон.


 

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