GB2331160A - Manufacture of optical fibres and gel in welded metal tube - Google Patents

Abstract

An optical element for transmission of data or other signals comprising optical fibres located in a metal tube is made by advancing a metal strip 6 and forming it into a tube 4, initially with an open slot 7 between its edges. The fibre(s) 2 are introduced through the slot together with a gel, eg through tube 14, and the tape edges are then welded together at section 4 to form a closed tube after which the product is taken up on a haul-off device 18 and allowed to relax. The gel is selected to apply a viscous drag sufficient to mechanically couple the (or each) fibre to the tube at some point located between the welding point 12 and the haul-off at 18. The temperature of the tube at a point upstream of that point and/or the initial tension in the fibre(s) is continuously controlled in response to determined values of the excess fibre length e.g using gauges 24,26. Temperature gauge 28, heater 30 and cooler 32 are shown.

Description

MANUFACTURE OF OPTICAL ELEMENTS This invention relates to the manufacture of optical elements in which one or more optical fibres are located within a metal tube. Such elements have wide application, for example in combined optical and electrical transmission cables as described in EP-A-O 286 804, and in many other fields of application such as temperature and/or strain sensing, data logging down boreholes.

It has been proposed, for example in EP-A-O 299 123, to manufacture such optical elements by a process comprising the steps of: (i) causing a strip of metal to move in the direction of its length through a strip shaping station in which the strip is shaped into the form of a tube having a longitudinally extending slot defined by opposed longitudinally extending edges of the strip; (ii) introducing one or more optical fibres and gel into the tube; (iii) welding the opposed longitudinally extending edges of the strip together to form a welded tube; and (iv) passing the tube to a haul-off device.

However, whenever such optical elements are manufactured, it is important to ensure that there is an excess length of optical fibre in the tube so that the tube can be subjected to tensile loading in use without straining the optical fibres. Thus, excess lengths of at least 0.05% (i.e. a fibre length at least 0.05% greater than that of the tube) are, in general desired, and more usually at least 0.1%. The degree of excess fibre length in the tube will depend principally on the use to which the optical element is put, and for any particular application will depend on a number of factors such as the expected loading of the cable, temperature variations in use, and the cable design (for example whether the tube will be stranded helically or whether it will extend centrally along the cable). For some applications, for example, an excess length of up to 0.6% may be required. While the minimum excess fibre length required will normally be determined by the strain to which the element will be subjected to in use, it is important that the excess fibre length is not too great since this may cause unacceptable attenuation in the fibres due to macrobending of the fibres in the tube if the fibres touch the internal wall of the tube.

Various methods have been proposed for generating such excess length of optical fibre within the tube. For example, EP-A-0 299 123 and WO 84/00820 refer to thermal contraction of the tube as it cools after the welding step, while EP-A-0 299 123 also refers to pushing the optical fibre into the tube by means of the gel, but neither of these effects are easily controllable.

EP-A-0 440 926 describes a method in which the metal strip is heated before the optical fibre is inserted, the degree of heating of the strip being controlled in accordance with the degree of excess length required, and the tube is cooled at the end of the process to generate the excess length. While this process would appear to have more control than that described in EP-A-0 299 123, the location at which the metal strip is heated is a considerable distance from the location at which the tube must cool in order to provide an excess length, and is separated therefrom by a number of locations at which operations are conducted that will introduce quantities of heat (the laser welding station, the die-drawing station), with the result that precise control of excess length will be difficult. In addition, EP-A-0 440 926 appears to take no account of the effect of any gel in the tube (no such gel is mentioned) or any account of the effect of passing the tube through the die-drawing stage in which its diameter will be reduced.

EP-A-0 456 836 describes another method of manufacturing such an optical element in which the optical fibre excess length is controlled by winding the tube about a haul-off device in the form of capstans, and then allowing the tube to relax before it is wound on a drum. Such a method has the disadvantage that it is not easy to continuously control the excess length because it is set by the geometry of the capstan.

The method has the further disadvantage that the optical fibre inside the tube is pulled along by engaging the inside surface of the tube as it passes over the capstan. This causes the optical fibre length to be about 0.09% smaller than the axis of the tube as they pass around the capstan. This may not seem a particularly large amount, but it causes the excess length to be reduced from 0.19% to 0.1%.

Another specification, EP-A-0 703 478, describes the manufacture of such an optical element in which, after the tube has passed through a die-drawing stage, the excess length is generated producing an elastic strain in the tube in the range of 0.1 to 0.6% between a clamping tool and a take-up device (a take-up reel), and then allowing the tube to relax (by releasing the elastic strain when the tube is on the take-up reel).

While that specification states that it is intended to improve on the method of EP-A-0 456 836, the effect of any gel in the tube is ignored (no such gel is mentioned), with the result that, as with EP-A-0 456 836, some of the excess length generated will be lost due to winding the tube around the take-up reel. Furthermore, it is not stated how the method improves control of the excess length of the optical fibre other than a reduction in the prestress (hence elastic strain) in the metal strip.

The present invention is intended to provide an improved method for forming such an optical element, in which the excess length of the optical fibre in the tube can be controlled more accurately controlled.

The method according to the present invention is characterised in that the gel applies a viscous drag on the or each fibre sufficient to cause the fibre or fibres mechanically to couple to the tube at some point located at or upstream of the haul-off device but downstream of the welding stage, and the method includes continuously controlling the temperature of the tube at a location upstream of the said point and/or the initial tension in the or each optical fibre before it is introduced into the tube in response to determined values of the excess length of fibre.

The method according to the invention thus provides a closed-loop mechanism for controlling the excess length of fibre in the tube (excess fibre length) and thereby has the advantage that the excess fibre length can be controlled so that it is independent of transient variations in process conditions and/or so that the excess fibre length can be varied along the length of the optical element. The method has the further advantage that different elements with different degrees of excess fibre length can be manufactured at different times on the same equipment without altering the equipment, but merely by altering the set point in the closed-loop feedback path.

Although not essential to the invention, the method will normally include the step of passing the welded tube through a die-drawing stage in which its diameter is reduced, before itXis passed to the haul-off device. The haul-off device thus must apply sufficient tension to the tube to pull it through the die drawing stage, and will therefore cause the tube to be stretched in the region of its elastic limit. After the tube has passed through the haul-off device it is allowed to relax, thereby to provide an excess length of fibre in the tube.

A significant part of the excess fibre length, and normally, the major part thereof will be determined by parameters set by the manufacturing equipment and/or the optical element, and will not be capable of being varied easily from one run to another. Such parameters include: (i) the optical element design and dimensions, for example the bore of the tube and the number of fibres; (ii) the diameter reduction of the tube as it passes through the die; (iii) the tension on the tube required to pass it through the die; (iv) the design of any injector tube employed in order to introduce the fibres into the tube, and in particular, the diameter of any such injector tube relative to the bore of the metal tube of the optical element after it has been drawn down; (v) the heat input caused by the welding step and/or caused by mechanical work during the die drawing step; and (vi) the line speed.

Even if such parameters can be varied from one run to another, they are normally set to optimise other conditions of the process and/or optical element, and are essentially open-loop. The temperature control and/or optical fibre back-tension control add to (or even subtract from) the excess fibre length given by the fixed parameters of the process. For example, altering the temperature of the tube over a range of 1000C can provide a contribution of up to 0.15% excess fibre length. Methods employed for controlling the temperature of the tube include: (i) hot or cold air blowing onto the tube; e (ii) induction heating; (iii) passing the tube through a bath of liquid or past a liquid spray at controlled temperature; (iv) heating or cooling the die employed in the die drawing stage.

Although the tension on the or each optical fibre required to draw it into the tube as the tube is passed through the die drawing stage is generated by the haul-off device acting on the tube (as it must be in all such methods), the tension is transmitted from the tube to the fibres by the viscous drag of gel at a region downstream of the die-drawing stage (if present), but preferably upstream of the haul-off device. This has the result that, at a location (hereinafter called the "lock point") upstream of the hauloff device, the optical fibres move at substantially the same speed as the tube, so that no optical fibre excess length is lost as the tube is wound around a capstan or other such means that provides the haul-off device. The haul-off device can in principle be a drum or spool on which the element is subsequently stored before use. However, in such a case, the element will still be under tension during storage. Thus, preferably, the element is released from the haul-off device and then passed to a take-up device such as a drum or spool once the tension in the element has been released. Normally the haul-off device will comprise a capstan around which the element is coiled for a few turns before it is released and fed to the take-up drum. It should be appreciated that the precise position of the lock point is not important to the invention, and it is not necessary to know exactly where it occurs in order to practise the method. Indeed, the position of the lock point will vary when the parameters listed above are altered, and will also vary slightly during manufacture of the element as the temperature and/or fibre tension is varied.

The method according to the invention has the further advantage that excess length can be provided by thermal means in addition to that provided by tension applied to the tube. While the temperature-control step can, in general, be located at a range of locations, it is preferred for it to be located upstream of the lock point so that thermal expansion or contraction of the tube will cause the speed at which the fibre is drawn into the tube to be altered. It is possible to incorporate any further cooling step beyond the lock point, but this will not affect the excess fibre length of the optical element. It will be appreciated that the step of heating the tube for controlling its temperature is in addition to any heating of the tube caused by other steps in the process, such as the welding step or due to mechanical work applied to the tube during the die drawing operation. The method is not limited to the use of a single heating and/or cooling step. For example, different methods of heating and/or cooling having different step responses can be employed in order to improve the control over the excess fibre length.

Accurate measurement of both the fibre speed at the input, measurement of the optical element speed at the output of the process is clearly important to the process.

Such measurement can be carried out using accurate capstans and shaft encoders or alternatively, and preferably, by means of non-contact transducers such as laser Doppler gauges. Both measurements are preferably referenced back to normal (ambient) temperature and zero strain.

One method in accordance with the present invention will now be described by way of example with reference with the accompanying drawing in which: Figure 1 is a schematic view of the optical element as it is formed during the method; and Figure 2 is a graphical representation of various parameters such as the tension in the strip, temperature and optical fibre strain during the method, drawn with an abscissa corresponding to figure 1.

Referring to the accompanying drawings, an optical element 1, which comprises a plurality of optical fibres 2 loosely housed within a stainless steel tube 4 of 3 mm diameter and 0.2 mm wall thickness, is formed from a strip 6 of stainless steel which is passed through a set of tube forming rollers (not shown - section 3) that fold the strip generally into the shape of a tube that has a longitudinally extending slot 7 defined by the opposed longitudinally extending strip edges. The tube is passed to a laser welding station (section 4) where the opposed edges are urged together by means of pinch rollers 8 to close the slot, and welded together to form an hermetically sealed semifinished tube 10, for example by means of a 500 W CO2 laser 12.

At the same time, the optical fibres 2 are inserted into the tube together with a quantity of water resistant gel. The optical fibres 2 and gel are introduced into the tube 10 by means of an injector tube 14 of smaller diameter than the bore of the tube 10.

The injector tube 14 starts upstream of the tube forming station (section 3) and extends within the tube 10 beyond the laser welding station so that the optical fibres are protected from any damage caused by the laser by means of the injector tube 14. A short distance beyond the end of the injector tube 14, the semifinished tube 10 is passed through a die drawing station 16 (section 6) where its diameter is reduced by up to 30% to form the finished tube 4.

The tube 4 is then passed through further stations (sections 7 - 9) that, together extend for a distance in the range of from 2 to 10 metres, before being wound around a haul-off capstan 18 which provides the tension in the tube 4 to pull it through the die drawing stage and the other stations downstream of the die drawing stage. The tube is only wound around the haul-off capstan for a few turns before being led off the capstan and wound onto a take-up drum after releasing the tension therein.

The design of the injector system is such that fibre is guided along the injector tube 14 with little or no back flow upstream. Along the bore of the injector tube 14, the gel flow imparts a drag force on the fibres which pulls the fibres into the injector tube against an back-tension applied to the fibres upstream. The tension and strain in the fibres are also held along the length of the injector tube 14. When the fibres emerge from the injector tube, the tension and strain in the fibres would be released. In the case of an isolated system, with a uniform injector tube, the fibre speed cannot be greater than the speed of the gel. However, when coupled to conditions downstream, the injector system exhibits three flow conditions: (a) The gel and fibre velocity are equal. There is therefore no effect on the fibre tension downstream.

(b) The gel velocity is greater than the fibre velocity. The gel now acts to reduce tension ig the fibre downstream, or even cause buckling where there is sufficient resistance downstream.

(c) The gel velocity is less than that of the fibre. In this case the fibre is pulled through the gel and therefore the gel imparts drag or back-tension on the fibre downstream.

The above conditions are determined by the number of fibres, the diameter and length of the injector tube 14, the properties of the gel and by the general flow characteristics of the injector system. The flow can also be affected by any suction effect of the welded tube being drawn over the end of the injector tube 14, particularly where there is a tight fit between the two. It is also affected by the upstream tension on the fibres due to resistance around pulley guides and back-tension from the fibre payoffs. All these factors allow a coarse tuning of the fibre and gel velocities for the next stage (sections 5 to 7).

There is a finite length (and thus travelling time) between the end of the injector tube 14 and the die 16 at which the tube diameter will be reduced by up to 30%. It is in this transition phase, after leaving the injector, when the gel fills or partly fills the welded tube 10. When the tube 10 reaches the die, the gel is forced into the final tube diameter. The fibres are also partly or fully immersed in the gel during this transition phase which, because it is relatively short, can be assumed to have negligible effect on the fibre tension. The flow rate of gel must be balanced between the injector and the final tube 4 diameter so that there is neither an overfilling in the transition space (creating back-pressure and/or forcing gel backwards between the strip and injector tube 14) nor an under-filling into the final tube (a small degree of under-filling is acceptable).

Considering the relative sizes of the injector tube 14 bore and that of the final tube 4, three flow conditions can be considered: (a) The injector tube 14 bore is equal to the bore of the final tube 4. In this case the gel velocities are equal (VG = V2) where VG is the gel velocity and V2 is the velocity of the tube 4 after draw-down.

In this case the fibre velocity (VF) is matched to the gel velocity (VF=VG).

(b) The injector tube 14 bore is smaller than the bore of the final tube 4, which requires a greater gel velocity: VG > V2 VG > VF (c) The injector tube 14 bore is greater than the bore of the final tube 4, which requires lower gel velocity: VG < V2 VG < VF Thus, the size of the optical element and the percentage draw-down are further parameters that enable coarse tuning of the fibre speed, tension and strain passing into the final tube.

The action of drawing the tube 10 through the die 16 will create a degree of heating (point 21 in Figure 2b) which is relative to the degree of draw-down and hence the degree of mechanical work applied to the tube. In addition, there will be some heat remaining in the tube from the welding operation (point 23 in Figure 2b). The temperature in the tube at the die 16 and for the first stage thereafter can be controlled by: (a) cooling the tube before the die in order to remove heat from the weld; (b) insulating the tube in order to maintain heat from the weld in the tube; (c) heating or cooling the die lubricant; and (d) heating or cooling the tube after the die 16.

Therefore, after the transition point the fibres enter a thermally expanded or contracted tube, at which stage, over a sufficient length, they are held in the tube 4 by the gel. The tube later returns to ambient temperature with a consequential change in effective length with respect to the fibres. The heat from the tube will be transmitted with a time lag to both the gel and the fibres themselves.

The drawing force needed to reduce the tube diameter as it passes through the die is created by the capstan 18. This causes plastic elongation in the tube material together with a resultant elastic elongation (as shown in Figure 2a). The strip need not be under tension before the die, although a back tension can also be created, for example by the tube forming rollers or other parts of the process, which requires greater tension from the capstan and increased overall strain in the tube. In practice, there will be a small tension before the die. The tension in the tube is effectively released after the capstan, causing shortening of the tube, although there may still be a small tension in the tube to enable a controlled winding onto the take-up drum (but this will also be released in later processes).

As described above, the fibres pass through the transition point (stage 5) at which the tube diameter is reduced, with a pre-determined tension and strain (or compression/buckling). They then pass into the tube at stage 6 along with the gel.

Hereafter the gel imparts a drag on the fibres, depending on its viscosity (generally inversely proportional to the gel temperature). The drag is also proportional to the length over which the gel and fibre are together within the tube, and this is balanced by the back tension of the fibres going into the reduced diameter tube (which strains the fibres) or the buckling force (where the fibres adopt a shallow serpentine route within the tube). A minimum length of gel and tube from stage 6 onwards (the "holding length" 20) is required when there is no back-tension in the fibres. If there is an initial tension in the fibres, the strain is locked into the fibres over the holding length (which needs to be longer), until released by thermal/mechanical relaxation of the tube.

Similarly, if the fibres are already buckled as they enter the tube, this buckling is held in this form until the tube changes further downstream, but then adds to the overall excess fibre length. The excess fibre length is created beyond the "lock point" 22 (i.e. beyond the end of the "holding length" 20) where the tube returns to ambient temperature and zero tension.

Fine control of the excess fibre length is achieved by means of varying the temperature of the tube 4 in the holding region (i.e. in sections 6, 7 and 8 before the "locking point" 22) and/or by controlling the back tension in the fibres.

A laser Doppler gauge 24 is provided in order to measure the speed of the welded tube 4 at a point downstream of the lock point, preferably just upstream of the haul-off capstan 18, and a further Doppler gauge 26 measures the speed of the optical fibres as they are fed into the injector tube 14. In addition, a temperature gauge 28 is provided at the same position as the Doppler gauge 24 in order to measure the temperature of the tube 4. The ratio of the speeds, when adjusted to ambient temperature and zero strain, gives the excess length of fibre in the tube. This is compared with the set point value of the excess length to generate an error value which is used to determine the degree of heating or cooling of the tube required. A heater 30, for example a hot air gun or induction heating coil, and a cooling means, for example a cold-air gun 32 are located in sections 8 and 9 respectively in order to heat and/or cool the tube 4 before it reaches the locking point 22. By this means, the tube can, for example, be subjected to a controlled degree of expansion before the fibres 2 are coupled to the tube 4 by means of the gel, and then allowed to contract as it cools to ambient temperature after the fibres are coupled to the tube.

In order to change the excess length of fibre (within limits) between runs, one simply adjusts the set point of the feedback loop. Adjustment of the set point of the feedback loop during a run will enable the excess length of fibre to be varied along the length of the optical element.

Claims (8)

1. Claims: 1. A method of forming an optical element that comprises one or more optical fibres located in a metal tube, in which element there is an excess length of optical fibre in the tube, which method comprises the steps of: (i) causing a strip of metal to move in the direction of its length through a strip shaping station in which the strip is shaped into the form of a tube having a longitudinally extending slot defined by opposed longitudinally extending edges of the strip; (ii) introducing one or more optical fibres and gel into the tube; (iii) welding the opposed longitudinally extending edges of the strip together to form a welded tube; (iv) passing the tube to a haul-off device; and (v) allowing the tube to relax after the haul-off device thereby to provide an excess length of fibre in the tube; characterised in that the gel applies a viscous drag on the or each fibre sufficient to cause the fibre or fibres mechanically to couple to the tube at some point located at or upstream of the haul-off device but downstream of the welding stage, and the method includes continuously controlling the temperature of the tube at a location upstream of the said point and/or the initial tension in the or each optical fibre before it is introduced into the tube in response to determined values of the excess length of fibre.
2. 2. A method as claimed in claim 1, wherein the excess length of fibre is determined from measurements of the speed of the fibres before introduction into the tube and measurements of the speed of the tube downstream of the said point.
3. 3. A method as claimed in claim 1 or claim 2, wherein the welded tube is passed through a die drawing stage in which its diameter is reduced before it is passed to the haul-off device.
4. 4. A method as claimed in claim 3, wherein the temperature of the tube is continuously controlled by heating or cooling the tube in the region of the die.
5. 5. A method as claimed in claim 3 or claim 4, wherein the temperature of the tube is continuously controlled by heating or cooling the tube downstream of the die.
6. 6. A method as claimed in any one of claims 1 to 5, wherein the temperature of the tube is continuously controlled by hot-air heating or by induction heating.
7. 7. A method as claimed in any one of claims 1 to 6, wherein the control of the temperature is varied with time in order to vary the excess length of fibre along the length of the element.
8. 8. A method as claimed in any one of claims 1 to 7, wherein the element is taken from the haul-off device to a take-up device, and tension in the element is released between the haul-off device and the take-up device.