Patent ID: 12252432

DETAILED DESCRIPTION

FIGS.1to3illustrates an apparatus10for drying and consolidating an optical fibre preform16, in an embodiment consistent with the present disclosure. The apparatus10comprises a furnace9, which can be of conventional type, the furnace9comprising a muffle tube11of substantially cylindrical shape. The furnace9and specifically the muffle tube11, when in use, extends along a vertical axis, indicated with axis Z, whereas double arrow L indicates the axial or vertical direction of the up or down movement of the preform16into or out of the muffle tube11.

The terms “upper” and lower” or “top” and “bottom” or “below” and “above” when referring to the apparatus10are defined with respect to the vertical direction L or to an axial direction substantially parallel to the vertical direction L. These terms are used to indicate the relative position of the elements to one another in their orientation during processing of the preform.

The terms “inner” and “outer” or “radially inner” and “radially outer”, with reference to the elements of the apparatus, are intended to refer to the relative radial position thereof.

The muffle tube11has an inner sidewall forming a hollow muffle chamber of substantially cylindrical shape. The muffle tube and thus the muffle chamber has a generally elongated shape configured to house an optical fibre preform16. In particular, the muffle tube11has an opening15at the top side of the muffle, indicated in the following as muffle opening, which defines the opening for the passage (insertion-extraction) of a preform16in the muffle tube11. In an embodiment, the opening15is centred about the Z-axis. Optionally, the muffle tube comprises an upper flange14extending radially outwardly from the muffle opening15. The up and down movement of the preform16into the muffle tube is along the Z-axis.

The muffle tube11is made of glass, in particular of quartz. Customarily, muffle tubes for processing silica-based preforms for the production of optical transmission fibres are made of highly pure quartz to avoid contaminations in the preform during heating.

The furnace9further comprises a first heater12surrounding the muffle tube11in an upper length of the muffle tube. The first heater12defines a first heating zone extending over the upper length of the muffle tube11and thus of the furnace chamber. In operation, the first heating zone is set at a first temperature suitable for dehydration and/or doping of the porous layers of the preform. For example, the first temperature is of from 1000° C. to 1350° C.

A second heater13, which is positioned below the first heater12in the axial direction L, surrounds the muffle tube11. The second heater13defines a second heating zone extending over a lower length of the muffle tube11. In operation, the second heating zone is set at a second temperature suitable for consolidation of the porous preform into a solid glass preform. The second temperature is higher than the first temperature and it is usually of from 1450° C. to 1600° C.

By “porous preform” in this application it is meant both a totally porous preform (e.g. for the formation of a glass core rod) and a partially porous preform comprising a porous soot layer for the formation of an intermediate cladding and/or an overcladding on the glass core rod.

In an embodiment, the overall length of the muffle tube11and thus of the furnace chamber, taken in the Z-axis, is greater than the length of the preform16so that the latter can move up and down, in the Z-axis, inside the furnace chamber. In an embodiment, the length of the furnace chamber is at least 1.5 times the length of the preform to be processed. For example, the preform has a length of from 2.0 to 3.5 meters.

Typically, first and second heaters12,13are ring-shaped to surround the muffle tube11. For example, each of heaters12,13comprises one or more annular heating elements.

The apparatus10further comprises a hollow extension tube19. The extension tube19has an inner sidewall to form a hollow extension chamber21of substantially cylindrical shape. The extension tube19has a top side and a bottom side.

The extension tube19is configured to encircle at least a length portion of preform16. For example, the length and width (diameter) of the extension chamber are sized to house at least a length portion of the preform. In an embodiment, the extension tube19is made of quartz.

Typically, the preform16is provided with a preform handle17made of quartz, which can be joined or integral to the preform.

Dimensions of the muffle tube11and of the extension tube19may be selected based on the dimension of the preform to be processed in the apparatus so as to allow a down and up movement of the preform16without the inner sidewall of the chambers touching the preform. The preform16is suspended and supported by a supporting handle18, typically made of quartz, which comprises a supporting rod18aand a holding portion18bpositioned at a bottom end of the supporting rod18aand configured to hold the preform handle17. In the non-limiting example ofFIGS.1-3, the holding portion18bis a C-shaped hook integral to the supporting rod18a. In this example, more clearly seen inFIG.3, suspension of the preform16makes use of an enlarged-width portion17aof the preform handle17, such as a ball-shaped portion, which is housed in the holding portion18bof the supporting handle18.

A gas-discharge hood30(herein indicated as “hood”), described in more detail in the following, is placed on top of the extension tube19and comprises a gas discharge port46, for example in a hood sidewall31. The hood30is removably fixed to the extension tube19.

The gas-discharge port46can be connected with an external exhaust pipe35(shown inFIGS.1-3), for the discharge of gases flowing during the processing of the preform16, when the extension tube19is connected with the muffle tube11.

In the example ofFIGS.1-3, the exhaust pipe35is placed at a height corresponding to the vertical position of the hood30for connection with the gas discharge port46, when the extension tube19is joined to the muffle tube11. For example, a shaft36is placed in the vicinity of the muffle opening15for holding the exhaust pipe35in position for the connection to the gas discharge port46of hood30. It is to be understood that the presence of elements35,36is optional since other configurations may be envisaged for the discharge of the gases from the extension chamber21during dehydration and/or consolidation of the preform.

In known ways, the supporting handle18is operatively connected to a moving system27, only partially shown in the figures. The moving system27is designed for the vertical movement of the preform16, e.g. insertion and extraction of the preform in/from the muffle tube11and for the rotational movement of the preform about the Z-axis. The moving system27is further designed for the vertical movement of the extension tube19, this vertical movement being independent from the vertical movement of the preform16. In the figures, only the up and down transport mechanism of the preform is schematically shown. The moving system27may be of the type described in more detail in WO 2018/177514 A1.

In an initial stage shown inFIG.1, the preform16is positioned above the muffle tube11, while the extension tube19is positioned in axial alignment with the preform16and, in the example shown in the figure, the extension tube19is placed above the preform16.

FIG.2shows a subsequent stage in which the preform16is partially inserted in the muffle tube11and the extension tube19is lowered onto the preform16so as to surround it.

The extension tube19may descend to surround an upper length portion of the preform16before or after the preform16is partly inserted in the furnace9.

In an embodiment, processing of the preform16starts with a lower length portion of the preform16inserted in the muffle tube11so as to be substantially positioned in the first heating zone defined by the first heater12. If the first heating zone is placed close to the muffle opening15of the muffle tube11, the length of the inserted lower portion of the preform16approximately corresponds to the length of the first heating zone, as schematically shown inFIG.2, so as an upper portion of the preform16lies outside of the muffle tube11.

In the embodiment shown inFIGS.1and2, a tower28acts as a supporting structure for the moving system27. The tower28may stand on a supporting plane29. In the non-limiting example shown inFIGS.1-2, the supporting plane29is a floor area having an opening for communication with the muffle tube, positioned below the floor area (not shown).

With reference toFIG.4, the extension tube19forming the extension chamber21is provided at one of its ends, at the bottom side, with a lower opening24configured to encircle the preform16. Opposite to the lower opening24, a cover plate22is provided at the top side of the extension tube19and includes an upper opening23in the form of a through-hole for the passing of the supporting rod18aof the supporting handle18. The upper opening23is centrally positioned in said cover plate22. In particular, the upper opening23is substantially centred about the vertical axis Z.

In the embodiments shown in the figures, the cover plate22includes an outer flange extending radially outwardly the extension tube19to ease fastening of the hood30to the extension tube19, as described in more detail hereinafter.

In an embodiment, the length of the extension tube19is sized to house at least a length portion of the preform16, specifically its upper length portion.

In an embodiment, the extension tube19includes a bottom rim flange25at the lower opening24, the bottom rim flange25extending radially outwardly from the lower opening24. The bottom rim flange25has annular shape.

In one or more embodiments, the extension tube19is one-piece, with tube, cover plate and flange(s) being made of quartz.

The hood30is removably fixed to the cover plate22so as to be attached to and detached from the extension tube19.

As already said above, in an embodiment, the muffle tube11comprises an upper flange14(as fromFIGS.1and3) extending radially outwardly the muffle opening15, at the top side of the muffle tube11. The upper flange14has annular shape. The muffle tube11is preferably one-piece with the upper flange14.

When the extension tube19is lowered onto the muffle tube11to join its upper flange14with the bottom rim flange25of the extension tube19, the upper surface of flange14and the lower surface of rim flange25are put in contact to one another so as, by joining these surfaces, a sealed chamber is formed by the extension chamber21defined by the extension tube19and the chamber of the muffle tube11.

In an embodiment, the muffle tube11and the extension tube19are made of glass, preferably of quartz, and joining of the muffle tube with the extension tube is a glass-to-glass seal between the joining surfaces of flanges14and25. While the weight of the extension tube19is suitable to provide an airtight contact with the muffle tube11, one or more O-rings or gaskets may be provided at the interface between of the flanges14and25as an optional safeguard for the glass material. In an embodiment, the O-ring's or gasket's may be made of a thermoresistant polymer like perfluoroelastomer, or of graphite.

In an embodiment, the diameter of the furnace chamber, i.e. the inner diameter of the muffle tube11, is substantially equal to the diameter of the extension chamber, i.e. the inner diameter of the extension tube19.

The furnace9comprises one or more gas inlets, only schematically indicated in the Figures with gas inlet part47(FIGS.1and2), which is connected with the hollow chamber of the muffle tube11for the supply of one or more gases passing through and/or around the preform16. In the embodiments shown inFIGS.1and2, gases enter the furnace chamber from the bottom of the muffle tube11and stream upwards. As described in the foregoing, in operation, namely during processing of the preform16, the extension tube19is joined to the muffle tube11to form a sealed chamber and gases are discharged outside the extension chamber21from the gas discharge port46of the hood30. Typically, the gas discharge port is positioned at the opposite side of the furnace with respect to the gas inlet part47. In the embodiment shown in the figures, the gas discharge port46is positioned at the opposite side of the sealed chamber with respect to the gas inlet part47, in particular with gas flowing from bottom to top.

The apparatus10is suitable for drying and consolidating a porous optical fibre preform. In an embodiment, the preform, before undergoing dehydration and/or consolidation, has a porous overcladding layer formed around a glass core rod.

When the extension tube19is joined to the muffle tube11and the hood30is mounted on the extension tube19, the sealed chamber is in communication with the exterior by means of a hood through-hole34positioned at the top of the hood, i.e. in hood cap33. The hood through-hole34is centrally positioned about the Z-axis for the passing of the supporting handle18and in particular of the supporting rod18ain the hood cap33.

In known ways, dehydration and consolidation of the preform16is achieved by moving the preform through the first heating zone of a relatively limited length, i.e. shorter than the length of the preform, and then to the second heating zone. The muffle tube11may extend below the second heating zone for a length configured to house the whole preform as a cooling zone. Typically, cooling is carried out after consolidation while flowing inert gas, such as helium, across the muffle tube.

The dehydration process starts by drying the portion of the preform positioned in the first heating zone. In an embodiment, the portion of the preform inserted in the muffle tube11lies above the second heating zone so as to prevent consolidation of the lowermost portion of the preform to take place before its drying. Typically, the first heating zone is at a temperature of from 1000° C. to 1200° C. The preform is maintained in this position for a certain time, such as from 60 to 90 minutes. Subsequently, the preform is gradually lowered through the first heating zone at a given rate so as to dehydrate the whole preform when the entire preform length has passed the first heating zone. The lowering rate may be of from 4 mm/min to 8 mm/min.

When the preform is down driven through the first heating zone, successive longitudinal portions of the preform are exposed to the first heating zone and subsequently to the second heating zone. The second heating zone is set at a temperature suitable for vitrification, typically of from 1450° C. to 1600° C.

Typically, during dehydration and consolidation, the preform rotates about its longitudinal axis in order to improve axial symmetry. In ways per se known, a rotation transmission mechanism (not shown) coupled to the supporting handle18transmits rotation to the preform.

In an embodiment, the porous glass soot preform is a silica-based porous glass preform for the fabrication a silica-based optical fibre of low attenuation for use in telecommunication systems. The porous soot layers can be formed by known methods, such as a flame hydrolysis process of silica-based soot or combustion of silica-based reactants as octamethylcyclotetrasiloxane (OMCTS, also called D4).

In an embodiment, the preform, before undergoing dehydration and/or consolidation, has a porous overcladding layer formed around a glass core rod. In ways per se known, after consolidation, the solid glass preform can be drawn in an optical fibre.

It is to be understood that the arrangement and length of the heating zones with respect to the preform may be different from those described above and shown in the embodiments ofFIGS.1and2. For example, in alternative to the illustrated embodiment, the furnace9may have a single heating zone for dehydration and/or consolidation.

After cooling, the muffle tube11is opened by lifting the extension tube19above the muffle tube11to allow the removal of the preform16from the muffle tube11.

FIGS.1-3illustrate an apparatus having a flow control assembly90according to an embodiment described in more detail with reference toFIGS.6and7, whereasFIG.5shows a flow control assembly91according to the embodiment described in more detail inFIGS.9-12. However, the description of the embodiments described with reference toFIGS.1-S can be applied to any embodiment of the flow control assembly herein disclosed.

As more clearly shown inFIGS.5-7,9,11-12, the hood30defines an inner space37when the hood is fixed on the extension tube19. The hood30includes a sidewall31and a hood cap or top plate33arranged on the sidewall31. The hood cap33includes the centrally positioned through-hole34for the passing of the supporting rod18a.

In an embodiment, the hood30is made of metal, for example aluminium or anodized aluminium.

In operation, the hood30is fastened to the cover plate22of the extension tube19.

In an embodiment, the hood cap33and sidewall31are two distinct pieces, as fromFIGS.5to10, the hood cap33being removably connected to the hood sidewall31. In the illustrated examples, the hood cap33is fastened to the sidewall31by means of screws38.

An O-ring seal32may be provided between the cap33and the sidewall31. For example, the O-ring seal32is lodged in a groove (not visible) provided around the joining surfaces of the cap33and sidewall31. By hindering gas leaking from the peripheral region of the hood, provision of an O-ring seal32allows a more accurate control of gas inflow/outflow from the hood through-hole34, as described in more detail hereafter.

In alternative, cap33and sidewall31of the hood30may be one-piece.

The hood30has a substantially cylindrical shape to fit onto the extension tube19. However, the shape should not be considered limitative. When the hood30is placed on top of the cover plate22of the extension tube, the hood through-hole34is in-axis with the upper opening23for the insertion of the supporting rod18athrough the inner space37of the hood30and entry into the extension tube19through the opening23.

In an operative position, the through-hole34is in communication with the inner space37of the hood. As indicated above, the through-hole34is opened at the top side of the hood to put in communication the muffle tube11, via the extension tube19, with the exterior.

In an embodiment, the through-hole34and the upper opening23have both a circular cross-section with a centre positioned substantially on the same axis (e.g. Z-axis).

In the non-limiting examples illustrated in the figures, the gas discharge port46is formed in the sidewall31of the hood30.

The hood30is connected to the extension tube19through its cover plate22. In the illustrated embodiments, the hood30comprises two ribs44a,44bpositioned at the bottom of the sidewall31. Rib44aprojects radially inwardly with respect to the hood sidewall31and rib44b, which is positioned below rib44a, projects both radially inwardly and outwardly with respect to sidewall31(FIG.6). The ribs44a,44bare vertically spaced to form a groove43in the inner of the hood sidewall31. The groove43is configured to seat the outer flange of the cover plate22thus connecting the cover plate22to the sidewall31of hood30in a tight configuration. In this way the hood30can be mounted on top of the extension tube19by encircling and gripping the outer flange of the cover plate22.

For example, the hood sidewall31can include two semi-circular sections having two vertical ends, each end being provided with a respective connecting plate protruding outwardly and fastened to one another by fixing elements (details not shown in the Figures).

The Applicant has observed that the temperature reached in the extension tube19, though lower than that in the muffle tube11, may cause damages at or in the vicinity of the cover plate22of the extension tube, which is made of quartz. This may be caused by the contact of the quartz cover plate22with the hood30, which is made of metal and thus prone to thermal expansion.

To help ensuring the integrity of the cover plate22, a gasket45(FIG.6) may be interposed between cover plate22and hood30(in particular, between the lower rib44band the lower surface of the flange of cover plate22). The gasket45may be made of a suitably thermoresistant polymeric material or of graphite.

The apparatus10comprises a flow control assembly90,91including a sealing assembly80directly or indirectly connected with the hood30. The flow control assembly90,91is designed to at least partly seal the through-hole34of the hood30in communication with the exterior of the apparatus10so as to prevent the inflow of gases, in particular of atmospheric air, from entering into the hood. Atmospheric air would dilute the processing gases flowing from the processing chambers housing the preform to the recycling process. At the same time, the flow control assembly90,91is designed to allow the vertical movement and the rotation of the supporting rod18aof the supporting handle18through the through-hole34of the hood30and the upper opening23of the extension tube.

When the hood30is connected with the extension tube19and the extension tube19is connected with the muffle tube11, a sealed chamber is formed by the joining of the extension tube19with the muffle tube11, the sealed chamber being in communication with the exterior through the hood through-hole34.

With reference to the embodiment ofFIG.8, the sealing assembly80comprises a ring-shaped seal82. The seal82is sized to surround the supporting rod18awhen the latter is inserted in the extended elongated chamber. In particular, the seal82has an inner diameter configured to directly contact the outer diameter of the supporting rod18a.

In an embodiment, the seal82includes a sealing lip82a(visible inFIGS.8and9) for engagement with the supporting rod18aof the supporting handle18. The seal82may also include a garter spring (not shown in the figures), for example coupled to the sealing lip82a, for a tighter contact with the supporting rod18awhile the latter axially moves and rotates about its axis.

In embodiments, the ring-shaped seal82is made of a thermoset elastomeric polymer, for example of a fluoropolymer (fluoroelastomer), which are generally suitable for continuous use at relatively high temperatures, up to 200-300° C.

The sealing assembly80further comprises an expansible member83configured to expand or contract in an axial direction, in particular in the Z-axis. The expansible member83has a tubular shape with an inner diameter configured to allow the passage of the supporting rod18a. In embodiments, the expansible member83comprises a pleated portion83aconfigured to expand and contract in the axial direction and a non-axially expandable portion83bcontiguous to the pleated portion83ain the axial direction. In an embodiment, the non-axially expandable portion83bis one-piece with the pleated portion83a.

The pleated portion83ahas a length varying, in the axial direction L, from a minimum value to a maximum value.

As described in more detail hereafter, the inner diameter of the expansible member83and in particular of the pleated portion83amay vary across its length, provided that its minimum inner diameter is equal to or larger than the inner diameter of the ring-shaped seal82so as to allow the passage of the supporting rod18a.

In embodiments, the minimum inner diameter of the expansible member83is larger than the inner diameter of the ring-shaped seal82so that the supporting rod18a, in typical conditions during processing of the preform, is in contact only with the seal82.

The non-axially expandable portion83bhas an annular shape with an inner diameter and an outer diameter in the radial direction. In the expansible member83, the minimum inner diameter of is sized so as to leave a radial gap around the supporting rod18afor an axial movement through the hood30with limited radial offset. In an embodiment, the minimum inner diameter of the expansible member83corresponds to the inner diameter of the non-expandable portion83b.

To facilitate the fastening of the sealing assembly80directly or indirectly to the hood30, the sealing assembly80comprises a base84for the expansible member83, in the following, expansible member base84, contiguous to the pleated portion83aand opposite to the non-expandable portion83bof the expansible member83(more clearly visible inFIG.8). Expansible member base84extends radially outwardly of the pleated portion83ato leave a hollow passage ranging from the minimum to the maximum inner diameter of the expansible member83. Expansible member base84may be one-piece with the pleated portion83a.

In embodiments, the expansible member83is one-piece.

According toFIG.8, the expansible member base84comprises a plurality of holes for the fastening of the sealing assembly80directly or indirectly to the hood30.

The ring-shaped seal82is operatively connected to the expansible member83and in particular to its non-axially expandable portion83b. In the embodiments shown in the figures, the ring-shaped seal82is housed within the non-expandable portion83bof the expansible member83. With reference toFIGS.6,7,9and11, the non-axially expandable portion83bincludes a radial cavity configured to house the ring-shaped seal82, which is located within the radial cavity so as to extend up to the most radially inward position of the sealing assembly80with respect to the central axis Z. In this way, the most radially inward position of the sealing assembly corresponds to the inner diameter of the ring-shaped seal82.

In an embodiment, the inner diameter of seal82is sized to have a tight connection with the supporting rod18a.

In embodiments, the ring-shaped seal82is held in place by the non-expandable portion83bby compression.

As the ring-shaped seal82is operatively connected with the supporting rod18aand the expansible member83, through the expansible member base84, is directly or indirectly connected to the hood30, the pleated portion83ais suitable to compensate any potential offsets between the fixed elements82and84. In an embodiment, the expansible member83is made of a thermoplastic polymer. For example, the expansible member is made of a thermoplastic fluoropolymer, such as PFTE (polytetrafluoroethylene), PCTFE (polychlorotrifluoroethylene) or PFA (perfluoroalkoxy alkane).

In a first embodiment of the present disclosure illustrated in more detail inFIGS.6and7, the apparatus10comprises a flow control assembly90including a sealing assembly80, of the type described with reference toFIG.8, the sealing assembly being indirectly connected with the hood30. According to the first embodiment, the flow control assembly90including the sealing assembly80is placed on top of the hood30, specifically on the hood cap33, in axis with the hood through-hole34. When the hood30is connected to the extension tube19, the disposition of the expansible member83is such that the non-expandable portion83band thus the ring-shaped seal82lies below the pleated portion83a.

Due to the pleated shape of the pleated portion83aand the associated constraints in the axial direction, the expansible member83has overall inner and outer diameter which may vary along its length in operation. The inner diameter of the pleated portion83amay vary in the axial direction as long as it allows the passage of the supporting rod18awithout a direct contact between the expansible member83and the supporting rod18a.

The hood through-hole34has a diameter larger than the outer diameter of the non-expandable portion83bso that the non-expandable portion83bcan be fitted inside the through-hole34.

In the embodiment ofFIGS.6-7, the flow control assembly90includes an open box51positioned on the top of the hood30, i.e. on hood cap33, and axially centred to the through-hole34. The open box51is open both at its bottom side to be in communication with inner space37of the hood30through the hood through-hole34and at its top side.

The box51can be made of metal, such as aluminium. In an embodiment, the box51has a generally cylindrical shape defined by sidewall51b. The open box51may further comprise an outer flange51acontiguous to the bottom side of the sidewall51band extending outwardly thereof. The open box51can be fixed onto the hood cap33by bolts, screws or clips at the outer box flange51a(details not shown in the figures).

The open box51comprises an inward flange51cpositioned at the top side of the open box and extending radially inward with respect to sidewall51b. The inward flange51cis contiguous to the box sidewall51band provides an inwardly protruding supporting surface, which the sealing assembly80can be fixed on. In an embodiment, the inward flange51cis one-piece with sidewall51b, as shown in the enlarged portion ofFIG.7.

The expansible member83of the sealing assembly80is fixed to the open box51and housed therein. The pleated portion83aand the non-expandable portion83bare placed within the sidewall51b.

The expansible member base84lays on and it is fastened to the box inward flange51cthrough rivets55. The box inward flange51cextends radially inwardly so as to form an annular top surface while leaving a top opening for the open box. The box top opening defined by the inward flange51cforms a box through hole.

The expansible member base84has an outer diameter larger than the diameter of the box through hole. In this way, by fastening the expansible member base84to the box inward flange51c, the sealing assembly80partly covers the box through hole but for the hollow tubular space of the sealing assembly80.

In the present embodiment, the expansible member83hangs by the expansible member base84into the open box51, the expansible member83being free to move axially (up and down) to a certain degree by contracting and expanding the pleated portion83a(and, to a lesser degree, radially).

Chiefly, the axial movement of the expansible member83takes place in the expandable pleated portion83a, whereas the non-expandable portion83bmay move axially across a shorter length. Being operatively connected to the supporting rod18a, the sealing assembly80provides for a sealed engagement with the supporting handle18that effectively reduces the escape of the processing gases from the interior of the sealed chamber (mainly from the hood through-hole34), without impairing the movement of the supporting handle18through this chamber.

The open box51can provide protection to the polymeric elements of the expansible member83from ambient dust.

By significantly reducing the inflow of ambient air, the design according to this embodiment increases the purity of helium-containing gases flowing through the discharge port46, thereby making helium recycling more efficient and profitable.

Detection pressure measurement devices (not shown) may be placed at the gas discharge port46to detect the outflow of the processing gases.

According to a second embodiment illustrated inFIGS.5,9-12, a flow control assembly91comprises a sealing assembly80as described in the foregoing. In this embodiment, the sealing assembly80is arranged, in the axial direction L, between the upper opening23of the extension tube19and the through-hole34of the hood30, within the sidewall31of the hood30(i.e. within the hood inner space37). The disposition of the expansion member83is such that the non-expandable portion83bhousing the ring-shaped seal82lies above the pleated portion83a. The hood through-hole34, the upper opening23as well as the expansible member83have respective diameters sized to leave a radial gap around the supporting rod18afor an axial movement through the hood30with limited radial offset.

In the embodiment ofFIGS.9-12, the flow control assembly91comprises a plurality of springs62(in the non-limiting illustrated example four springs) extending in the axial direction L, the springs62radially surrounding the expansible member83. In an embodiment, the springs62are mounted on respective vertical supporting elements, such as vertically arranged rivets63. Springs62have a spring length and rivets63extend in the vertical direction for a length portion of the spring length.

In an embodiment, the springs62are fixed, at one end, by the rivets63on a supporting flange61, which is placed on the cover plate22of the extension tube19, within the inner space37of the hood30. The supporting flange has a generally cylindrical shape.

The springs62, which may be coil springs, extend from the supporting flange61to the inner surface of the hood cap33in a partially compressed condition, which is pre-set depending on the expected processing parameters, in particular the pressure of the gas outflow. The pre-set compressed condition is slight in absence of the processing gases and/or at the beginning of the process of dehydration or consolidation of the preform. In particular, in these conditions, the hood cap33of the hood30slightly urges the springs62down in the axial direction. The springs62absorb the stress otherwise exerted on the sealing assembly80and the supporting flange61and thus against the cover plate22which, being made of quartz, could be damaged.

In the case of presence of higher overpressure in the muffle tube11and extension chamber21during processing of the preform, e.g. due to blocking of the exhaust pipe35, the springs62allow the outflow of gases to outflow from the extension chamber21underneath the supporting flange61through the through-hole34of the hood30.

Supporting flange61has a ring shape with a central axis, which is substantially coincident with the central axis of the upper opening23of the cover plate22, on which the flange61is provided (i.e. Z axis). Supporting flange61has an outer sidewall61afacing the exterior of the flange and an upper surface61b.

The supporting flange61can be made of metal, such as aluminium.

The pleated portion83aof the expansible member83is fastened to the supporting flange61through the expansible member base84. Specifically, the base84is fastened to the upper surface61bof the supporting flange61. Thus, the sealing assembly80and in particular the expansible member83is vertically constrained only at one end, but anywhere else the expansible member83can freely move axially. The ring-shaped seal82, operatively surrounded by the non-axially expandable portion83b, tightly engages the supporting rod18athereby closing through-hole34of cap33to the exterior, with consequent reduction the outflow of gases. In an embodiment, a washer65, for example made of fluoropolymer like polytetrafluoroethylene, is disposed between the cover plate22and the outflow flange61in order to reduce the risk of damaging the cover plate22, which is made of quartz. The supporting flange61is secured to the cover plate22by virtue of the compression exerted by the springs62onto the hood cap33.

In an embodiment, the upper surface61bof the supporting flange61has respective axial holes61c(shown inFIG.10) for the insertion of the rivet63to be mounted on the supporting flange61. Rivets63may be made of metal, for example of aluminium.

In one or more embodiments, the supporting flange61has a hollow body limited in the radial direction by the outer sidewall61a. In an embodiment, the supporting flange has no inner sidewall.

Opposite to the upper surface61b, the supporting flange61has a lower surface (not visible) in contact with the washer65.

In one or more embodiments in which the supporting flange61is hollow, its outer sidewall61ahas a radial through-opening68(FIGS.10and11). The radial through opening68can lodge a connecting pipe71(FIGS.11and12) to carry an outflow of the gases exiting the extension tube19from upper opening23to the outside of the apparatus10, for example to a gas collector (non shown) for the recycling of the processing gases, in particular helium.

When the expansible member base84is fixed on upper surface61bof the hollow supporting flange61, the base84covers the upper opening23of the extension tube, except for the tubular hollow space within the expansible member83. However, in view of the presence of the ring-shaped seal82in a tight connection with the supporting rod18a, the outflow of the gases exiting the extension chamber19is substantially fully conveyed out through the opening68of the outflow flange61.

The Applicant has considered that the outflow of gases from the muffle tube11through the sealed chamber11,19by means of an exhaust port of a relatively large outflow cross-section leads to a fast extraction of gases, thus to a fast extraction of helium. Provision of a pipe of relatively small cross-section for the outflow decreases the extraction velocity and/or the flow rate of the exhaust gases, thereby decreasing the usage of helium during the processing of the preform.

The connecting pipe71is provided to collect the gases exiting the furnace from upper opening23. To this end and according to the embodiment shown in the Figures, the exit of the radial opening68is connected to the connecting pipe71by means of a pipe coupler69, which couples the radial opening68with the pipe71(FIG.11). The connecting pipe71exits the hood30through gas discharge opening46and has a pipe outlet74. The pipe outlet74can be connected to an external exhaust pipe, such as to exhaust pipe35(FIGS.1-3), for the discharge of gases or to a different exhaust pipe or system.

As from theFIGS.11and12, the diameter of the connecting pipe71is smaller than the diameter of the discharge opening46. In an embodiment, the connecting pipe71has an orifice of cross-sectional area/diameter of 6 to 10 mm.

In an embodiment, the pressure of the outflowing gasses is controlled to estimate the usage of the gas, such as helium or a gas mixture containing helium. To this purpose and according to an embodiment, a differential pressure cell (DP cell)72is arranged along the pipe71to intercept the gas flow for measurement of a gas differential pressure across the pipe orifice. In known ways, the DP cell72is in communication also with the external air to determine the values of differential pressure from the values of ambient pressure and of the outgoing gas (FIG.12).

In an embodiment, downstream the DP cell72, pressure of the outflowing gas is controlled by a mass flow controller (MFC)73or through a control valve (not shown), which also maintains the desired pressure. The helium pressure may range between 3 and 15 torr.

In a gas having He of 1 to 8 slpm, Cl2from 0.02 to 2 slpm, and O2from 0.1 to 1 slpm, preferably, the pressure inside the processing chamber is of from 1 and 15 Torr, more preferably from 3 to 7 Torr, for example 5 Torr.

In case of damage of the supporting flange or of undesired inflow of atmospheric air, the outflow of gases may be aspirated through the discharge port46to be conveyed to an exhausted gas system (not shown).

Example 1

Several glass optical fibre preforms were fabricated by using an apparatus as described in WO 2018/177514, indicated as comparative apparatus A, or by using an apparatus, which is consistent with the embodiment described with reference toFIGS.6-7and indicated as apparatus B. Except for the provision of the flow control assembly described in the foregoing, the apparatus B corresponds to the apparatus A. The inner diameter of the muffle tube and of the extension tube, namely the diameter of the extended chamber, was of 330 mm. Helium was flown through the extended chamber at various flow rates. For apparatus A, the exhaust pressure (measured in the exhaust pipe35) was −1.5 Torr, whereas for apparatus B the pressure was −0.2 Torr. The quality of the fibres drawn from the optical fibre preforms was checked by determination of DCDR values. A DCDR value is herein defined as the ratio between the sum of all the fibre length portions shorter than 18 m of the drawn optical fibre found, while drawing, to be defective in the diameter measurements, and the overall drawn fibre length.

TABLE 1He flow (slpm)DCDR Apparatus ADCDR Apparatus B25100%<2%30100%<1%35<2%<1%40<1%<1%

With reference to Table 1, at relatively low values of He flow up to 30 slpm (standard liter per minute), apparatus A exhibits an unsatisfactory performance with very high DCDR values, in the practice all of the fibre length portions shorter than 18 m are defective and there is no fiber length portion longer than 18 m without defects. In contrast, at the same helium flow, apparatus B shows a satisfactory performance with DCDR values of less than 1%. While the standard apparatus A needs to use more than 30 splm to provide a valuable optical fibre, apparatus B in accordance with the present disclosure provides a valuable drawn fibre even with a lower Helium flow (25 splm).

Example 2

Several glass optical fibre preforms were fabricated by using the apparatus as described in WO 2018/177514, indicated as comparative apparatus A, or by using an apparatus consistent with the embodiment described with reference toFIGS.5,9-11and indicated as apparatus C. Except for the provision of the flow control assembly, the apparatus C corresponds to the apparatus A. The diameter of the muffle tube and extension tube (extended chamber) was of 330 mm. Helium was flown through the extended chamber at the range rate indicated in Table 2. For apparatus A, the exhaust pressure (measured in the exhaust pipe35) was −1.5 Torr, whereas for apparatus C the pressure was + 1/15 (0.06) Torr. The quality of the fibres drawn from the optical fibre preforms was checked by determination of DCDR values.

TABLE 2He flow (slpm)Apparatus A DCDRApparatus C DCDR3 ÷ 5100%<1%

At very low values of He flow, i.e. 3 to 5 slpm, apparatus A exhibited an unsatisfactory performance with very high DCDR values. In contrast, within the same range of helium flow, apparatus C shows a satisfactory performance with DCDR values of less than 1%.

In addition, due to the very low value of He flow rate, the preforms consolidated in the Apparatus C needed an outgassing process for a length of time that was 40%-60% shorter than that needed with a standard apparatus.

Example 3

The composition of the outflow gas mixture was measured in the exhaust pipe35(consolidation end) of apparatus B of the present disclosure and after various purification steps. The composition is set forth in Table 3.

TABLE 3He %Cl2%O2%N2%@ consolidation end95.41.92.70.005after Cl2removal97.3—2.70.005after O2removal99.995——0.005

The amount of He in the composition of the outflow gas mixture makes its recovery economically convenient as the amount of the gases other than He are low and their elimination yields a substantially pure He.

By applying the apparatus B of the present disclosure, it was possible to recover He at a higher purity. With the comparative apparatus A, due to the entrance of atmospheric air in view of the absence the sealing assembly of the present disclosure, the recovery was not industrially convenient with the same recovery apparatus. In the comparative apparatus A, the amount of 02 was much higher (14%) and He was very diluted.

Previously described embodiments refer to an apparatus for dehydration and consolidation using an extended chamber formed by the connection of the furnace chamber with an upper extension tube. In an operative condition of the furnace, the extension tube is at a temperature lower than the temperature within the furnace because of the axial distance from the heaters12,13. This is particularly the case for the top portion of the extension tube at which the sealing assembly is arranged. Typical temperatures at the top portion of the extension tube19of length of from 500 to 1500 mm, are not higher than 400° C. Polymeric materials included in the sealing assembly according to the present disclosure are selected to withstand these temperatures.