Methods and apparatus for forming heat treated optical fiber

A method for forming an optical fiber includes drawing the optical fiber from a glass supply and treating the fiber by maintaining the optical fiber within a treatment temperature range for a treatment time. Preferably also, the fiber is cooled at a specified cooling rate. The optical fiber treatment reduces the tendency of the optical fiber to increase in attenuation due to Rayleigh scattering, and/or over time following formation of the optical fiber due to heat aging. Apparatus are also provided.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for forming optical fiber and, more particularly, to methods and apparatus for forming optical fiber having improved characteristics.

BACKGROUND OF THE INVENTION

Attenuation and sensitivity to heat (or thermal) aging may be critical attributes of optical fibers, particularly for high data rate optical fibers. In making optical fibers, it may be necessary or desirable to minimize attenuation loss in the intended window of operation for the fiber. Attenuation in an optical fiber can increase after fabrication of the fiber because of a phenomenon called “heat aging.” Heat aging is the tendency of some optical fibers to increase in attenuation over time following formation of the fibers due to temperature fluctuations in the fiber's environment. Typically, the attenuation change from heat aging may be apparent at approximately 1200 nanometers (nm) with increasing effect up to about 1700 nm in a spectral attenuation plot. Furthermore, attenuation may be the result of Rayleigh scattering loss. Therefore, improved methods that reduce fiber attenuation due to heat aging and Rayleigh scattering are desired.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide methods and apparatus for forming an optical fiber, such as a doped optical fiber. As optical fiber is drawn from an optical fiber preform at certain draw speeds and draw tensions, undesirable defects such as heat aging defects are induced into the optical fiber. Likewise, certain draw conditions produce more micro-scale density variations that lead to increased Rayleigh scattering. To combat these defects, the optical fiber is treated in accordance with the invention by maintaining the optical fiber within a treatment temperature range for a treatment time. In particular, it is desired to subject the optical fiber, as drawn, to a specified cooling rate. The phenomena of heat aging is best minimized by performing slowed cooling, preferably, while maintaining the optical fiber within a treatment tension range. Likewise, the phenomena of Rayleigh back scattering is reduced by subjecting the fiber to a specified cooling rate. Thus, advantageously, the invention herein reduces the tendency of the optical fiber to increase in attenuation over time following formation of the optical fiber, i.e., it reduces the so-called heat aging effect. Further, the invention herein further decreases the micro-density variations that contribute to Rayleigh scattering and therefore reduces the attenuation of the optical fiber.

The glass preform, and thus the optical fiber, may be doped with a dopant selected from the group consisting of germanium, fluorine, phosphorous, chlorine or combinations thereof. In particular, certain fiber refractive index profiles are found by the inventors to be more susceptible to heat aging, for example, fibers with high amounts of dopants are found to be very susceptible. All refractive index profiles exhibit attenuation from Rayleigh scattering.

In the various embodiments, the optical fiber is drawn from a draw furnace apparatus. In one embodiment, the drawn optical fiber is passed through a treatment furnace. The treatment furnace is preferably disposed substantially immediately downstream from the draw furnace. Most preferably, the treatment furnace is attached directly to the end of the draw furnace at a location where the fiber exits therefrom such that a seal is preferably formed therebetween. This minimizes unwanted entry of air into the draw furnace.

In further embodiments, the optical fiber is drawn from a draw furnace such that the drawn fiber is initially surrounded by a first gas. The drawn optical fiber may be treated by passing the drawn optical fiber through a passage or chamber of a passive muffle (lacking an active heating element). The passage or chamber preferably contains a second gas having a lower thermal conductivity than the first gas. Preferably, the gases mix and are discharged out of the end of the passive muffle.

According to one embodiment of the invention, the cooling rate of the fiber within the chamber containing the second gas is controlled thereby minimizing the induced heat aging effect. It has been found that a cooling rate of between 840° C./s and 4000° C./s between the temperature range of between about 1100° C. to about 1500° C. is desirable for controlling heat aging of the fiber.

According to other embodiments of the present invention, methods are provided for treating an optical fiber following being drawn. In particular, the treatment advantageously reduces the heat aging effect where the fiber has been formed under such conditions where attenuation thereof tends to increase over time following optical fiber formation. The optical fiber is treated by maintaining the optical fiber within a treatment temperature range for a treatment time while maintaining the optical fiber within a treatment tension range to reduce the tendency of the optical fiber to increase its attenuation over time following formation of the optical fiber.

According to further embodiments of the present invention, apparatus are provided for manufacturing an optical fiber having reduced heat aging defect. In one embodiment, a draw furnace contains a doped glass preform from which the optical fiber can be drawn at a draw speed and a draw tension sufficient to introduce a heat aging defect in the optical fiber. A treatment device is positioned downstream of the draw furnace. The treatment device is operative to treat the optical fiber by maintaining the optical fiber within a treatment temperature range for a treatment time while maintaining the optical fiber within a treatment tension range to reduce the tendency of the optical fiber to increase in attenuation over time after the optical fiber has been formed.

According to further embodiments of the present invention, apparatus are provided for forming and treating an optical fiber. A draw furnace includes an exit wall and is adapted to form the optical fiber such that the optical fiber exits the draw furnace at the exit wall. A treatment furnace is secured to the draw furnace housing adjacent the exit wall and defines a passage therein. The treatment furnace is configured and positioned such that the optical fiber enters the passage as it exits the draw furnace. Preferably, the passage and all passages through which the fiber passes have a minimum dimension of 12 mm such that the gob may drop therethrough.

According to further embodiments of the present invention, apparatus are provided for forming and treating an optical fiber. A draw furnace includes an exit wall and is adapted to form the optical fiber such that the optical fiber exits the draw furnace and the exit wall. The draw furnace contains a first gas, such as Helium, for example. A passive muffle (see definition below) is disposed adjacent the draw furnace and defines a passage. The passage contains a second gas having a lower thermal conductivity than the first gas, such as Argon, for example. The passive muffle is joined to the exit wall such that ambient air cannot enter the draw furnace or the passive muffle at the joinder therebetween. The first and second gasses mix in the passive muffle and exit at an end thereof.

According to further embodiments of the invention, a method of manufacturing an optical fiber at high speed is provided that comprises the steps of drawing the optical fiber from a heated glass supply, such as optical fiber preform, at a draw rate of greater than or equal to 10 m/s, followed by heat treating the optical fiber by maintaining the optical fiber in a heated treatment zone for a residence time greater than 0.07 seconds and less than 0.25 seconds while subjecting the optical fiber to an average cooling rate in the heated treatment zone of greater than 1,200° C./s and less than 5,000° C./s.

According to further embodiments of the invention, a method of manufacturing an optical fiber is provided that comprises the steps of providing a heated glass preform having a germainia-doped central core region and a substantially pure silica cladding region, drawing the optical fiber from a heated glass preform at a draw rate of greater than or equal to 15 m/s and at a draw tension between 25 and 200 grams, and heat treating the optical fiber in a heated treatment zone having an atmosphere containing helium flowing at greater than 10 liters/minute, and having an entry temperature of the optical fiber into the heated treatment zone is greater than 1,600° C., an exit temperature of the optical fiber from the heated treatment zone between 1,300° C. and 1,400° C., and the optical fiber is maintained in the heated treatment zone for a total residence time of greater than 0.07 and less than 0.15 seconds while controlling an average cooling rate of the optical fiber in the heated treatment zone to be greater than 2,000° C./s and less than 3,500° C./s.

Further features and advantages of the present invention will be appreciated by those of ordinary skill in the art from a reading of the figures and the detailed description of the preferred embodiments which follow, such description being merely illustrative of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes methods for treating and for forming and treating drawn optical fibers to reduce the heat aging sensitivity (defects) of the treated optical fibers. As used herein, “heat aging” means a defect in the optical fiber that causes attenuation in the fiber to increase over time subsequent to the initial formation of the fiber. As will be better understood from the description that follows, the methods and apparatus of the present invention may allow for relatively high speed, high tension formation of drawn, doped, optical glass fibers having reduced heat aging sensitivity as compared to like fibers which have been drawn at such speeds and tensions but without the treatment step of the present invention.

With reference toFIG. 1, in accordance with method embodiments of the present invention, an optical fiber is drawn, for example, from a suitable doped glass blank or preform, at a selected speed SDand a selected tension FDthat is sufficient to introduce a heat aging defect in the drawn optical fiber (Block10). Either or both of the core and the cladding (if any) of the drawn fiber may be doped. Typically, the core is doped and may include multiple segments therein, i.e., include a segmented core structure.FIGS. 5–7illustrate delta (%) versus radius (μm) for several fiber refractive index profiles that appear to be sensitive to heat aging and which benefit from being formed and treated in accordance with the present invention. The draw speed SDis preferably maintained between about 2 m/s and 35 m/s during draw. More preferably, the draw speed SDis between about 6 m/s and 25 m/s. Draw speeds SDof greater than about 6 m/s induce some defect for most Dispersion Compensating (DC) fibers, for example, although, in some fibers, the defect may occur for draw speeds as low as 2 m/s or more. The draw tension FDis preferably in the range of between about 25 grams and 200 grams, and more preferably, in the range of between about 90 grams and 200 grams. It has been found that heat aging is typically induced in doped fibers, such as DC fibers, that are drawn at a draw speed of greater than about 6 m/s while being maintained at a draw tension of greater than 90 grams.

It should be noted that in some cases, it is possible to decrease the heat aging effect by operating with different draw conditions, such as operating at a lower draw speed or at a higher draw tension. However, some of these conditions are undesirable for either economic reasons or because the fiber attributes would be undesirable. The present invention allows the production of optical fiber more economically, and with better attributes such as strength, attenuation and uniformity while still producing an optical fiber with less attenuation increase due to heat aging in comparison to untreated optical fibers.

As is shown inFIGS. 6 and 7, such DC fibers14typically have a core including a central core15, a moat16and a ring17. The central core15and ring16typically include germania doping, while the moat typically includes fluorine doping. The delta values for the core15are typically greater than 0.8% and preferably range between about 0.8 to 3.0%, whereas the deltas of the rings17are typically greater than 0.2% and preferably range from between about 0.2 to 1.0% for such DC fibers14. The deltas of the moats16are typically less than −0.2% and preferably range from between about −0.2 to −1.0%. Other fiber types, such as fiber18shown inFIG. 5are also sensitive to heat aging and may include a core15and a ring17.

The heat aging defect induced in the foregoing manner may be detected and measured by the following heat aging test method. First, the drawn fiber is cooled to about 20° C. and thereafter the fiber is heat cycled. The fiber is heat cycled by maintaining the drawn fiber at 200° C. for 20 hours and then cooling the fiber back to 20° C. The attenuation of the drawn fiber is thereafter measured (eg, using an optical bench such as a PK 2500 spectral bench available from Photon Kinetics or an Optical Time Domain Reflectometer (OTDR) apparatus) at the wavelength of interest (typically between 1000 nm–1700 nm). The fiber, when drawn (Block10) and measured in this manner, exhibits an attenuation in the wavelength of interest that is increased by at least 0.03 dB/km to 0.25 dB/km or more in the heat cycled fiber as compared to the cooled fiber prior to heat cycling (un-heat aged fiber) when measured at 1550 nm. Thus, it should be recognized that it is highly desirable to reduce the heat aging effect by treating the fiber in accordance with the invention thereby minimizing any undesirable increase in attenuation.

In order to combat the aforementioned heat aging defect, the temperature TTof the drawn fiber is maintained within a selected temperature range T1to T2for a selected time tTand preferably at a selected tension FT(Block12). Typically, the draw tension FDis the same as the treatment tension FT. In this manner, the heat aging defect present in the drawn fiber prior to the treatment step may be reduced significantly or may even be effectively eliminated.

The foregoing method may be better appreciated from the more detailed description that follows. Suitable and preferred materials and parameters for executing the foregoing steps are set forth below. Additionally, apparatus according to the present invention for conducting the foregoing and other methods are described hereinbelow.

With reference toFIG. 2, an optical fiber forming apparatus100according to embodiments of the present invention is shown therein. The apparatus100includes, generally, a draw furnace120, a treatment furnace150and a tensioning station170, shown as a tractor assembly, for applying tension to the drawn fiber. The apparatus100may be used to form a treated optical fiber110A from a doped glass preform102, for example. More particularly, the draw furnace120may be used to form a drawn optical fiber strand110(hereinafter “the drawn fiber110”) and the treatment furnace150may thereafter be used to treat the drawn fiber110to form a treated optical fiber strand110A (hereinafter “the treated fiber110A”). The treated optical fiber110A being treated so as to minimize the heat aging effect. The tensioning station170serves to control and maintain the desired tension in the fiber110,111A. Additional conventional process steps may be included, such as non-contact diameter measurement apparatus, further fiber cooling apparatus, fiber coating and curing apparatus for applying and curing the primary and secondary fiber coatings, and spool winding apparatus. Such additional process steps are conventional and not shown for clarity. Additionally, an iris or moveable door mechanism may be employed at the bottom of the treatment furnace to minimize the amount of air entry into the treatment furnace.

The glass preform102is preferably formed of a doped silica glass. The preform102may be formed such that either the core or the cladding (if present) of the drawn fiber is doped, or such that both the core and the cladding of the drawn fiber are doped. The silica glass may be doped with one or more of germanium, fluorine, phosphorous or chlorine, or combinations thereof, for example. Other suitable dopants may be used as well. Germanium doped fibers, such as shown inFIGS. 5–7, were found by the inventors to exhibit heat aging under most manufacturing conditions. Methods and apparatus for forming the preform102are well known and are readily appreciated by those of skill in the art. Such methods include IVD, VAD, MCVD, OVD, PCVD and the like.

The draw furnace120preferably includes a housing122surrounding the preform and having a flange123secured on the lower end thereof, the flange123serving as the exit wall of the draw furnace120. An axial opening124is defined in the flange123through which the fiber110passes and through which the previously dropped glass gob may pass. An annular sleeve-like susceptor126(which may be, for example, formed of graphite) extends through the draw furnace120and defines a passage130therein. The passage130includes an upper section adapted to receive and hold the optical fiber preform102and a lower section through which the drawn fiber110passes as glass is melted and drawn off from the preform102. The gob, formed at the initiation of drawing also passes through this section. The lower section of the passage130communicates with the opening124. A hollow exit cone139is preferably positioned over the opening124. An annular insulator132and an induction coil(s)136surround the susceptor126.

A suitable inert forming gas FG, most preferably helium, is introduced into the passage130at about 1 atmosphere of pressure through a suitable flow inlet138and flows downwardly and out of the draw furnace120through the opening124. The draw furnace120, as described and illustrated, is merely exemplary of suitable draw furnaces and it will be appreciated by those of skill in the art that draw furnaces of other designs and constructions, for example, using other types of heating mechanisms, susceptors and insulation, etc. may be employed.

With reference again toFIG. 2, opposed flow passages148extend radially through the flange123and terminate in openings at the upper surface123A thereof. The passages148also extend vertically through the flange123and terminate adjacent the outer periphery of the cone139. Forming gas FG is additionally fed through the openings of the passages148and flows up around the cone139and back down through the center opening of the cone139. The forming gas FG may be, for example, helium gas (He), nitrogen gas (N2), Argon gas (Ar), or any other suitable inert gas. Most preferably, the forming gas FG is helium gas.

The treatment furnace150is positioned below, and preferably interconnected to, the flange123. The treatment furnace150includes a heating unit160with one or more annular heating elements168therein. The heating element may be, for example, an electrical resistance or an induction heating coil. Openings152A and154A are defined in the upper and lower ends of treatment furnace152and154, respectively. The openings along the draw path are sufficiently large to enable the glass gob to drop through upon initiation of draw. The ends152,154and the sleeve146serve as the housing for the treatment furnace150. However, it will be appreciated that other housing configurations and components may be employed. The treatment furnace150is preferably secured to flange123of the draw furnace120by suitable means such as fasteners.

A generally cylindrical quartz spool162is disposed in the heating unit160. The spool162defines a passage162A and has a pair of quartz flanges162B located on opposed ends thereof. The flanges162B may be, for example, flame welded to the ends of a quartz tube to form the spool162. A first graphite gasket164is interposed between the lower surface of the flange152and the upper flange162B. A second graphite gasket164is interposed between the lower flange154and the lower flange162B.

Gas rings166having feed passages166A surround the graphite gaskets164and have small perforations adapted to direct a purge gas PG toward the graphite gaskets164. The purge gas PG is provided to reduce or prevent exposure of the graphite gaskets164to air and may be, for example, helium (He), Argon (Ar), nitrogen (N2), or any other suitable inert gas.

A purge gas member159is affixed to the lower surface of the flange154. A purge gas PG is pumped into the purge tube passage159A to prevent air from entering the passage162A from below.

The passage162A of the quartz tube162preferably has a diameter dimension D of greater than 12 mm at all places along its length, and preferably between about 12 mm and 80 mm, and more preferably between 45 mm and 80 mm to allow the glass gob formed at the initiation of drawing to readily drop therethrough. The length L of the treatment zone of the treatment furnace150extending between the upper surface of the flange152and the lower surface of the flange154is preferably between about 0.2 m and 3 m, and more preferably between 0.5 m and 1.0 m. The preferred length L will depend on the draw speed of the fiber110and the preferred ranges above are for a draw speed of from about 2 m/s to 35 m/s, and more preferably between 6 m/s and 25 m/s.

The tensioning station170may be any suitable device for controlling the tension in the drawn fiber110. Preferably, the tensioning device170includes a microprocessor which continuously receives input from one or more fiber tension and/or diameter sensors (not shown) and is operative to apply the tension of the fiber110as needed. In a preferred embodiment, the tension commanded is based upon controlling the diameter to equal a set diameter stored in memory.

The apparatus100may be used in the following manner to manufacture a treated optical fiber110A. The furnace induction coil136is operated to heat the tip102A of the optical fiber preform102to a preselected draw temperature TD. Preferably, the draw temperature TDis in the range of between about 1800° C. and 2200° C. More preferably, the draw temperature TDis in the range of between about 1900° C. and 2050° C. The preform tip102A is maintained at the selected draw temperature TDso that the drawn fiber110is continuously drawn off of the tip102A in a draw direction V, which is preferably vertically downward. The fiber110is maintained at a calculated draw tension FDas described above by the tensioning device170or other suitable tension applying apparatus such that the set diameter (typically 125 μm) of the fiber is met within a predefined tolerance band. The forming gas FG (e.g., helium) is pumped from the upper inlet138and through the passages130,124,152A,162A,154A and out through the purge tube passage159A.

In this way, the drawn fiber110is drawn off from the preform102at a selected draw speed SDas described above. The selected draw temperature TDand the draw tension FDused to manufacture the fiber causes the fiber110to have the undesirable heat aging defect. That is, as a result of the draw temperature TDand the draw tension FDused to draw the fiber110at the desired speed SD, the drawn fiber110will exhibit a sensitivity to heat aging.

Because the treatment device150is secured substantially immediately adjacent the opening124of the draw furnace120, the drawn fiber110is not quenched by cooler ambient air as the fiber110exits the draw furnace120. Further, the possibility of oxygen getting into the draw furnace is reduced, thereby minimizing possible degradation of the graphite susceptor126. In the present invention, the drawn fiber110passes through the passage124and is substantially immediately heated by the heating unit160. The heating unit160maintains the temperature of the fiber110at a treatment temperature TTwithin a selected temperature range T1to T2. The lower temperature T1is preferably between about 1100° C. and 1400° C. and the upper temperature T2is preferably between about 1200° C. and 1800° C. More preferably, the lower temperature TTis between about 1200° C. and 1350° C. and the upper temperature T2is between about 1300° C. and 1450° C. Also, as the fiber110passes through the passage162A, the fiber110is maintained at a selected treatment tension FT. Preferably, the treatment tension FTis between about 25 and 200 grams. More preferably, the treatment tension FTis between about 90 and 170 grams. The length L of the treatment zone is selected such that the drawn fiber110is maintained within the selected temperature range T1to T2for a selected resident treatment time tT. The treated fiber110A exits the treatment furnace150through the bottom opening154A and preferably continues downwardly to additional processing stations (additional cooling, measurement, coating, etc.).

The above-described treatment temperature TT, treatment tension FTand resident time tTare cooperatively selected to reduce or eliminate the heat aging defect or sensitivity in the fiber110. Accordingly, the treated fiber110A so formed will have a lesser heat aging defect or sensitivity as compared to an optical fiber110which has not been suitably treated in the manner described above (i.e., using the step of Block12inFIG. 1), but which has otherwise been formed in the same manner. The foregoing methods and apparatus thus allow for relatively high speed drawing of optical fiber with reduced heat aging defects as compared to untreated fibers drawn at the same speed.

Preferably, the draw furnace120and the treatment furnace150are relatively configured and secured and the gases are supplied such that they provide an air-tight path from the passage130to the opening159A.

With reference toFIG. 3, an optical fiber forming apparatus200according to further embodiments of the present invention is shown. The apparatus200includes a draw furnace220corresponding to the draw furnace120. In place of the treatment furnace150, the apparatus200includes a passive treatment assembly250. The assembly250is “passive” in that it does not include a heating device corresponding to the heating module160in any portion thereof. In other words, the fiber is cooled at a controlled rate without the aid of an active heating module.

The apparatus200includes a draw furnace220and a tensioning station270corresponding to the draw furnace120and the tensioning station170, respectively. Preferably, the draw furnace220is of the type having a graphite susceptor. The passive treatment assembly250includes a tubular muffle252having an upper flange254. The muffle252is affixed directly to the lower end wall223of the furnace220by bolts or other fasteners (not shown for clarity) that extend through holes in the flange254and engage the end wall223. The muffle252is preferably formed of metal, such as stainless steel or aluminum.

The muffle252defines an upper opening256at a first end, an opposing lower opening258at a second end and a passage252A extending therebetween. Preferably, the diameter E of the passage252A is substantially uniform and greater than 12 mm, more preferably between about 12 mm and 80 mm, and most preferably between 45 and 80 mm. The upper opening256communicates with the lower opening224of the draw furnace220. A plurality of axially spaced supply ports259are formed in the side wall of the muffle252and communicate with the passage252A along its length.

A treatment gas flow system260is operatively and fluidly connected to the muffle252. The treatment gas flow system260includes a treatment gas supply261that is fluidly and operatively connected to each of the ports259by a manifold or conduits262. The treatment gas supply station261includes a supply of a selected treatment gas TG, and a pump or the like operative to pressurize the treatment gas TG sufficiently to force it through the conduits262and the feed ports259and into the passage252A. The treatment gas supply station261may optionally include a heating unit to heat the treatment gas TG. However, preferably the treatment gas is supplied at 20° C.

The apparatus200may be used in the following manner to form a treated optical fiber210A. Using the draw furnace220and the tensioning device270, a fiber210corresponding to the fiber110is drawn from a preform202corresponding to the preform102in the manner described above with regard to the apparatus100, at a draw temperature and a draw tension sufficient to introduce a heat aging defect. As the fiber110is being drawn, a forming gas FG is introduced through an inlet identical to that shown inFIG. 2. The forming gas flows through the passage230about the preform202and the fiber210, through the opening224in the furnace end wall223and into the first end of the passage252A through the opening256.

The drawn fiber210enters the passage252A of the muffle252immediately upon exiting the furnace220. As the fiber210passes through the passage252A, the treatment gas TG is pumped from the treatment gas supply261into the passage252A through the at least two axially spaced supply ports259as indicated by the arrows inFIG. 3. The treatment gas flows into the passage252A at the various stages and mixes with the forming gas FG. Preferably, the treatment gas TG has a thermal conductivity k of less than about 120×10−6cal/(sec)(cm)2(° C./cm), and more preferably less than about 65×10−6cal/(sec)(cm)2(° C./cm) at 25° C. The mixture of the treatment gas TG and the forming gas FG flows through the passage252A and exits through the second end opening258.

The treatment gas TG has a lower thermal conductivity than the forming gas FG. Preferably, the thermal conductivity of the treatment gas TG is less than 40% of, and more preferably less than 20% of, the thermal conductivity of the forming gas FG. The treatment gas TG is preferably nitrogen or argon. More preferably, the treatment gas TG is argon. The forming gas FG is preferably helium.

As the drawn fiber210is drawn through passage252A, the drawn fiber210is maintained at the selected treatment tension FT, and the treatment temperature TTof the fiber210while in the passage252A is maintained in the selected temperature range T1–T2for the selected residence time tTas discussed above with respect to the apparatus100. In the manner described above with respect to the apparatus100, the selected treatment tension FT, temperature range T1to T2and residence time tTare cooperatively selected such that they reduce or eliminate the heat aging defect in the fiber210, thereby providing a treated fiber210A corresponding to the treated fiber110A. In the case of the apparatus200, the length M of the passage252A of the passive treatment device250is selected to provide the desired residence time tTin view of the draw speed of the fiber210.

The lower thermal conductivity of the treatment gas TG slows heat transfer from or cooling of the drawn fiber210so that the fiber210is maintained in the selected temperature range T1–T2while in the passage252A. The flow rate, turbulence and temperature of the treatment gas TG may be selected as appropriate to provide the desired cooling rate. In accordance with this embodiment of the invention, the desired cooling rate in the treatment furnace250is between 2500° C./sec and 3500° C./sec in a temperature range of between 1200° C. to 1500° C.

With reference toFIG. 4, an optical fiber forming apparatus300according to further embodiments of the present invention is shown therein. The apparatus300includes a draw furnace320of the type having a graphite susceptor. The apparatus300corresponds to the apparatus200except as follows and may be used in the same manner except as follows.

The muffle250is replaced with a multi-piece muffle assembly349defining a continuous passage349A. The muffle assembly349includes an annular upper muffle section351including a flange354for securing the muffle assembly349to the exit wall323of the draw furnace320. A second annular muffle section353is affixed to the lower end of the muffle section351and defines a passage353A. An outlet port357is formed in the side of the muffle353and communicates with the passage353A. A third annular muffle section352is affixed to the lower end of the muffle section353and defines a passage352A. A fourth annular muffle section355is fixed to the lower end of the muffle section352and defines a passage355A. A feed port359is formed in the muffle355and communicates with the passage355A. The diameter F of the passage349A is preferably substantially uniform and preferably greater than 12 mm, more preferably between about 12 mm and 80 mm, and most preferably between 45 and 80 mm and is preferably of substantially constant diameter along its length N. The length N of the muffle assembly349is preferably between about 0.2 m and 1.0 m.

Additionally, in the apparatus300, the treatment gas flow apparatus260is replaced with a treatment gas flow system360. The flow system360includes a treatment gas supply361corresponding to the treatment gas supply station261. The treatment gas supply station361is fluidly connected to the feed port359by a conduit362. The flow system360further includes a pump364fluidly connected to the outlet port357by a conduit363. The pump364is preferably a Venturi pump that is provided with a supply of compressed air from inlet365A as illustrated.

In use, the treatment gas TG is introduced from the treatment gas supply361through the conduit362and the feed port359into the passage355A. The pump364provides a sufficient vacuum and resultantly draws at least a portion of the treatment gas TG up through the passages352A and353A, through the outlet port357and the conduit363, and out through an outlet365B. Simultaneously, the vacuum generated by the pump364draws the forming gas FG from the draw furnace320through the passage353A, the outlet port357and the conduit363, and out through the pump outlet365B as well. This is beneficial, because it prevents the mixing of the two gasses in the lower end of the passage349A.

Using a draw furnace, a negative dispersion germania-doped optical fiber having a profile including a core and a ring as shown inFIG. 5was drawn from a doped preform at a rate of 14 meters per second (m/s) with a tension of 150 grams. Thereafter, the fiber was cooled to 20° C. and then subjected to the heat aging test as described above. Following this test, the measured attenuation increase in the untreated fiber at 1550 nm was 0.0830 dB/km.

A second fiber was drawn from an identical preform in the same manner as described just above. The second fiber was passed through a treatment apparatus in accordance with the invention as described inFIG. 4immediately after the fiber exited the draw furnace. The length and operating parameters of the treatment furnace were selected such that the temperature of the second fiber was maintained at a desired temperature for a desired amount of time. In particular, the length M of passage was about 0.615 m. Thus, the fiber was maintained at a temperature of from about 1700° C. to about 1525° C. for a residence time of about 0.044 seconds while the tension in the fiber was maintained at 150 grams. The forming gas FG was helium and the treatment gas TG was argon at 23° C. Thereafter, the fiber was cooled to 20° C. and then subjected to the same heat aging testing as heretofore described. The measured attenuation in the fiber subjected to the treatment increased only 0.027 dB/km at 1550 nm. Thus, for this fiber type as shown inFIG. 5, a 67% reduction in the heat aging was obtained by subjecting the fiber to the additional treatment step in accordance with the invention.

Using a draw furnace, a negative dispersion germania and fluorine doped optical fiber having a profile including a core, moat and a ring as shown inFIG. 6was drawn from a preform at a rate of 14 meters per second (m/s) with a tension of 150 grams. Thereafter, the fiber was cooled to 20° C. and then subjected to the heat aging test as described above. Then testing revealed that the measured attenuation increase in the fiber at 1550 nm was 0.285 dB/km following heating for 20 hours at 200° C.

A second fiber was drawn from an identical preform in the same manner as described just above. The second fiber was subjected to the treatment apparatus and method in accordance with the invention described inFIG. 4herein immediately after the fiber exited the draw furnace. The length and operating parameters of the treatment furnace were selected such that the temperature of the second fiber was maintained at the conditions identified in Example 1. Thereafter, the fiber was cooled to 20° C. and then subjected to the same heat aging testing as heretofore described. The measured attenuation increase in the fiber subjected to the treatment was only about 0.033 dB/km at 1550 nm. Thus, for this fiber type, a dispersion compensating fiber having a positive delta core, a negative delta moat and a positive delta ring, it should be recognized that a dramatic reduction (88%) in the heat aging was obtained by subjecting the fiber to the additional treatment step. The cooling rate applied in the previous two examples was approximately 3980° C./s.

Using a draw furnace, a germania and fluorine doped silica glass optical fiber having a negative dispersion and dispersion slope and a profile as shown inFIG. 5was drawn from a preform at a rate of 14 meters per second (m/s) with a tension of 150 grams. A helium forming gas was used in the draw furnace. Thereafter, the fiber was cooled to 20° C. and then subjected to the heat aging testing where the fiber is maintained at 200° C. for 20 hours. At the end of this period, the fiber was cooled to 20° C., the measured attenuation increase in the fiber at 1550 nm was 0.420 dB/km.

A second fiber was drawn in the same manner as described just above from an identical fiber. The second fiber was passed through a heated treatment apparatus as shown inFIG. 2immediately after the fiber exited the draw furnace. The length of the muffle was 0.4 m and its inside diameter was 60 mm and the temperature was selected such that the temperature of the second fiber was maintained at from about 1700° C. to about 1525° C. for a residence time of about 0.028 seconds while the tension in the fiber was maintained at 150 grams. The second fiber was heat aging tested as before and the measured attenuation increase in the fiber at 1550 nm was 0.0015 dB/km. Thus, the present invention resulted in a 96% reduction in heat aging.

Other actual experimental examples are illustrated in Table 1. Listed are the Example Number (Ex.), the attenuation change with (With Treat) and without (W/O Treat) the heat aging reduction treatment, the % reduction in heat aging when treated (% Red.), the fiber profile (Prof.) of the fiber treated, the dopants present in the treated fiber (Dop.), the draw tension used (Tens.), the draw speed used (Draw Speed), the apparatus used (App.), and whether the apparatus included a heater (Heater).

Table 1 illustrates the results for the various example.

Another embodiment of the invention is shown and described with reference toFIGS. 8–11. In accordance with this embodiment, a method for high speed drawing (at draw rates of greater than or equal to 10 m/s) and heat treatment of optical fiber is provided. As best shown inFIG. 11, a treated optical fiber is produced at high speed from an optical fiber forming apparatus400according to embodiments of the present invention. The apparatus400includes, generally, a draw furnace420, followed downstream by, and preferably mechanically coupled to, a heat treatment furnace450. The apparatus400may be used to form a heat treated optical fiber410A by drawing it at high speed from a heated glass supply402, such as a optical fiber preform for example, and then subjecting the drawn fiber to a defined temperature profile (a time-temperature profile, for example, as shown inFIG. 10) such that attenuation of the fiber due to Rayleigh Scattering is reduced. The method described herein is particularly effective at producing, at high speed, germanium-doped central core optical fibers having reduced attenuation due to reduced Rayleigh scattering loss. In particular, the method is well adapted to providing low attenuation (less than or equal to 0.187 dB/km at 1550 nm and/or less than or equal to 0.327 at 1310 nm) in optical fibers having germanium-doped central cores. One such fiber is a single mode step index fiber example as shown inFIG. 9having a germanium-doped central core414and a substantially pure silica cladding415surrounding and abutting the core.

More particularly, the draw furnace420may be used to form a drawn optical fiber strand410(hereinafter “the drawn fiber410”) at high speed and the treatment furnace450may thereafter be used to heat treat the drawn fiber410, thus formed at high speed, to produce a treated optical fiber strand410A (hereinafter “the treated fiber410A”). The treated optical fiber410A is heat treated to preferably reduce the attenuation due to Rayleigh back scattering over an operating wavelength (with attenuation, for example, of less than or equal to 0.327 dB/km at 1310 nm, and preferably less than or equal to 0.187 dB/km at 1550 nm).

As should be recognized, additional apparatus may be included for performing subsequent conventional process steps after the heat treating step. For example, a non-contact diameter measurement apparatus404for measuring a representative diameter of the fiber may follow after the heat treatment step. Further, a fiber cooling apparatus406may be provided for even further cooling the treated fiber410A to a sufficiently low temperature (for example, less than about 100° C.) to allow a protective polymer coating(s) to be applied to the outer periphery of the treated fiber410A. A fiber coating apparatus408A and curing apparatus408B for applying and curing the primary polymer coating may also be provided. Furthermore, additional coating and curing apparatus may be provided for applying and curing a secondary polymer coating (not shown). Tensioning apparatus470are preferably provided for applying the desired draw tension to the fiber after it is coated. Finally, a spool winding apparatus471and reciprocating guide469may be provided for winding the heat treated and coated optical fiber onto a winding spool473, such as a shipping or bulk spool. Additionally, an iris or moveable door mechanism472may be employed at the bottom of the treatment furnace450to minimize the amount of air entry into it from the exit.

In operation, the method in accordance with embodiments of the invention comprises the steps of drawing the optical fiber410from a heated glass supply, such as an optical fiber preform402(preferably including a germania-doped central core region and a substantially pure silica cladding region—corresponding to, and forming when drawn, an optical fiber having a germania-doped central core and a cladding of substantially pure silica), at a draw rate of greater than or equal to 10 m/s, followed by heat treating the optical fiber by maintaining the optical fiber in a heated treatment zone412for a residence time (preferably greater than 0.07 and less than 0.25 seconds) while subjecting the optical fiber410to an average cooling rate in the heated treatment zone412of greater than 1,200° C./s and less than 5,000° C./s; more preferably greater than 2,000° C./s and less than 5,000° C./s, and in some embodiments, greater than 2,000° C./s and less than 3,500° C./s. The average cooling rate is preferably greater than 2,000° C./s and less than 5,000° C./s, for example, when the draw speed is greater than or equal to 20 m/s. The average cooling rate in the heated treatment zone412is defined as the fiber surface temperature at the entry point “A” (the fiber entry surface temperature) of the fiber minus the fiber's surface temperature at an exit point “B” (the fiber exit surface temperature) of the fiber divided by the total residence time of the fiber in the treatment zone.

The method in accordance with embodiments of the invention is particularly well suited for reducing the attenuation due to Rayleigh scattering of an optical fiber410, such as the standard step index single mode fiber shown inFIG. 9. The method is particularly well suited for manufacturing an optical fiber having a central core414at the fiber's centerline including a germanium dopant and a cladding415including substantially pure silica (with no appreciable refractive index altering dopants). As used herein, the term “central core” refers to the portion of the fiber where the majority of the light is confined when in operation, which is at the center part of the fiber, and which has a higher refractive index portion as compared to an outermost glass cladding portion. The cladding415is that part of the fiber410that surrounds and abuts the central core and which extends to the outside diameter of the glass portion of the fiber (to a diameter of about 125 microns) and which has a lower refractive index than the central core414. As should be recognized, the heated glass source preform402also includes a core region414A and a cladding region415A (shown partially cut away inFIG. 11) whose physical proportions and composition roughly corresponds to the central core414and cladding415of the fiber drawn therefrom. In other words, the core region414A is doped with at least germanium and the cladding region415A is formed of substantially pure silica.

The method in accordance with embodiments of the invention will be further described with reference toFIGS. 10 and 11.FIG. 10illustrates one preferred cooling profile for forming the heat treated optical fiber410A in accordance with the invention. Preferably, after being drawn from the heated glass supply preform402(having a root portion heated to about 1800–2200° C.), the drawn fiber410enters into the treatment furnace450at time equals 0.0 seconds such that the fiber410has a fiber entry surface temperature preferably between an upper temperature411A of 1700° C. and a lower temperature411B of 1,200° C. at the point of entry into the treatment zone412of the furnace (designated “A”); more preferably between 1,550° C. and 1,700° C.; and in some embodiments greater than 1600° C. The fiber410is then heated and slow cooled in the treatment zone412and spends a sufficient total residence time tr in the treatment zone412(between 0.07 and 0.25 seconds) such that partial fiber annealing (slowed but non-equilibrium reordering of the glass on an atomic scale) takes place. The surface temperature of the fiber at the exit (designated “B”) from the heat treatment zone is between an upper temperature413A of 1,450° C. and a lower temperature413B of 1,250° C. Annealing in accordance with the invention herein reduces the Rayleigh back scattering loss and, therefore, reduces the attenuation of the treated optical fiber410A at the wavelengths of interest (e.g., 1310 nm and 1550 nm) as compared to untreated fiber.

As is shown inFIG. 11, the draw furnace420and treatment furnace450are preferably configured to form a continuously enclosed path for the optical fiber as it passes between the drawing and heat treating steps. For example, the treatment furnace450may mount directly to a lower flange420B of the draw furnace420or attach to an interconnection member420C such as tube shown. The fiber is disposed in an inert atmosphere and is free from air exposure while it passes between the draw furnace420and the treatment furnace450. This advantageously minimizes air exposure to the graphite muffle tube432of the draw furnace420that may cause degradation thereof.

As shown inFIG. 8, during the steps of drawing403and heat treating405, an atmosphere preferably containing an inert gas is provided. The inert gas may be helium, nitrogen, argon, or mixture thereof Preferably, the inert gas (preferably a helium gas) is supplied at draw furnace inlet420A at the top of the draw furnace. The inert gas travels alongside of the glass supply preform402and exits the draw furnace420at a lower end thereof along with the fiber410. The gas then travels along with the fiber (but generally at a different rate) through the passage413of the treatment furnace450and exits through a lower end (at point “B”) of the treatment furnace. The flow rate of the inert treatment gas (preferably helium) through the passage413of the heated treatment zone412of the treatment furnace415during the step of heat treating is preferably greater than 10 liters/minute, and most preferably between 10 and 50 liters/minute.

Optionally, the draw furnace520may be configured such that it is separated from, i.e., the exit end of the draw furnace is not directly connected to the entrance end of the heat treatment furnace550, as shown in partial view ofFIG. 12. In this configuration, the gas atmosphere disposed in the treatment furnace550may contain a different gas than in the draw furnace. For example, an atmosphere containing argon only, or mixture of both helium and argon, may be provided inside the passage513during the step of heat treating such that the fiber is disposed in an inert atmosphere. Preferably, for example, a draw gas, such as helium, is provided to flow through the draw furnace520during the step of drawing, while a treatment gas (such as substantially pure nitrogen, pure argon, or a mixture of substantially pure argon and substantially pure helium) is provided to the treatment furnace550during the step of heat treating. For example, as shown inFIG. 12, the treatment gas in the treatment furnace550may be provided at input port511and extracted at the bottom of the treatment furnace at515. The flow rate of the inert treatment gas through the passage513of the heated treatment zone512of the treatment furnace550during the step of heat treating is preferably greater than 10 liters/minute, and most preferably between 10 and 50 liters/minute. The additional process components (e.g., additional measurement, cooler apparatus, coating/curing apparatus, and winding apparatus) are not shown for clarity inFIG. 12.

In a preferred embodiment ofFIG. 11, the optical fiber410is drawn through the heated treatment zone412at a draw rate of greater than or equal to 10 m/s; more preferably greater than or equal to 15 m/s; and in some embodiments, greater than or equal to 20 m/s. Preferably, the fiber410is a single mode step index fiber such as is shown inFIG. 9having a germanium dopant in the central core414and a substantially pure silica cladding415. However, it should be recognized that the method described herein is equally useful and adapted for treating any optical fiber having a germanium-doped central core. Preferably, the germanium is present in the core in a sufficient amount to provide a relative refractive index percent of at least 0.3% as compared to the cladding. The fiber410is preferably drawn by heating the preform402to a flowing consistency (1800–2200° C.) at its draw root and applying a draw tension to the coated optical fiber by using a tensioning apparatus470set to provide a tension of between about 25 grams to about 200 grams; more preferably between about 60 and 170 grams; and most preferably about 90–150 grams. Drawing at high speed and tension enables production of large volumes of the optical fiber which is then heat treated in accordance with aspects of the present invention to further minimize the attenuation of the produced fiber as compared to untreated fiber.

The drawn fiber410is maintained in the heated treatment zone412for a total residence time of greater than 0.07 seconds and less than 0.25 seconds; more preferably greater than 0.07 and less than 0.15 seconds; and in some embodiments less than 0.1 seconds. Following treatment, the fiber then exits the zone412at the exit to the treatment zone412(point “B”). The average cooling rate for the fiber410while passing through the treatment zone412is preferably greater than 1,200° C./s and less than 5,000° C./s; more preferably greater than 2,000° C./s and less than 5,000° C./s; and in some embodiments, greater than 2,200° C./s and less than 3,500° C./s. During the treatment step, the walls414of furnace's treatment zone412are heated and maintained at an appropriate temperature to provide a passage temperature (at the center of the passage413where the fiber travels) in at least a portion of the heated treatment zone412of greater than 1,300° C.; more preferably between 1,400 and 1,600° C. The heat treating step is accomplished by one or more heaters, which may be resistance-type heaters, for example.

In a preferred embodiment, the treatment furnace450includes a plurality of individual heaters (c–h) spaced along the axial length of the treatment furnace450. Each of the heaters encircles the fiber, and each is preferably individually controlled by a controller417. During the step of heat treating, the fiber is subjected to heat from multiple heating zones; at least one of the heating zones (each zone roughly corresponding to the physical size of the heaters (c–h)) of the multiple heating zones is set to different temperature as compared to another of the multiple heating zones. Preferably, the temperature of the wall414of each heater is controlled by a controller417such that at least one of the heating zones c–h has a passage temperature of between 1,400° C. and 1,600° C. In a preferred mode of operation, a first zone (example c) closer to the draw furnace420is controlled to have a passage temperature at its center (at point “c′”) of between 1,100° C. and 1,300° C., while a second zone (example h) further away from the draw furnace is controlled to have a passage temperature (at point “h′”) of between 1,400° C. and 1,500° C. The actual wall temperatures will be set such that the desired fiber exit surface temperature condition is achieved to provide the desired cooling rate. If the gas used is other than helium, for example, the wall temperature would be set to a lower temperature because the thermal conductivity of Argon and mixtures of Agron and Helium would have a lower coefficient of thermal conductivity and, therefore, more of a temperature difference is required between the furnace's passage temperature and the fiber temperature to achieve the same cooling rate.

In accordance with embodiments of the invention, it is preferable to configure and locate the treatment furnace450to provide an fiber entry surface temperature of the optical fiber, as it enters the treatment zone412of between 1,200° C. and 1,700° C. at point “A”; more preferably between 1,550° C. and 1,700° C.; and in some embodiments, greater than 1,600° C. Preferably also, it is desired to configure the length and operating temperature of the treatment furnace450to provide an exit temperature of the optical fiber410A at an exit of the treatment zone412at point “B” of between 1,250° C. and 1,450° C.; more preferably between 1,300° C. and 1,450° C.; and most preferably between 1,325° C. and 1,425° C.

According to one embodiment illustrated inFIG. 11, it is desirable to provide an entry temperature of the optical fiber410as the fiber enters the treatment zone450(at “A”) of between 1,550° C. and 1,700° C.; more preferably between 1,600° C. and 1,700° C., and provide an exit temperature of the heat treated optical fiber410A as the treated fiber exits the treatment zone412(at “B”) of between 1,300° C. and 1,450° C., and more preferably between 1,325° C. and 1,425° C.

The muffle tube416of the treatment furnace450can be preferably manufactured from a substantially pure silica quartz glass, ceramic and/or carbon materials. The heating elements of the treatment furnace are preferably molydisilicide high temperature heating elements available from Kanthal. The inner diameter of the tube416is preferably about 60 mm. The construction of the heating furnace ofFIG. 12is such that it has the same components as described forFIG. 11.

As shown inFIG. 12, fiber510is drawn from and heat treated by apparatus500. The fiber510is drawn from the heated glass supply502at a draw speed of greater than 10 m/s and at a draw tension between 25 and 200 grams. A helium atmosphere is provided in the draw furnace520. An air entry preventer572such as a gas shield, moveable iris, or door mechanism is preferably employed at the lower end of the draw furnace to minimize intrusion of air that may into the furnace chamber that may cause degradation of the graphite muffle tube526. A multi-element heat treatment furnace 550 is provided downstream from the draw furnace 520. The structure is identical to that described with respect toFIG. 11, except that it is physically separated from the draw furnace 520 by a space where the fiber passes through air. Preferably, the cooling profile in the heated treatment zone 512 of the heat treating furnace 550 is arranged and configured the same as taught inFIG. 10such that the total residence time in the zone is between 0.07 and 0.25 seconds, and the average cooling rate in the zone 512 is preferably greater than 1,200° C./s and less than 5,000° C./s; more preferably greater than 2,200° C./s and less than 3,500° C./s. Likewise, the heat treatment furnace 550 is configured and positioned such that the fiber entry surface temperature at point A is between 1,400° C. and 1700° C. (between 1,550° C. and 1,700° C. for draw speeds greater than or equal to 15 m/s) and an fiber exit surface temperature of the heat treated optical fiber510A as the treated fiber exits the treatment zone 512 (at point “B”) of between 1,325° C. and 1,425° C.

EXAMPLES

Table 2 below illustrates the results for various experimental examples (13–18) of fiber actually produced using the treatment apparatus ofFIG. 11.

FIG. 11shows the apparatus400used for producing the treated fiber410A of Example 13, except that the treatment furnace in this example included only two heater elements. From the preform402, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9and the profile core delta and radius is selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and provides and enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperatures of the two heater elements were set to 1,250° C. The treatment zone412of the furnace450was 1.19 m long and the muffle tube416of the treatment furnace450was pure quartz having an inner diameter of 60 mm. The optical fiber410was drawn at a draw rate of 10 m/s and passed through the treatment furnace450such that the total residence time was 0.119 seconds in the zone412. The fiber's entry surface temperature was 1440° C. and the fiber's exit surface temperature was 1,270° C. Accordingly, the average cooling rate in the treatment zone412was 1,430° C./s. The attenuation of the fiber produced in accordance with the method was measured to be 0.327 dB/km at 1310 nm, and 0.186 dB/km at 1550 nm.

Again,FIG. 11shows the apparatus400used for producing the treated fiber410A of Example 14. In this example, the heating elements c–d, e–f, and g–h were wired together in pairs such that the combined elements c–d, for example, act as a single heater element. Likewise, e–f and g–h are also wired together, thereby producing three independently controllable heating elements. From the preform402, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9and was selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and thereby forms an enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperature of the heater elements c–d, e–f, and g–h were set to 1,250° C., 1,450° C., and 1,450° C., respectively. The treatment zone412of the furnace450was 1.77 m long and the muffle tube416of the treatment furnace450was pure quartz tube having an inner diameter of 60 mm. The optical fiber410was drawn at a draw speed of 15 m/s and passed through the treatment furnace450such that the total residence time was 0.118 seconds in the zone412. The fiber's entry surface temperature was 1,560° C. and the fiber's exit surface temperature was 1,370° C. Accordingly, the average cooling rate in the treatment zone412was 1,610° C./s. The attenuation of the fiber produced for Example 14 was measured to be 0.322 dB/km at 1310 nm, and 0.185 dB/km at 1550 nm.

The configuration of the apparatus of Example 3 was the same as described for Example 13 (having only two heater elements). From the preform402, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9and its core delta and radius was selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and forms an enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperatures of the two heater elements were set to 1,150° C. The treatment zone412of the furnace450was 1.19 m long and the muffle tube416of the treatment furnace450was pure quartz having an inner diameter of 60 mm. The optical fiber410was drawn at a draw rate of 15 m/s and passed through the treatment furnace450such that the total residence time was 0.079 seconds in the zone412. The fiber's entry surface temperature was 1,560° C. and the fiber's exit surface temperature was 1,270° C. Accordingly, the average cooling rate in the treatment zone412was 3,670° C./s. The attenuation of the fiber produced in accordance with the method was measured to be 0.326 dB/km at 1310 nm and 0.185 dB/km at 1550 nm.

FIG. 11shows the apparatus400used for producing the treated fiber410A of Example 16. The configuration of the heating elements was the same as described for Examples 13 and 15. From the preform402, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9was selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and forms an enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperature of the two heater elements employed were set to 1,300° C. The treatment zone412of the furnace450was 1.19 m long and the muffle tube416of the treatment furnace450was pure quartz having an inner diameter of 60 mm. The optical fiber410was drawn at a rate of 15 m/s and passed through the treatment furnace450such that the total residence time was 0.079 seconds in the zone412. The fiber's entry surface temperature was 1,560° C. and the fiber's exit surface temperature was 1,360° C. Accordingly, the average cooling rate in the treatment zone412was 2,530° C./s. The attenuation of the fiber was measured to be 0.326 dB/km at 1310 nm, and 0.184 dB/km at 1550 nm.

The apparatus used for producing the treated fiber of Example 17 is the same as described in Example 14. From the preform, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9was selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and provides and enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperature of the heater elements c–d, e–f, and g–h were set to 1,150° C., 1,150° C. and 1,450° C., respectively. The treatment zone412of the furnace450was 1.77 m long and the muffle tube416of the treatment furnace450was pure quartz having an inner diameter of 60 mm. The optical fiber410was drawn at a draw rate of 24 m/s and passed through the treatment furnace450such that the total residence time was 0.074 seconds in the zone412. The fiber's entry surface temperature was 1,690° C. and the fiber's exit surface temperature was 1,360° C. Accordingly, the average cooling rate in the treatment zone412was 4,460° C./s. The attenuation of the fiber produced in accordance with the method was measured to be 0.325 dB/km at 1310 nm and 0.187 dB/km at 1550 nm.

The apparatus used for producing the treated fiber of Example 18 is the same as described in Examples 14 and 17. From the preform402, a single mode step index fiber was drawn having a core of germanium-doped silica and a cladding of substantially pure silica. The fiber was drawn at a draw tension of 100 grams. The refractive index profile of the fiber is shown inFIG. 9was selected to provide a total dispersion of the fiber between 16 and 22 ps/nm/km at 1550 nm. The heat treatment furnace450is coupled directly to the draw furnace420and provides and enclosed pathway for the flow of about 23 liters/minute of substantially pure helium treatment gas from the inlet420A around the preform402, through the passageway413, and exiting at point B. The temperature of the heater elements c–d, e–f, and g–h were set to 1,150° C., 1,150° C. and 1,550° C., respectively. The treatment zone412of the furnace450was 1.77 m long and the muffle tube416of the treatment furnace450was pure quartz having an inner diameter of 60 mm. The optical fiber410was drawn at 24 m/s and passed through the treatment furnace450such that the total residence time was 0.074 seconds in the zone412. The fiber's entry surface temperature was 1,690° C. and the fiber's exit surface temperature was 1,380° C. Accordingly, the average cooling rate in the treatment zone412was 4,190° C./s. The attenuation of the fiber was measured to be 0.325 dB/km at 1310 nm and 0.186 dB/km at 1550 nm.

Table 3 below illustrates the calculated results for various theoretical examples (Examples 19–22) using the treatment apparatus ofFIG. 11.

In the theoretical examples provided in Table 3, it should be recognized that for high speed treatment of fibers having germania-doped central core and substantially pure silica cladding as shown inFIG. 9, the desired cooling rate is preferably greater than 1,200° C./s; and more preferably greater than 1,200° C./s and less than 5,000° C./s. Preferably, the total residence time in the heated treatment zone is between 0.07 and 0.25 seconds; and more preferably between 0.07 and 0.15 seconds.