DE3686891T2 - MANUFACTURE OF OPTICAL FIBERS. - Google Patents

Description

This invention relates to the production of fibers suitable for the transmission of radiation in the optical spectral range, that is to say radiation of wavelengths in the ultraviolet, visible and infrared ranges.

The ability to introduce small amounts of impurity dopants, such as rare earth or transition metal ions, into the core or cladding of an optical fiber is useful for a number of reasons.

The manufacture of fiber optic amplifiers or lasers with, for example, neodymium or erbium as an impurity dopant is possible. An example of a neodymium fiber laser is described in our co-pending international patent application No. WO 87/01246, which has a priority date of August 13, 1985.

It is known from US-A-4 597 787 to deposit dopant in a chamber and evaporate it from there into a gas stream containing glass-forming material and then deposit the glass to create a preform for drawing an optical fiber. The preamble of claim 1 is known from this publication.

It is well known that the introduction of, for example, terbium ions (Tb³⁺) into the silica matrix increases the Verdet constant of the glass, and this is beneficial for fiber devices or sensors that interact with magnetic fields.

To indicate the temperature of a medium surrounding the fiber, a distributed temperature sensor can be constructed which exploits the temperature dependent changes in either the absorption spectrum or the fluorescence decay time of a rare earth or transition metal ion, for example Nd³⁺ or Cr³⁺. See for example Snitzer, Morley and Glenn "Fiber optic rare earth temperature sensors" Proc. Ist Int. Conf. on Optical Fibre Sensors, London 1983, pp. 79-92.

It is known that both the Kerr effect and the nonlinear optical coefficients of the glass are increased by the insertion of, for example, Nb³⁺ ions into the silicate glass matrix.

The introduction of certain ions, for example cerium, into the glass allows the construction of scintillation counters by converting the energy of an incident high-energy particle or beam into an optical signal that propagates within the fiber. Doping of optical fibers with cerium oxide is described in JP-A-56-155036.

We have developed a new manufacturing technique that allows the production of optical fibers containing controllable, low (< 1 wt.%) amounts of one or more impurity dopant ions in either the core or cladding glass of an optical fiber, or both. The process allows the use of precursors, such as rare earth halides, that have a high melting point and have not been useful heretofore because they have a very low vapor pressure at the temperatures typically found in reactant delivery systems for producing optical fibers. This temperature is usually limited to around 250ºC, a temperature at which the polytetrafluoroethylene components used in the rotating seal connecting the precipitation tube to the reactant delivery system begin to deform. Our process is also applicable to liquids that have a low vapor pressure at low temperatures (< 250ºC).

The impurity dopant or dopants introduced into the glass using the process can themselves create the refractive index difference or differences required for the fiber to function as a light guide. Alternatively, the difference in refractive index can be achieved in combination with commonly used optical fiber dopants such as boron trioxide, fluorine, germanium oxide, phosphorus pentoxide and titanium oxide. The technique is unique in that it allows the manufacture of long lengths of fiber containing, for example, rare earth ions that have relatively high absorption in the visible/near infrared region, while essentially retaining the low loss properties of communications grade fiber at other wavelengths.

According to the present invention there is provided a method of producing a preform for the manufacture of optical fibers containing doped glass, wherein a dopant material is deposited in a doping chamber and subsequently evaporated from the chamber and deposited on the inner surface of a tubular glass member, characterized in that the method comprises the sequential steps of: heating the doping chamber to fuse the dopant to the chamber wall, subsequently heating said chamber to cause said dopant to vaporize at a predetermined rate, flowing a gaseous source material through said support chamber to mix said dopant with the source material, precipitating a mixture of solid components from the mixture of said source material and said dopant, and fusing said solid components for forming a doped glass for said preform.

The method preferably includes the successive steps of

(i) one or more dopants is or are deposited in one or more dopant carrier chambers,

(ii) the chamber(s) is heated in a dehydrating atmosphere to purify the dopants and, in the case of solid dopants, to melt them to the chamber walls,

(iii) the dopant or dopants are heated within the chamber(s) to cause them to vaporize at a predetermined rate, while gaseous source substances are passed through the carrier chamber(s) to mix the dopant or dopants with the source substances,

(iv) a mixture of solid components is deposited from the mixture of said source substance(s) and said dopant(s),

(v) the precipitated mixture is heated in a dehydrating atmosphere to purify the precipitated mixture,

(vi) the solid components are melted to form a precipitated glass,

(vii) the hollow tube is caused to collapse into a solid rod,

(viii) the rod is pulled to form an optical fibre.

Of the above, steps (i) and (ii) may be carried out either before mounting the tube in a lathe for producing the preform or on the tube mounted in the lathe. The steps (iii) and (iv) are always carried out with the precipitation tube mounted in the production lathe. Steps (v) to (vii) may be carried out either with the tube mounted in the production lathe or during the fibre drawing process (step (viii)).

The invention will now be described with reference to the attached drawings. It shows:

Fig. 1 shows the chemical vapor deposition device used in the manufacture of optical fibers;

Fig. 2 shows the absorption spectrum of a fiber containing 30 ppm Nd³⁺;

Fig. 3 the fluorescence spectrum of a fiber containing 300 ppm Nd³⁺;

Fig. 4(a) the local attenuation of a Nd³⁺-doped single-mode fiber;

Fig. 4(b) the corresponding absorption of a reference fiber;

Fig. 5 is an absorption spectrum showing losses due to hydroxyl ion absorption; and

Fig. 6 the absorption spectrum of a single-mode fiber that is codoped in the core region with Tb³⁺- and Er³⁺- ions.

A specific embodiment of the invention will now be described with reference to Fig. 1. It allows the use of a single starting dopant which has a melting point > 250°C (i.e. is a solid at temperatures below the melting point of polytetrafluoroethylene) and which is to be inserted into the core of a single-mode fiber.

Before deposition, a deposition tube is prepared by introducing the required dopant into a dopant carrier chamber 1, where it is purified by heating in a dehydrating atmosphere, for example an atmosphere containing either gaseous chlorine or gaseous fluorine, using a stationary heat source, for example burner 2. This step also fuses the dopant to the chamber wall so that particles of the dopant are prevented from migrating along the tube and forming bubbles in the subsequently deposited glass. The inside of the deposition tube 4 is then cleaned by gas phase etching using fluorine produced by thermal decomposition of a fluorine-containing compound, for example sulphur hexafluoride (SF₆) or a halocarbon, for example CCl₂F₆, to remove any dopant which has deposited during the drying process. The cladding glass is then deposited. During subsequent core deposition, the dopant support chamber is heated to a temperature at which the solid impurity dopant either sublimates or becomes a liquid with a partial vapor pressure on the order of a few hundred Pascals (a few millimeters of mercury), typically at 900 to 1200°C. This produces small amounts of an impurity vapor which is entrained downstream by the flow of reactants where it is then oxidized in the hot region 6 created by the deposition burner and deposited unmelted at a low temperature (typically < 1600°C) together with other core forming materials, e.g. SiO₂, P₂O₅, GeO₂. The porous glass layer is then further dried by heating in a dehydrating atmosphere and thereafter melted to give a transparent, non-porous layer 8. The tube is then caused to collapse to form a solid rod and is then drawn into a fiber.

Another specific embodiment of the invention is the production of mono- and multimode optical fibers that are doped in the core area, for example, with neodymium ions, Nd³⁺.

Referring to Figures 2 to 4, the deposition process was carried out in a similar manner to that generally described above, with the source of dopant vapor being hydrated neodymium trichloride, NdCl3.6H2O (99.9% pure, melting point = 758°C). This was then hydrated and purified by heating within the tube in a chlorine atmosphere, after which the tube was cleaned by gas phase etching using a fluorine-releasing vapor, e.g. sulfur hexafluoride (SF6). A number of low loss cladding layers were then deposited until a cladding diameter to core diameter ratio for the collapsed preform of greater than 7:1 was achieved. This was necessary to avoid excessive loss due to OH- ingress. and other impurity ions from the support tube into the regions of the fiber where there is significant field penetration by the conducting modes. The fiber core was then deposited in an unmelted state while the dopant support chamber was heated as before so that the NdCl3 vapor generated in the dopant support chamber was oxidized to Nd2O3 within the warm region. The core layer was then dried in a chlorine atmosphere and sintered before the tube was allowed to collapse into a solid rod.

Absorption measurements on single- and multimode fibers show that neodymium is incorporated into the glass matrix as the trivalent Nd³⁺ ion. Fibers with absorption peaks (at 590 nm) ranging from 30 dB/km to 30000 dB/km (corresponding to dopant levels of 0.3 to 300 ppm Nd³⁺) have been fabricated. The absorption spectrum for a length of 500 m of neodymium-doped fiber with a dopant level of approximately 30 ppm is shown in Fig. 2(a). It clearly shows the very high absorption levels in the visible and near infrared range of up to 3000 dB/km. Despite this high loss, it is It is noteworthy to note the presence of a low loss window between 950 and 1350 nm of < 2 dB/km, a figure not very different from that observed in conventional fibers. The low OH- absorption peak at 1390 nm gives an indication of the success of the procedures used to dry the neodymium compounds both before and during deposition.

The fluorescence spectrum of a fiber doped with 300 ppm Nd3+ is shown in Fig. 3, where broad fluorescence bands with peak wavelengths of 940, 1080 and 1370 nm are clearly visible. Due to the high silica host glass, the bands are shifted to slightly lower wavelengths than the corresponding bands in layered glasses used for conventional lasers. Measurements of the 1/e fluorescence lifetime using a pump wavelength of 590 nm yielded a figure of 450 µs for the 940 nm as well as the 1080 nm transition. In addition, the persistence of the incorporation of the dopant along the fiber length has been solved by measuring the local attenuation along the fiber length using an OTDR technique. The source wavelength of 620 nm is chosen to be at the tail end of the 590 nm absorption band to achieve manageable attenuation. The results are shown in Fig. 4(a) and indicate good uniformity of dopant incorporation along the length of the fiber, reflecting a high degree of control in the fabrication process.

According to another specific embodiment of the method, we have created highly birefringent and singly polarizing fibers containing Nd³⁺ ions in the core region. These were obtained by combining the previously described method for introducing impurity dopants into the fiber core with the known gas phase etching method for producing asymmetric, highly stretched "bow-tie" fibers as described in the Patent application No. 8218470. Fibers with an overlay length of only 2 mm in combination with dopant levels of > 200 ppm Nd³⁺ have been obtained using this process.

As another specific embodiment, we have produced mono- and multimode fibers containing Er3+ ions in concentrations of up to 0.25 wt.% Er3+. This is explained with reference to Fig. 5, which shows the absorption spectrum of a monomode fiber doped with 2 ppm Er3+ in the core region.

The fiber was prepared as described previously for the Nd3+ doped fibers, but the precursor material was hydrous erbium trichloride, ErCl3.6H2O (99.9% pure, melting point = 477°C). The rest of the fabrication process was as described above except that very few low loss cladding layers were deposited so that the resulting fiber drawn from the preform had a cladding diameter to core diameter ratio of 2:1. As a result, there was penetration of the support tube by the field which was nominally limited to the fiber core, giving a higher than expected loss due to OH- absorption at 1390 nm, as shown in Figure 5. Using this method, fibers containing up to 0.25 wt.% Er3+ were prepared. which results in losses of less than 40 dB/km in the range between 1 um and 1.3 um, although there are peaks in the absorption band of > 50 dB/m.

Another specific embodiment relates to a fiber containing terbium ions, Tb³⁺, codoped with erbium ions, Er³⁺, in the core region. This was produced as described above, except that a mixture of terbium trichloride and erbium trichloride was added to the dopant carrier chamber before deposition began. Fibers drawn from such a preform have similar absorption properties shown as shown in Fig. 6, where the large absorption peak at 486 nm and the broad absorption centered at 725 nm are due to the presence of the Tb³⁺ ion, while the absorptions at 518 and 970 nm are due to the presence of the Er³⁺ ion. This illustrates the possibility of doping the deposited glass with multiple impurity ions simultaneously.

The process can also be used to incorporate ions of other rare earths and transition metals into optical fibers.

It should be noted that the process described above is applicable to all types of fibers that can be produced by the so-called modified chemical vapor deposition (MCVD) process, namely:

(i) Single-mode fibers

(ii) Multimode fibers

(iii) highly birefringent and singly polarizing fibers

(iv) circular birefringent fibres (with helical core)

(v) Multi-core fibres

(vi) Fibres with ‘toroidal core’.

Claims (8)

1. A method of producing a preform for the manufacture of optical fibers containing doped glass, wherein a dopant material is deposited in a doping chamber and subsequently evaporated from the chamber and deposited on the inner surface of a tubular glass member, characterized in that the method comprises the following sequential steps: heating the doping chamber (1) to melt the dopant to the chamber wall (3); subsequently heating said chamber to cause said dopant to change to vapor form at a predetermined rate; flowing a gaseous source material (SiCl₄, GeCl₄, O₂) through said carrier chamber to mix said dopant material with the source material; precipitating a mixture (8) of solid components from the mixture of said source material and said dopant material; and melting said solid components to form a doped glass for said preform. 2. A method for producing a preform according to claim 1, characterized in that it further comprises a step of heating the dopant material carrier chamber while maintaining a dehydrating atmosphere therein. 3. A method for producing a preform according to claim 1, characterized in that said doping material contains a rare earth element or a transition metal. 4. Method for producing a preform according to claim 3, characterized in that neodymium is provided as the rare earth element. 5. Method for producing a preform according to claim 3, characterized in that erbium is provided as the rare earth element. 6. Method for producing a preform according to claim 3, characterized in that terbium is provided as the rare earth element. 7. A method for producing a preform according to one of claims 1 or 2, characterized in that the dopant material contains a plurality of impurity ions. 8. A method for producing a preform according to claim 7, characterized in that the doping material contains terbium and erbium.