NL8400455A - METHOD FOR MANUFACTURING AN OPTICAL FIBER - Google Patents

Description

t VO 5342

Title: Method for manufacturing an optical fiber.

The invention relates to a method used in the manufacture of glass optical fibers.

In the manufacture of optical fibers, a glass preform consisting of a selectively doped glass tube S is manufactured by a process such as MCVD (modified chemical vapor deposition), MCVD coated with a plasma, or PCVD, described by D. Kuppers and others, in the Journal of the Electrochemical Society, 423, 1079 (1976). The preform is either narrowed or sealed at one end, compressed into a solid body, and at the same time not compressing or after compression, an optical fiber is drawn from the solid body. Bound OH proportions in the preform and in the resulting fiber absorb into the wavelength range, which is used more particularly in optical communication systems, and significantly increase the signal attenuation in such systems. Therefore, during the formation of the preform, great care is taken to substantially exclude the incorporation of CH shares, for example SiOH, in the precor form. Generally, the major source of OH proportions includes hydrogen-containing units, which are more particularly converted to water at the compression temperature. The water, in turn, reacts with the preform to provide bound OH proportions. Therefore, in order to maintain the quality of the fiber finally obtained, cell-rich measures are also taken during the compression of the preform to exclude hydrogen-containing units.

Sr has been an important method used to prevent the recording of OH shares during the compression of the preform.

To this procedure, described by KL, Walker and others, "Reduction of Hydroxyl Contamination In Optical Fiber Preforms," Third International Conference on Integrated Optics and Optical Fiber Communications, San 30 Francisco, California, April 27-29, 1981, (New York; IEEE , 1981), WA4, 86-38 (1981), chlorine is introduced during the collapse procedure.

The chlorine generally reacts with water, for example water, formed from hydrogen-containing units, and provides hydrochloric acid via the reaction H 2 O + Cl 2 t ==, 2HC1 + 1/202- (1)

Si Λ / ζ c 'V *' * ii ^ ^

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The resulting HCl is not preformed and is removed in the effluent. It has been found that this molecular chlorine collapsing procedure results in fibers with relatively low losses due to OH absorption losses, generally in the range of 0.3-2 dB / km at the OH absorption peak wavelength of 1 39 μm. Nevertheless, in spite of the acceptable level of losses already achieved, it is certainly advantageous to significantly reduce the losses to even more desirable levels. 10 The loss due to the incorporation of OH shares is significantly reduced compared to other methods by introducing carbon tetrahalide compounds, such as carbon tetrachloride, into the environment of the glass body, which is modified for ultimate use in the manufacture of glass optical fibers, for example, in the embodiment of a preforming environment, in which collapse occurs. More specifically, OH absorption losses of 0.05 dB / km at 1.39 µm have been obtained. Therefore, by using this method according to the invention it is possible to significantly improve the quality of the fiber finally obtained compared to that which can be obtained using gases, such as molecular chlorine.

The figure illustrates an embodiment of the method according to the invention.

The losses caused by OH proportions in optical fibers are significantly reduced if a carbon tetrahalide based material is, for example, carbon tetrachloride, CBrCl. , CB ^ C ^, or mixtures of these compounds, in the environment of a glass body, which is modified either physically or chemically. More particularly, the addition of carbon tetrahalide compounds of the invention is useful in reducing OH uptake during a wide variety of processes, modifying a glass body, for example, altering its physical shape, such as during preform collapse process and a chemical modification, such as during the preform fabrication process. In both cases, the presence of a carbon tetrahalide during the modification of a glass 8400455 -3- • »<

body to a very low level of losses in the end from I.

this body manufactured fiber. Although the method of the invention generally applies to procedures in which glass bodies were modified in processes leading to fiber manufacture, in this explanation the preform collapse procedure is used as a teaching aid to indicate the parameters, which play a role in the method according to the invention. Nevertheless, the same parameters apply to other modification procedures of a glass body.

The desired carbon tetrahalide compound is supplied to the vitreous environment, for example, in the interior cavity 10 of the pre-tube during collapse. (The inner cavity is preferably chosen since in most collapsing processes, the inner glass region forms the light guide region where it is most critical to limit OH proportions.) It is particularly desirable carbon tetrachloride (generally, but not necessarily, with a carrier gas) as the carbon traisLide based material. The carbon tetrahalide compound is simply supplied to the preform environment by conventional means, such as by passing a carrier gas through a bubble device containing the desired carbon tetrahalide material and then feeding the carrier gas with the carbon tetrahalide material to the preform environment. . (When a combination of CC14, CBrCl2 and / or CB ^ C ^ is desired, the material is obtained by combining gas streams from separate bubble devices or by using a bubble device} containing all components. In the former case, the mole fraction of each introduced component depends on the individual gas flow rates through each bubble device and the temperatures thereof In the latter case, the mole fraction of each material in the gas phase depends on but is not equivalent to its corresponding mole fraction in the bubble device or the temperature thereof In both cases, a control sample is used in a simple manner to determine the proper conditions for obtaining the desired ratio in the final gas flow, although the use of CBrC 2 and CB 2 C 2, either separately or in combination is not excluded, it is more desirable to use this carbon tetrahalide (if not all) in combination with CCl ^ use 8400455 * 4 -4-x. An explanation for this result is that HBr (the reaction product of a bromine-containing material with water) is less stable than HCl. This relative instability of HBr requires the use of relatively high concentrations of the halogen contributing species 5 to obtain equivalent results.

The concentration of the carbon tetrahalide compound, which is used to remove hydrogen-containing units in the preform collapse process, affects other processes, which also use wounds in some situations during the preform collapse process. For example, it is desirable to introduce a dopant compensator, for example germanium tetrachloride, together with oxygen in the preform environment during collapse at certain times. The oxygen reacts with the GeCl4 to maintain the desired GeO2 concentration on the inner surface of the preform. When this procedure is used, the halogen released from both the germanium tetrahalide and the carbon tetrahalide affects the concentration of GeO 2 of the chemical equilibrium of the reaction represented by the following equation:

GeX4 + O2 <- Ge02 + 2X2 (2) 20 (X is a halogen). Therefore, if a dopant compensation is to be used in combination with a carbon tetrahalide, the resulting equilibrium shift must be eliminated by a corresponding increase in the amount of germanium tetrahalide material used.

Likewise, processes leading to a 1) large amount of halogen from a source other than a carbon tetrahalide or 2) oxygen in the vicinity of a preform also have a potential impact on equilibrium, indicated in equation 2) and therefore in turn affect the amount of carbon tetra-halide needed to remove the desired amount of water. for example, if oxygen is introduced, for example, as a carrier gas, the equilibrium (equation 1) is shifted to the left. Therefore, the minimum amount of carbon tetrahalide needed (for a given amount of hydrogen-containing unit) to avoid an OH presence in the fiber increases with the increasing presence of oxygen. (In contrast, lift gas such as helium or another inert gas, little influence.) 8400455 c * -5-

If a halogen from sources other than carbon tetrahalide is also present, less carbon tetrahalide is required. Although temperatures often affect equilibrium conditions, the temperature applied during the preform collapse, for example 2000 to 2200 ° C, has practically no influence on the required level of the carbon tetrahalide and therefore on the level of OH absorption in the fiber, which is formed from a preform obtained using a carbon tetrahalide.

Generally, the process of the invention is not used as a rough removal procedure for hydrogen containing units. There are other precautions.;. such as purification of reaction materials, used to significantly reduce the level of the hydrogen-containing units. For this reason, sufficient carbon tetrachloride, even in the presence of oxygen, is introduced in a simple manner to avoid the losses that occur at the hydrogen-containing levels. units, which are present after these precautions have been taken.

(In general, 1-10 weight ppm of contaminating hydrogen, though bound, is present). Nevertheless, as discussed above, oxygen affects the minimum amount of carbon tetrachloride required for a given level of the hydrogen containing unit.

Oxygen is generally present during collapse, at levels up to 0.1 atm, even if not introduced intentionally. For such oxygen levels, desired results are obtained for typical hydrogen unit levels when a partial pressure of 0.015 atm or more of carbon tetrachloride is added. the environment of the glass body is applied, if oxygen is intentionally introduced and therefore the oxygen level is above 0.1 atm, it is more particularly desirable to maintain the fraction 30 at levels below 20. (p_ and P_ are the partial pressures of and introduced CCl ^ respectively. When lower than normal hydrogen unit levels are present (less than 1 ppm) or when less than a substantially total removal is acceptable, a correspondingly smaller 1/2 is used. Amount of carbon tetrahalide or P / p. (In ° 2 CC1d processes other than the MCVD preform collapse, it is possible that a background of less than 0.1 atm oxygen is present. For such 8400455-6 cases, a accordingly lower level of carbon tetrahalide introduction also to desired results.)

A control sample is used to determine the precise amount of carbon tetrahalide required to achieve the desired level of OH attenuation.

The presence of oxygen sources and halogen sources, which are different from carbon tetrahalides, are the primary influences introduced by processes not directly related to the process of the invention. However, it is possible that other materials may be introduced for purposes outside of the process of the invention which may affect the reaction equilibrium between water (the material resulting from hydrogen-containing units) and the carbon tetrahalide composition and thus an adjustment of require parameters used in the method of the invention. A control sample is used in a simple way to determine the corrections required for each particular situation.

Regardless of the considerations discussed earlier, certain precautions should be taken. In the case of using CCl, it is generally desirable to limit the chlorine concentration, expressed as molecular chlorine in the preform environment, to less than 0.3 atm. Above these levels, the high concentration of chlorine tends to form bubbles in the preform and therefore leads to unacceptably large losses. In addition, oxygen in the presence of carbon tetrahalides tends to avoid carbon deposits, and this oxygen induces the formation of gases such as carbon dioxide, carbon monoxide and, in some circumstances, phosgene.

Also, the carbon tetrahalide material should not have an excessively hydrogen-containing unit contamination level, ie a level greater than 40 ppm, expressed as a weight fraction of H. Therefore, the carbon tetrahalide composition having hydrogen levels greater than 40 ppm should preferably be cleaned by conventional methods such as photochlorination and flow through with a dry inert gas to remove HX (X = Cl or Br) and ^ 0. A cleaning, which provides less than 6 ppm, is preferred. (A long-term photochlorination of CBrCl 2 and CB ^ C ^, if necessary to obtain the desired purity level, sets a portion of however, the bound Br 1 to 1 in Cl. The resulting carbon tetrahalides, as discussed above, are relatively acceptable for use in the process of the invention. The examples below are illustrative of the invention.

Example I "

Preforms obtained by the MCVD process as described by S.R. Nagel and others in IEEE Journal of Quantum 10 Electronics, OE-13 (4), 459-476 (1982). These preforms were first sealed at one end and then collapsed by repeatedly passing a torch 20 longitudinally held at a temperature between 2000 and 2200 ° C.

Before sealing one end of the preform, a gas stream of 330 cc per minute of oxygen was passed through a carbon tetrachlorocarbon bubble device, which was maintained at a temperature of 40 ° C. This CCl 2 containing gas stream was combined with a second flow oxygen of 1000 cc per minute. The combined gas stream was introduced at one end of the preform, 25, and maintained for a time sufficient to clean the tube.

The end of the preform opposite the point at which the gas was introduced was then sealed. The gas flow during sealing was gradually reduced to avoid a large pressure increase in the preform. This gradual decrease resulted in a flow of oxygen through the bubble of 25 cc per minute and a secondary oxygen flow of 75 cc per minute. The torch was moved along the length of the mold at speeds ranging from 6-10 cm / min. . During each run, the pressure in the tube was controlled by controlling either the discharge IS, from the preform of the gas introduced, or the rate at which the combined gas stream was introduced. The pressure was controlled by these aids so that too rapid collapse was avoided but total collapse occurred after approximately 5-7 runs. Then, a fiber was drawn from the preform in a standard manner, such as that described by L.L. Blyler, Jr and others in Proceedings of IEEE 68, 1194-1198 (1980). The losses in the 8400455 * *.

-3-

The losses in the resulting fiber were measured using a procedure described in Chapter 11, Optical Fiber Telecommunications, issued by S.E. Miller and Others, Academy Press (1979). The resulting fibers exhibit OH absorption losses from 0.05 dB / km to 0.1 dB / km at 1.39 µm.

Example II

The same procedure, as described for Example I, was performed except that the preform was not initially sealed.

As described in Example I, the initial flow rate of oxygen through the bubble device was 330 cc / min and the second oxygen flow rate was 1000 cc / min. This ratio of flow rates between the carbon tetrachloride with oxygen and the oxygen free of carbon tetrachloride was maintained. However, the combined total flow rate was decreased to an extent that allowed the total collapse to occur in 7 passes. The resulting fibers exhibited OH absorption losses from 0.05 dB / km to 0.1 dB / km at 1.39 µm.

Example III

The procedure of Example II was followed except that germanium tetrachloride was simultaneously fed to the preform environment. This was effected by passing oxygen at a rate of 15 cc per minute through a germanium tetrachloride bubble maker maintained at a temperature of 40 ° C. The flow rate through the germanium tetrachloride bubble device was practically unchanged throughout the collapse procedure. The resulting fiber had a measured QH absorption loss of approximately 0.1 dB / km at 1.39 µm.

8400455

Claims (10)

2. Process according to claim 1, characterized in that the carbon tetrahalide contains at least one of the carbon tetrachlorides, CClgB 10 CCl3Br. A method according to claim 1 or 2, characterized in that the liquid comprises carbon tetrachloride and at least one of CClB br and CClBr. 4. A method according to claims 1, 2 or 3, characterized in that the carrier gas comprises oxygen. A method according to any one of the preceding claims 1-4, characterized in that said gas comprises GeCl4. 6. A method according to any one of the preceding claims 1-5, characterized in that gas is supplied to a cavity in the body by bubbling a carrier gas through a liquid. Method according to any one of the preceding claims 1-6, characterized in that the modification comprises collapsing a glass preform. 8. Method according to any one of the preceding claims 1-7, characterized in that the fiber is pulled out of the collapsed body. Method according to any one of the preceding claims 1-8, characterized in that the high temperature is in the range from 2000 to 2200 ° C. 10. Optical glass fiber manufactured in accordance with the method according to any one of the preceding claims 1-9. 8400455