JP5484380B2 - Evaluation method for optical fiber preform - Google Patents

Description

  The present invention is based on the RIT method (rod-in-tube method), which is manufactured by heating and fusing a base material for an optical fiber mainly used for communication, particularly a rod-shaped member and a large cylindrical member. The present invention relates to an evaluation method for a manufactured optical fiber preform.

In general, an optical fiber includes a core portion that transmits light and a cladding portion that surrounds the core portion. The core part generally has a higher refractive index than the clad part. The optical fiber is drawn to a desired thickness by heating and softening the optical fiber preform in an electric furnace.
In many cases, the optical fiber preform is manufactured by first manufacturing a core rod including a core portion and, in some cases, a part of the cladding portion, and further adding a cladding portion to the outside of the core rod.

  For the production of the core part, methods such as VAD, OVD, MCVD, and PCVD are used. The clad part to be applied to the outside of the core part is deposited directly on the core rod by the OVD method, etc. and applied by transparent vitrification in a heating furnace, or by applying a separately manufactured cylindrical body to the core rod There is.

  The latter method of covering the core rod with a cylindrical body is called the RIT (rod-in-tube) method or the RIC (rod-in-cylinder) method. Usually, a separately manufactured cylindrical body is stretched to have a desired thickness is called a tube, and the manufactured cylindrical body itself is called a cylinder. In other words, the size of the cylindrical body used by the RIC method is larger than that of the RIT method, but technically the size is the same, and the boundary is ambiguous. Therefore, it is generically called RIT method hereinafter.

  In the RIT method, a core rod is inserted into a cylindrical body, and heating and integration are performed. However, when the integration is performed, the core rod may be stretched to a desired thickness, or only integration may be performed. . Of course, it is simpler in terms of equipment to simply perform integration. On the other hand, although it is complicated in terms of equipment, there is an advantage that a base material having a size suitable for the subsequent drawing furnace can be produced by performing the stretching process at the same time as the integration. In the latter case, the size of the original cylindrical body and the core rod can be increased without being limited by the size of the drawing furnace, so that the productivity can be increased compared to the former. In some cases, drawing to the optical fiber may be performed simultaneously with the integration.

  An OVD method or the like is used for manufacturing a synthetic quartz cylindrical body. When using a large cylindrical body, heating and integration are often performed in an electric furnace. Specifically, for example, after inserting the core rod into the cylindrical body, one end thereof is inserted into the electric furnace while purging the gap between the cylindrical body and the core rod with a clean gas. As the temperature of the electric furnace increases, the cylindrical body and the tip of the core rod soften over time. Here, when the purge with the clean gas is switched to the pressure reduction process with the pump, the gap between the softened portions of the cylindrical body and the core rod is crushed and integrated. Thereafter, the entire heating area is gradually shifted to be fused and integrated. Since the cylindrical body and the core rod are sufficiently softened, it is possible to extend to a desired outer diameter by adjusting the lower take-up and the upper feed speed.

  When a base material is manufactured by the RIT method, a transmission loss abnormality may be rarely seen. In such a case, as shown in FIG. 1, the transmission loss tends to be higher than usual as the wavelength is particularly long. It is known that such a transmission loss abnormality occurs when the interface between the core rod and the cylindrical body is contaminated, and the cause is considered to be absorption loss due to contaminants or microbending loss. This is considered that the core rod or the cylindrical body was originally contaminated, or that the gas in the furnace was contaminated by entering the gap between the core rod and the cylindrical body due to poor purge. The former case can be prevented by etching the surface of the core rod and the cylindrical body with an HF solution.

  When drawing in-house and using optical fiber as a product, pass / fail can be determined by measuring the optical fiber transmission loss and comparing it to the reference value. It is uneconomical. In addition, when a base material is sold as a product and a customer performs drawing, it is necessary to determine pass / fail at the base material stage.

Since the transmission loss cannot be measured at the base material stage, it is necessary to measure and determine the transmission loss by drawing a part of the base material when the transmission loss may be increased.
When a large core rod and a cylindrical body are used for integration and stretching at the same time as the RIT method, the completed base material is appropriately cut, and a plurality of base materials are manufactured in one batch. If the transmission loss is stable for the entire base material, you can determine whether the entire base material is acceptable or not by sampling and drawing from one location. In order to use it effectively, it is necessary to draw a plurality of points, and there is a problem that the base material discarded by sampling increases, and the labor and cost of drawing the sample increase.

  In view of such circumstances, it is an object of the present invention to provide an optical fiber preform evaluation method that economically evaluates an optical fiber preform manufactured by the RIT method.

  The optical fiber preform evaluation method of the present invention includes a first step of preparing a first member made of rod-shaped glass and a second member made of cylindrical glass, and a center hole of the second member. A second step of inserting the first member therein, a third step of purging a gap between the second member and the first member with a clean gas, the first member and the first The fourth step of heating and softening the member formed by combining the two members from one end, the fifth step of switching the purge with the clean gas to the decompression process by suction with a pump, and the softening by the decompression process A sixth step of fusing the first member and the second member, and moving the heating area little by little, while heating and fusing the entire effective portion of the first member and the second member in sequence. Stretch to an outer diameter of a predetermined range, and cut multiple lengths within a predetermined range. The optical fiber base material manufactured through the seventh step of the fiber base material is drawn from the side near the end where heating is started, and when there is a predetermined length, transmission loss is measured to determine whether there is an abnormality. It is characterized by determining.

Of the plurality of cut optical fiber preforms, the base material closest to the heating start end is drawn from the side closest to the end where heating is started, and the transmission loss is measured. It is determined that there is no abnormality in all the base materials that have been processed.
When abnormality is recognized in the measurement of the transmission loss, the drawn base material is rejected, and the number of bright spots observed when observing the base material with light exceeds a predetermined number. In this case, all the remaining optical fiber preforms that have been cut are rejected.

  In addition, when abnormality is recognized in the measurement of the transmission loss, the drawn base material is rejected, but the number of bright spots seen when observing the base material with light is a predetermined number. If it does not exceed, draw the wire sequentially from the base material next to the heating start end next to the base material (second base material from the heating start end side), repeat the pass / fail judgment by the measurement result of transmission loss, An acceptable range of a plurality of cut optical fiber preforms is determined.

  According to the procedure of the evaluation method of the present invention, a part of the base material is drawn and evaluated, so that the evaluation of the optical fiber base material cut into a plurality of parts can be performed economically.

The wavelength distribution of the difference in transmission loss between a normal optical fiber and an optical fiber having an abnormal transmission loss is shown.

  For the base material with abnormal transmission loss, we increased the number of lines and investigated the distribution of transmission loss in the base material. As a result, the transmission loss may be large on the side where heating is started at the time of heating and fusing, and may tend to gradually decrease toward the opposite end, while the transmission loss may be large overall. I understood.

  Among them, the one with a large transmission loss as a whole was confirmed to have more bright spots than usual in the layer corresponding to the interface between the core rod and the cylindrical body. This is the process performed at the beginning of heating and fusing, and the core rod and the cylindrical body are softened by heating in the furnace while purging the gap between the core rod and the cylindrical body with a clean gas. When the gas in the furnace drifts into this gap due to turbulence in the air flow for purging, etc., impurities such as metals contained in the furnace gas accumulate in the gap between the core rod and the cylindrical body, resulting in transmission loss. This is thought to have caused the bright spot.

Moreover, when the contamination by such a path | route is not so remarkable, it does not appear as an abnormality in the appearance called a bright spot, but it is thought that only a transmission loss may be raised. In such a case, it was considered that the risk of high transmission loss was highest at the end close to the heating start side where the gap between the core rod and the cylindrical body was exposed in the furnace.
Therefore, when a large number of base materials are manufactured in a single batch by drawing and cutting to a predetermined length simultaneously with integration by the RIT method using a large core rod and cylinder, the transmission loss of these base materials The following rules were established to determine pass / fail of characteristics.

First, sampling is performed from the heating start end side of the base material closest to the heating start end, and the transmission loss is measured by drawing. If this is below the reference value, the whole is accepted. A transmission loss value of 0.195 at 1550 nm was used as a reference value.
Secondly, if the number of bright spots exceeds the standard in the layer corresponding to the interface between the core rod and the cylindrical body, the rest of the cut base material is not passed if it does not pass based on the first rule. It will be rejected. The number of bright spots was 35 as a reference number per meter.
Third, if the bright spot is below the reference number and does not fail based on the second rule based on the first rule, the base material closest to the heating start end is rejected. Sampling is performed from the heating start end side of the base material close to the heating start end next to the base material, and the determination is performed in the same manner as the first rule. If it passes, all the remaining cut base materials are accepted, and if it is not accepted, the drawn base material is rejected, and then sampling is similarly performed from the base material close to the heating start end. . This is repeated to determine the pass range.
According to the above rules, it is possible to reduce the number of base materials with good characteristics that are discarded wastefully while keeping the sampling amount to a minimum.

  A core rod manufactured by the VAD method was drawn to a 64 mm outer diameter with a glass lathe using an oxyhydrogen flame, and a cylindrical body of synthetic quartz finished with mechanical grinding and polishing with an outer diameter of 195 mm and an inner diameter of 68 mm was prepared. . Each effective length is 1500mm. Each of them was connected with a handle for gripping, etched with about 5 wt% HF solution for 5 minutes, washed with pure water, and dried.

A core rod was inserted into the cylindrical body, a jig capable of sealing was attached while connecting the handles, and dry nitrogen was allowed to flow from a separately provided port. The flowing dry nitrogen passes through the gap between the core rod and the cylindrical body and flows out from the opened lower part.
A set combining the core rod and cylindrical body is installed in a drawing furnace equipped with a vertically movable device, an electric furnace, and a take-up roller, and the position is adjusted so that the tip is located in the electric furnace. It was.

When the temperature of the electric furnace is raised and heated at 2000 ° C. for a while, the core rod and the tip of the cylindrical body soften and begin to descend in the form of a thin cylinder and rod. When it is sufficiently lowered, a tip made of silicone resin is plugged at the tip, and the purge with nitrogen is switched to a pressure reduction treatment with a pump. Thereby, the space | gap of a softening part is crushed and a core rod and a cylindrical body are integrated.
While adjusting the feeding speed of the rod and the cylindrical body, the take-up by the roller was started, and each speed was adjusted and cut with a predetermined length, and a base material having a desired outer diameter could be obtained. . In one batch, six base materials having an outer diameter of 80 mm and a length of 1400 mm were obtained. Similarly, a total of 100 batches were performed to obtain 600 base materials.

About these base materials, it sampled according to the following rules and judged each pass / fail.
First, sampling is performed from the heating start end side of the base material closest to the heating start end among the plurality of base materials cut to a predetermined length, and the drawing and the transmission loss are measured. If the transmission loss is below the reference value, all the base materials of the batch are accepted.
Secondly, if the number of bright spots over the reference number is confirmed in the layer corresponding to the interface between the core rod and the cylindrical body, it is determined that all the cut base materials are rejected. To do. This is based on past experience that furnace gas inhalation can cause bright spots and increase transmission losses. If there are other causes of bright spots, it may not lead to an increase in transmission loss. Therefore, only the result of the appearance inspection of the number of bright spots does not provide a basis for whether there is an abnormality in the transmission loss.

  Third, if the bright spot is not more than the reference number and the second rule is not rejected based on the first rule, the mother that is closest to the heating start end after drawing is drawn. The material is rejected, sampling is performed from the heating start end side of the base material close to the heating start end next to the base material, and the determination is performed in the same manner as the first rule. If it passes, all the remaining cut base materials are accepted, and if it is not accepted, the drawn base material is rejected, and then sampling is similarly performed from the base material close to the heating start end. This is repeated to determine the pass range.

  For sampling, a length of about 150 mm was cut out from a base material having a length of 1400 mm. As a result, abnormal transmission loss was observed in 5 out of 100 batches. Of these, 2 batches had more than 50 bright spots for each base material, and were rejected without further sampling. The remaining three batches were evaluated by performing sampling drawing from the side closer to the heating start end of the second base metal from the heating start end side, rejecting each base material that was sampled. As a result, no abnormality in transmission loss was found in all three, so the remaining base materials were accepted.

As described above, 103 sampling lines were drawn from 600 base materials of 100 batches, 15 base materials were rejected, and a base material having a high transmission loss could be prevented from being shipped as a product. Regardless of the present invention, when the sampling position was not set, there was a high possibility that the three base materials would be shipped despite the high transmission loss.

Claims (4)

A first step of preparing a first member made of rod-shaped glass and a second member made of cylindrical glass, and a second step of inserting the first member into a central hole of the second member A member formed by combining a step, a third step of purging a gap between the second member and the first member with a clean gas, and the first member and the second member is heated from one end. A fourth step of softening, a fifth step of switching the purge with the clean gas to a decompression process by suction with a pump, and the first member and the second member softened by the decompression process are fused. In the sixth step, the heating region is moved little by little, and the entire effective portion of the first member and the second member is stretched to an outer diameter within a predetermined range while sequentially heating and fusing the entire region. Through a seventh step of cutting each length of the optical fiber to form a plurality of optical fiber preforms. An optical fiber preform characterized in that a manufactured optical fiber preform is drawn from the side near the end where heating is started, and when a predetermined length is drawn, transmission loss is measured to determine whether there is an abnormality. Evaluation method. Of the plurality of cut optical fiber preforms, the base material closest to the heating start end is drawn from the side closest to the end where heating is started, and the transmission loss is measured. The optical fiber preform evaluation method according to claim 1, wherein it is determined that all the preforms are not abnormal. Among the plurality of cut optical fiber preforms, the base material closest to the heating start end is drawn from the side closest to the end where heating is started, and the transmission loss is measured. If the drawn base material is rejected and the number of bright spots observed when the base material is irradiated with light exceeds a predetermined number, all the optical fiber bases that have been cut are left. The optical fiber preform evaluation method according to claim 1, wherein the material is rejected. Among the plurality of cut optical fiber preforms, the base material closest to the heating start end is drawn from the side closest to the end where heating is started, and the transmission loss is measured. Indicates that the drawn base material is rejected, and when the number of bright spots observed when the base material is irradiated with light does not exceed a predetermined number, it is close to the heating start end next to the base material. The optical fiber preform evaluation method according to claim 1, wherein the optical fiber preform is successively drawn from the side preform, and the pass / fail judgment based on the measurement result of the transmission loss is repeated to determine a pass range of the plurality of cut optical fiber preforms. .

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