A method of manufacturing an unclad infrared optical fiber

Abstract: The optical fiber for infrared transmission has a composition ratio of silver chloride and silver bromide in the range of 30 to 70 percent by weight of silver chloride, and has a diameter of 0.35 mm or less. The infrared optical fiber has high mechanical strength, and thus does not burn due to degradation in the laser beam transmission capability, even after 10,000 times of repetitive bending with a curvature radius of 20 mm, which is the condition necessary for use within the interior of a body, and it exhibits excellent radiant optical characteristics. Also, by applying an appropriate load to the optical fiber preform, an infrared optical fiber of straight shape free from curving and having excellent radiant optical characteristics, can be produced stably within a short manufacturing time period. The appropriate applied load is a tensile load greater than the yield stress but smaller than the rupture stress of the unclad optical fiber, and is applied during the preform drawing operation.

Description: Technical Field The present invention relates to a method of manufacturing an unclad infrared optical fiber used as an optical transmission line in a laser beam machine or a laser medical instrument. Background Art In medical operations using laser cutting and laser beams, various kinds of laser beams are currently used according to the purpose of the operation. In the latter case, a carbon dioxide (CO 2 ) laser beam is used because of its high absorptivity in the living organism and its superior incising and vaporizing capabilities. As a means for transmitting the CO 2 laser beam to the target, a mirror articulated optical waveguide using a combination of mirrors has previously been used, because quartz optical fibers as used for telecommunications cannot transmit the CO 2 laser beam, which has a wavelength of 10.6 µm and so falls within the category of medium infrared radiation. However, since the mirror articulated optical waveguide lacks operatability for precise medical operations, many efforts have been made to replace it with an infrared optical fiber. As a result, a number of kinds of infrared optical fibers have been put into practical use for general surgery, but it still cannot be said that these infrared optical fibers have sufficient flexibility. In particular, there has recently been a need for a so-called "CO 2 laser endoscope" that enables the treatment of internal organs to be performed, without an external medical operation for cutting open the body, by inserting the infrared optical fiber along with the endoscope into the interior of the body to direct the CO 2 laser beam to the diseased part, but the current infrared optical fibers completely lack flexibility for this application. The infrared optical fiber used in this application is required to have sufficient flexibility to be easily bent within the interior of the body as well as the power transmission capability for incising and vaporizing the diseased part. As to the conditions of use for the endoscopes currently used, it is considered that what is required is flexibility which can retain the power transmission capability against repeated flexing of approximately 10,000 times with a curvature radius of 20 mm. It is also important for the infrared optical fiber to have excellent optical characteristics such as a transmittance and a radiation spread angle of the laser beam. Metal halides such as thallium halide, silver halide and cesium halide are previously used as infrared optical fiber materials which can transmit the CO 2 laser beam with high efficiency, but these materials generally have inferior mechanical bending characteristics and are easily breakable. Since the infrared optical fiber formed from silver halide is relatively flexible and not easily breakable, various researches are currently underway for improvement. A report has been made of an example of infrared optical fibers formed from materials with improved elongation at rupture by adding 0.01 to 10 percent by weight of silver chloride to silver bromide or of silver bromide to silver chloride for improved mechanical bending characteristics. However, while the infrared optical fiber of this composition does not break with bending of a radius of as small as 20 mm because of its great elongation rate, there is the problem that it is susceptible to plastic deformation with bending because of its small yield strength, causing fusing at the deformed portion during transmission of the laser beam or resulting in degradation in the optical characteristics. There has also been a report of an example of infrared optical fibers formed to have a diameter of 0.9 mm from materials with mechanical strength improved by using silver chloride and silver bromide in an equivalent mole ratio in its composition, but it has the problem that after a slight number of repetitive bending with a radius of 20 mm, it is fused during transmission of the laser beam; the optical characteristics deteriorate; and sometimes, the optical fiber ruptures. Furthermore, with regard to the method of manufacturing infrared optical fibers, the conventional hot extrusion (c.f., Figure 5) of metal halide materials has the problem that the infrared optical fibers are molded in a corrugated form and that the extrusion speed is slow. This tendency is particularly noted in the extrusion of silver halide materials with improved mechanical strength, and therefore, the extrusion of optical fibers with excellent optical characteristics has not been possible. A method of extruding a halide crystal and rectifying the fiber by applying vertical tension can be seen in JP-A-61 128 208 of the same applicant. Disclosure of the Invention In view of the above problems it is the object of the present invention to provide a method of manufacturing an unclad infrared optical fiber with high flexibility and excellent optical characteristics. The present invention is basically directed to a method of manufacturing an unclad infrared optical fiber substantially free of foreign particles with a diameter of more than 3 µm, wherein by hot extrusion with the use of dies an optical fiber of linear shape having a diameter of 0,5 mm or less is formed from a mixture of silver chloride and silver bromide with the amount of silver chloride in the mixture being in the range from 30 to 70 % by weight, and wherein the formed infrared optical fiber is subjected to a tensile load which is greater than the yield stress, but smaller than the rupture stress of said unclad infrared optical fiber. An unclad infrared optical fiber manufactured by this method will not yield plastic deformation with bending of a curvature radius of 20 mm which is an operating condition for practical use as a "laser endoscope". Also, because the infrared optical fiber is produced by hot extrusion with a tensile load greater than the yield stress being applied thereto, the infrared optical fiber can be formed in a straight shape. Brief Description of the Drawings Figure 1 is a schematic cross-sectional view of an extruder used for the method of manufacturing an infrared optical fiber in one example of the present invention. Figure 2 shows characteristic curves of the relationship between the diameter of the infrared optical fiber in the above example and the secular change of the transmittance thereof in a repetitive bending test. Fig. 3 shows characteristic curves for the rupture stress of the infrared optical fiber and the yield stress and rupture stress of the silver chloride - silver bromide single crystal with respect to the concentration of silver bromide. Figure 4 is of characteristic curve showing the relationship between the composition ratio and the radiation spread angle of the infrared optical fiber. Figure 5 is a schematic cross-sectional view showing a conventional extruder. Best Mode for Carrying out the Invention The following describes in detail a method of manufacturing an unclad infrared optical fiber according to the present invention, as well as the various characteristics of the infrared optical fiber, with reference to the accompanying drawings. Silver chloride and silver bromide are mixed with each other to have a composition ratio in the range of 30 to 70 percent by weight of silver chloride, and single crystals thereof are prepared by the Bridgman method. Then, an infrared optical fiber 1 is produced by use of an extruder as shown in Figure 1. A preform 2 is a base crystal obtained by forming the single crystal in a cylindrical shape with a diameter of approximately 8 mm. The reference numeral 3 is a pressurizing ram, and the reference numeral 4 are dies having a nozzle 5 to determine the fiber diameter. In this example, a nozzle with a diameter of 0.3 to 0.5 mm is used. The dies 4 are formed from silicon nitride ceramics which has high hardness and excellent corrosion resistance to halide materials and sodium thiosulfate(Na 2 S 2 O 3 ) for cleaning halide materials. The reference numeral 6 is a container for housing the preform, and the reference numeral 7 is a heater for heat controlling the container 6. A weight 8 is used for applying a tensile load to the infrared optical fiber. Next, the procedure for extrusion of the infrared optical fiber will be explained below. First, the extruding temperature is set within the range of 150 o to 220 o C. After the temperature has stabilized, a pressure of 5 to 15 ton/cm 2 is applied to the preform 2 by the use of a hydraulic press, and while applying an additional load to the infrared optical fiber 1 by the weight 8, the infrared optical fiber 1 is produced. The weight 8 has a weight that can apply a load greater than the yield stress and smaller than the rupture strength of the infrared optical fiber. A weight of 300 g within the above load range is suitable for an infrared optical fiber with a diameter of 0.3 mm. In a conventional method in which no tensile load is applied, infrared optical fibers are molded in a corrugated form as shown in Figure 5, thus having the problem that the laser beam is radiated from the infrared optical fiber over a wide angle of 20 to 30 degrees and that the production speed is slow. This is because the hole of the nozzle 5 is too small for precise machining of the hole, hence difficulty in obtaining an ideal hole shape, and also because the silver chloride - silver bromide materials exhibit great mechanical strength. By forming while applying the tensile load of 300 g, it is possible to produce the infrared optical fiber, having a straight shape, no corrugations, and excellent radiant optical characteristics with a radiation spread angle of 14 degrees, as compared with 20 to 30 degrees in the prior art, at the speed two times faster than the conventional method. The following will now describe the relationship between the diameter of the infrared optical fiber 1 thus produced and the repetitive bending characteristics thereof. For the infrared optical fiber 1 subjected to the bending test, silver chloride - silver bromide fibers were used with a composition ratio corresponding to 43 percent by weight of silver chloride, of three different diameters of 0.5 mm, 0.35 mm, and 0.3 mm, and of a length of 1.5 m. As shown in Figure 2, the fibers were bent with a bending radius of 20 mm and a bending angle of 180 degrees to approximate the actual operating conditions for a laser endoscope, and the change in the transmittance was observed. As shown in this figure, the infrared optical fiber with a diameter of 0.5 mm showed a drop in the transmittance after 2,000 times of bending, and burnt at the bent portion during the measurement of the transmittance after 3,000 times of bending. The infrared optical fiber with a diameter of 0.35 mm showed no drop in the transmittance even after 10,000 times of bending, but burnt at the bent portion during the measurement of the transmittance after 12,000 times of bending. With the infrared optical fiber with a diameter of 0.3 mm, no burning was observed even after 30,000 times of bending, and the drop in the transmittance was less than 2 percent. As a guideline for the required flexibility, supposing that the life of the optical cable housing the infrared optical fiber is one year, that the times of bending for one medical treatment is 50, and that 200 treatments are performed per year, it is considered that the infrared optical fiber is required to have the flexibility that withstands about 10,000 times of bending. The infrared optical fiber with a diameter of 0.35 mm or less is capable of, under the actual operating conditions, withstanding more than 10,000 times of bending and yet retaining the power transmission capability with negligible drop in the transmittance and other radiant optical characteristics, and is expected to satisfactorily meet the requirements for use within the interior of a body. The following description deals with materials for the infrared optical fiber from the viewpoint of the composition ratio. Figure 3 shows the rupture stress of the infrared optical fiber and the yield stress and rupture stress of the silver chloride - silver bromide mixed crystal with respect to the concentration (percent by weight) of silver bromide. The infrared optical fiber exhibits a greater mechanical strength as the composition ratio approaches the center of the graph. This is also true of the optical characteristics. That is, because degradation in the optical characteristics is caused by excessive plastic deformation, it is considered that the composition ratio at the center of the graph where the silver chloride - silver bromide material exhibits great yield stress and resistance to plastic deformation is suitable for the material used for the infrared optical fiber. To prove this, infrared optical fibers of various composition ratios were produced to evaluate their optical characteristics. Figure 4 shows the relationship between the composition ratio and the radiant optical characteristics. As shown in this figure, the radiation angle of an infrared optical fiber containing 3 percent by weight of silver bromide takes an extremely great value of 30 degrees, and the radiation angle becomes smaller as the composition ratio approaches the center of the graph, thereby attaining more excellent radiant optical characteristics. Although not shown, the tendency was noted that a greater transmittance was also provided as the composition ratio approached the center of the graph. In the above example, the description has been mainly directed to the degradation in the radiant optical characteristics in connection with bending in the actual use of the optical fiber. However, because the degradation in the radiant optical characteristics such as an increase of the radiation spread angle and a decrease in the transmittance can also be caused by the stress applied during the production of the infrared optical fiber such as the extrusion and end polishing processes, the most suitable infrared optical fiber is obtained in the concentration range of 30 to 70 percent by weight for the silver chloride - silver bromide where great yield stress is provided. The following will now describe the laser beam absorption coefficient and power transmission capability of the infrared optical fiber. The absorption coefficient was obtained from the elongation of the infrared optical fiber by the calorimetric measurements, while the power transmission capability was measured by increasing the power of laser beam introduced into the infrared optical fiber and measuring the decayed laser beam output. As a result of the comparison of these two characteristics, it was found that the laser beam output of 10 W required the absorption coefficient of the infrared optical fiber of 10 -3 (1/cm) or less. From this, it can be seen that an infrared optical fiber having an absorption coefficient of 10 -3 (1/cm) or less is required to obtain a laser beam transmission power of 10 W which is considered necessary for the incision and vaporization of a diseased part. Industrial Applicability According to the present invention, when a composition ratio which provides great yield stress is selected for the material used for the infrared optical fiber and the diameter of the thin infrared optical fiber is set to 0.35 mm or less, it is possible to obtain an infrared optical fiber which has a great mechanical strength, so that it does not burn because of degradation in the laser beam transmission capability, even after 10,000 times of repetitive bending with a curvature radius of 20 mm which is the actual operating condition, and it exhibits excellent optical characteristics such as a transmittance and a radiation spread angle. Also, by applying an appropriate tensile load to the infrared optical fiber when extruding it, an infrared optical fiber, which is of a straight shape free from curving and which has excellent optical characteristics, can be produced stably with a short manufacturing period of time. List of the Reference Numbers in the Drawings 1 INFRARED OPTICAL FIBER 2 PREFORM 3 PRESSURIZING RAM 4 DIES 5 NOZZLE 6 CONTAINER 7 HEATER 8 WEIGHT

Claims: A method of manufacturing an unclad infrared optical fiber (1) substantially free of foreign particles with a diameter of more than 3 µm, wherein by hot extrusion with the use of dies (4) an optical fiber (1) of linear shape having a diameter of 0,5 mm or less is formed from a mixture of silver chloride and silver bromide with the amount of silver chloride in the mixture being in the range from 30 to 70 % by weight, and wherein the formed infrared optical fiber is subjected to a tensile load which is greater than the yield stress, but smaller than the rupture stress of said unclad infrared optical fiber (1). A method of manufacturing an unclad infrared optical fiber according to claim 1, wherein the dies (4) have a diameter of 0,35 mm or less.

Inventors: IKEDO MASARU, KAYASHIMA KAZUHIRO, OHSHIMA KIYOKO, TATEISHI FUMIKAZU, IKEDO, MASARU, KAYASHIMA, KAZUHIRO, OHSHIMA, KIYOKO, TATEISHI, FUMIKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: C03B37
IPC codes: G02B6, C03B37, C03C13