Glass Material for Optical Fibers Used in Infrared Region

1. Field of the Invention.

This invention relates to a chalcogenide glass material used as optical fibers capable of transmitting infrared rays, particularly the ray of 10.6 .mu.m in wavelength.

2. Description of the Prior Art.

Optical fibers have heretofore been made of materials based on silica glass. Silica glass, however, has a disadvantage in that it is capable of transmitting light having a wavelength of up to approximately 2 .mu.m, but it absorbs substantial amount of a ray having a longer wavelength owing to the lattice vibration. Accordingly, conventional optical fibers of the silica glass type cannot transmit, for example, a ray of 10.6 .mu.m in wavelength which corresponds to that from CO.sub.2 -laser beam used in laser surgery or laser welding. For this reason, in the past searches for a material which transmits rays having a wavelength in the range of from 2 .mu.m to 20 .mu.m was extensibly carried out. As a consequence, chalcogenide glass was found promising as a useful material in said wavelength range. Of the chalcogenide glasses, the selenium-based glass has its lattice-vibrational absorption in the long wavelength region exceeding 20 .mu.m and, hence, can transmit the CO.sub.2 -laser beam of 10.6 .mu.m in wavelength. However, upon examination of the optical absorption characteristics of a germanium (Ge)-selenium (Se) glass as a typical example of Se-based glass, it is seen that, as shown in FIG. 1 attached hereto, there exists a strong absorption at a wavelength of 12.8 .mu.m due to the vibration of Ge-O bond formed by the contamination with oxygen. The tail of this vibrational absorption extends to a wavelength of 10.6 .mu.m. Owing to this absorption, the temperature of Ge-Se glass is elevated, leading to an abrupt increase in the absorption by free electrons. As a consequence, optical fibers consisting of such a type of glass have an important defect of a large absorption loss so that it is not practically used.

An object of this invention is to provide a chalcogenide glass composition which is free from the aforementioned problems of conventional chalcogenide glass such as impurities, particularly contamination with oxygen and which can be used in producing low-loss optical fibers for transmitting infrared rays.

To achieve the object, the infrared optical fiber of this invention is characterized by containing 2 ppm or more and 100 ppm or less of at least one of the aluminum (Al), gallium (Ga), and indium (In) in order to decrease the absorption loss due to Ge-O bond, which adversely affects transmission characteristics of the C0.sub.2 -laser beam at 10.6 .mu.m.

FIG. 1 is a diagram showing the light transmission characteristics of a Ge(30 mole-%)-Se(70 mole-%) chalcogenide glass.

FIG. 2 is a diagram which shows the difference of light transmittance between a chalcogenide glass containing aluminum and that containing no aluminum.

FIG. 3 is a diagram representing the dependence of absorption at 12.8 .mu.m due to the vibration of Ge-O bond upon the aluminum content.

The invention will be described in detail hereunder with reference to the drawings.

Examination was carried out on the relationship between the absorption due to the Ge-O bond in Ge-Se glass and the amount of Al, Ga or In added to the glass. FIG. 2 represents, as an example, the results obtained in the case of aluminum. In the glass composition, Ge:Se=20:80 (molar ratio). In FIG. 2, where the relationship between transmittance and wavelength is plotted, curve 1 relates to the glass containing no aluminum, and curve 2 the glass containing 10 ppm of aluminum, the maximum transmittance being about 70% owing to the surface reflection. It is seen from FIG. 2 that the absorption at 12.8 .mu.m due to the vibration of Ge-O bond becomes substantially diminished by the addition of aluminum. This fact points out that a chalcogenide glass containing aluminum is effective in transmitting with minimum loss the C0.sub.2 -laser beam of 10.6 m.mu. in wavelength.

In FIG. 3 are shown the results of quantitative examination on the relationship between the absorption due to vibration of the Ge-O bond and the amount of aluminum added to the glass composition, wherein Ge:Se=20:80 (molar ratio). It is seen from FIG. 3 that the absorption decreases with the increase in aluminum content of the chalcogenide glass until the content reached 2 ppm and then remains constant. This shows that aluminum of an amount of 2 ppm or more is required in order to minimize the absorption. Therefore, the lower limit of aluminum content was decided to be 2 ppm. On the other hand, it was found that the presence of more than 100 ppm of aluminum exerts adverse effect on the glass making because of the reaction which takes place between the aluminum and the silica glass used during glass making. The range of effective amounts of aluminum, therefore, is from 2 to 100 ppm. This effective range was found to be common to selenium-base glasses (e.g. Ge-As-Se glass and Ge-Sb-Se glass). Nearly the same range as above was found effective with other metals, that are gallium and indium.

Ingot germanium of 99.99999999% purity and selenium shots of 99.999% purity were used as starting materials. These materials were weighed out so that the ratio of Ge:Se may become 20:80 (molar ratio) and sealed together with 10 ppm of 99.999% pure aluminum in a silica glass ampoule at a vacuum of 10.sup.-5 torr. The sealed ampoule was stirred in a vibrating electric furance at 800.degree. C. for 38 hours, and then air-cooled to yield 100 g of a glass block. The resulting block was fabricated into a rod, 10 mm .phi..times.10 cm, by means of a grinder and the rod was drawn through a die to an unclad optical fiber, 1 mm .phi..times.5 m. The light transmission loss at a wavelength of 10.6 .mu.m was found to be 3 dB/m, indicative of a large reduction in loss as contrasted with 10 dB/m of aluminum-free reference glass.

An optical fiber was prepared in the same manner as in Example 1 using the same Ge-Se composition, except that 20 ppm of gallium was added in place of the aluminum. The resulting optical fiber showed a transmission loss at 10.6 .mu.m of 4 dB/m, indicative of a large reduction in loss, as compared with 10 dB/m of the gallium-free reference glass.

An optical fiber was prepared in the same manner as in Example 1 using the same Ge--Se glass composition, except that 30 ppm of indium was added in place of the aluminum. The resulting optical fiber showed a transmission loss at 10.6 .mu.m of 3.5 dB/m, indicating a large reduction in loss as compared with 10 dB/m of the indium-free reference glass.

It is seen from the Examples above that the optical fiber glass material for infrared rays, according to this invention, which is a selenium-base chalcogenide glass containing 2 to 100 ppm of at least one of the aluminum, gallium, and indium, shows a large reduction in the deterioration of light transmission characteristics caused by the contamination with oxygen. The present invention is widely adaptable to a technical field utilizing the optical energy, such as laser machining and laser surgery, thereby contributing much to the improvement in those techniques.