GB2056965A - Optical fibers - Google Patents

Abstract

An optical fiber comprising a quartz glass having an oxide of an element of Group V of the Periodic Table incorporated as a high refractive index portion and a quartz glass having at least fluorine incorporated therein as a low refractive index portion. P2O5, As2O3, Sb2O5 and Bi2O5 are given as examples of such Group V oxides. The optical fiber is particularly suitable for image transmission and is less prone to damage or influence by radiation than conventional optical fibers.

Description

SPECIFICATION Optical fibers This invention relates to an optical fiber and, more particularly, to an optical fiber which is capable of faithfully and efficiently transmitting visible light and which is less damaged or influenced by radiation than previously proposed optical fibers.

Recently, an image transmitting system fortransmitting an image in the visible region ofthe spectrum using an optical fiber has increasingly been employed in various optical systems.

For the optical fiber for use in such image transmission, it is required that the transmission loss is low and, moreover the dependence of transmission loss on wavelength in or near the visible region of the spectrum or in the vicinity thereof is small.

Furthermore, it is required in image transmission in the neighborhood of a nuclear reactor or under conditions where radiations, such as 40,,an, X-rays are emitted, for the optical fiber to be less damaged or influenced by radiation.

Conventional optical fibers have failed to meet all the requirements as described above. The reasons for this are believed to be that the conventional optical fibers have been studied and developed mainly for light communication and, therefore, they have designed to reduce the loss in a specific wavelength region corresponding to that of the light being transmitted than to improve the dependence thereof on wavelength, and that the wavelength of a gallium-arsenic based illuminant element, which is promising as a light source for light communication, is more than 0.8 ,am, i.e., in the infrared region.

For example, there is known a quartz glass-based fiber wherein a pure quartz glass (SiO2) is used in the high refractive index portion and a quartz glass having F or B203 incorporated therein is used in the low refractive index portion. While this fiber is less damaged or influenced by radiation, it suffers from the disadvantage that absorption (color center) due to defects in the glass structure tends to occur in the red region (wavelength 0.63 ,tom), since pure quartz glass is used in the high refractive index portion.

Therefore, when the optical fiber is used for the image transmission, the transmitted light has a bluish tint and it is not possible to transmit an image faithfully. Furthermore, in such a combination of quartz glass having F or B203 incorporated therein and a pure quartz glass, the difference in refractive index is at most 1% and it is therefore difficult to improve the launching efficiency with a light source by increasing the numerical aperture of the fiber.

Further, a quartz glass-based fiber comprising a quartz glass having incorporated therein GeO2 used in the high refractive index is known. For this fiber, no color center can be seen in the visible region.

However, since the wavelength of the U. V. absorption edge of GeO2 is 363 nm, the loss in the near ultraviolet region becomes high. This naturally results in an increase of the loss at the short wave band in the visible region. In addition, it has experimentally been confirmed that the fiber is greatly damaged or influenced by radiation.

Furthermore, an optical fiber wherein a polymer material such as plastics is employed in the low refractive index portion is known. However, in such an optical fiber wherein the high refractive index portion is made of glass and the low refractive index portion is made of a polymer material, the reliability of the resulting optical fiber is generally low since the polymer material is easily invaded by water.

In general, from the standpoints of transmission characteristics, reliability, strength, etc., the most preferred material for an optical fiber is at present a glass mainly composed of quartz. Therefore it is considered that a quartz glass based fiber is most preferably used as an optical fiberforthe direct image transmission. The limitations of conventional optical fibers are such that, although they can be used for optical communications, they are not always suitable for use in image transmission.

An object of the present invention is therefore to provide an improved optical fiber for use in image transmission.

Another object of this invention is to provide an optical fiber which is capable offaithfully and efficiently transmitting light in regions of the spectrum including the visible region and the near ultraviolet region.

A further object of the present invention is to provide an optical fiber which is less damaged or influenced by radiation.

According to the present invention, there is provided an optical fiber comprising a quartz glass having an oxide of an element of Group V of the Periodic Table incorporated therein as a high refractive index portion, and a quartz glass having at least fluorine incorporated therein as a low refractive index portion.

In the accompanying drawings:- Figure 1 is a vertical sectional view of an optical fiber according to one embodiment of the present invention.

Figure 2 is a view illustrating the process for producing an optical fiber according to the present invention.

Figure 3 is a graph in which transmission loss is plotted against wavelength for an optical fiber of the present invention.

Figure 4 is a graph in which transmission loss is plotted against wavelength for a conventional optical fiber, and Figure 5 is a graph in which the increase in transmission loss at 0.83 ,am is plotted against radiation time for various optical fibers.

In a quartz glass comprising SiO2, the ratio of oxygen to silicon is stoichiometrically small, although the reason therefor is not clearly understood from the stand point of glass structure, and therefore, structure defect due to the deficiency of oxygen occurs. This structure defect produces the transmission loss. However, this can be compen sated by adding an oxide of an element of the Group V of the Periodic Table in which oxygen is present in a stoichiometrically excessive amount whereby the deficiency of oxygen is eliminated.

The cause of the color center forming in a pure quartz glass is assumed to be that positive holes trapped by unbonded oxygen atoms produce the defects. Forthe compensation ofthe defect, a dopant which serves as an electron donor is preferably used. It is believed that the oxide of an element of the Group V of the Periodic Table serves as an electron donor, and no color absorption in the visible region can be seen in the optical fiber according to this invention.

In the case of step fiber, the difference in refractive index An in the optical fiber of this invention can be represented by the following equation: An = An1 + An2 (1) wherein An, = (refractive index of SiO2 doped with the oxide of an element of the Group V of the Periodic Table) - (refractive index of SiO2) An2 = (refractive index of SiO2) - (refractive index of SiO2 doped with at least fluorine) Therefore, in comparison with a conventional optical fiber in which the core is formed from pure quartz glass, the difference in refractive index can be increased by An, and the launching efficiency with a light source can be increased.

The optical fiber of this invention wherein quartz glass with the oxide of an element of Group V of the Periodic Table added thereto is used in the high refractive index portion and quartz glass with at least F added thereto is used in the low refractive index portion and quartz glass with at least F added thereto is used in the low refractive index portion has the following advantages: The dependence oftransmission loss on wavelengths in the visible region and the near ultraviolet resion can be reduced; the difference in refractive index can be increased; and the optical fiber is less damaged or influenced by radiation.

Examples of fluorine sources which can be used for incorporating fluorine into a quartz glass is a low refractive index portion are F2, SiF4, CmClnF, wherein m is an integer of 1 to 5, n is 0 or an integer of 1 to 10, and I is an integer of 1 to 10 (such as CCI2F2, and CF4), BF2 or SF6.

Examples of oxides of an element of Group V of the Periodic Table are oxides of P, As, Sb and Bi such as P2Os, As203, Sub206, and Biros and preferably P2Os.

The present invention is further explained in greater detail with reference to the accompanying Figures and the following example.

Referring not to Figure 1 the optical fiber illustrated therein is a step index optical fiber and comprises a core 1 and a cladding 2. The core 1 which forms the high refractive index portion is formed from a quartz glass having P2Os incorporated therein and the cladding 2 which forms the low refractive index portion is formed from a quartz glass having fluorine incorporated therein.

A method of producing such an optical fiber is illustrated in Figure 2. In one embodiment, high fre quench electric power with the frequency of about 4 MHz and a voltage of 20 KW is supplied from the outside of a quartz tube 3 by means of a high frequency coil 4 to generate a high frequency induced plasma 5 in the quartz tube 3, thereby forming a reaction system in which-the plasma is used as a heat source. Thereafter, CCl2F2 as a gaseous fluorine compound, and SiCI4 as an easily vaporizable halogen compound, are introduced into the quartz tube 3 where they are subjected to the combustion reaction and SiO2 containing F is deposited in the quartz tube 3 to form a layer 6.

Afterthe layer 6 forming the low refractive index portion, has been formed to the required predetermined thickness, POCI3 and SiCI4 gases are introduced into the quartz tube 3 where they are subjected to the combustion reaction and SiO2 containing P2O5which forms the high refractive index portion, is deposited.

In the above embodiment, if BF3 gas and SiCI4 or another silicon-halogen such as SiF4, are introduced into the quartz tube instead of CCI2F2 and SiC 4 used above and then subjected to the oxidation reaction as described above, SiO2 containing F and B203 can be formed the low refractive index portion. Also, if AsCls and SbCl6 gases are used instead of POCI3, SiO2 containing As203 and SiO2 containing Sub205 can be formed, respectively, as a high refractive index portion. The quartz tube 3 in which the high refractive index portion and the low refractive index portion have been laminated are then consolidated and drawn to form an optical fiber.

Of course, it is possible to produce the optical fiber by an external coating method.

With the thus obtained optical fiber of this inven ron, the refractive indices of the high and low refractive index portions can be controlled to the desired values by changing the concentrations of gases to be used in the reaction. For example, when the refractive index of the high refractive index portion is made higher than that of quartz glass by increasing the concentration of P2Os and the refractive index of the low refractive index portion is made lowerthan that of quartz glass by increasing the concentration of fluorine, the difference in refractive index can reach 0.019. This is greater than that of a conventional opticalfiberwherein pure quartz glass is used in the high refractive index portion, and the launching efficiency with a light source increases.

The graph of Figure 3 relates to an optical fiber of the present invention prepared as described above.

The graph of Figure 4 relates to a conventional optical fiber wherein the cladding is a quartz glass having fluorine incorporated therein and the core is pure quartz glass.

As apparent from the Figures 3 and 4, no color center in the 0.63 , zm wavelength band is observed forthe optical fiber of this invention Moreover, as is apparent from the fact that the wavelength of the absorption edge of P2Os is 145 nm, the loss in the near ultra-violet region is controlled to a low level and, as a result, the dependence of loss on wavelength in the visible and near ultraviolet reg ions is improved.

With respect to the stability of optical fibers against the radiation, Figure 5 shows experimental results on properties of various optical fibers under irradiation of y-rays. In Figure 5, the abscissa indicates the radiation time in terms of radiation rate at 106 RIH and the ordinate indicates the increase in transmission loss as determined at a wavelength of 0.83 Fm. The optical fibers shown in Figure 5 by the numbers 1 to 6 used in the experiments have the following composition.

Optical Fiber 1: GeO2-P2O5-SiO2 core (MCVD*1) Optical Fiber 2: GeOrP2O5-P202 core (MCVD) Optical Fiber 3: GeO2-P2O5-SiO2 core (MCVD) Optical Fiber 4: SiO2 core with a fluorine-doped clad (PCVD*2) Optical Fiber 5: SiO2-P20s core with fluorine-doped clad (PCVD) Optical Fiber 6:SiO2-GeO2 core (VAD* 3) *' MCVD = Modified Chemical Vapor Deposition *2 PCVD = Plasma Chemical Vapor Deposition *3 VAD = Vapor-phase Axial Deposition As is apparent from the results shown in Figure 5, the optical fiber comprising a core containing an oxide of an element of Group V of the Periodic Table (P2O5) with a fluorine-doped clad exhibits the minimum transmission loss upon irradiation with frays.

With reference to the example wherein P205 is used as an oxide of an element of the Group V of the Periodic Table, other oxides such as As2O3, Sub205 and Bi205 can be used with similar results. For increasing the refractive index, it is preferred to use the oxides of those elements having high atomic weights, but such oxides tend to increase the loss in the near ultraviolet region. Therefore, it is necessary to selectthe dopantdepending upon the required wavelength.

While only the step index optical fiber has been explained in the above example, of course, this invention can be applied to the so-called graded index type optical fiber.

As explained above in greater detail, the optical fiber of this invention comprises a high refractive index portion which is formed from a quartz glass having incorporated therein the oxide of an element of Group V of the Periodic Table and a low refractive index portion which is formed from a quartz glass having incorporated therein at least fluorine. The resulting optical fiber is advantageous in that the increase of transmission loss in the visible and near ultraviolet regions is reduced; the difference in refractive index can be increases; and the damage or influences by radiation can be reduced.

The application of the optical fiber of this invention to, for example, an optical fiber for image transmission permits the faithful and efficient transmission of images.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims (5)

1. An optical fiber comprising a quartz glass having an oxide of an element of Group V of the Periodic Table incorporated therein as a high refractive index portion and a quartz glass having at least fluorine incorporated therein as a low refractive index portion.

2. An optical fiber as claimed in claim 1, wherein said fluorine is provided from a fluorine source of F2, SiF4 or CmClnFI wherein m is an integer of 1 to 5, n is O or an integer of 1 to 10 and I is an integer of 1 to 10.

3. An optical fiber as claimed in claim 1 or 2, wherein said oxide is P20s, As203, Sb2O5 or Bi2O5.

4. An optical fiber as claimed in claim 1 or 2 wherein said oxide is P205.

5. An optical fiber as claimed in claim 1, substan- tially as hereinbefore described.