FI94560B - Process for coupling a thin-cored optic fibre, for example an Er-alloyed optic fibre, with a standard uniform fibre, and a thin-cored optic fibre - Google Patents

Description

94560

A method of attaching a thin core, for example an Er-doped optical fiber, to a standard monofilament fiber and a thin core optical fiber

The invention relates to a method 5 according to the preamble of claim 1 for connecting a thin-core, for example Er-doped, optical fiber to a standard monofilament fiber.

The invention also relates to a thin-core optical fiber.

10 The incorporation of Er fiber into standard fiber has been described in several publications. Such connections are necessary e.g. in the case of optical amplifiers, where the Er-doped amplifier fiber is connected to branch fibers and insulator made of standard single-mode fibers or to the tail fibers of the connectors. Er-doped fiber has a thinner core than standard monofilament fibers because the high refractive index and thin core improve the efficiency of the fibers.

According to the publications, the diffusion of the Er fiber core can be exploited to reduce the coupling attenuation. However, high temperature and more than 20 times are required to diffuse the Er fiber structure according to the prior art. In addition, both time and temperature are critical in these methods: too long a diffusion time or too high a temperature can cause the core to expand so large that it degrades the coupling efficiency.

The object of the present invention is to obviate the shortcomings of the technique described above and to provide a completely new type of method for connecting a thin core, for example an Er-doped optical fiber, to a standard monofilament fiber and a thin core optical fiber.

The invention is based on the fact that the thin Er core of the Er-doped fiber is surrounded by a phosphorus-doped (P205 glass) region with an outer diameter corresponding to the diameter of the core of the standard fiber. The phosphorus-doped region accelerates the diffusion of the core into the surrounding material and allows a good coupling efficiency between the thin-core fiber and the standard monofilament fiber.

94560 2 More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The thin-core optical fiber according to the invention is in turn characterized by what is set forth in the characterizing part of claim 3.

The invention provides considerable advantages.

With the solution according to the invention, the diffusion takes place simultaneously during the normal welding process when the fibers are joined together. A separate diffusion step is not required. Fast connection and simultaneous diffusion substantially reduce costs. The outer diameter of the phosphorized area precisely defines the outer boundary of the diffusion, so that in a welding situation, too long a welding time does not cause a core that is too wide in the thin-core fiber and thus a poor connection efficiency. With the method according to the invention 15, the welding time can be reduced to 1-10 seconds.

Thanks to diffusion, the connection attenuation is of the order of 0 to 0.4 dB. Without diffusion, the coupling attenuation would be on the order of 1 to 3 dB.

The invention will now be examined in more detail with the aid of the exemplary embodiments according to the accompanying Figures 20. In the figures, the size of the fiber core is exaggerated for clarity.

Figure 1a shows a cross-sectional end view of an Er fiber according to the invention.

Figure 1b shows a cross-sectional side view of the Er fiber of Figure 1a.

Figure 2a shows a cross-sectional end view of a standard monofilament fiber according to the prior art.

Fig. 2b shows a cross-sectional side view of the standard monofilament fiber of Fig. 2a.

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Figure 3 shows a cross-sectional side view of a joint of an Er fiber and a standard monofilament fiber according to the invention.

The phosphorus-doped glass region 2 of the Er fiber 1 of Figure 1a is fabricated by an MCVD-5 device by passing O 2 gas through a liquid POCl 3 source and then passing evaporated POCl 3 with correspondingly vaporized SiCl 4 to a quartz tube (substantially larger in diameter than the final fiber). The diameter of the pipe is typically a few centimeters. The gases passing through the tube are heated by a flame from the outside to form P 2 O 5 -alloyed SiO 2 glass. 10 Phosphorus-doped glass is formed as cylindrical layers on the inner surface of a quartz tube. Inside the phosphorus-doped region, the fiber core is grown in the normal manner. Finally, the assembly is drawn into a thin fiber 1. The MCVD technique is described, e.g., in "Optical Fiber Communications, Vol. 1 Fiber Fabrication", edited by Tingye Li, PP 2-6.

15

The core 4 of the Er fibers made by the method of the invention is typically 2 to 8 μm in diameter. The core 4 is surrounded by a phosphorus-doped layer 2, the thickness of which in the fiber 1 is such that the outer diameter of the P 2 O 5 -doped region corresponds to the diameter 7 (about 6-10 μm) of the core 7 of the standard monofilament 6 according to Figs. In this case, the thin core 4 of the Er fiber 1 diffuses (widens) rapidly up to the outer boundary 5 of the phosphorus-doped region 2, in the surrounding unalloyed SiO 2 Iase 3 the diffusion is slow. The expansion of the Er-doped core 4 is thus automatically limited so that the diameter of the core 4 at the joint is similar in both weldable fibers 1 and 6, whereby a small joint damping is achieved. The outer diameter of fibers 1 and 6 is typically 125.

In order for the core to diffuse rapidly during butt welding, it is preferred to use a P 2 O 5 alloy of about 0.4 mol% -2.1 mol% (1-5% by weight) around the core 4. In this case, the core 4 is allowed to diffuse into the P205-doped softer glass layer 2 when heated. The increase in the refractive index around the core 4 caused by phosphorus can be compensated by fluorine doping in the phosphorus region 2.

4,94560 This technique is described in "Optical Fiber Communications, Vol. 1 Fiber Fabrication", edited by Tingye Li, PP 13-14.

According to Figures 2a and 2b, the standard monofilament fiber 6 consists of a core 7 acting as a light channel 5 and a surrounding shell part 8. The outer diameters of both the thin-core fabric 1 and the normal fiber 6 are approximately equal. The material of both the shell portion 8 of the normal fiber and the shell portion 3 of the thin core fiber 1 is typically SiO 2.

According to Figure 3, a conical transition region 9 is formed inside the Er fiber in the vicinity of the joint area by heating the joint area from the side, for example by means of an arc. The shape of the transition region 9 is substantially dependent on the temperature profile caused by the welding. With commercial welding equipment, the welding heat is strongly concentrated at the joint, resulting in the conical shape shown in Fig. 15 3. The welding temperature is typically about 1400-1800 ° C.

The interface 5 of the phosphorus-doped region 2 acts as a diffusion limiter because the diffusion of the core 4 to the fiber shell region 3 is substantially slower than to the phosphorized region 2.

The invention can be used generally to join thin-core fibers to standard monofilaments.

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Claims (4)

6 94560 Claims: A method for joining a thin core, for example an Er-doped optical fiber (1), to a standard monofilament fiber (6) by welding, in which method the thin core fiber (1) is placed opposite the standard monofilament fiber (6) and the joint is heated, characterized in that - such a thin core fiber is used (1), around the core (4) of which a phosphorus-doped region (2) is formed, the outer diameter of which corresponds to the diameter of the core (7) of the standard monofilament fiber (6) to be joined. Process according to Claim 1, characterized in that a thin-core fiber (1) is used, the doping of the region surrounding the core (4) of P 2 O 5 -15 being about 0.4 mol% -2.1 mol% (1-5% by weight). . A thin core, for example an Er-doped optical fiber (1), comprising a core (4) acting as a light channel and a surrounding shell part (3), characterized in that a phosphorus-doped region (2) is formed around the core (4), having an outer diameter 20 corresponding to the diameter of the core (7) of the standard monofilament fiber (6) to accelerate the connection of the thin core fiber (1) with the standard monofilament fiber. * Optical fiber (1) according to claim 3, characterized in that the P 2 O 5 doping of the region surrounding the core (4) is about 0.4 mol% -2.1 mol% (1-5% by weight). • · 6 94560