BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing, showing a Cr:YAG crystal fiber with a silica capillary inserted in a sapphire tube and a laser beam applied to the sapphire tube for the growth of a double-clad optical fiber according to the present invention.
FIG. 2 is a 2D temperature distribution chart obtained by means of insertion of a silica capillary and a Cr:YAG crystal fiber in a sapphire tube and heating of the sapphire tube with laser subject to finite element method according to the present invention.
FIG. 3 is a single dimensional temperature distribution chart obtained by means of insertion of a silica capillary and a Cr:YAG crystal fiber in a sapphire tube and heating of the sapphire tube with laser subject to finite element method according to the present invention.
FIG. 4 illustrates the growth of crystal and the related equipment subject to LHPG method according to the present invention.
FIG. 5 is a core diameter-laser power curve obtained from the fabrication of a double-clad crystal fiber by means of direct heating according to the present invention.
FIG. 6 is a core diameter-laser power curve obtained from the fabrication of a double-clad crystal fiber by means of sapphire tube radiation heating according to the present invention.
FIG. 7 is a core diameter-core variation curve obtained from the fabrication of a double-clad crystal fiber with and without sapphire tube according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An indirect heat type double-clad optical fiber fabrication method in accordance with the present invention is as follows:
Prepare a crystal fiber at first, and then insert the crystal fiber into a silica capillary, and then attach a sapphire tube to the periphery of the part of the silica capillary to be heated, and then apply a laser beam to the sapphire tube to increase the temperature of the sapphire tube and to further fuse the silica capillary with thermal radiation so that the fused silica capillary is wrapped about the crystal fiber, forming the desired double-clad optical fiber.
Referring to FIG. 1, before actual growth, finite element method was employed to simulate 2D temperature distribution by means of insertion of a silica capillary 20 and a Cr:YAG crystal fiber 30 in a sapphire tube 10 and heating of the sapphire tube with laser R. In this test, the sapphire tube has the length of 1.5 millimeters, the outer diameter of 1.2 millimeters, and the inner diameter of 0.48 millimeters. The silica capillary has the length of 100 millimeters, the outer diameter of 0.32 millimeter, and the inner diameter of 0.076 millimeter. Cr:YAG crystal fiber has the diameter of 0.076 millimeter. Because focused laser beam is applied to an annular heating zone of 0.44 millimeter on the sapphire tube, the temperature distribution is axis symmetrical. The result was as shown in FIGS. 2 and 3. As illustrated, the temperature variation from the inside toward the outside is within 1° C. This explains an excellent thermal capacitive effect that helps smooth heating of the silica capillary to grow a double-clad optical fiber having a uniform diameter.
FIG. 4 illustrates the growth of crystal and the related equipment subject to LHPG method. By means of the application of the related equipment to match with different drawing and pushing speed ratios, different contraction ratios can be obtained. The invention also uses this equipment. The actual performance is as follows:
At first, prepare a Cr:YAG crystal fiber having the diameter of 0.07 millimeter, then insert the Cr:YAG crystal fiber into a silica capillary of outer diameter 0.32 millimeter and inner diameter 0.076 millimeter, and then set a sapphire tube of length 1.5 millimeter, outer diameter 1.2 millimeter and inner diameter 0.48 millimeter in the laser focus, and then apply CO2 laser to the sapphire tube within the height about 0.44 millimeter to produce thermal radiation, thereby heating the silica capillary, causing the fused silica capillary to be wrapped about the crystal fiber, and therefore the desired double-clad optical fiber is thus obtained.
The crystal fiber material can be glass or crystal. According to the present invention, the Cr:YAG crystal fiber is used as a thermal capacitor that stabilizes wave motion of CO2 laser, and is free from the wave motion of laser power for heating the silica capillary indirectly to grow a double-clad crystal fiber having a uniform diameter and a core smaller than 10 micrometer. The laser power required for heating silica capillary indirectly with thermal radiation is about one tenth of the laser power required for heating silica capillary directly with laser beam.
The change of core diameter relative to laser power in the growth of double-clad crystal fiber with or without sapphire tube is observed. When growing a double-clad crystal fiber without a sapphire tube, as shown in FIG. 5, it shows a change of 58.9% in core diameter when laser power is changed by 1%. When growing a double-clad crystal fiber with a sapphire tube, as shown in FIG. 6, it shows a change of 5.88% in core diameter when laser power is changed by 1%. Apparently, the thermal capacitance effect of the sapphire tube effectively improves the problem of significant variation of the core diameter with the variation of the applied laser power.
Referring to FIG. 7, the application of the present invention can grow a double-clad crystal fiber of core diameter 5±25 μm. Reducing core diameter to below 5 μm has the benefits of: (a) reducing the number of modes and lowering transmission loss, and (b) improving the conversion efficiency of Cr:YAG crystal fiber in spontaneous emission to amplify light source. Further, when fused with glass fiber, it greatly reduces insertion loss due to a different core sectional area. In general, Cr:YAG double-clad crystal fiber can be used for making high-efficiency, low transmission loss spontaneous emission optical amplifier for all-optical network.
Patent Application
20080047303
