Preparation method of rare earth-doped optical fiber preform with large core-cladding ratio

The invention relates to a preparation method of a rare-earth-doped optical fiber preform with a large core-cladding ratio. The preparation method comprises the following steps: shrinking a quartz reaction tube into a rare-earth-doped optical fiber preform; drilling a core part doped with rare earth in the middle and integrally taking out the core part to form a rare earth-doped small-diameter core rod, and welding a quartz rod with the diameter larger than that of the rare earth-doped small-diameter core rod at each of the two ends to serve as a supporting rod; performing high-temperature flame polishing treatment on the rare-earth-doped small-diameter core rod welded with the supporting rod by using oxyhydrogen flame; a new quartz reaction tube is connected into the MCVD deposition lathe again, the core part of the prefabricated rod is deposited according to the method, the treated rare-earth-doped small-diameter core rod is placed in the center position of the quartz reaction tube from the left side of the