Patent ID: 8375749

Claim:
A method for fabricating a porous silica preform having a core region and a cladding region, comprising: a step of, when depositing glass particles to be an effective region of the porous silica preform, while a starting rod is rotated about its axis and shifted upward, supplying gas-A containing silicon to a core burner to obtain glass particles for core, depositing the glass particles for core on a lower portion of the starting rod in the vertical direction to form the core region, supplying the gas-A containing silicon and gas-B containing fluorine to a cladding burner so that a ratio of the number of fluorine atoms to the number of silicon atoms satisfies the following Formula (3) to obtain glass particles for cladding, and depositing the glass particles for cladding around the core region to form the cladding region: {(number of F atoms)/(number of Si atoms)}>0.3 (3); and a step of, when forming a tail of the porous silica preform below the effective region after depositing the glass particles to be the effective region of the porous silica preform, while the starting rod is rotated about its axis and shifted upward, forming a hardened layer which has a high powder density by supplying the gas-A containing silicon and the gas-B containing fluorine to the cladding burner so that a ratio of the number of fluorine atoms to the number of silicon atoms satisfies the following Formula (2) and increasing the temperature of the flame of the cladding burner to obtain glass particles which are used to form the hardened layer, and depositing the glass particles which are used to form the hardened layer below the effective region: {(number of F atoms)/(number of Si atoms)}<0.3 (2).