Patent ID: 7647792

Claim:
A method for fabricating a porous silica preform having a core region and a cladding region, comprising: a step A of, before 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 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 (1) to obtain fluorine-doped silica glass particles, and depositing the fluorine-doped silica glass particles around the starting rod: {(number of F atoms)/(number of Si atoms)}≦0.1 (1); and a step B of, when depositing glass particles to be the effective region of the porous silica preform, while the starting rod is rotated about its axis and shifted upward, supplying the 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 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 (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).