A method of preparing an optical preform includes the steps of: a) etching an optical preform to remove a portion of an oxide material deposited on the preform by using a gas comprising an etchant gas containing fluorine at a sufficient temperature and gas concentration to create a redeposited germanium containing compounds contamination such as GeOx in the remaining oxide material; and b) cleaning the etched preform using a cleaning gas containing at least one halogen gas at a sufficient temperature and gas concentration to remove the redeposited germanium containing compound contamination without any substantial further contamination of the remaining deposited oxide material. Preferably the halogen is either chlorine or bromine.