c followed by applying an x ray excited electron spectral analysis xps to the heat treated doped glass also the same doped glass was subjected to a heat treatment for an hour under a nitrogen gas atmosphere of 1000 u00b0 c containing 10 of hydrogen followed by applying an xps to the heat treated doped glass it has been found that strong peaks corresponding to a 2 o 3 and as 2 o 5 appear in a bonding energy region of about 45 ev in the case of a doped glass subjected to the heat treatment under a nitrogen gas atmosphere in the doped glass thermally reduced under an atmosphere containing hydrogen however the peak in a bonding energy region of about 45 ev was weakened also a strong peak corresponding to arsenic appeared in the bonding energy region of about 41 ev it follows that the oxidized state of an impurity in the present invention is defined to denote the state of a high bonding energy in xps likewise the reduced state of an impurity is defined to denote the state of a low bonding energy in xps in the present invention hydrogen constitutes a typical example of a diffusion control substance serving to reduce the impurity contained in the doped glass hydrogen may be supplied in the form of a hydrogen molecule h ion or hydrogenated ion such as sih3 the ionized hydrogen is bonded to an oxygen atom in the sio 2 network in place of an.