Hydrogen as origin of thermal fixing in LiNbO3: Fe

Protons are identified as the mobile ions, which neutralize the electronic space charge fields during thermal fixing of holograms in LiNbO₃:Fe. The protons form OH^? centers and can be observed via vibrational absorption bands. In crystals with reduced OH^? content holograms cannot be fixed thermally. In crystals containing fixed patterns the OH^? concentration shows a spatial variation corresponding to the electric charge distribution.