The present invention includes an exposure chamber configured to contain a passivating gas having a selected hydrogen concentration, the exposure chamber further configured to contain at least one NLO crystal for exposure to the passivating gas within the chamber, a passivating gas source fluidically connected to the exposure chamber, the passivating gas source configured to supply passivating gas to an interior portion of the exposure chamber, and a substrate configured to hold the NLO crystal within the chamber, the substrate further configured to maintain a temperature of the NLO crystal at or near a selected temperature, the selected temperature being below a melting temperature of the NLO crystal.