A high-intensity femtosecond (fs) laser can fabricate complicated three-dimensional microstructures inside photosensitive glass with high spatial resolution. In this work, the mechanism of the photoreaction of the photosensitive glass to the infrared fs laser is investigated. We examine the photoinduced electron excitation process on the basis of the determination of the critical dose and a change of the optical-absorption spectrum after the fs laser irradiation. The photoreaction mechanism is discussed in comparison with the case of an ultraviolet nanosecond laser irradiation. Finally, the successive interband electron excitation through defect levels by multiphoton absorption is proposed.

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