The Photonics Research Institute at the National Institute of Advanced Industrial Science and Technology (AIST) and the Research Group under Professor Isamu Miyamoto of the Department of Manufacturing Science at the Graduate School of Osaka University have been successful in the formation of the world's most heat-resistant diffraction grating in an optical waveguide. The diffraction grating is formed by a very simple process involving the use of heat treatment after laser irradiation. The practical applications of the optical waveguide obtained with this type of diffraction grating include its use in temperature sensors and variable filters exploiting the thermo-optical effect.

When the interference fringe of an ultraviolet laser is shone onto an optical fiber or optical waveguide the result is that the refractive index of the region strongly exposed to the light increases, with the formation of a diffraction grating whose structure corresponds to the pitch of the interference fringe pitch. The optical signal propagating region generally known as the core consists of GeO₂-SiO₂ glass, and the light-induced change in the refraction index is attributed to the structural change occurring in the vicinity of the germanium atom. The problem, however, is that this is only a momentary, transient change that can easily be reversed by heating with decreasing the diffraction grating strength.

It has been known for a long time, however, that the addition of B₂O₃ to the GeO₂-SiO₂ glass imparts a high sensitivity to laser light with increase in the amount of change in the refraction index. In this study, a glass was prepared with the addition of B₂O₃ by the plasma CVD method. The new phenomenon was discovered in the process of investigating the heat resistance of this diffraction grating. It was found that a diffraction grating was formed by irradiating a laser on to these GeO₂-B₂O₃-SiO₂ glass film using the familiar method. After subsequent heating to 500 ^oC, the diffraction grating was observed to disappear completely as with the boron oxide free GeO₂-SiO₂ glass. Yet, when heated further to 600 ^oC the diffraction grating, however, a new grating formed with exactly the same period but more than ten times the diffraction effect. The new lattice will not vanish provided that the glass is not reheated to above or above its softening point of 800 ^oC.

The precise principle and the role of B₂O₃ in the process leading to the formation of the heat-resistant diffraction grating is not known. The glass film with B₂O₃ addition develops a strong absorptivity specifically in the ultraviolet region when heat-treated at 600 ^oC. This entails a significant increase in its refractive index. Its absorption intensity is diminished after irradiation with a laser beam prior to the heat treatment. This means that the thermally induced increase in the refractive index is greater without irradiation with a laser beam. This appears to suggest that, as shown in Fig. 1, the refraction index pattern associated with the laser-induced grating is the reverse of the pattern associated with the thermally induced diffraction grating. This phenomenon does not readily occur unless the diffraction grating has a period of a less than a few hundred nanometers. It can therefore be described as a unique phenomenon that is only well-pronounced in the extremely small range at the nano-level.

The AIST has formed a channel type waveguide core from the newly developed heat-resistant diffraction grating and substantiated that is has the high stability it had been expected to present, as shown in Fig. 2. The discovery open up a new potential for the practical realization of the variable filters for optical communications that have to meet extremely high reliability requirements and of the temperature and pressure sensors for which heat-resistance is critical.

Fig. 1 Model Showing the Formation of a Heat-resistant Diffraction Grating with a Pattern of Periodic Change in Refractive Index

Fig. 2 Model view (left) of prototype waveguide filter and actually measured spectrum. The spectrum exhibits no change even after heating to 400 oC.