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Update(MM/DD/YYYY):12/17/2002

Successful Development of the World's Most Heat-Resistant Optical Waveguide Filter

- New discovery of the Laser Induced Effect -

Key Points

  1. The laser write-in type diffraction grating used in optical communications has a heat resistance of only 100 oC or less and its reliability has therefore been too poor for practical use.
  2. We have been successful in developing a high-reliability diffraction grating with a very high heat-resistance temperature of 400 oC or more.
  3. We have fabricated an optical waveguide device provided with the newly developed heat-resistant diffraction grating.


Outline Description

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.

Figure1

Fig. 1 Model Showing the Formation of a Heat-resistant Diffraction Grating with a Pattern of Periodic Change in Refractive Index
Figure2(1)   Figure2(2)
Fig. 2 Model view (left) of prototype waveguide filter and actually measured spectrum. The spectrum exhibits no change even after heating to 400 oC.






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