Permanent Photoinduced Refractive-Index Changes for Bragg Gratings in Silicate Glass Waveguides and Fibers
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devoted to this topic (and to poling of sil-
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ica, a field reviewed by W. Margulis in this issue).2 In the following, a brief history of the main breakthroughs that led to the widespread use of photosensitivity will be given and followed by a quick survey of the devices made possible and of their applications. The remainder of this article will deal with some of the materialsscience aspects important in understanding the mechanisms of photosensitivity. Of special interest is the fact that several different processes can be called upon to explain how ultraviolet (uv) light can induce a permanent index change in doped silica. The glass composition and fabrication procedure, the light source used, and the details of the irradiation conditions all have an influence on the characteristics of the induced refractive-index changes. This in turn has an impact on the type of device that can be fabricated in a given waveguide or fiber and on tire reliability of devices in the field. For the reader interested in a more exhaustive description of photosensitivity, Bragg gratings, and their applications than can be given here, a good up-todate reference is a recent book chapter by Russell and Archambault.3
Brief History of Photosensitivity in Silica Waveguides and Fibers To be specific, the topic addressed here is not the kind of sensitivity to uv light experienced by all kinds of materials with an optical bandgap large enough to retain some transparency in the uv. For instance the optical degradation of uv-grade pure silica exposed to ArF excimer-laser light entails refractiveindex changes of the order of only 1CT6. Nor are we considering photosensitivity in the sense of the visible photodarkening or induced color center bands that are observed in certain doped glasses. What is described here is the rather circumscribed phenomenon by which (1) a doped or modified silica glass with weak absorption in the uv experiences relatively large refractive-index changes (10~4 to 10~2) in the visible and infrared parts of the optical spectrum upon exposure to uv light, (2) the refractive-index changes are "permanent" at room temperature, and (3) the optical absorption at infrared wavelengths does not increase significantly. Three distinct types of photosensitivity have been observed as follows: (a) Type I is observed at uv intensities well below the damage threshold of the materials involved. The refractiveindex change is gradual, positive, and monotonic. (b) Type II or "one-pulse" corresponds to the process that occurs when the laserlight intensity reaches the damage threshold of the material.4 The full index change is achieved suddenly with a single pulse of excimer-laser light. This is very practical for the expedient writing of gratings but lacks the uniformity and control necessary to produce the high-spectral-quality gratings needed in many applications. (c) Type IIA is observed in fibers and waveguides with relatively large germanium concentrations (typically a few 10s of percent) and corresponds to a regime in which a positive in
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