Gratings

The rapidly increasing demand for highly wavelength selective elements can be met by using Bragg gratings written directly in the core of optical fibers or waveguides. For this we use UV light at a wavelength around 248nm and a diffractive optical element called a phase mask. The goals of this research project are to implement advanced UV writing facilities which can induce almost any given index modulation and obtain a deeper understanding of the whole writing process from the basic glass physics via the involved optics to the final UV-written components.
In 1999 we have carried out further development of single-mode distributed feedback (DFB) fiber lasers to enable these to operate at longer wavelengths. We have obtained operation up to 1618nm and demonstrated low-noise transmission at 1613nm.
In order to produce UV-written gratings in many different kinds of glass it is often necessary to use a method to enhance the UV sensitivity of the material. The preferred method today is to load the glass with deuterium gas at high pressure. However, the use of loading may lead to a reduced stability of the UV-induced index change. We have developed a new isochronal anneal method to make accelerated testing of grating stability.
In addition to Bragg gratings we have studied long period gratings (LPG) which couple light between different forward propagating modes (e.g. cladding modes). We have particularly studied the influence of core concentricity error on the performance of LPG. We have found some new effects, which may be important for the bending sensitivity of LPG.