Abstract

As the demand for bandwidth in optical communication systems continues to grow much effort is put into a more efficient use of the available wavelength span through the wavelength division multiplexing (WDM) technique. While the next generation electronic components will be able to handle 40 Gbit/s signals, the development in WDM systems is pushing the ITU channel spacing down to 50 GHz and in time 25 GHz. As the channel spacing decreases, the spectral properties of the individual components such as Bragg gratings becomes crucial. Bragg gratings working as channel selective components must have high bandwidth utilization and low out-of-band reflection in order to avoid cross-talk induced problems. In order to avoid an increasing bit-error rate when cascading several gratings the variation in the signal reflection and dispersion in the stop band must be small. The transmission dip must be 30 dB and the side lobe suppression in the reflection spectrum must exceed 30 dB. Gratings with simple apodization profiles, such as Gaussian and Blackmann apodizations are not able to satisfy all these demands. The Sinc grating on the other hand has almost a square filter function with a flat stop band and little reflection outside this. Problems with dispersion can be reduced to acceptable levels by assymmetric modification of the Sinc profile.¹ Sinc gratings require several phase shifts, which is not possible to induce with the dual-scan method.² A frequently used method to write Bragg gratings with multiple phase shifts is the dithering method³ where fiber and/or phase mask are moved during the exposure. This method requires interferometric control of the relative position of the fiber and phase mask and is hence very sensitive to vibrations in the setup. We present a method, which is based on a spatial separation of the s- and p-po-larizations of the UV beam.⁴ Using this method we have been able to write 80 dB strong gratings with good agreement between simulations and written gratings.

© 2002 Optical Society of America