Abstract

FBGs are promising devices for DWDM networks [1]. Phase mask (PM) technique is one of the most versatile techniques for massive production of FBGs. However, ever increasing requirements of DWDM put stringent requirements on FBG sidelobe suppression and spectral skirt sharpness. In this paper, we report a detailed comparison of different apodization profiles with emphasis on narrow linewidth, sharp spectral skirt and a large figure of merit which is the ratio of bandwidth at -0.5 dB to that at -25 dB. Effective apodized PM was fabricated by using e-beam writing and reactive ion-milling (RIM) with stitching error-free. For a gaussian apodized FBG. it has a gaussian refractive index modulation, an inverse gaussian index background and a constant average index change along the FBG with no unwanted dispersion. FBG apodization using PM dithering technique suffers some drawbacks for pulsed UV excimer lasers and for very high UV photosensitive fibers. Scanning writing apodized FBG technique needs double UV exposures for FBG writing and background index change resulting in a lower control accuracy and a lower fabrication efficiency. Using apodization aperture and amplitude masks have the similar problems of double exposure and lower control accuracy resulting unwanted dispersion. Apodized PM technique is an efficient method for qualified DWDM FBGs production with a simple setup and an excellent fabrication repeatability.

© 1998 IEEE