Abstract

A phase mask is a phase diffraction grating usually recorded in photoresist and then etched in a bulk substrate as a one-dimensional uniform and periodic surface relief pattern[1,2]. When illuminated at normal incidence with coherent or partially coherent light, the phase mask produces in its near-field self-interference between the first order diffracted beams. This interference fringe pattern will have a spatial frequency that is twice the spatial frequency of the phase mask, and will be photoimprinted in the photosensitive medium placed in close proximity of die phase mask grating. This will generate a uniform periodic modulation of the refractive index acting as a Bragg filter. The Bragg spectral response will then have a narrow-band reflectivity peak at a design wavelength, accompanied by a series of sidelobes at adjacent wavelengths. In today’s global communication, based on wavelength division multiplexing, there is a requirement for Bragg grating with a spectral response having reduced noise and sidelobes to avoid cross talk. Bragg gratings with reduced sidelobes in their spectral response can be generated by recording an index modulation with a spatially varying modulation amplitude[3]. The condition for an efficient sidelobe suppression is that the modulation level decreases continuously to zero at both limits of the Bragg grating. It can be achieved using appropriate apodization profiles[4]. Different approaches have been investigated to realize Bragg gratings that generate a spectral response without sidelobes, with or without the use of a phase mask [5,6,7,8,9]. We propose a new fabrication process leading to improved results in the spectral response of the recorded apodized Bragg grating, suitable for mass production: simplicity, reliability, low cost and short fabrication time.

© 1998 Optical Society of America