Abstract

The architecture of this new type of optical band-pass filter is shown in Fig. 1. It consists of a pair of transmission-type volume gratings¹ G₁, G₂ with identical structures in the upper part and a pair of reflection-type volume gratings G₃, G₄ with identical structure in the lower part. The K-vector diagram² of each volume grating is shown in circle near that grating. The output of the upper part is the input of the lower part, and the performance of the lower part is opposite to that of the upper part. For convenience, the performances of these two parts are written together with a slash. The input wave is normally incident on g₁/g₃, and it can be diffracted by G₁/G₃ into the substrate. The diffracted angle is proportional/inversely proportional to wavelength in a transmission-type/reflection-type volume grating, respectively. The diffracted angle corresponding to the central wavelength λc is designed so that it is equal to the critical angle. Then the component with wavelength larger/smaller than λc is partly reflected and attenuates obviously as the wave is guided through the substrate. Because the structure of G₂/G₄ is the same as that G₁/G₃, the diffracted wave of G₂/G₄ will be parallel to the input wave, that is, the output wave passes normally through the substrate. Hence, the upper/lower acts as a low/high pass filter. Consequently, it can act as a band-pass filter and its bandwidth is determined by the number of total internal reflection and dispersive properties of the volume gratings.

© 1995 IEEE