Transmission filters with multiple flattened passbands based on chirped moire gratings

In-fiber chirped moire gratings (CMG´s) are attractive as wavelength selective elements, and in particular as multiple passband transmission filters, for optical communications. However, as with single phase-shifted gratings, the passbands exhibit nonideal filter characteristics such as a Lorentzian shape with narrow transmission peaks, and rounded, possibly broad bottoms, and are not suitable for system design. We show that we can flatten the passbands by introducing regions of constant refractive index within the CMG. By choosing the number, lengths, and location of the regions with constant refractive index, passbands with steep slopes and ripples less than 0.5 dB can be achieved.