  We propose a new method to narrow the linewidth of entangled photons
generated from spontaneous parametric down conversion (SPDC). An internal Bragg
grating is incorporated onto a nonlinear optical crystal waveguide. We study
theoretically the spectral characteristics of SPDC under two Bragg grating
structures. We show that using the Bragg grating with a midway pi-phase
shifter, it is a promising way to generate narrow-line entangled photons.
