This paper studies the beat frequency characteristics of photonic crystal waveguide directional couplers of the two parallel straight waveguides by using the coupled-mode theory and acquires the analytical expressions for the light intensity distributions in the waveguides when two incident single-frequency waves with similar frequency into the input ports of the two waveguides and calculates the mode field distribution of the coupled waveguides by applying the time-difference time-domain method.The results show that the beat frequency envelope of the incident wave can be detected in the two parallel waveguides by the coupling action of the waveguides and its time-domain characteristics conform to the theoretical analysis.Finally

it presents a frequency transition device for the lightwave to millimeter wave transition by using photonic crystal directional coupled waveguides.