**4. Conclusions**

240 Optical Communications Systems

trimming in the design. Because the add operation is the "time-reversed" process of the channel drop operation, the tunneling-based channels add and drop operation can be combined into a compact form as shown in Fig. 28. The wavelength-selective reflection section locates in the central of the structure, and it ensures full power transfer between the bus waveguide and channel-add/drop sections. The top takes the narrow-band signal out of the bus waveguide while the bottom one couples signal from the transmitters into the bus-

Fig. 28. The steady state wave propagation at the resonant frequency

CDF. (a) Drop mechanism. (b) Add mechanism. (c) Add/drop mechanism.

*c a* of a passive channel add/drop filter designed based on proposed

Re <sup>1</sup> 0.36076 (2 / ) *<sup>s</sup>*

waveguide.

In summary, the resonance frequencies and the field distributions of 2D-PC have been investigated. PC hybrid waveguides with quasi-flat and Lorentzian transmission spectrum were analyzed and modeled by using FDTD and CMT methods. The theoretical results derived by CMT were in good agreement with FDTD simulation results. It was shown that when the phase-shift of the electromagnetic waves traveling between two adjacent PC coupled cavities ( ) , is close to ( 1/2) , *k* the transmission spectrum of the hybrid waveguide is quasi-flat. A modified HW3 with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used in implementation of WDM filters. It was shown that in this case is close to zero. Transmission of ultra-short pulses through the hybrid waveguide was also investigated. A low cross-talk and wideband PC waveguide intersection design based on two orthogonal hybrid waveguides in a crossbar configuration was proposed. Also, it has been shown that when ( 1/2) , *k* optimum performance results for the intersection can be achieved. In addition, it has been clearly proved that simultaneous crossing of ultra-short pulses through the intersection is possible with negligible interference. The transmission of a 200-fs pulse at 1550 nm was simulated by using the FDTD method, and the transmitted pulse showed negligible cross-talk and very little distortion. A three-port high efficient CDF with a coupled cavity-based wavelength-selective reflector, which can be used in WDM optical communication systems, was proposed. The CMT was employed to drive the necessary conditions for achieving 100% drop efficiency. The FDTD simulation results of proposed CDF which was implemented in 2D-PC, showed that the analysis was valid. The simulation results show that the designed CDF has a line-width of 0.78*nm* at the center wavelength 1550 , *nm* and also a multi-channel CDF with channel spacing around 10*nm* (1 ) *nm* with inter-channel crosstalk below 30*dB* ( 15 ) *dB* is possible. These characteristics make the proposed CDF suitable for use in WDM optical communication systems.
