DESIGN AND CHARACTERIZATION OF A BASED ON PHOTONIC CRYSTAL WAVEGUIDES COUPLING-STRENGTH-CONTROLLED DIRECTIONAL COUPLER (2002)

Numerical and experimental studies on photonic crystal waveguide directional couplers (DCs) with coupling-strength-control defects are reported here. The basic structure consists of two parallel-coupled single-defect waveguides separated by a triple-line air-hole array, with some air holes in the middle of the triple-line eliminated. Fundamental operation as the DC at a wavelength of -1.3 pm is observed with a short coupling length. 2 A two-dimensional photonic crystal (2DPC) has the potential of a small photonic integrated circuit (PIC) with light sources, optical switches, and waveguide components (I). Coupled waveguides are among the important elements of PC-based PICs, as they can be applied to various devices, such as optical directional couplers (DCs) (2) and optical switches. This paper specifies the calculated and measured transmission characteristics of light passing through the 2DPC-based optical DC with coupling-strength-control defect (CCD) regions. Through proper design, fundamental operation as the DC at a wavelength of -1.3 pm has been demonstrated. The basic structure of a coupling-strength-controlled (CC)-DC, as shown in the inset of Fig. 1, consists of two parallel-coupled single-defect waveguides (PSWs) separated by a triple-line air-hole array in 2DPCs. By eliminating some air holes in the middle of the triple-line, the overall coupling length can be dramatically reduced by 113. The transmission spectra and electro magnetic-field distributions were calculated by the 2D finite-difference time-domain (FDTD) method. The lattice constant (a), air-hole diameter, and effective refractive index used in the calculation were 360 nm, 216 nm, and 2.81, respectively. The selected lengths of the PSW and CCD were 20a and loa, respectively. Simulated results indicate that 20% of the input optical beam is reflected at the entrance (port a in Fig. I), and the remainder is coupled along the PSWs and finally divided equally between output port e (39%) and port d (38%). This configuration is therefore designed as the 3-dB divider rather than the 100% DC. An air-hanging-type AIGaAs-2DPC slab CC-DC device was fabricated using high-resolution electron-beam lithography, dry etching, and selective wet-etching techniques (3). The core thickness, lattice constant and hole diameter are 250 nm, 360 nm, and 220 nm, respectively. The waveguides consist of a missing row of air holes within a triangular PC block. The PC area is sandwiched by 2-pm-wide and 0.4-mm-long input/output stripe waveguides. Figure 1 shows the measured transmission spectra for TE-like mode. The solid line, dotted line, and thin solid line indicate spectra for ports a to c for CC-DC, ports a to d for CC-DC, and a 60-degree double-bend waveguide, respectively. For calibration of the transmittance, a nominally identical sample without a PC was used. From this result, the transmittance spectra for each port are found to be almost the same, including the peak value of 30 %. This result indicates that the observed optical beam corresponds to the DWG-related waveguide mode for the 2DPC with the D-WG. These results show that this CC-DC serves as a good 3-dB beam-splitter. In conclusion, we have designed and characterized the CC-DC. Throueh Droner desien. fundamental oneration as an an 0.278 0276 0.274 0.272 0.21 0.268 0.266