FDTD simulation of femtosecond optical gating in nonlinear optical waveguide utilizing intersubband transition in AlGaN/GaN quantum wells

The propagation and the gate operation of femtosecond pulses in nonlinear o ptical waveguides utilizing the saturation of the intersubband absorption a t 1.55 mu m in nitride multiple quantum wells are simulated for the first t ime. The calculation was carried out by a one-dimensional finite-difference time-domain (FD-TD) method combined with three-level rate equations descri bing the intersubband carrier dynamics. The absorption recovers within Ips when the pulse width is less than 200 fs, which will allow 1-Tb/s operation . However, the pulse shape may be deformed with the propagation due to the coherent effect and the interference between the signal and the control pul ses, and thus, optimization of the pulse widths and the incident timing is required. Since the transparent window (width of the control pulse) becomes shorter according to the propagation, the width of the control pulse shoul d be set broader than that of the signal pulse. As an example, we assume th e case where a 1.6-mu m, 100-fs signal pulse is gated by a 300-fs control p ulse at a wavelength of 1.5 mu m in a 500-mu m length waveguide. A 140-fs g ated signal pulse with a smooth envelope is expected to appear after the ba nd-pass filter. The extinction ratio is expected to be greater than 15 dB.