Optical pulse processor based on achromatic time lens assisted by compound phase modulation

The time-lens based optical pulse processors have been widely applied in ultrafast optical processing and quantum optics. Conventionally, the time-lens systems used to adopt a sinusoidal waveform to approximate quadratic temporal phase modulation. It inevitably restricts the duration of a linear chirp, and thus limiting the temporal aperture to realize perfect Fourier transform. In this paper, we propose what we believe to be a novel achromatic time lens to expand the temporal aperture by using compound phase modulation. The achromatic time lens first compresses an optical pulse via a traditional time lens, and then introduces a linear chirp over the pulse duration by applying compound waveform consisted of the weighted fundamental and second harmonics. Numerical results proved that the synthesized linear chirp can be maintained over the pulse duration, when the duty cycle of the incident pulse increased from 15% to 72%. It indicates the effective temporal aperture to be expanded significantly. Experimentally, we have applied the achromatic time lens system to process a Gaussian pulse without distorting the pulse shape. The pulse width is compressed from 34 ps to 2.2 ps, while the spectral bandwidth is broadened from 0.32 nm to 1.6 nm. Both the temporal waveform and spectral envelope are maintained. The achromatic time lens scheme exhibits potential integration for supporting on-chip pulse processing with large time aperture.