On-chip pulse generation at 8µm wavelength

The mid-infrared wavelength range is very interesting for spectroscopy because most molecules have a specific signature in this wavelength range. Frequency combs are particularly interesting for this purpose. In the mid-infrared, they rely on quantum cascade lasers (QCL) which have already been used for dual comb spectroscopy. However, QCL comb temporal profile is almost continuous. Generating ultrashort pulses from a QCL comb would be very interesting for ultrafast spectroscopy. Notably, QCL combs are chirped making them well-suited for pulse compression as it has already been demonstrated using a free space external compressor that compensated for the chirp of the QCL comb. This work thus presents an integrated external pulse compression device in the mid-infrared. We use here graded-Silicon-Germanium (SiGe) waveguides to benefit from the Ge wide transparency window in the mid-infrared. We demonstrate a photonic integrated circuit based on chirped Bragg gratings to compensate for the chirp of a QCL comb and perform pulse generation from this source. Pulses as short as 1.39 ps wide were obtained. This result lays the foundation for fully integrated photonic circuits to generate short pulses from a QCL comb source.