A Rapid Swept-Source Mid-Infrared Laser

In this paper we describe the design and realization of a swept-source EC-QCL. Since the moving mass of a galvanometer based mirror is less than that of a diffraction grating, a Littman external cavity was constructed to enable rapid sweep rates. We demonstrate a 400Hz repetition rate with a tuning speed of 45,000cm-1/s. Both figures of merit are higher than those achieved in conventional Littrow configuration EC-QCLs. The tuning speed is not limited by the cavity design or gain chip but by the inertia of the moving mirror used. We go on to assess the fundamental limit to the sweep rate of our swept-source. A key factor is the number of round trips of the cavity that are required to reach saturation. This photon build up time is dependent on the cavity length, the round trip gain, the cavity linewidth, and the ASE power within the spectral window of the cavity [4]. To the best of our knowledge, this is the first assessment of QCL gain chips for rapid swept-source applications. We demonstrate that >100kHz repetition rate should be possible from sweptsource EC-QCLs.