ACTIVE-COMB

Frequency combs (FCs) are coherent light sources that emit a broad spectrum consisting of discrete, perfectly spaced modes, each having an absolute frequency measurable within the accuracy of an atomic clock. After 20 years of development in the near-infrared and visible domains, FCs have revolutionized frequency metrology with strong impacts in other fields ranging from astronomy to communications. From these developments, mid-infrared (MIR 3-12 µm) FCs have recently become of significant interest for molecular spectroscopy. This is a result of the extraordinary large absorption of molecular vibrational and rotational modes – the spectroscopic ‘finger-print’ region - and the ability to implement superior comb-based spectroscopy with dramatically increased speed, resolution, spectral bandwidth, sensitivity and precision, compared with classic approaches. A myriad of approaches for MIR FCs are currently being investigated, each promising to bring new discoveries to MIR spectroscopy. FC operation in the crucial mid-wave infrared (MWIR) 3 µm to 5 µm region, nonetheless, remains considerably underdeveloped compared to other parts of the MIR. The MWIR is critical as chemicals based on hydrocarbons as well as oxygen- or nitrogen-containing organic compounds have strong spectroscopic signatures, highlighting the MWIR importance in environmental and industrial emission control. This MWIR region, however, is the sweet-spot operation for Interband Cascade Lasers (ICLs). These semiconductor sources combine the interband transition of a conventional diode laser with the voltage-efficient cascading scheme introduced by the quantum cascade laser (QCL). Importantly ICLs typically have laser threshold powers that are an order of magnitude below QCLs making them ideal for extremely compact, stable and battery powered systems. Furthermore the relatively ‘slow’ gain dynamics of ICLs, unlike QCLs, make them simultaneously adapted for both passive and active modelocking, facilitating the possibility of ICL FCs. These three major points of i) emission in the range 3 – 5 µm, ii) low electrical power requirements and iii) inherent dynamics make them ideal for miniature MWIR FC operation. The ACTIVE-COMB project will breakthrough this spectral technology gap to realize ICL based frequency combs in the MWIR range with milli-watts of average output power, using simultaneously passive and active techniques within a monolithic and therefore extremely compact geometry. This will be based around new bandstructures, waveguides and dispersion control to realize this far-reaching objective, as well as state-of-the-art ultrafast techniques to fully characterize the ICL comb nature.