Broadband mid-infrared optical circulator for quantum cascade laser frequency combs.

We present the design and implementation of a free-space broadband mid-infrared optical circulator, particularly suitable for applications involving quantum cascade laser (QCL) frequency combs (FCs). The circulator is based on a phase-retarding mirror (PRM) and achieves an isolation level of 29.7 dB with a single-mode QCL and 25.2 dB with a QCL-FC, which significantly attenuates unwanted optical feedback (OF) for these devices. The optical circulator also offers an insertion loss of less than 0.59 dB with a QCL-FC operating at ∼10 µm, which greatly improves the optical throughput of a QCL-FC-based remote sensing system in addition to suppressing OF. As a proof-of-concept demonstration, we show that the sensitivity and stability of an open-path dual-comb spectrometer can be improved by an order of magnitude when the optical circulator is incorporated into its telescope transceiver. This circulator can be easily opto-mechanically tuned for mid-infrared sources at different wavelengths (tested down to 9.9 μm with PRM designed for 10.6 μm) and can be used with FCs as well as other broadly tunable laser sources such as external cavity QCLs.