Toward Stand-Alone Alkali-Based Mid-Infrared Frequency References

Highly accurate and precise spectroscopy in the mid-infrared (MIR) spectral region is a versatile tool that finds great interest in fundamental scientific research as well as in mitigating myriad applications in diverse fields. A major driving force for the rapidly increasing interest in MIR spectroscopy is the emergence of quantum cascade lasers (QCLs). Yet, as of today, such QCLs are still experiencing significant free-running frequency fluctuations which makes their frequency stabilization challenging. Here, we propose and experimentally demonstrate a method enabling development of frequency standards in the MIR, based on high excited stats of alkali vapors. The technique is based on the generation of a Doppler-free gain spectrum in the MIR by copropagating two stabilized pump near-infrared (NIR) lasers. Using this technique, the frequency of a distributed feedback (DFB) QCL at 5.23 μm is stabilized to the corresponding 6 2 P 3/2 –5 2 D 5/2 transition of rubidium (Rb) vapors, showing a substantial stability improvement compared to the free-running operation. Stabilization to an atomic MIR transition can be implemented in a rather simple scheme, with diverse applications. For example, the approach may enable the development of highly accurate self-calibrated optical spectrometers and can also be used for direct stabilization of MIR frequency combs.