Quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and 1  ×  10^−13-level frequency instability

Abstract

We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, via upconversion to the near-IR spectral range, at a level of 1 × 10 −13 . The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability < 1 × 10 −13 and inaccuracy 5 × 10 −13 , using a frequency comb phase-stabilized to an independent ultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated by fiber noise. Active suppression of fiber noise could result in sub-10 Hz linewidth.High-resolution spectroscopy in the mid-IR has been a crucial tool for the elucidation of fine details of molecular structure and dynamics for many decades. Earlier, mostly gas lasers (CO 2 , CO, He-Ne) with narrow linewidth (kHz to sub-Hz, e.g. [3]) have been used as sources, permitting spectral resolutions below the Doppler width of gaseous room temperature samples, by employing appropriate nonlinear spectroscopic techniques. The molecular gas lasers are only line-tunable, in some cases restricting severely their applicability. As a remedy, the generation of tunable microwave sidebands has been successfully implemented [7], but the method has the drawback of the low power in the sidebands. Another approach is the difference-frequency generation (DFG) from near-IR sources which allows a wide tuning range, but generally suffers from low mid-IR power [12].Quantum cascade lasers (QCLs) offer both high power and relatively wide tuning. The free-running linewidth on the timescale of 1 s is typically on the order of 1 MHz and has been the subject of several studies [2,11]. Linewidth narrowing, alongside absolute frequency stabilization and finally absolute frequency measurement, are therefore important tasks for rendering QCLs usable for high-resolution spectroscopy.It has already been shown that the QCL linewidth can be dramatically reduced, by locking to a reference cavity [10], by phase-locking to a narrow-linewidth CO 2 reference laser, reaching less than 10 Hz relative linewidth [9], or by phase-locking to the DFG wave generated from two near-IR cw lasers [5]. This latter approach however limits the spectral coverage of the QCL, since it relies on particular reference lasers.The upconversion approach [1] appears as particularly suitable as a general approach for QCL frequency metrology and linewidth narrowing [8]. References [5,8] also performed absolute frequency measurements. In our own work [6], we upconverted radiation from a 5.4 µm QCL to 1.2 µm by mixing with a cw 1.5 µm fiber laser. These two near-IR waves were simultaneously measured by and stabilized to a standard Er:fiber comb referenced to a hydrogen maser. No linewidth narrowing was implemented at the time.In the present work, we extend significantly the performance of QCL frequency metrology, by demonstrating absolute frequency stabilization, linewidth narrowing, and absolute frequency measurement at ...

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