Absolute frequency referencing in the long wave infrared using a quantum cascade laser frequency comb.

Area of Science:

• Quantum optics and spectroscopy
• Mid-infrared photonics
• Metrology and frequency standards

Background:

• Optical frequency combs (OFCs) based on quantum cascade lasers (QCLs) have revolutionized mid-infrared (mid-IR) spectroscopy.
• However, QCL-OFCs have not been utilized as broadband absolute frequency references.

Purpose of the Study:

• To demonstrate the possibility of using QCL-OFCs as broadband absolute frequency references in the mid-IR.
• To perform comb-calibrated spectroscopy using a QCL-OFC locked to a molecular transition.

Main Methods:

• Development and implementation of a quantum cascade laser optical frequency comb (QCL-OFC) system.
• Comb-calibrated spectroscopy performed at a wavelength of 7.7 µm (1305 cm^-1).
• Locking the QCL-OFC to a stable molecular transition for frequency referencing.

Main Results:

• Achieved a relative frequency stability of 1.5 × 10^-10 (at 100-s integration time).
• Obtained a relative frequency accuracy of 3 × 10^-9.
• Demonstrated sub-Hz-level stability for hours when locked to a reference.

Conclusions:

• QCL-OFCs can be successfully employed as broadband absolute frequency references in the mid-infrared.
• The achieved performance is comparable to state-of-the-art methods relying on nonlinear frequency conversion.
• QCL-OFCs show significant promise as metrological tools for the mid-infrared spectral region.