10  kHz linewidth mid-infrared quantum cascade laser by stabilization to an optical delay line

Area of Science:

• Quantum Optics
• Laser Spectroscopy
• Mid-Infrared Technology

Background:

• Quantum cascade lasers (QCLs) are crucial for mid-infrared applications but suffer from broad linewidths.
• Frequency stabilization is essential for high-resolution spectroscopy and metrology in this spectral range.
• Existing stabilization techniques often face limitations in complexity or applicability to mid-infrared QCLs.

Purpose of the Study:

• To demonstrate a novel frequency stabilization technique for mid-infrared quantum cascade lasers (QCLs).
• To significantly reduce the linewidth and frequency noise of a mid-infrared QCL.
• To report the first application of free-space optical delay line stabilization in the mid-infrared.

Main Methods:

• Stabilization of a mid-infrared QCL to a free-space optical delay line.
• Utilizing an imbalanced Mach-Zehnder interferometer with a 1 m path length difference as a frequency discriminator.
• Implementing a self-homodyne configuration with feedback applied to the QCL current for frequency locking.

Main Results:

• Achieved a sub-10 kHz full width at half-maximum (FWHM) linewidth (at 1 s integration time).
• Reduced the frequency noise power spectral density by nearly 40 dB below 10 kHz Fourier frequency.
• Demonstrated a linewidth reduction by a factor of approximately 60 compared to the free-running laser.

Conclusions:

• The presented method effectively stabilizes mid-infrared QCLs, achieving unprecedented narrow linewidths.
• This technique offers a simple yet powerful approach for enhancing laser performance in the mid-infrared.
• The study assesses the current limitations and discusses future potential of this stabilization setup.