Sub-10-kHz Linewidth Quantum Cascade Laser Achieved by Stabilization to a Short Free-Space Delay Line

The use of an unbalanced interferometer has been shown to be an interesting alternative to high-finesse optical cavities to reduce the frequency noise of a laser by active stabilization. In a self-heterodyne configuration, the resulting frequency-to-amplitude noise discrimination obtained when demodulating the heterodyne beat signal scales with the pathlength difference between the two interferometer arms, so that long fiber delays up to a few kilometers have typically been used in the near-infrared [1]. Here, we apply this approach to reduce the frequency noise of a quantum cascade laser (QCL) in the mid-infrared (MIR) spectral region. Owing to the poor availability of frequency-shifter modulators and low-loss single-mode optical fibers in the MIR, we temporarily restricted the experimental scheme to a self-homodyne configuration with a short freespace pathlength difference of 1 m acting as a frequency-to-amplitude noise converter. With this preliminary scheme, we achieved a reduction of the QCL frequency-noise power spectral density (PSD) by almost 40 dB over a large range of Fourier frequencies and a resulting linewidth narrowed below 10 kHz at 1-s integration time, compared to almost 500 kHz for the free-running laser.