Strong Optical Feedback Stabilized Quantum Cascade Laser

A narrow-line-width quantum cascade laser (QCL) is highly demanded for high-resolution spectroscopy of narrow molecule absorption lines. This work shows a low-noise QCL stabilized by strong optical feedback without any feedback phase control. Although optical feedback is usually detrimental to the stability of laser diodes, the QCL is highly stable against optical feedback. The stabilized QCL line width is proved to be insensitive to the feedback phase, as long as the feedback strength is strong enough. Therefore, fine control of feedback length on the subwavelength scale is not required, which significantly simplifies the experimental setup. Using this simple approach, we demonstrate that the QCL line width is narrowed from 7.6 MHz down to 107 kHz. Meanwhile, the frequency noise below 100 kHz Fourier frequency is reduced by about 40 dB.