Towards the full frequency stabilization of quantum cascade laser frequency combs

Optical frequency combs (OFCs) generated by quantum cascade lasers (QCLs) have recently been proven in the mid infrared [1] and they have already been applied to spectroscopy in free-running operation [2]. It is clear however that for more demanding applications such as high-resolution and accuracy spectroscopy and metrology the performance obtained in free-running operation is not enough, since a consistent amount of frequency noise is present. The first attempt of frequency stabilizing a quantum cascade laser comb (QCL-comb) against a metro-logical difference-frequency-generated comb through a single phase-locking chain using the laser bias current as actuator [3] gave a reduction of the single QCL-comb tooth linewidth from 500 kHz down to values ranging from 1 to 23 kHz in a 40-ms time scale. This is still a partial result since for a total stabilization and control a second actuator is needed. Here we propose the use of feedback from a controlled mirror as a second actuator. With two actuators (at least partially independent) it is possible to correct both for offset and spacing frequency fluctuations. In fig. 1, left the experimental setup is shown. The QCL-comb emits around 4.70 μ m from both the facets. The output of one facet (left in fig. 1, left) is sent back to the laser by a mirror mounted on a piezoelectric actuator. The spacing between the longitudinal modes of the laser is fs ≃ 7 GHz given by the laser waveguide length and refractive index. The beat note between the modes can be detected by placing a bias tee on the laser bias cord. Then this signal is down-mixed to be used in a phase-locked loop (PLL) employing the mirror piezoelectric mounting as actuator to compensate for fs fluctuations. The very same signal is observed on a spectrum analyzer. In fig. 1, right a comparison between the beat note in free running operation and in locking operation is shown. The resolution-bandwidth-limited peak clearly states the effectiveness of the locking.