Frequency Locking of a 3.5 Thz Quantum Cascade Laser Using a Gas Cell

Abstract

We report a frequency locking experiment of a 3.5 THz third-order distributed feedback quantum cascade laser (QCL) by using a molecular absorption line of methanol (CH[subscript 3]OH) gas. With the help of the absorption line, the frequency noise of the QCL is transformed into measurable amplitude fluctuation. We first present the study of the noise of the THz QCL with the contribution from both the frequency and amplitude domain, by using a NbN superconducting hot-electron bolometer as a power detector. We then present the frequency locking measurement with a lock-in amplifier registering the derivate curve of the absorption line and a proportional-integral-derivative (PID) controller generating the feedback signal. The linewidth of the QCL in the free-running mode was found to be around 900 KHz. This linewidth is reduced to below 17 KHz (full width at half maximum) with a Gaussian-like shape when the control loop is active. Because of the frequency stabilization the noise power spectral density of the QCL shows a reduction of more than 20 dB at frequencies below 30 Hz.