Frequency and phase-lock control of a 3?THz quantum cascade laser

Abstract

We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

“…This blue shift is most likely due to the frequency pulling of a Stark-shifted gain spectrum [20]. This observation indicates that the QCL behaves as a voltage controlled oscillator for the bias range of interest, which is required for phase-locking [7], [8]. To close the phase-lock loop (PLL), the beat signal, as shown in Fig.…”

“…In the case of frequencylocking, the laser's average frequency is fixed, but its linewidth remains equal to the laser's intrinsic linewidth. Until now, only two experiments [7], [8] to stabilize a THz QCL have been published. One is the frequency locking of a 3.1 THz QCL to a far-infrared (FIR) gas laser [7], the other is the phaselocking of the beat signal of a two lateral modes of a THz QCL to a microwave reference [8].…”

“…Until now, only two experiments [7], [8] to stabilize a THz QCL have been published. One is the frequency locking of a 3.1 THz QCL to a far-infrared (FIR) gas laser [7], the other is the phaselocking of the beat signal of a two lateral modes of a THz QCL to a microwave reference [8]. These experiments have suggested the feasibility of phase-locking, but have not led to a practical scheme for a LO.…”

Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser

“…Recent work has demonstrated frequency locking of a QCL to a far-infrared ͑FIR͒ gas laser line at 3.105 THz. 5 This same work demonstrated a lasing LW of 65 kHz, which could be maintained indefinitely as a result of the frequency stabilization. The LWs of QCLs that were reported earlier than Ref.…”

“…The LWs of QCLs that were reported earlier than Ref. 5 were unstabilized and could be measured only for a short sweep time of ϳ3 ms. They were measured using roomtemperature Schottky diodes to mix signals from a terahertz QCL and a FIR gas laser, 6 two terahertz QCLs, 7 or two longitudinal emission modes of a single QCL.…”