Phase locking of 2.324 and 2.959 THz quantum cascade lasers using a Schottky diode harmonic mixer

The 23rd and 31st harmonics of a microwave signal generated in a novel THz balanced Schottky diode mixer were used as a frequency stable reference source to phase lock solid-nitrogen-cooled 2.324 and 2.959 THz QCLs. Hertz level frequency stability was achieved, which was maintained for several hours. Steady improvements in THz QCL performance over the last decade [1] has generated considerable interest in their use as compact sources and local oscillators for both coherent (e.g., radar) and incoherent (e.g., high resolution astronomical spectra) THz heterodyne detection systems [2, 3]. One of the remaining obstacles is the development of a compact QCL frequency stabilization technique. Several frequency and phase locking methods have been applied to improve temporal coherency of THz QCLs since 2005, when Betz published the first paper on QCL frequency stabilization [4]. In most cases, their added complexity compromises the value of employing the QCL source. A summary of those methods can be found in [5]. It is widely understood that the high harmonic of an ultra-stable microwave source is an ideal frequency reference source for THz QCL frequency or phase locking since microwave synthesizers are compact and readily available. However, the signal-to-noise (S/N) of a beat signal decreases rapidly with increasing harmonic number and conventional THz Schottky diode (SD)-based devices, whether open resonator (e.g., corner-cube mounted) or planar waveguide-mounted, cannot produce a sufficiently strong intermediate frequency (IF) signal for laser frequency or phase locking at frequencies above, roughly, 1.5 THz [6]. A breakthrough came in 2012 when a 2.324 THz QCL was frequency-locked to the 21st harmonic of a microwave signal [5] using a novel room temperature 1.5 THz waveguide-mounted SD mixer [7, 8], based on a THz MMIC chip, which was fabricated at JPL [9]. Less than a year later, an alternative practical approach was reported by Hayton et al. [10]. The authors phase locked 3.4 THz DFB laser to the 18th harmonic of a 190.7 GHz reference source employing a room temperature GaAs/AlAs superlattice diode. The obstacle to THz QCL frequency stabilization using microwave sources has therefore been overcome with both the THz SD MMIC chip and the superlattice diode. Motivated by the high-frequency performance of the 1.5 THz SD and with available MMIC chips designed and fabricated for 2 and 3 THz, a THz fundamental waveguide mixer block containing a 2.0-2.5 THz SD balanced mixer chip was assembled. In the work described here, the mixer …