Frequency Stabilization of a 4.7 THz Quantum Cascade Laser Using a Delay Line Frequency Discriminator

Astronomical heterodyne spectrometers, such as upGREAT on the SOFIA telescope, use a powerful quantum cascade laser (QCL) as local oscillator (LO) to observe the atomic oxygen line at 4.7 THz. The free-running laser, although temperature stabilized, is prone to broad effective linewidth and frequency drifts and lacks an absolute frequency reference. In this work, our successful frequency locking of a 4.7 THz QCL by employing a delay line as a means of frequency discrimination is presented. We use a subharmonically pumped superlattice mixer to downconvert the QCL signal to approximately 400 MHz. From this signal the delay line frequency discriminator produces an error signal, which is fed back to the QCL supply current to correct the frequency instabilities. Our method reduces the laser linewidth from 10 MHz in free running mode to 780 kHz when locked. The locking scheme is robust and has been demonstrated to operate down to signal to noise conditions as low as 2 dB.