We report on the performance of a high sensitivity 4.7 THz heterodyne receiver based on a NbN hot electron bolometer mixer and a quantum cascade laser (QCL) as local oscillator. The receiver is developed to observe the astronomically important neutral atomic oxygen [OI] line at 4.7448 THz on a balloon based telescope. The single-line frequency control and improved beam pattern of QCL have taken advantage of a third-order distributed feedback structure. We measured a double sideband receiver noise temperature (Trec(DSB)) of 815 K, which is ~ 7 times the quantum noise limit (hν/2kB). An Allan time of 15 s at an effective noise fluctuation bandwidth of 18 MHz is demonstrated. Heterodyne performance was further supported by a measured methanol line spectrum around 4.7 THz.
We report on the application of a new technique for actively stabilizing the power of a far infrared gas laser as the local
oscillator (LO) in a superconducting hot electron bolometer (HEB) heterodyne receiver system at 2.5 THz. The
technique utilizes PID feedback control of the local oscillator intensity by means of a voice-coil based swing arm
actuator placed in the beam path. The HEB itself is used as a direct detector to measure incident LO power whilst
simultaneously continuing to function as heterodyne mixer. Results presented here demonstrate a factor of 50
improvement in the measured total power and spectroscopic Allan variance time. Allan times of 30 seconds and 25
seconds respectively are shown for large and small area HEB's with a measured effective noise fluctuation bandwidth of
12 MHz. The technique is versatile and can be applied to any LO source and at any LO frequency.
We report a new experiment on a high-resolution heterodyne spectrometer using a 3.5 THz quantum cascade laser
(QCL) as local oscillator (LO) and a superconducting hot electron bolometer (HEB) as mixer by stabilizing both
frequency and amplitude of the QCL. The frequency locking of the QCL is demonstrated by using a methanol molecular
absorption line, a proportional-integral-derivative (PID) controller, and a direct power detector. We show that the LO
locked linewidth can be as narrow as 35 KHz. The LO power to the HEB is also stabilized by means of swing-arm
actuator placed in the beam path in combination of a second PID controller.
High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7
THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron
bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution
heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a
2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH3OH) gas as a signal
source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different
pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial
demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental
demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is
in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.