We report on a heterodyne receiver designed to observe the astrophysically important neutral atomic oxygen [ OI ] line at 4.7448 THz .
The local oscillator is a third-order distributed feedback Quantum Cascade Laser operating in continuous wave mode at 4.741 THz .
A quasi-optical , superconducting NbN hot electron bolometer is used as the mixer .
We recorded a double sideband receiver noise temperature ( T ^ { DSB } _ { rec } ) of 815 K , which is \sim 7 times the quantum noise limit ( \frac { h \nu } { 2 { k _ { B } } } ) and an Allan variance time of 15 s at an effective noise fluctuation bandwidth of 18 MHz .
Heterodyne performance was confirmed by measuring a methanol line spectrum .