http://repository.tudelft.nl/view/ir/uuid:1311361d-f737-40a5-98ef-aa19ad5e21f0/
Ren, Y.
High-resolution spectroscopy at super-terahertz frequencies (2-6 THz) can play a vital role in astronomical observation and atmospheric remote sensing. It provides unique and important information on the history of our universe and its evolution, by getting into the insight of the physical and chemical conditions. Moreover, it can help to address questions about our own atmosphere such as ozone layer depletion and climate change problems. However, up to now this frequency region has rarely been accessible for high-resolution spectroscopy due to the lack of suitable local oscillator technology. The recently developed terahertz quantum cascade lasers become the most promising candidate as a novel solid-state terahertz source.
Terahertz quantum cascade lasers, after one decade development from the first demonstration in 2002, are now capable of delivering milliwatts or more of continuous-wave coherent radiation over the terahertz frequency range. For the local oscillator application, a heterodyne sensitivity measurement has been performed, which proved a prominent power stability with no additional inher- ent noise for a terahertz quantum cascade laser. However, a finial and crucial step to demonstrate a heterodyne receiver system is a direct high-resolution heterodyne spectroscopic experiment, which is also an important approach to characterize the performance of the entire system.
In this thesis, we have realized a high-resolution heterodyne spectrometer by introducing a terahertz quantum cascade laser as a local oscillator, a super- conducting hot electron bolometer as a mixer and a Fast Fourier Transform Spectrometer as a back-end spectrometer. The first molecular spectrum by using a terahertz quantum cascade laser as local oscillator was obtained at a frequency of 2.9 THz. We push further this heterodyne spectrometer up to 3.5 THz with a more advanced terahertz quantum cascade laser, where the first 3.5 THz methanol (CH3OH) spectra were obtained with ∼1 GHz tuning range from the local oscillator. Excellent agreement between the measured spectra to the theoretical calculation was achieved with respect to both line intensity and frequency.
Furthermore, we have explored the frequency locking capability of such terahertz quantum cascade lasers by using a terahertz molecular absorption line as a reference frequency. Based on a compact gas cell and a power detector, the frequency stabilization is achieved with a minimal linewidth of 18 kHz and a Gaussian-like shape. Such kHz linewidth with a compact locking scheme is favorable for any space- or ballon-borne instrument.
For actual observation applications, the effective integration time is a cru- cial issue that determines the efficiency of the observation. A robust exper- imental scheme has been demonstrated to simultaneously stabilize the fre- quency and amplitude of a terahertz quantum cascade laser. The frequency stabilization has been realized using a methanol absorption line, a power de- tector and a proportional-integral-derivative loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, and using the direct detection output of a super- conducting mixer in combination with a 2nd feedback loop. As a result, a fully stabilized heterodyne spectrometer at super-terahertz freqeuencies was demon- strated, with improved Allan Variance times, and also supported by measured heterodyne molecular spectra.
Based on all this work, terahertz quantum cascade lasers become techno- logically much more mature and convincing to be used as local oscillator. A di- rect outcome is a new NASA mission: Galactic/Xgalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory (GUSSTO), in which terahertz quantum cascade lasers have been proposed as local oscillators for the 4.7 THz receiver channel. Within the Phase-A-Concept study period, in collaboration with Q. Hu’s group at MIT and C. Walker’s group at University of Arizona, we demonstrated a heterodyne receiver using an advanced third- order distributed feedback quantum cascade laser as a local oscillator, whose emission frequency is only a few GHz away from the OI line at 4.7448 THz. Excellent receiver sensitivity together with a heterodyne spectrum have been demonstrated. All these efforts should lead soon to the first realization of a terahertz quantum cascade laser for astronomical application in a telescope. Also the local oscillator technology described in this thesis, offers the technique for other instruments such as Oxygen Heterodyne Camera (OCAM) proposed on SOFIA and also creates new mission opportunities in the future.
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