4.7-THz Local Oscillator for SOFIA Based on a Quantum-Cascade Laser

Kurzfassung

Abstract— We report on the development of a 4.7-THz local oscillator (LO) for the heterodyne spectrometer GREAT on SOFIA. The design of the LO and its performance in terms of output power, frequency accuracy, frequency stability, and beam profile as well as its implementation in GREAT will be presented. I. INTRODUCTION H ETERODYNE spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. It allows for the study of the chemical composition, the evolution, and the dynamical behaviour of astronomical objects such as molecular clouds and star-forming regions. For frequencies beyond 2 THz, SOFIA, the Stratospheric Observatory for Infrared Astronomy, is currently the only platform which allows for heterodyne spectroscopy at these frequencies. One example is the OI fine-structure line at 4.7448 THz, which is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, on board of SOFIA. II. RESULTS The local oscillator (LO) combines a quantum-cascade laser (QCL) with a compact, low-input-power Stirling cooler. The 4.7-THz QCL is based on a hybrid design and has been developed for continuous-wave operation, high output powers, and low electrical pump powers [1]. Efficient carrier injection is achieved by resonant longitudinal optical phonon scattering. This design allows for an operating voltage below 6 V. The amount of generated heat complies with the cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power [2]. The QCL has a lateral first-order distributed feedback (DFB) grating, which is optimized for 4.745 THz. This yields single-mode emission over most of the driving current of the laser. Outcoupling is achieved through one of the end facets of the single-plasmon waveguide. The beam of the QCL is shaped with a dedicated lens and a spatial filter into an almost Gaussian profile. The M2 value which can be achieved with this method is approximately 1.2 [3]. The peak output power of the QCL is 0.5 mW. Frequency stabilization is achieved by using a low-noise current source for the QCL and a dedicated temperature stabilization of the heat sink of the QCL. In this way, a frequency stability better than 1.6 MHz (full width at half maximum, FWHM) is achieved. This can be further reduced by locking the emission from the QCL to an absorption line of CH3OH at low pressures (approximately 1 hPa). Using a pyroelectric detector and a proportional-integral-derivative controller, an additional improvement of the frequency stability is achieved [4]. Using this scheme, the FWHM of the laser line is below 0.5 MHz within 30 minutes measurement time. The absolute frequency of the LO has been determined by measuring the absorption spectrum of CH3OH and comparing this with data from the literature. It has been found that the LO emits in a range of ±4 GHz around the OI line. III. SUMMARY We have developed an LO for the heterodyne spectrometer GREAT on board of SOFIA. The LO is based on a QCL with a lateral DFB grating and a single-plasmon waveguide. The LO provides up to 0.5 mW output power in an almost Gaussian beam with an M2 value of 1.2. Its frequency is tunable by current and temperature within approximately ±4 GHz around the OI line. The LO fulfills all requirements and will be operated on SOFIA in 2014 for the first time. REFERENCES [1]. L. Schrottke, M. Wienold, R. Sharma, X. Lü, K. Biermann, R. Hey, A. Tahraoui, H. Richter, H.-W. Hübers, and H. T. Grahn, “Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz,” Semicond. Sci. Technol. vol. 28, 035011 (2013). [2]. H. Richter, M. Greiner-Bär, S. G. Pavlov, A. D. Semenov, M. Wienold, L. Schrottke, M. Giehler, R. Hey, H. T. Grahn, and H.-W. Hübers, “A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler,” Opt. Express vol. 18, pp. 1017710187 (2010). [3]. H. Richter, A. D. Semenov, S. G. Pavlov, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, K. S. Il’in, M. Siegel, and H.-W. Hübers, “Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler”, Appl. Phys. Lett. vol. 93, 141108 (2010). [4]. H. Richter, S. G. Pavlov, A. D. Semenov, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, and H.-W. Hübers, “Submegahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line”, Appl. Phys. Lett. vol. 96, 071112 (2010).