Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz

We report on the development of a THz quantum-cascade laser (QCL), which will be used as the local oscillator in the heterodyne spectrometer GREAT (German Receiver for Astronomy at Terahertz Frequencies) on board of SOFIA, the Stratospheric Observatory for Infrared Astronomy, for the investigation of inter-stellar atomic oxygen (OI) by the detection of its fine structure line at 4.745 THz.

Figure 1 Schematic view of the subband structure for the QCL.

Figure 2 Frequency tunability and beam profile of a DFB QCL. The grey horizontal solid line indicates the frequency of the OI line. The frequencies were measured with a Fourier-transform infrared spectrometer by varying the current through the laser and keeping the temperature at 42–47 K. The beam profile was measured after a TPX lens, which was used for beam forming.

This work was supported in part by the Investitionsbank Berlin (grant no. 10146488 and 10146490) within the EFRE program of the European Union and by the German Federal Ministry of Economics and Technology (grant no. 50OK1104).

The local oscillator is required to emit a single mode with a tuning range of about 5 GHz either below or above 4.745 THz. The required optical power is about 1 mW under continuous-wave (cw) operation. For the necessary operation at cryogenic temperatures, cooling by a mechanical cryocooler is mandatory since cooling within a liquid-helium flow cryostat is not an option for such airborne applications because of aircraft safety reasons. In this configuration, the electrical driving power of the QCL has also to meet the cooling power provided by the mechanical cooler.

 

For single-mode operation, we combine the QCL with a lateral first-order grating. Tuning is achieved by adjusting the effective refractive index of the resonator via changes of the internal temperature of the QCL. This temperature is defined by selecting an appropriate operating temperature and electrical driving power. Therefore, the design of the QCL has to exhibit the gain maximum at 4.745 THz with sufficiently large wall-plug efficiency over a rather wide range of electrical pumping power density and, at the same time, a negligible spectral shift.

 

The QCLs are based on a bound-to-continuum transition, in which the carrier injection is assisted by a transition resonant to the longitudinal-optical (LO) phonon energy as shown in Fig. 1. The samples were grown by molecular beam epitaxy on semi-insulating GaAs substrates containing 88 periods and single- plasmon waveguides. Distributed-feedback (DFB) lasers with first-order lateral gratings have been fabricated by dry etching. Figure 2 shows the frequency tuning of a DFB QCL mounted in a mechanical cooler. The temperature of operation was 42–47 K. The emission covers the frequency range which is required for the observation of the OI line. The output power in this configuration is about 0.5 mW. The beam profile shown in the inset was measured with a microbolometer camera after a polymethylpentene (TPX) lens, which is almost circular with some fringes due to diffraction.

 

 

Publication
1 Autor L. Schrottke , M. Wienold , R. Sharma , X. Lü , K. Biermann , R. Hey , A. Tahraoui , H. Richter , H.-W. Hübers , H.T. Grahn
Titel

Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz

Source Semicond. Sci. Technol. , 28 , 035011 ( 2013 )
2335 Cite : Bibtex RIS
L. Schrottke, M. Wienold, R. Sharma, X. Lü, K. Biermann, R. Hey, A. Tahraoui, H. Richter, H.-W. Hübers, and H.T. Grahn