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.
|1||Author||L. Schrottke , M. Wienold , R. Sharma , X. Lü , K. Biermann , R. Hey , A. Tahraoui , H. Richter , H.-W. Hübers , H.T. Grahn|
Quantum-cascade lasers as local oscillators for heterodyne spectrometers in the spectral range around 4.745 THz
|Source||Semicond. Sci. Technol. , 28 , 035011 ( 2013 )|