Terahertz free space communications demonstration with quantum cascade laser and quantum well photodetector

Abstract

An all photonic terahertz communication link operating at 3.8 THz using a quantum cascade laser and quantum well photodetector has been demonstrated. The link consists of a quantum cascade laser transmitter and a quantum well photodetector receiver. The link was used to transmit audio through 2 m of room air. Carrier strength at the photodetector was 100 times greater than the noise level measured.It is anticipated that many new applications of the terahertz (THz) spectrum are possible if simple compact sources and photodetectors were readily available. Many groups around the world are in the process of developing semiconductor sources and photodetectors for the THz spectrum [1][2][3][4]. In this Letter, we report on a demonstration of a free space link we have constructed to show the feasibility of laser generated free space communications at THz frequencies. We show that the basic characteristics needed for a simple telecommunications link can now be performed easily using a quantum cascade laser as a source and a quantum well photodetector as a receiver. A previous demonstration of a sub-THz analogue transmission system was reported by Jastrow et al. at 300 GHz using an electronic system [5]. The work we report here operates at 3.8 THz and is an all photonic design for THz generation, radiation, collimation and detection. Fig. 1 is a schematic representation of the link. At the left a quantum cascade laser housed in a vacuum dewar provides 3.8 THz radiation which is collected and collimated by a parabolic mirror labelled M1. The laser transmitter was constructed from a multiple quantum well structure described earlier [6] with a 1 mm-long and 100 mm-wide surface plasmon waveguide formed on a semi-insulating GaAs substrate. A reflecting mirror was formed on the back facet by first coating the facet with an aluminium oxide insulator, to prevent short circuiting the electrodes, followed by evaporating a gold layer over the facet. The exit coupler of the laser is the cleaved surface. M1 is a 50 mm focal length off-axis parabolic reflector while M2 is 76 mm focal length. Both mirrors are 50 mm in diameter. The laser was mounted on an aluminium cold finger in a liquid nitrogen dewar with the laser facet approximately 2 mm from the low density polyethylene window. This permitted collecting a large fraction of the diverging beam with an off-axis parabolic mirror. A temperature sensor was mounted near the base of the QCL die and showed that the base temperature was maintained at 78 or 77 K for all of the work we report here. MOD LNA MP3 player 77k M1 M2 2 m path AM radio Fig. 1 Schematic of link showing quantum cascade laser at left and quantum well photodetector on right

Terahertz wireless communications

“…One should also note that limited THz communication demonstrations have been performed with 3.8 THz QCLs. 69 …”

“…69 Analog audio data is transmitted over a 2 m path length. Under normal operation, a custom-made pulse generator generates 8 ns pulses at a repetition rate of 455 kHz.…”

Ultrafast terahertz detectors based on three-dimensional meta-atoms

“…This demonstration has been possible exploiting the beating between Fabry-Perot modes of a 6-mm-long THz quantum cascade laser (QCLs). Before that, the best measured speeds were limited to the MHz/sub-MHz range [17], [18]. However, to further reduce the RC time constant and develop an architecture with the potential of response bandwidths in the tens of GHz at THz frequencies, an ultra-small, micron-sized resonator geometry is crucial.…”

Analysis and Approach for Key Technique Based on THz Wireless Transmission

“…Finally, the signal is demodulated by AM module in radio set and is restored baseband signal. The dates show as table 1 [5][6][7][8][9]. This paper conducts related project research.…”