I. Introduction
Millimeter and terahertz (THz) waves, which cover the frequency range from 30 GHz to 10 THz, have gained much attention in recent years for their application in the ultra-fast wireless communications [1], [2]. Introduction of photonics technologies for millimeter/THz wave generation, modulation and detection is very effective to enhance the bandwidth and/or the data rate. So far, a real-time error-free (bit error rate: BER ) transmission with the highest data rate of 40 Gbit/s [3] and 48 Gbit/s [4] has been demonstrated using a single polarization system and a polarization multiplexed system, respectively, based on on-off keying (OOK) modulation and direct detection at 300 GHz carrier frequency. One of the practical and effective strategies to further increase not only the bit rate, but also the link distance is to introduce multi-level modulation techniques such as quadrature phase-shift keying (QPSK), using coherent carrier signals together with a coherent detection scheme.