FREE SPACE TO CHIP EM WAVE COUPLING FOR TERAHERTZ PHOTONIC INTEGRATED CIRCUITS

Abstract

The THz band is a frequency range that lies between 0.1 THz to 10 THz. The large frequency of these waves provides a large bandwidth for wireless devices. The coupling of THz waves from free space to the THz PICs can be achieved by either in-plane or out-of-plane couplers. Owing to the benefits of free-space coupling demonstrated by outof- plane couplers, we have designed two grating couplers based on the SOI platform. The devices are designed using an HRFZ-Si core on the silica substrate to work on an operational frequency of 0.75 THz. The designed couplers have a smaller footprint than the previous works. The radiation efficiency and directionality of the partially etched GC are found to be 55.6% and -2.54 dB respectively. The structure radiates THz waves at an angle of 9◦. The grating coupler has a 3dB bandwidth of 71 GHz with a beamwidth of 7.7◦. The partially etched grating coupler suffers from huge back-reflections when the radiation angles are close to the normal which further reduce the radiation efficiency of the device. To reduce the back-reflections at small radiation angles of a partially etched grating coupler, a chirp parameter was introduced to the grating which resulted in 2% back-reflection at a radiation angle of 4◦. The directionality and efficiency of the device are found to be -2.82 dB and 52.2% respectively with a 3dB bandwidth of 62 GHz and a beamwidth of 17◦. The introduction of the chirp parameter in the grating design significantly reduces back-reflections at small radiation angles, with an increase in the coverage area. The designed grating couplers exhibit large radiation efficiencies with minimal back-reflections at 0.75 THz, which makes them suitable for THz applications.