# An Overview of Terahertz Imaging with Resonant Tunneling Diodes
## 4 Compact RTD THz Imaging and Other NDT Systems  
The RTD THz transceiver has been widely reported for next generation ultrahigh speed wireless communication system towards 100 Gbps [23; 24; 25; 57; 59]. The application of RTD technology in THz imaging systems and other non-destructive testing (NDT) applications is just emerging but is under fast development. A prototype compact 300 GHz RTD camera was proposed in [60]. The size of the camera is comparable to a coin as shown in Figure 5a. Inside the package, the InP RTD transceiver is mounted on Si hemispherical lens as shown in Figure 5d. The camera package integrates a collimation lens to improve the beam profile. Both transmission and reflection imaging systems have been proposed using the RTD camera as shown in Figure 6. The system setup details can be found in [60]. The proposed 300 GHz RTD reflection system successfully mapped the concealed objects inside a plastic box with a 1.0 mm spatial resolution. Also, the imaging system based on a Michelson interferometer was demonstrated for the first time with estimated depth resolution of around 1 \(\upmu\)m. These results make RTDs a very promising candidate for compact THz imaging systems. Figure 5: (**a**) Front view of the RTD camera package. (**b**) Rear view of the RTD camera package. (**c**) 300 GHz RTD chip on Si-hemispherical lens. (**d**) Schematic diagram of the package. Reprinted with permission from Ref. [60]. In [61], a simplified 345 GHz RTD reflection imaging system was proposed. The system diagram is shown in Figure 7. Compared with traditional coherent tomography (OCT) imaging system [62], the proposed system significantly simplifies the setup by eliminating the use of integrated reference mirror, THz source, beam splitter, and detector. Instead, it only uses one single RTD transceiver. The working principle of RTD transmitter and detector (coherent) has been explained in Sections 2 and 3. The RTD transceiver was placed 30 cm in front of an elliptical mirror reflector and the imaging stage was placed around 40 cm apart. The detected signal was extracted through a bias-tee and then amplified by a low noise amplifier (LNA), a lock-in amplifier, and then final analog-digital (A/D) converter to digitize the signals. The preliminary results demonstrated 2D imaging with high resolution of 0.1 mm, which is smaller than a quarter of the wavelength at 345 GHz [61]. Although the mechanical chopper was employed in the setup in Figure 7, the RTD source can be on-off modulated simply through the DC bias. The modulation speed is up to tens of GHz [23, 24], which will make the imaging system more practical for industrial applications by eliminating the chopping mechanism. Another important emerging application is RTD-based THz radar, including amplitude-modulated continuous-wave (AMCW) where the THz wave amplitude is modulated by sinusoidal signal superimposed on the DC bias of RTD [63, 64, 65, 66], and subcarrier frequency modulated continuous wave (FMCW) radar, where again the amplitude of THz is modulated with linearly sweeping frequencies [26, 67]. Figure 6: (a) THz transmission imaging system using RTD transceiver. (b) THz reflection imaging system using RTD transceiver. Reprinted with permission from Ref. [60]. Figure 7: Schematic diagram of a single RTD transceiver THz imaging system. Adapted with permission from Ref. [61]. For AMCW, the reflected signal is demodulated by envelope detector (SBD) with the phase difference measured. The phase shifting carries the depth information and matches the movement of the target. For FMCW, the operating principle is different from normal FMCW radar in which the carrier frequency is swept [68]. The reason is due to the difficulty of linearly sweeping the RTD carrier frequencies using current technology. The preliminary experimental results in [63; 64; 65; 66; 26] demonstrate that RTD radars are capable of precise distance measurement with very low ranging error (\(\upmu\)m range).