Next Generation Terahertz Systems using Nonlinear Phenomena
Creators: name: Naghavi, SeyyedMohammadHossein
Publisher: name: My University
Year: 2023
Subjects: subject: Terahertz
subject: Nonlinear Phenomena
subject: FMCW radar
subject: Super-resolution sub-terahertz radar
subject: Traveling-wave energy enhancement devices
subject: Plasma wave
subject: Electrical Engineering
subject: Engineering

description: Terahertz frequencies have opened a plethora of unique applications in the fields of sensing, spectroscopy, imaging, and communications. The short wavelength and availability of wide bandwidth in terahertz frequencies increase the resolution of sensing and imaging systems, as well as the data capacity of communication networks. However, terahertz systems suffer from the immaturity of available technologies in generating and sensing terahertz frequencies. The current advanced silicon processes (CMOS, BiCMOS, FDSOI) have a maximum operation frequency limit fmax of about 500 GHz, which limits the device activity close to fmax. The maximum frequency limitation comes with weaker output power and restricted bandwidth of transmitters, as well as poor sensitivity of receivers due to the lack of low-noise amplifiers. In addition, harmonic oscillators and frequency multiplier chains are two common ways of generating THz power that utilize the higher harmonics of microwave or millimeter-wave oscillators. Employing higher harmonics comes with higher phase noise of THz signals, which again restricts the performance of THz receivers. To overcome these constraints of the THz band, we leverage the nonlinear phenomena of THz electronics and show how nonlinear techniques can significantly enhance the output power, phase noise, and sensitivity of THz systems. To prove the point, we focus on THz frequency-modulated continuous-wave (FMCW) radars as imaging systems and THz amplifiers as the fundamental component in the THz band. First, we review three conventional FMCW radars at 100, 220, and 250 GHz and use them as imaging devices. Then we focus on a soliton self-injection locked (SSIL) autodyne frequency modulated (AFM) radar at 250 GHz that uses two nonlinear phenomena (self-injection locking and quadratic dissipative soliton) to significantly improve the system performance compared to other reported works. The self-injection locking phenomenon remarkably enhances the phase noise of the FMCW transmitter and brings the range accuracy of the radar to the order of 3.4 μm. Moreover, the formation of soliton at the IF signal because of the parametric nonlinearity of varactors breaks the limit of range resolution of the FMCW radars ΔRmin = c/(2B). It creates a frequency comb in the radar’s IF spectrum, where each frequency line in the comb has an effective bandwidth proportional to the actual radar bandwidth times to the frequency line order N. Working with higher-order frequency lines comes with a better range resolution of the radar by adding integer coefficient N to the denominator of the FMCW radar’ range resolution formula ΔRmin = c/(2NB). Furthermore, to fundamentally overcome the conventional frequency limitations of field-effect active devices, especially CMOS and FDSOI transistors, we leverage the nonlinear term in the equation of motion of electrons inside the device channel that becomes strong enough at THz frequencies. This approach blends device physics, plasma wave propagation phenomena, electromagnetics (E/M), and ideas inspired by microwave vacuum tubes (Klystron and traveling-wave tubes) into a coherent theme that is fully compatible with standard CMOS processes. In the proposed technique, since the array of transistors operates in a space-charge wave mode (plasma wave), the conventional definition of fmax, as the measure of the activity of individual devices, does not limit the fundamental frequency of operation. The embedded nonlinearity in the equation of motion of plasma waves transfers the energy from DC electric field inside the channel to the AC E/M waves.