Passive mode-locking and terahertz frequency comb generation in resonant-tunneling-diode oscillator

Abstract

Optical frequency combs in the terahertz frequency range are long-awaited frequency standards for spectroscopy of molecules and high-speed wireless communications. However, a terahertz frequency comb based on a low-cost, energy-efficient, and room-temperature-operating device remains unavailable especially in the frequency range of 0.1 to 3 THz. In this paper, we show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb. The standard deviation of the spacing between the comb lines, i.e., the repetition frequency, is reduced to less than 420 mHz by applying external bias modulation. A simulation model successfully reproduces the mode-locking behavior by including the nonlinear capacitance of RTD and multiple optical feedback. Since the mode-locked RTD oscillator is a simple semiconductor device that operates at room temperature and covers the frequency range of 0.1 to 2 THz (potentially up to 3 THz), it can be used as a frequency standard for future terahertz sensing and wireless communications.

Fundamentals and recent advances of terahertz resonant tunneling diodes

“…127,[144][145][146][147] Mode locking of RTD oscillators with external feedback has also been studied. 148) The effects of the external feedback are analyzed using the transmission line model, 127,144) in which the radiation space is equivalently expressed by a transmission line with the characteristic admittance being the radiation conductance G , rad as shown in Fig. 24(b).…”

Free‐Electron‐Driven Frequency Comb

“…Then, the THz frequency comb, which operates at room temperature and covers the frequency range of 0.1 to 2 THz, is demonstrated based on the passively mode‐locked resonant‐tunneling‐diode oscillator. [ 43 ] In addition to these approaches, the coherent synchrotron radiation based on short electron bunches also enables decade‐spanning THz frequency combs with high resolutions over broadband. [ 44 ] Regardless, exploring methods to generate functionalized and stable THz frequency combs with flexible tunability across a broad frequency range remains an urgent need for bridging the THz gap.…”