Researchers at the University of Michigan have developed a high-bandwidth analog-to-digital converter (ADC) that uses a terahertz (THz) signal, which is inherently broadband, as a carrier frequency. A major challenge in the ADC design space is achieving larger operation bandwidth and higher sampling speed. Traditional ADC designs based on electrical sampling and quantization face plateauing improvements in operating speeds due to the relatively large temporal jitter of sampling signals. Given the intrinsically large bandwidth of optical frequencies and the ultra-stable sampling pulse trains available from mode-locked lasers, optical ADCs overcome this issue. However, optical ADCs suffer from a mismatch between photonic devices and integrated circuit components. Additionally, the optical bandwidth cannot be fully utilized due to the lack of resonant conversion between photons and electrons. Operation speed of optical ADCs is further limited by the modulation bandwidth of the E-O modulator stage and the limited repetition rate of the mode-locked lasers.
The ADC design developed at the University of Michigan overcomes the challenges associated with traditional and optical ADCs. Since THz frequencies interact coherently with high-speed electronics, the design is ideal for next-generation high-speed integrated systems. A spoofed-surface-plasmon-polariton (SSPP) architecture is employed as the waveguiding structure for the THz signals. The active-controlled SSPP architecture in the form of a Mach-Zehnder interferometer (MZI) is also used as the modulator. A thin layer of doped GaAs is incorporated on the inner wall of the groove region of the SSPP waveguide. By controlling changes in the refractive index of the GaAs, the phase of the THz signals can be shifted with respect to the analog modulation signal, thereby forming a spatially shifted radiation pattern in the far field. A detector array arranged in a binary manner converts the deflected THz radiation pattern into a digitized multi-bit output. The proposed ADC achieves an operation of up to 50GS/s, state-of-the-art among today’s ADC designs.
- High-speed analog-to-digital converters, a basic component in high-speed THz circuits
- High-speed and high-bandwidth operation
- Coherent operation with high-speed electronics