Extending the Unambiguous Range of THz - TDS Using Stepped - Frequency Intensity Modulation for Photonic Radar Applications

Ambiguity in radar and imaging applications— particularly at short ranges—poses significant challenges for conventional techniques such as frequency-modulated continuous-wave (FMCW) and pulsed radar systems. FMCW radars suffer from low beat frequencies and increased noise at close distances, while pulsed systems require impractically high sampling rates for fine resolution. Terahertz time-domain spectroscopy (THz-TDS) achieves micrometer-scale precision but is constrained by an unambiguous range (UR) limitation imposed by the pulse repetition rate of the laser source. This trade-off complicates applications demanding both high precision and extended range.To overcome these limitations, we present a photonic terahertz radar system that combines THz-TDS with stepped-frequency intensity modulation. The THz-TDS mode delivers micrometer-level accuracy (±15 µm) within its 3 mm UR, while the modulated mode extends the UR significantly, achieving a resolution of 15 mm and an accuracy of ±1.47 mm using a 10 GHz modulation bandwidth. This hybrid approach bridges the gap between high resolution and long-range detection, offering a versatile solution for non-destructive testing, imaging, and short-range radar applications in the terahertz frequency range.