45 km ROTDR with 0.5 m/0.11 °C via complex-domain square-wave width-chirp pulse compression

Raman optical time-domain reflectometry (ROTDR) inherently balances sensing range, spatial resolution, and temperature accuracy through the pulse duration dictated by the OTDR position principle. However, optimizing one metric conventionally degrades the others, forming a theoretical trade-off. This work introduces complex-domain square-wave width-chirp pulse compression to break that physical limitation. The steep edges and rich high-order harmonics of complex-domain square-wave width-chirp pulse undergo matched filtering, producing a compressed δ-pulse whose full width at half maximum, rather than the original pulse duration, now governs sensing spatial resolution. Complex-domain matched filtering, implemented via a conjugate time-reversal filter, achieves a 15.09 dB gain in signal-to-noise ratio, while the complex-domain envelope extraction method isolates and removes Raman phase noise. The proposed scheme simultaneously achieves 45 km sensing distance, 0.5 m spatial resolution, and 0.11 °C temperature accuracy, demonstrating complete decoupling of these metrics from the pulse duration. The proposed framework offers a new paradigm for long-range, high-precision distributed temperature sensing and is extensible to Brillouin and Rayleigh scattering systems.