Modulation instability in long distance φ-OTDR system

•We comprehensively explored MI characteristics in the φ-OTDR system, considering input power, detection distance, and background noise.•Experiments confirmed an intrinsic spectral component of MI surrounding the signal in long-distance detection, previously unreported.•We have developed a fitting expression through numerical simulation to optimize the input power in the φ-OTDR system, surpassing the accuracy achieved by traditional formulas.