A theoretical study of a new application of a simple π-phase-shifted waveguide Bragg grating (PSWBG) in reflection mode as a high-speed optical dark-soliton detector is presented. The PSWBG consists of two concatenated identical uniform waveguide Bragg gratings with a π phase shift between them. The reflective PSWBG, with grating reflectivities equal to 0.9, a free spectral range of $1.91\mathrm{THz}$ , and a nonlinear phase response, can convert a $40\mathrm{Gbit}∕\mathrm{s}$ noisy dark-soliton signal into a high-quality $40\mathrm{Gbit}∕\mathrm{s}$ return-to-zero signal with a peak power level of $\sim 17.5\mathrm{dB}$ greater than that by the existing Mach–Zehnder interferometer with free spectral range of $1.91\mathrm{THz}$ and a linear phase response.