  The laser spectroscopy with a narrow linewidth and high signal to noise ratio
(S/N) is very important in the precise measurement of optical frequencies.
Here, we present a novel high-resolution backward resonance Bragg-scattering
(RBS) spectroscopy from a population difference grating (PDG). The PDG is
formed by a standing-wave (SW) pump field in thermal 87Rb vapor, which
periodically modulates the space population distribution of two levels in the
87Rb D1 line. A probe beam, having the identical frequency and the orthogonal
polarization with the SW pump field, is Bragg-scattered by the PDG. Such
Bragg-scattered light becomes stronger at an atomic resonance transition, which
forms the RBS spectrum with a high S/N and sub-natural linewidth. Using the
scheme of the coherent superposition of the individual Rayleigh-scattered light
emitted from the atomic dipole oscillators on the PDG, the experimentally
observed RBS spectroscopy is theoretically explained.
