Brillouin-gain-spectrum measurement along an optical fiber has recently gained a lot of interest owing to its potential for strain monitoring in installed cables and for distributed temperature sensing. We present a method that has the following advantages: (1) few optical elements are required, (2) a single laser source is used, and (3) a single fiber end is accessed. A schematic diagram of the experimental setup is shown. The operation of this setup relies on two features. First, the pump and probe signals are pulses that both propagate back and forth through the fiber by using a reflection at the far end. The second and more important feature is the use of a guided-wave intensity modulator for pulsing the laser light to form the pump signal and for generating the probe signal