Faded channels in a distributed acoustic sensing system can be mitigated using a phase modulator. A first pulse and a second pulse of an optical signal can be determined. A phase modulator can modulate the first pulse to have a different wavelength than the second pulse. The first pulse can be launched into a sensing fiber that extends into a wellbore. A first backscattered signal can be received from the sensing fiber in response to the first pulse being launched into the sensing fiber. The second pulse can be launched into the sensing fiber and a second backscattered signal can be received from the sensing fiber. Data about an environment of the wellbore can be determined by processing the first backscattered signal and the second backscattered signal to compensate for fading in the first backscattered signal or the second backscattered signal.