We demonstrate the working principle of a one-dimensional intensity-based fiber-optic displacement sensor. The sensor consists of one receiving fiber, which is moved laterally in the optical field emitted by an emitting fiber. It is shown numerically that the sensor response is highly linear (nonlinearity error of 0.1 to 2%) for a wide range of travel (2.24 to 860 microm). The sensor response is also simulated experimentally using a highly precise robot, the results of which correspond very closely to numerical ones. Linearity, travel, and sensitivity are experimentally determined for different gaps between the emitting and the receiving fibers (10 microm to 10 mm). A design chart that includes the nonlinearity error (0.5% to 2%), the travel (2.78 to 860 microm), the sensitivity (0.032 to 0.37 dB/microm), and the gap distance (1 to 10 mm) is finally proposed.