The hypothesis of cosine distributing light intensity is put forward based on analyzing several models of emitting fiber light intensity distribution.Single-fiber mathematic model is founded and coaxial optical fiber bundle displacement sensor is designed for measuring radial runout of impeller pump rotor.And the mathematic model of fiber bundle sensor modulating function is set up and output characteristic of the sensor is emulated and analyzed.The result of dynamic testing for the stability of the system indicates that the stability within 5 min is better than ±10 μm for 5 000 r/min

which meets the design demand.It is obvious that the hypothesis of cosine distributing light intensity meets the practice

which has guiding significance for the design of intensity modulated fiber sensor.