Abstract: The problem of optical field chaos in a storage ring free-electron laser oscillator hasbeen discussed, by using a phenomenal model. From the theoritical analysis and numerical simulation, it can be found that the variation of laser intensity versus time can be both periodical and chaotic when there is a weak gain modulation in the optical cavity. The correspondent leading Lyapunov characteristic exponent goes to a negative and a positive real number as time increases. Afurther research is carried out, and a chaotic transition via period-doubling bifurcation has been found by varing the modulation parameters.

Key words: free-electron laser, phenomenal theory, optical field chaos