Recent applications of laser chaos, e.g. in random number generation, cryptography and sensing motivate the search for the widest band chaos while keeping the laser system relatively simple. We analyze here the chaos bandwidth of a laser diode subjected to a phase conjugate feedback (PCF) both experimentally and theoretically. We demonstrate that a commercial laser diode whose frequency of relaxation oscillations is 3.9 GHz can, thanks to a single PCF, generate a chaotic signal with up to 18.5 GHz chaos. The chaos bandwidth is defined as the span of frequencies which contains 80 % of the frequencies of the chaos. We report that the chaos bandwidth of a laser diode first increases linearly with the feedback strength but then saturates. Numerical simulations reproduce this observation and highlight the influence of the finite penetration depth in the phase-conjugate mirror.
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