  By frequency-band extracting, we experimentally and theoretically investigate
time-delay signature (TDS) suppression and entropy growth enhancement of a
chaotic optical-feedback semiconductor laser under different injection currents
and feedback strengths. The TDS and entropy growth are quantified by the peak
value of autocorrelation function and the difference of permutation entropy at
the feedback delay time. At the optimal extracting bandwidth, the measured TDS
is suppressed up to 96% compared to the original chaos, and the entropy growth
is higher than the noise-dominated threshold indicating that the dynamical
process is noisy. The effects of extracting bandwidth and radio frequencies on
the TDS and entropy growth are also clarified experimentally and theoretically.
The experimental results are in good agreements with the theoretical results.
The skewness of the laser intensity distribution is effectively improved to
0.001 with the optimal extracting bandwidth. This technique provides a
promising tool to extract randomness and prepare desired entropy sources for
chaotic secure communication and random number generation.
