  Time delay signature (TDS) of a semiconductor laser subject to dispersive
optical feedback from a chirped fiber Bragg grating (CFBG) is investigated
experimentally and numerically. Different from mirror, CFBG provides additional
frequency-dependent delay caused by dispersion, and thus induces
external-cavity modes with irregular mode separation rather than a fixed
separation induced by mirror feedback. Compared with mirror feedback, the CFBG
feedback can greatly depress and even eliminate the TDS, although it leads to a
similar quasi-period route to chaos with increases of feedback. In experiments,
by using a CFBG with dispersion of 2000ps/nm, the TDS is decreased by 90% to
about 0.04 compared with mirror feedback. Furthermore, both numerical and
experimental results show that the TDS evolution is quite different: the TDS
decreases more quickly down to a lower plateau (even background noise level of
autocorrelation function) and never rises again. This evolution tendency is
also different from that of FBG feedback, of which the TDS first decreases to a
minimal value and then increases again as feedback strength increases. In
addition, the CFBG feedback has no filtering effects and does not require
amplification for feedback light.
