Frequency noise (FN) characteristics of a semiconductor laser, taking into account multiple reflections in the external cavity, are investigated theoretically and through simulation in weak and strong feedback. Excellent agreement of the theoretical predictions with simulation estimates of FN spectrum is achieved in all cases of stable laser operation. Strong feedback operation is investigated extensively. It is shown that strong feedback leads to extended cavity operation, which is evidenced by peaks in the FN spectrum located at the inverse of the round-trip time in the compound cavity. Effects of the dynamic instability on the FN spectrum are shown to lead to sharp peaks, located in the FN spectrum approximately at the frequency of the compound cavity and its harmonics. Operation on a kink in the steady-state characteristics is shown to lead to a remarkable linewidth enhancement. The existence of a double-peaked structure in the FN spectrum is reported for the first time. It is also shown that the splitting of relative intensity noise (RIN) spectrum peaks may not to be accompanied by the splitting of FN spectrum peaks, because the effects of the phase-amplitude coupling between laser diode and external cavity on the FN and RIN spectra can be distinct.
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