The behavior of semiconductor lasers subject to external optical feedback has been well documented,1 although many aspects of this behavior are not well understood. Strongly coupled external cavity semiconductor lasers (>1% power feedback) are especially useful for such purposes as linewidth narrowing and modelocking. It is not clear whether such external cavity lasers will be more or less sensitive to additional parasitic optical feedback from sources other than isolated laser diodes. Here we consider a double external cavity configuration as illustrated in Fig. 1, where the mirrors M1 and M2, respectively, represent the primary external cavity and an extra cavity for additional feedback.
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