The optical spectrum and coherence properties of a semiconductor laser are drastically influenced by optical feedback. When only a fraction of 10−5 of the output light of the laser is coupled back by a distant external mirror, the coherence length of the light drops from a few meters to a few millimeters and the optical spectrum broadens from ≈60 MHz to ≈20 GHz. This phenomenon was named the coherence collapse and was described theoretically for the first time by Lenstra et al1; the theory was refined and extended in Ref. 2. In this treatment the light coupled back by the external mirror is considered as an effective noise source. Such a noise source determines the autocorrelations of the laser output light and thus also the autocorrelations of the feedback light, which must be made self-consistent with the effective noise source.

© 1990 Optical Society of America

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