Line broadening up to 25 GHz in a single-mode semiconductor laser with relatively strong optical feedback is reported and theoretically analyzed. Measurements of the coherence function were performed using a Michelson interferometer and demonstrate that the coherence length decreases by a factor 1000 (to approximately 10 mm) due to optical feedback. A self-consistent theoretical description is given, which is based on the view that coherence collapse is maintained due to optical-feedback-delay effects, in which quantum fluctuations play no role of importance. A connection with recently suggested chaotic behavior is made. The theoretical results obtained are in good qualitative and reasonable quantitative agreement with measurements.