Comprehensive investigation of noise characteristics and radiation spectrum with special attention to the mode-hopping effect of Fabry-Perot (FP) multiple quantum well laser diodes (LDs) have been carried out: laser radiation spectra, optical and electrical fluctuations and cross-correlation factor between them have been measured under stable and mode-hopping operation. At the mode-hopping that occurs at particular operation conditions (injection current and temperature) LD radiation spectrum is unstable in time, very intensive and highly correlated Lorentzian-type optical and electrical fluctuations are observed. Characteristic time of physical processes that take part in LD structure during mode-hopping is ranged from microseconds to milliseconds. These processes are caused by the carrier generation and recombination in the centers formed by the defects in barrier layer (in the active region). (C) 2012 Elsevier Ltd. All rights reserved.