the present invention discloses a low - cost light source for optical transmission systems and optical networks based on wavelength - division multiplexing technology . a light source in accordance with the present invention is implemented by externally injecting a narrow - band incoherent light into a fabry - perot laser diode . after injection of narrow - band incoherent light , the output of f - p ld becomes wavelength - selective rather than multi - mode and the output wavelength of f - p ld coincide with the peak wavelength of the injected incoherent light . multi - channel wdm light sources according to the present invention can be implemented using a single broadband incoherent light source and plurality of f - p lds . an optical transmission system for upstream signal transmission in an passive optical network using the light source according the present invention is also disclosed .