An optical receiver able to generate the LOS alarm reliably is disclosed. The optical receiver module in the optical receiver provides a level monitor that monitors the average of the optical input, compares it with a reference and generates a decision signal that is provided to the squelching unit. The squelching unit, by receiving the decision signal, isolates the outputs of the output buffer from the inputs thereof and fixes the outputs of the receiver module in the level “0” or the level “1”. Even the main amplifier is coupled with the receiver optical module in the AC mode; no complicated response appears in the input of the main amplifier.