A light emitting device, particularly a super luminescent diode, includes an active layer provided between upper and lower electrodes for injecting electric current into the active layer. The active layer functions as an optical waveguide and has first and second edge faces for emitting light. The device further includes first and second light receiving sections for receiving light emitted from the first and second edge faces respectively and generating first and second pieces of optical information respectively and a control section for controlling the current injection amount into the active layer from the upper electrode according to the first and second pieces of optical information. The optical output and the spectral shape of the device can be easily, accurately and reliably controlled in a short period of time.