Optical amplifier

An optical amplifier comprising:

• an input terminal where signal light enters;
• an output terminal where the signal light is outputted;
• a transmission line which is arranged between the input terminal and the output terminal and on which the signal light propagates;
• an optical amplification device arranged on the transmission line and having an amplification transmission medium for amplifying the signal light and a pumping light source for supplying pumping light to the amplification transmission medium;
• an amplification controller for controlling at least the pumping light source;
• and
• a light detection circuit arranged in at least either one of a section on the transmission line between the input terminal and the optical amplification device and a section on the transmission line between the optical amplification device and the output terminal,
• wherein the light detection circuit has:
• a light splitting unit for extracting a partial light of the signal light as split light from the transmission line;
• a light receiving unit including a photoelectric conversion element and adapted for outputting a voltage signal according to a light quantity of the split light received by the photoelectric conversion element;
• an extraction unit for extracting a voltage component in a high frequency region included in a voltage component of the signal light, from the voltage of output signal from the light receiving unit;
• a signal component detection unit for detecting the voltage of output signal from the light receiving unit;
• a high frequency component detection unit for detecting a voltage of output signal from the extraction unit;
• and
• a detection controller for detecting, at least, whether there is an increase of dark current in the photoelectric conversion element included in the light receiving unit, based on respective detection results by the signal component detection unit and by the high frequency component detection unit.