Laser apparatus and extreme ultraviolet light generation system

A laser apparatus comprising:

• a master oscillator configured to output a laser beam to travel along a laser beam path;
• a plurality of amplifiers configured to amplify the laser beam outputted by the master oscillator on the laser beam path;
• a photodetector including a first photodetector configured to detect energy of a light beam traveling back along the laser beam path and a second photodetector configured to detect power of the light beam traveling back along the laser beam path;
• and
• a controller configured to receive an energy detection signal from the first photodetector and a power detection signal from the second photodetector, monitor intensities of the energy detection signal and the power detection signal, determine that a return beam is generated when the intensity of the energy detection signal exceeds a first threshold, determine that a self-oscillation beam is generated when the intensity of the power detection signal exceeds a second threshold, and store determination results about the return beam and the self-oscillation beam to a memory.