SYSTEM FOR MEASURING OPTICAL OUTPUT FROM LIGHT EMITTING ELEMENT, AND CIRCUIT USED THEREFOR

PROBLEM TO BE SOLVED: To provide a system and a circuit used in a structure of three-terminal monolithic integration laser/photodetector capable of operating at a low bias voltage with good electric separation between the laser and the photodetector. SOLUTION: In a laser/photodetector structure 50, a VCSEL type light emitting element 520 and a hetero-junction phototransistor 510 as a photodetector joined optically with the light emitting element 520 are provided. The light emitting element 520 and the hetero-junction phototransistor 510 are formed in a monolithic integration technology in a way that the hetero-junction phototransistor 510 is overlapped on the light emitting element 520. In this case, a contact part of the light emitting element 520 functions also as an emitter of the hetero-junction photodiode 510, so bias applied to the light emitting element 520 and to the hetero-junction phototransistor 510 are made independent electrically, and at the same time the necessary bias voltage is made relatively low.