A circular cylindrical broil apparatus includes an outer cylinder and a core part. As the optical element back-face output power detected by PD6 depends on the optical element driving current amount, the electronic absorption amount of the EA optical modulator monitored by the electronic absorption monitor depends on the optical output of the optical element 5 and the EA optical modulator driving point of the driving part 8. Because it includes a prism instead of a more complex and delicate optical assembly, such an optical coupler can often be less expensive and more rugged, and can often have a higher connection density, than prior optical couplers. In some configurations, the holder can incorporate a peripheral device.