According to examples, a channel checker optical time-domain reflectometer (OTDR) may include a laser source to emit a laser beam. An optical switch may be optically connected to the laser source to receive the laser beam and to selectively transmit the laser beam to a circulator that is optically connected to a device under test (DUT). A first coupler may be optically connected to a first photodiode and to the circulator. A second coupler may be optically connected to the first coupler, the optical switch, and a second photodiode.