Optical transceivers have loopback and pass-through paths for diagnosing transceiver components and optical networks connected to the optical transceiver or for routing data out of the transceiver in a pass-through mode. The lamp is installed within a rectangular parallelepipedal housing, and an exhaust fan is disposed to the rear of the lamp. When a request is present, an intake valve characteristic is adjusted to be larger than when the request is not present.