Abstract: A circuit which improves the linearity of a high power laser diode is disclosed. A electrical diode is coupled to the anode of the laser diode and a filter/bias network back biases the laser diode and forward biases the PN diode. At lower current levels the amount of current flowing through the laser diode is lowered. At higher current levels, rolloff in the PN diode allows more current through the laser diode. The rolloff performance of the laser diode and PN diode mirror each other closely enough to improve the laser's linear performance by 3 dB to 5 dB.

Abstract: A substantially non-interfering, low-intensity sweeping test signal is transmitted along a network to be tested. Means are provided for receiving and tracking the test signal at the opposite end of the network. A reference signal is transmitted along with said test signal for alerting and synchronizing the operation of the tracking means. The reference signal is at a frequency outside of the range of operating or normal signals transmitted over the network. Desirably the reference signal comprises a sweeping and a fixed frequency signal. These are then used to generate the sweeping test signal. Also, the reference signal is used by the tracking means for processing the received test signal.