Analysis of the all-electrical automatic gain control of travelling-wave semiconductor laser amplifi - Optoelectronics, IEE Proceedings-

Semiconductor laser amplifiers have been subject of great interest for several years now. Travelling wave semiconductor laser amplifiers have recently been implemented in optical fibre communication systems as optical repeaters and optical preamplifiers. Automatic gain control (AGC) of laser amplifiers is required in more reliable optical communication systems to compensate for the effect related to the drift in the signal polarisation, the input power, the temperature and the device aging. Intuitively, the actual optical gain of an amplifier can be obtained by extracting fractions of its input and output signals and comparing their power values. The signal proportional to the actual gain can then be compared to a reference value in a differential amplifier, and eventually the resulting error signal can be used to control the bias current of the laser amplifier. Both automatic gain and power control can be performed in this way [l]. However, this method requires two directional couplers inserted in the optical transmission line and two photo detectors for the signal detection. It is somewhat complicated especially in systems with two or more laser amplifiers. The AGC method proposed [2] would appear more simple. It extracts error signal information from the junction electric voltage of the amplifier because the junction voltage is related to the optical power through the carrier density. However, since the junction voltage also contains the voltage drop across the junction ohm resistance, which is temperature dependent and even larger than the useful signal voltage, a pilot tone has to be applied on the input optical signal in order to increase the sensitivity. Very recently, a new scheme that makes use of a multielectrode laser has been realised experimentally to