Variable gain optical amplifier and control method

#This invention generally relates to optical amplifiers and is specifically concerned with a simpler and less expensive control system and method that requires fewer signal monitoring components.

#Erbium doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise a pair of pump lasers whose outputs are optically coupled to the inputs of two, serially connected coils of erbium-doped optical fiber. In operation, the outputs of the pump lasers excites the atoms of erbium dopant within the serially connected coils of fiber. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output.

#When such EDFAs are used simply as amplification relay stations along a single, long-distance optical circuit, there is little need for a capacity to specifically control the amount of gain that the amplifier imparts on the incoming optical signal, as there is typically little change in the strength of the incoming signal. However, as optical systems have become more complex, the need for such a gain controller in the amplifier has increased. Such a need may arise, for example, when an optical network is installed around an urban area. Under such circumstances, the distances between the optical amplifiers may be very different, If the EDFAs in the system all have the same amplification capacity, this capacity must be adjusted by way of a gain control device so that the signal strength remains uniform throughout all branches of the network.

#FIG. 1 is a schematic diagram of a variable gain optical amplifier that does not embody the invention;

#FIG. 2 is a schematic diagram of a first embodiment of the amplifier of the invention;

#FIGS. 3A and 3B illustrate the transient performance for the embodiment of the invention illustrated in FIG. 2 by comparing how a sudden increase in the power of the input signal affects the power of the output signal over time, and

#The invention is an improvement of the optical amplifier 1 illustrated in FIG. 1. This amplifier includes an input 3 and an output 5 and a pair of amplifier stages 7 and 9 serially connected via a variable optical attenuator 11, and a gain flattening filter 13. The variable optical attenuator 11 includes a movable controller 14 which operates to increase or decrease the amount of attenuation, depending upon the position it is moved to. The s variable optical attenuator 11 and gain flattening filter 13 cooperate to provide a flat gain spectrum of the channels amplified by the amplifier 1.

#Each of the amplifier stages 7 and 9 includes a coil 15a, b of erbium-doped gain fiber. Laser-powered light pumps 17a, b are optically coupled to input ends of the coils 15a, b via wave division multiplexers 19a, b. Each of the light pumps 17a, b is in turn powered by an electrical power source 21a, b. Gain detecting circuits 23, 25, 27, and 29 are provided to measure the strength of the signal at points T1, T2, T3, and T4 in the amplifier 1. Each of these gain detecting circuits includes an optical tap 31a-d that diverts a small percentage of the signal light from the amplifier to a photodiode 33a-d. Each of these gain detecting circuits includes a transimpedance amplifier 35a-d that converts the electrical signals produced by the photodiodes 33a-d from the light they receive into a signal indicative of the signal strength at the particular point T1, T2, T3, and T4 that the gain detecting circuit is coupled to.

#The amplifier 1 further includes a dynamic controller 37 largely formed from a digital signal processor 39. The outputs of each of the transimpedance amplifiers 35a-d of the gain detecting circuits 23, 25, 27, and 29 are connected to an input of the processor 39. The output of the processor 39 is in turn connected to the electrical power sources 21a, b of the light pumps 17a, b of each of the amplifier stages 7,9. The dynamic controller 37 of the optical amplifier 1 may operate to control the output of the amplifier in three different modes, i.e., in accordance with a preselected gain setpoint Gspi in accordance with a preselected power setpoint Psp, or in accordance with a preselected current level for the pump lights 17a, b. However, the first mode of operation is the most preferred as it is the most useful for practical applications of the amplifier with an optical network.