Optical pre-amplifier apparatus and method for receiver performing gain control according to los declaration

Abstract

A receiver node includes an optical amplifier optically coupled to the input of a receiving device such as a PIN photodiode. A controller inputs received power level values from the receiver to perform gain control. The input signal of an optical receiving device, is pre-amplified by an optical amplifier such that the amplified signal is within a dynamic range of the receiving device. Upon the occurrence of a LOS (loss of signal) event, the controller sets the gain to a Home Gain value. Thus, when the signal is reestablished, a strong signal will not be over amplified and damage the receiver. Gain control is reestablished when the LOS event is over. Various methods are used to declare LOS including comparing the received power level or commanded pump power against a threshold. The LOS declaration may also be received from an external device or service channel.

Description

[0001] 1. Field of Invention[0002] The invention generally relates to optical communication receivers. The invention more particularly relates to optically pre-amplified receiver nodes.[0003] 2. Description of Related Art[0004] In conventional long-haul optical communications applications, the loss between the laser transmitter and the receiver is typically fixed or otherwise substantially invariant. Therefore, a gain block with fixed optical gain serves the purpose of ensuring that the receiver will receive a signal within the receiver's dynamic range.[0005] In some applications such as metropolitan optical ring networks, however, the loss between the laser transmitter and the receiver often changes due to changes in the span loss or due to channel upgrades. During channel upgrades, extra OADMs (optical add drop multiplexers) are added into the optical path of existing channels, therefore the loss between the transmitter and the receiver changes.[0006] Furthermore, a complete fiber...

AI Technical Summary

Benefits of technology

[0088] The use of such windows is particularly advantageous in the optical signaling and reception environment. Adjusting the pump 40 power excessively may introduce noise and cause the BER (bit error rate) to rise. Any increase in the BER is unacceptable as the specifications for BER are extremely demanding (on the order of 10.sup.-15 bit errors per second being an acceptable BER). The use of windows reduces the amount of pump 40 power level changes and possible race round conditions due to minor variations in power and thereby prevents an increase in the BER.

[0092] As will be further described in relation to the flowchart of FIG. 10, when the received power level is below -X (relative to the target) then the gain change rate is steep. In this way, the gain of optical amplifier 20 may be quickly adjusted and brought closer to the desired range (.+-.W1). Since the signal is far outside the desired range the effect on BER is much less important and may be ignored particularly when the signal is so far out of range as to be undetectable or not detectable with accuracy. When the received power level is below -Y but above -X (relative to the target) then the gain change rate is adjusted to be more shallow that the first gain change rate. The gain change rate may again be adjusted when the signal is above -Y so that this third gain change rate is shallower than the second.

[0094] Although instantaneous received power levels may be utilized as basis of control, it is preferable to use filtered value that smoothes local variations. More specifically, a filtering algorithm such as a moving average or moving mean may be used to track the received power level. In this way, local variations in the received power level may be smoothed and thereby prevent excessive pump power level changes and thereby reduce the BER.

[0110] Another alternative is to use the received power level to decide whether to declare a LOS event. The structures shown in FIGS. 3 and 4 are quite useful for this alternative. As described above in relation to FIG. 3, the controller 70 may also receive power levels detected by PIN receiver 30. FIG. 4 shows a specific architecture for providing received power levels (RX Power) to controller 70. Such received power levels may be compared against a threshold by the controller 70 to declare a LOS event. To minimize the effect of noise (e.g. on the LOS declaration decision and on the gain tracking feature described below), an optical filter 25 may be used as further illustrated in FIG. 3 or the selector 530 may be used as further illustrated in FIG. 4.

Problems solved by technology

Adjusting the pump 40 power excessively may introduce noise and cause the BER (bit error rate) to rise.

Any increase in the BER is unacceptable as the specifications for BER are extremely demanding (on the order of 10.sup.-15 bit errors per second being an acceptable BER).

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