In optical networks, channels will suffer from output power transients caused by a link fail- ure, a network reconfiguration or from the nature of the traffic of the network. These power transients (which will be modelled and analysed in the following chapters), affect the per- formance of the networks. The QoS provided will be unacceptable to the users. The speed of the power transients is proportional to the switching time of the optical power which is input into the EDFAs in the networks. Therefore, the recovery technique which is required to protect against such power transients must be extremely fast for large networks. Several schemes to protect the cascaded EDFA network links against the fast power transients have been developed in recent years [52].
1.9.1
Pump control
The gain of an EDFA can be adjusted by the control of its pump current. One approach con- trols the pump current in the time scales of the spontaneous lifetime of EDFAs [8]. Another has used low-frequency feed forward compensation with a low frequency control loop [56]. Further approaches have been proposed after the discovery of fast power transients, e.g. pump control on short time-scales has been demonstrated to limit the power transients of the surviving channels [57]. The automatic pump control in a two-stage EDFA operating on a
time scale of microseconds has been implemented experimentally, 7 channels were added or dropped in an 8-channel WDM system. The power transients exceeded 6 dB without the application of a gain control technique. When the pump control technique was added to the experimental set-up, the power transients reduced to less than 0.5 dB both for power dropped and power added conditions. The speed of the control circuit was within 7-8 µs, and in this way, the power transient effects on the surviving channels decreased [58, 59].
Several other approaches have also been implemented to mitigate the effect of the EDFA optical power transients on optical networks, and have been demonstrated by both Mehta and Karkasek, including the gain control technique, to enhance the performance of all-optical EDFAs networks during power transients [60–62].
1.9.2
Link control
The disadvantage of the pump control technique mentioned above, is that it requires protec- tion at every EDFA in the network. The word “link” here refers to a segment between two network elements or nodes, where channels can be added or dropped. Another technique to control the gain of amplifiers employs a control channel in the transmission band. An approach compensating the power transients in an EDFA at low frequencies (< 1 kHz) by
use of an idle compensation signal has been reported [63]. Another link control technique demonstrated by Srivastava and Zyskind uses the fast link control protection of surviving channels in the multi-wavelength optical networks. In this approach, a control channel is added before the first optical amplifier in a link (the output amplifier of a network element). The control channel is removed at the next network element. This technique protects the surviving channels on a link-by-link basis. The power of the control channel is such that total power of the signal channels and control channel is kept constant at the input of the first amplifier. This will maintain constant loading at the input of all EDFAs in the link.
Desurvire and Zirngibl have demonstrates a link control scheme [63]. The experiment used 8 channels including the control channel. The fast feedback circuit was used to align the line control channel power to maintain constant total power. The fast feedback circuit with a response time of 4 µs was used to adjust the power of the line control channel to keep the total power of the communication system constant. The signal and control channels are transmitted through seven cascaded EDFAs. The BER was measured on one of the
signal channels. It was found that, in the absence of a control channel, and when 5 out of 7 channels were added or dropped at a rate of 1 kHz, the surviving channels suffer power transients exceeding 2 dB and deterioration in BER. This power penalty is decreased to a few tenths of a dB, and BER adjusted to within the acceptable limit of network performance when the fast link control circuit was switched on [52]. Another approach, such as that used by Dimopoulos added an additional channel with the input signal channels to compensate for the increase or decrease of the input power during the single channel failures to maintain an acceptable level of survivability at low cost [64].
1.9.3
Laser control
A new scheme for link control based on laser gain control has been developed. A com- pensating signal in the first amplifier is generated using an optical feedback laser loop and then propagates down the link [65]. This forces the EDFA to go into lasing oscillation at an out of the band frequency, limiting the gain of the amplifier independent of the total input power. Once the power transients start to occur, the lasing signal is treated as an additional signal in the link. The laser gain control scheme has been demonstrated by Zirngibl [66]. Another approach using this technique was developed by Jackel and Richards [65]. In their approach, they succeed in combining laser control with the link control scheme mentioned above. The system stabilized within a few microseconds and the power penalty decreased to a few tenths of a dB after 6 EDFAs in the link. The draw-back of this scheme is that the speed of the response time is directly related to the laser relaxation oscillations which are generally on the order of tens of microseconds or slower [67, 68]. Homogeneous broadening of EDFAs, with the resulting spectral hole burning, can create gain variations at the signal wavelength, which will limit the range of control from this technique. This problem also applies to the link control scheme.
Much research, based on the above three basic schemes, have been implemented to mit- igate the effect of the EDFA power transients in optical networks, e.g. Pavel has developed an approach both for transient power control across optical communication links, and for spectral power control at EDFA sites [69]. It is obvious that the two parameters of optical power control are: transient power control at optical line amplifier sites, and spectral power control (equalizing the optical powers of each channel in the spectrum at dynamic EDFA
sites). A transient control strategy was proposed based on a combination of feed-back and feed-forward control. A spectral control approach based on decoupling the control loops by using time-constant layering [69].
Many comparative investigations of techniques to control power transients in wavelength routed optical networks have been carried out. One of these was by Olivers [70], who anal- ysed extreme conditions for the addition or dropping of channels. His results showed these techniques performed satisfactorily. However, differences were noticed regarding the effi- ciency, complexity, and implementation cost of each technique.
In addition to channel addition or removal in optical networks, there are two other types of signal disturbances that affect the performance of the optical networks: pilot tones and bursty traffic. Pilot tones are used to track and monitor the performance of each channel. Binary data can be frequency modulated upon the pilot tone, and this is important for signal routing. Tone frequencies are typically set above EDFAs natural gain dynamics, and below the links payload, i.e., between 10 and 100 kHz. Tone amplitude is below 10% of average power to avoid large penalties [71, 72]. However, due to cross-gain modulation these pilot tones induce ghost tones on surviving channels which may be mistaken as pilot tones at the receiver. Optical Burst Switching (OBS) allows for efficient resource sharing amongst numerous users which have burst-mode traffic. In bursty traffic input channel powers are turned ON and OFF for random lengths of time. As input traffic approaches self-similarity, burst lengths become comparable to EDFAs natural response time, leading to large output power swings.
The methods used to control EDFA power transients involve maintaining a constant av- erage input power to each amplifier in the network. This can be obtained in optical networks such as SONET or SDH, that use electrical multiplexing where a continuous bit stream is transmitted along the optical link. When the traffic in the network is in burst-OFF periods, idle codes are transmitted that maintain the average power of the optical channel almost con- stant. Hence, EDFA power transients are not a problem for this type of network. However, the current trend is to develop and evolve these networks with all optical systems that support optical burst switching (OBS), optical packet switching (OPS), and optical circuit switching (OCS). In these type of networks, there can be large time intervals where the average power in a specific channel is zero during the network operation. The dynamic channel power and
cascades of EDFAs can produce significant transients.
A method has been developed for resolving the effects of optical power transients gener- ated by rapid changes in the input power of EDFAs [73]. Because the amplitude and duration of the optical power transient provided by the EDFA is affected by how rapidly the input op- tical power to the EDFA varies, if the switching time for the input power is increased, the amplitude and duration of the power transient generated by a chain of EDFAs is decreased, see Section 6.2.3.