Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2002-06-26
2003-12-23
Nguyen, Matthew V. (Department: 2838)
Optical communications
Multiplex
Wavelength division or frequency division
Reexamination Certificate
active
06668137
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical power control arrangements for optical transmission systems, to receivers, optical interfaces, demultiplexers, add drop multiplexers, optical switches, all incorporating power control arrangements, to distributed power control arrangements, to optical power controller components, to methods of controlling optical power, to methods of coupling wavelengths via an optical interface, and methods of offering a data transmission service over optical transmission systems.
BACKGROUND TO THE INVENTION
It is known to control optical power level in optical WDM (Wavelength Division Multiplexed) transmission systems, and it is very important to control it accurately and consistently. Optical power is a principal factor limiting reach in amplified systems and therefore determining how many repeaters or regenerators are needed. This affects capital cost and maintenance costs. Optical power level also limits data rate and therefore capacity. Optical power can be considered in terms of total optical power of all WDM channels in the same optical path, or power per channel. If total power is too high, effects such as Stimulated Raman Scattering can lead to distortion of channel powers (see copending US patent application entitled “CONTROL OF SRS INDUCED TRANSIENTS” Nortel Networks ref 15240). If per channel powers are too high non-linear effects such as Self Phase Modulation (SPM) can limit the achievable bit error rate, non-linear effects such as SPM cannot be compensated for and hence must be avoided. If power per channel is too low, then at the channel receiver at the end of the optical path, the signal will be harder to distinguish from noise, and unwanted bit errors will occur.
Conventionally, power variations were known to include long term variations in total power level, and short term or transient variations. The longer term variations were compensated by amplifier control loops, both feed forward and feed back loops, based on total optical power detected before and after the amplifier, or by adjustment of the transmitter power via feedback control systems. However, the gain of optical amplifiers can only be controlled for all channels, not separately for individual channels. The speed of amplifier gain control is also limited by the characteristics of the control loop and the optical behaviour of the amplifier, similarly changing the power of the transmitter is a slow process due to the long feedback path to instruct the laser to change its power.
So short term variations or transients, are usually not compensated, but are tolerated by providing margins in the power budget at the design stage, and/or by providing for error correction of bits at the receiver for example. U.S. Pat. No. 6,222,652 (Roberts) shows how sources of transients can be detected, and discusses their causes.
Erbium doped fibre amplifiers can cause amplitude transients when amplifying several wavelengths at once. Consider the simple example of two wavelengths, if one wavelength is instantaneously removed while the pump power remains constant, then the output power at the other wavelength will increase by approximately 3 dB. The speed of the output transient is determined by the pump power and by the response of the Erbium doped fibre to changes in input power and is measured in microseconds. Addition of a second wavelength causes a similar 3 dB drop in the output power of the first wavelength present. In networks equipped with Optical Add/Drop functionality the number of wavelengths propagating through a given amplifier can change during the lifetime of a network, due to reconfiguration of the network or through failures within the network. In addition amplifiers and other optical elements have a polarisation dependent gain or loss and changes in polarisation thereby cause changes in power level. Even vibrations and mechanical movement of optical fibres, connectors and other components of the optical path can alter the loss and so cause variations in the optical power.
Further problems concerning optical power level control on a network wide basis are set out in U.S. Pat. No. 6,392,769. Here, it sets out that in each amplifier node of the WDM system it is desirable to maintain the transmitted power within a small range (typically about 1 dB variation). To accomplish power management at a node, it is possible to use an automatic level controller consisting of a power tap into a detector and an adjustable loss element (optical attenuator) with a simple feedback control loop which continuously compares the power level to a desired value, and adjusts the attenuation to maintain the power level at the desired value. However, real WDM networks include cascades of (typically nonlinear) fiber, amplifiers, and loss elements. The resulting system is nonlinear and potentially chaotic, such that when the simple control algorithm described is independently utilized in each span, the result can generate oscillations in power levels which are much worse (larger in amplitude and faster in frequency) than the original level fluctuations which the system is intended to correct. Further, while this kind of problem can occur in single wavelength networks, it is compounded by inter-wavelength interactions in WDM networks, and grows increasingly difficult to predict as the number of wavelengths increases. Finally, networks which deliberately add and drop wavelengths from a WDM span will need to be dynamically reconfigured. This patent proposes addressing the problem by eliminating feedback instabilities arising from interactions between automatic control loops at the amplifier nodes of an optical network by restricting operation to a single amplifier node at a time. Amplifier node activation is accomplished using a global control signal which is sequentially passed from an upstream node through all of the nodes of the system. This can be a very slow process as only one node can change its behaviour at a time.
U.S. Pat. No. 6,304,347 shows some known optical power management strategies for managing signal power levels in an optical network. In one power management strategy, a consistent output power per wavelength is maintained between neighboring network elements in an OBSLR (optical bidirectional switched line ring) network. Consistent means that the signal power level between network elements will not change significantly enough, over any switching condition in the network, to affect the ability of the network to carry traffic. This localizes power management within each node since input power levels to the nodes remain constant. As a result, power management for the network becomes a function of each node's internal component configuration and optical path variations. In this strategy all switching scenarios are folded into a small set of operating modes.
In another power management strategy shown in this patent, signal power parameters for different network switching scenarios are tracked. Thus, it is possible to optimize the available signal-to-noise ratio (SNR) in the network at the cost of calculating, storing and exchanging signal power parameters around the optical network. In another power management strategy, signal power parameters for different network switching scenarios are pre-computed and stored. The pre-computed values provide a way for network elements to quickly react to switching events without necessarily having to re-compute parameters as each event occurs.
U.S. Pat. No. 6,268,945 (Roberts) entitled “Stable power control for optical transmission systems”, shows an optical transmission system, in which changes in optical power are anticipated and damped by controlling the transmitter output power, or an external damping element. For WDM systems, wavelengths can be added or removed without causing rapid changes in total power which would otherwise disturb the output of downstream optical amplifiers, and cause bit errors.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved apparatus and methods. According to a first aspect
Barnes & Thornburg
Nguyen Matthew V.
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