Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-09
2001-07-31
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06268945
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods of operating optical transmission systems, to methods of controlling transmitters, to methods of controlling an optical power damping element, to optical transmission systems, to transmitters, and to damping systems.
BACKGROUND TO THE INVENTION
The power level of an optical signal in an optical transmission system limits the distance between regenerators or amplifiers, and needs to be controlled carefully to avoid errors in the detected bits.
A signal with too high an optical power is subject to nonlinear effects in the fibre such as Self-Phase-Modulation that can seriously degrade the signal. This causes bit errors or loss of frame in the signal. These nonlinear effects are especially severe at bit rates at and above 10 Gb/s. The onset of the nonlinear degradations can be quite sharp, in that only one or two dB of increase in power level can push a signal from optimum performance to a failed state.
A signal with too low an optical power is subject to noise degradations after attenuation by the optical link.
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 removed while the pump power remains constant, then the output power at the other wavelength will increase by 3 dB. The speed of this transient is determined by the pump power and by the response of the erbium doped fibre, 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 a wavelength division multiplex system new wavelengths commonly need to be added to systems that are in service. This may be due to an upgrade or may be caused by replacing a unit. Wavelengths also need to be removed when replacing a unit or reconfiguring the system.
Rapid changes in the power of an optical signal at one wavelength can move another signal away from its optimum power level towards too high or too low a power. Power margin must be allocated in the design of the optical system so that during a worst case transient, when combined with other worst case conditions, the data remains error free.
Allowing this margin reduces the available performance of the system, for example, reducing the maximum allowed amplifier spacings.
Various optical elements are sensitive to power changes, including receivers. Even if remaining within an appropriate static power range, rapid power transients can still cause bit errors. For example if the transient is faster than the response rate of automatic gain control in a receiver then the receiver electronics could be momentarily overloaded. These distortions can cause errors. During a transient the electrical signal, at the decision comparator will be larger or smaller than anticipated. The eye between logic levels will move, which places the decision threshold at the wrong location in the eye which causes bit errors.
Furthermore, amplitude transients can cause phase transients in clock recovery circuits that can exceed allowable jitter ranges, even to the extent of causing bit errors.
It is important that the signals carried by the wavelengths other than those being added or deleted remain error free.
It is known from U.S. Pat. No. 5,088,095 (AT & T) that gain clamping by out-of-band lasing in an optical amplifier can improve further the response to power transients in that amplifier. However, this requires a lot more pump power than a normal amplifier which is expensive to provide.
It is also acknowledged in U.S. Pat. No. 5,088,095 (AT & T) that it was known to stabilise amplifier output by detecting power changes at an amplifier input, and induce opposing compensatory changes in the pump power control circuitry.
Cooling an optical amplifier in liquid nitrogen has been shown in Journal of lightwave technology vol. 13, No 5, May 1995, pages 782-790 “Inhomogenously Broadened Fibre-Amplifier Cascades for Transparent Multiwavelength Lightwave Networks” by Goldstein et al, to allow separate saturation of the different wavelengths and so suppresses crosstalk of the power transient between wavelengths. However this is not practical for field equipment.
These three methods attempt to eliminate or minimise the transient effect of one channel on another in an optical amplified system, by improved gain control, once the transient reaches the sensitive element.
SUMMARY OF THE INVENTION
The invention aims to improve on such methods. According to a first aspect of the invention there is provided a method of operating an optical transmission system, comprising the steps of: transmitting at least one optical signal to an optical element which is sensitive to changes in the optical power of the signal; determining that the optical power is going to change; and damping the change in the signal input to the element, in response to the determination.
In contrast to the conventional methods, by determining that the optical power is going to change, and damping the change, the system performance can be improved, and power margins reduced, since power changes are suppressed before they reach the sensitive elements.
Advantageously, an instruction to change the power level is intercepted. This enables the instructed change to take place gradually, to reduce the disturbance which may be caused by the change.
Advantageously the determination of change is carried out by measuring the optical power, and extrapolating from the measurement. This enables unforeseen changes to be damped before they cause disturbances.
Advantageously the optical signal comprises a plurality of wavelengths and the damping step involves damping of one or more wavelengths individually. This enables the total power to be maintained without disturbing changes. Notably, optical amplifiers are sensitive to changes in total power. Advantageously a power change in one wavelength band is determined, and the damping step comprises the step of damping the change in total power by applying a gradually reducing compensating change to the power level of at least one other of the wavelength bands. This means the compensating change, which may be rapid, can be divided across a number of wavelengths, or that a dummy wavelength band can be used purely for the purpose of maintaining a stable overall power.
Advantageously the damping step involves controlling the output power of an optical source of the optical signal. This means existing control circuitry can be used with little modification.
Advantageously the method further comprises the step of controlling an output power of the optical source using a control loop with a predetermined maximum rate of change, wherein in the damping step is carried out with a lower rate of change.
Advantageously the optical element is an optical amplifier and the method further comprises the step of controlling the amplifier power output in response to the power in a subset of the wavelength bands. This enables the power margins to be reduced further.
Advantageously the method comprises the step of determining a desired rate of damping, according to the amount of change and a desired time to complete the change. This enables the rate of damping to be controlled to suit the circumstances.
Advantageously the damping step comprises the step of controlling the power level of a stabilising optical signal, to compensate for the change.
According to another aspect of the invention, there is provided a method of controlling a transmitter of an optical transmission system comprising the steps of: determining when an instruction to change an output power of the transmitter has been received; and changing the output power according to the instruction, at a rate slower than a usual response rate of the transmitter. This enables undesirable rapid changes to the damped.
According to another aspect of the invention, there is provided a method of controlling an optical power damping element in an optical transmission system comprising the steps of: determining when there is an optic
Crane John D.
Nortel Networks Limited
Pascal Leslie
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