Telefonaktiebolaget Lm Ericsson (publ)

Transmode Systems Ab

Johan Sandell

Sven Wingstrand

Edgard Goobar

Gunnar S Forsberg

Dag Bonnedal

Magnus &Ouml;berg

Magnus Sundelin

Edgar Goobar

Fredrik Lid&eacute;n

The invention relates to a method for conveying management information in a WDM system from a number of wavelength converters to a central management unit, wherein a management information signal is superimposed on the WDM signal from the respective wavelength converter. A fraction of the optical signal in the common optical transmission line is tapped off to a detector and the different management information signals are recovered by a receiver unit which is connected to the detector. The invention also relates to a WDM system and a pluggable WDM wavelength converter.

Method for conveying management information

The invention concerns a method and apparatus for controlling the signal output power of an optical amplifier (). A target output power utilised to monitor the amplifier output power comprises the sum of th desired signal output power and an estimation of the ASE generated at this power. The estimation of ASE at any gain is made using a predetermined relationship between the gain of an amplifier and the generated amplified spontaneous emission (ASE). In a simple implementation of the invention, the relationship is approximated as a linear dependence, when it is determined by measuring the ASE generated at two different gains. The received ASE is ascertained by measuring the received output signal power and deducting this from the total received output power. A more extensive measurement of the relationship may also be conducted. The determined target output power can be used to stabilise the amplifier output.

WDM optical communication system

An optical amplifier arrangement is provided that includes an amplifier () elements for measuring the gain applied to the total input power () and a control unit () adapted to determine the degree of variation of gain with wavelength, i.e. the gain tilt, on the basis of a measurement of total gain. This is achieved by using a predetermined relationship between the wavelength dependent gain variation and total gain. This relationship is determined on production by measurement of the amplifier or of a batch or class of amplifiers. In this way a precise figure for gain tilt is provided for each amplifier. This may be used to eliminate gain tilt. Alternatively, if the system requirements tolerate limited gain tilt, the figure may be used to monitor the system.

Gain control in optical amplifiers

An amplifier arrangement includes an optical amplifier ( ) with a source of injected power ( ) for controlling amplifier gain. A feed forward gain control loop is provided with a weighting arrangement ( ) capable of weighting the signal power at selected wavelengths input to the amplifier. In this way non-uniform spectral gain of the amplifier may be compensated for by selecting appropriate weighting factors. Thus the influence of wavelengths having a greater than average impact on the decay rate of the excited population within the amplifier is increased and vice versa. By tailoring the weighting arrangement to the specific class of amplifier utilised, and possibly to a specific amplifier within a class, amplifier gain can be precisely controlled and traffic signal power reliably held at a constant level regardless of link configuration changes.

Optical communication system

The invention concerns a method and apparatus for determining noise power caused by amplified spontaneous emission in a signal output by an optical amplifier ( ). The amplifier is connected downstream of further optical amplifiers or components that generates noise power due to amplified spontaneous emission (ASE). The amplifier is connected to a control channel and receives via this channel information about the proportion of ASE power contained in the amplifier input signal. The amplifier further includes a control unit for calculating the ASE power generated in the amplifier, for example using a predetermined relationship between the gain of an amplifier and the generated amplified spontaneous emission (ASE). The ASE power in the amplifier output signal is then determined as the sum of the generated ASE power and the power resulting from propagated ASE. The determined target output power can be used to stabilise the amplifier output.

Power stabilization in an optical communication system

An optical communication line having optical fiber amplifiers including a pump laser can be configured to suppress the transmission of optical signals by reducing the power of the pump laser. In order that the portion of the line downstream of the amplifier can be monitored for fiber breaks causing only negligible crosstalk with a parallel working line, and with the elimination of the risk of mistaking reflections from a working line for light emitted by the standby amplifier, the light emitted by the standby amplifier is modulated by a low frequency control tone. The modulated laser light causes modulation of the amplified spontaneous emission (ASE) generated in the amplifier to create a control tone. Monitoring means disposed at the end of the optical line detect this control tone and use this as the criteria for a functional connection downstream the amplifier.

Path monitoring in optical communication systems

An optical fiber amplifier has a length of active optical fiber into which pump light from a pump laser is injected. The pump laser is locally controlled by a laser monitor diode, the pump laser and the laser monitor forming a pump laser module. The overall amplification of the amplifier is controlled by two control loops: a feed forward loop and a feed back loop. These two control loops also control the pump laser. The feed forward loop gives a fast response and receives an input signal from an input monitoring diode. The feed back loop gives a slower response and sets the overall gain of the amplifier. The feed back loop receives an input signal from an output monitoring diode. In the feed forward loop, the input power level is biased and controlled by a set offset values from sources to provide a signal corresponding to a pump power reference level which maintains the desired gain. The offsets of characteristic curves indicating an overall behavior of the optical amplifier. The pump power reference level is further modified in accordance with the pump monitor signal locally controlling the pump laser. This gives a very fast and accurate control in spite of the non-linear behavior of the optical fiber amplifier.

Optical fiber amplifier control

An optical amplifier device intended for WDM light signals comprises a preamplifier ( ) and a power amplifier ( ) of the optical fiber amplifier type, which are connected in series and have gain curves dependent on the wavelength. A controllable attenuator ( ) is connected between the amplifiers ( and ) and is controlled by a control device ( ). The signal input to the preamplifier ( ) has a low power and is amplified to provide an output signal where the signals of the input channels have been amplified with different gains. The input signal of the power amplifier ( ) then has a larger power resulting in a gain characteristic different from that of the preamplifier ( ). By adapting the attenuation of the attenuator ( ) for variations in the input signal the gain characteristic of the power amplifier ( ) can be set so that the signal output from the amplifier device has a spectral dependence that is the same as the original spectral dependence obtained without variations. Also, the same spectral dependence can be maintained when the pump light power of the power amplifier is changed and then the total amplifier gain is changed, by adapting the attenuation of the attenuator ( ) so that the gain of the power amplifier is not changed. In the amplifier the lower possible noise from the preamplifier stage is added to the input signal and the full output power is always available from the power amplifier. If the input signal is e.g. increased the SNR of the signals passing through the amplifier is correspondingly improved. This improves the performance of an optical network in which the amplifier is used.

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Enskede, Sweden

Johan Sandell