Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2001-02-13
2004-10-12
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S160000, C398S177000, C398S034000, C398S037000
Reexamination Certificate
active
06804464
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical network wavelength management and configuration and, more specifically, to a programmable optical processor.
BACKGROUND TO THE INVENTION
The unprecedented growth of the Internet, E-commerce, private networks, high resolution digital video, and voice over IP is dramatically changing the demand for high speed broadband networks. Dense Wavelength Division Multiplexing (DWDM) technology has been deployed and widely adopted not only for long-haul backbone networks, but also for the metropolitan network market. The fundamental idea of DWDM technology is to simultaneously transmit multiple wavelengths over the same fiber. Systems based on DWDM often have optical amplifiers, especially Erbium doped fiber based amplifiers, to compensate for transmission losses incurred during transmission. However, theses optical amplifiers often cannot amplify different wavelength channels equally, resulting in signal strength differences among channels.
The evolution of optical networks has also brought a new kind of system topology, where an individual or a sub-group of several wavelength channels, among the whole group, may be switched, added, or dropped at a certain central office (referred as optical nodes). This may result in different transmission paths with individual or sub-groups of several wavelength channels transmitting through different network elements. Both result in different transmission losses (by fiber and/or devices) for different channels or sub-groups of channels.
Whether due to amplification, and/or to differences in transmission loss, when these channels reach a receiver, they do not have the same signal strength. The difference in signal strength can result in a situation where some channels overload the receiver because of too much power while others cause detection errors because of a weak incoming signal.
As an example, if group A channels are being sent to receiver A
1
and group B channels are being sent to receiver B
1
and both group A and B channels transit through the same amplifier, problems may occur. If receiver A
1
is much closer to the amplifier than receiver B
1
, then group A signals may arrive with more power at receiver A
1
than group B signals when they arrive at receiver B
1
. This is primarily because both group A and B share the same amplification medium and the amplifier cannot adjust its amplification of certain channels or groups individually travelling on the same fiber.
Currently, two methods exist which can compensate for such problems. The first is termed the pre or post tilting method (also called padding)—each channel is provided with a tilt either before the channel reaches the receiver or after the channel has left the transmitter. This method implies the use of separate optical attenuators for each channel. These attenuators can be set such that channels with a stronger signal strength are attenuated or given more loss than channels with a weaker signal strength. The signal strengths of the channels are thus equalized by the time the channels reach the receiver.
The second method involves gain flattening of optical amplifiers. Gain flattening filters are used with an optical amplifier so that stronger channels (channels with a higher signal strength) have their gain or amplification curtailed to equalize the signal strength among all the channels.
Network designers currently choose one or both of the two above methods. Both of these methods rely on a single concept—the attenuation of the signal strength of stronger channels. Unfortunately, attenuation can prove to be costly. The pre-tilt method necessitates a large number of attenuators. Each attenuator can be costly and, for a typical 40 channel system with one attenuator assigned to each channel, the cost rises accordingly for the whole system. For the gain flattening approach, the cost can also be correspondingly high. Not only does the system designer have to factor in the cost of the amplifier but also the complexity of the structure, waste of pump power, more components and lower manufacturing yield.
While there are gain flattened amplifiers now available, the physics of Erbium doped fiber amplifiers renders gain flattening quite difficult, especially to the level that an optical network requires. This difficulty causes a low yield rate when manufacturing gain flattened fiber amplifiers.
Also, when using gain flattened fiber amplifiers, a much larger pump laser (one providing more power) is required. This is because more pump power is required to compensate for the decreased amplification caused by the attenuation through the gain flattening filter. For the above reasons, gain flattened amplifiers, even passively flattened ones, are about 30-50% more expensive than non-gain flattened amplifiers for the same amplification factor.
There is therefore a need for a new approach which allows controllable amplification and/or attenuation of selected channels. Such a new method, and the apparatus which implements it, must necessarily overcome the problems associated with the known techniques as outlined above.
SUMMARY OF THE INVENTION
The present invention overcomes the problems discussed above by providing a method and devices for individually controlling the signal strength of single or multiple optical channels. A controller module monitors the signal strength of channels and amplifies those that need amplifying while attenuating those that are too strong using the same Erbium doped fiber amplifier. A controllable compensation module receives at least one channel and, when required, can either amplify or attenuate the signal strength of the channel(s). The module can be constructed out of a single fiber with an associated pump laser. If the laser provides insufficient pumping power, the fiber acts as an attenuator. If the laser provides a higher level of pump power, the fiber acts as an amplifier.
By independently controlling each compensation module, the signal strength of each channel or group of channels can be independently increased or decreased. This provides a level of control for each channel or channel group that is unprecedented. Because of this, costly pre-tilt and/or gain flattening methods of channel equalization are avoided.
In a first aspect the present invention provides a system for controlling the signal strength of multiple optical channels prior to said channels being received by an optical receiver. The system comprises an optical demultiplexer receiving a compound input (multi-wavelength) optical signal and demultiplexing the input optical signal into the individual or groups of optical channels, a plurality of controllable compensation modules with each module receiving at least one channel or channel group, a channel strength monitor which monitors the signal strength of each of the multiple optical channels, a controller coupled to the monitor and to each of the compensation modules, the controller causing each of the modules to amplify or attenuate the signal strength of the channels or channel groups received by each of the modules based on an output from the monitor to the controller, and an optical signal multiplexer receiving the multiple optical channels from the compensation modules. The multiplexer multiplexes the multiple channels into a compound output signal to be received by the receiver. Each of the compensation modules controls the signal strength of the channels or channel groups based on an input from the controller and is capable of both attenuating or amplifying at least one optical channel or channel group.
In a second aspect the present invention provides a programmable optical processor capable of changing a signal strength of an input optical signal. The processor comprises a controllable compensation module receiving the input optical signal and producing an output optical signal, a controller controlling the compensation module and a signal strength monitor coupled to the controller and coupled to determine the signal strength of the input optical signal and of the output optic
Dowslake Microsystems Corporation
Maclean Cassan
Pascal Leslie
Singh Dalzid
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