Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-02-13
2002-01-22
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06341021
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the art of multi-channel power equalization in wavelength-division-multiplexed (WDM) optical networks. It finds particular application in conjunction with data transmission for local and wide area optical networks and will be described with particular reference thereto. It is to be appreciate, however, that the invention is also amenable to other applications wherein high-speed data transfer among multiple users and/or long-distance data transmissions are desirable.
In the implementation and development of local and wide area optical networks, data transmission systems, routing networks, and the like, it is desirable to provide high-capacity, high-speed, and/or long-distance data transmissions among multiple users. Two valuable technologies useful for achieving the aforementioned goals include wavelength-division-multiplexers (WDM) and erbium-doped fiber amplifiers (EDFA). In conventional WDM systems, many different wavelength channels are simultaneously transmitted along the optical fibers or data transmission lines. A typical system will carry 4-16 different wavelength data channels with wavelength spacings varying between 0.8 and 4.0 nm. This approach dramatically increases the capacity of the transmission system and permits wavelength dependent optical network routing. EDFA's offer several advantages including high gain, low-additive noise, and fiber compatibility. Most relevant for WDM applications is the EDFA's ability to amplify multiple wavelengths over a wide band width, typically 1 MHz. Additionally, for robust networks and/or systems, it is advantageous that power differentials between the various WDM channels remain small so as to minimize interchannel cross talk and ensure adequate gain for all the channels. Moreover, stability to dynamic changes in the system parameters is paramount to the successful implementation of a high-speed WDM optical network.
Traditionally, two obstacles in implementing WDM networks are: (1) the non-uniformity of EDFA gain, and (2) dynamic changes in channel powers. Since the EDFA gain is not uniform with wavelength, the lower gain channels progressively lose power relative to the higher gain channels. Amplifier chains impart a significant power and signal-to-noise ratio (SNR) differential among the various channels, significantly limiting the transmission distance and usable amplifier band width. Conventionally, the usable band width can be as small as approximately 5 nm after an EDFA cascade. In addition, as channel powers vary in a dynamic network, several system complications may arise which can potentially cause network failure. Variables which are troublesome include: changes in the input signal powers; drift in component wavelength selectivity; changes in link losses; and, changes in amplifier gain. Variable insertion losses, neighboring channel addition and deletion, unstable laser power, non-uniform EDFA gain, and microsecond long gain transients in EDFA cascades are other system parameters whose change can have deleterious effects on signal power.
In the past, passive channel power equalization methods such as long-period fiber grading, end-to-end telemetry, and the like have been employed to at least partially address the above-mentioned concerns. However, while passive gain equalization schemes can provide performance improvement for a point-to-point static or slow-changing link, they are not satisfactory for a dynamically reconfigurable network or for a static network with potential parameter changes. This is because the individual channel powers may vary significantly due to the dynamic and distributed characteristics of the WDM network and the EDFA gain non-uniformity varies with dynamic input load. Even an end-to-end telemetry technique, in which the output determines a wavelength selective attenuation of the input, is not adequate for systems that may change faster than tens of milliseconds. In another previous undertaking employed to address the above-mentioned concerns, erbium-doped fluoride fibers having a smaller gain non-uniformity were utilized. However, such a system cannot accommodate the dynamic changes in the network.
The present invention contemplates a new and improved dynamic network equalization module that overcomes the above-referenced problems and others. Moreover, it equalizes WDM channel powers to ensure robust network operation, a high gain, and a high SNR for all channels.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a dynamic power equalization module for an optical network is provided. It includes a wavelength-division-multiplexer having an optical input for receiving an optical data transmission containing a plurality of different channels each with correspondingly different wavelengths. The wavelength-division-multiplexer spectrally separates the optical data transmission received by the optical input into parallel optical outputs which correspond to the different channels. A parallel array of acousto-optic modulators driven by RF acoustic signals is connected to the optical outputs of the wavelength-division-multiplexer. A coupler passively combines optical outputs from the parallel array of acousto-optic modulators such that the combined optical outputs from the parallel array of acousto-optic modulators is relayed to a plurality of coupler outputs corresponding to a feedback loop for the different channels and at least one coupler output corresponding to the dynamic power equalization module output. A plurality of optical filters are connected to the coupler outputs corresponding to the feedback loop. Each optical filter filters-out wavelengths that do not correspond to the channel in which they are being transmitted. A plurality photodetectors each receive an output from corresponding optical filters such that the plurality of photodetectors produce signals representative of powers of the different channels. A dynamic control circuit compares the signals produced by the plurality of photodetectors to determine the relative power of the different channels and generates the RF acoustic signals that drive the parallel array of acousto-optic modulators. The parallel array of acousto-optic modulators are driven such that each acousto-optic modulator dynamically controls each channels' transmission and reduces its power level to be substantially the same as that of a reference channel.
In accordance with another aspect of the present invention, a dynamic power equalization module for an optical network includes a module input for receiving an optical transmission containing a combination of different channels. Each channel has a different characteristic wavelength and different varying powers. A power equalization device that is polarization insensitive is connected between the module input and a module output. The power equalization device is driven by RF acoustic control signals. A feedback loop which receives at least a portion of output from the module output and generates, based upon a comparison of relative powers of the different channels, the RF acoustic control signals which drive the power equalization device. The power equalization device is driven such that it dynamically controls transmission of the different channels to substantially equalize power differentials therebetween.
In accordance with a more limited aspect of the present invention, the power equalization device is a polarization independent acousto-optic tunable filter.
In accordance with a more limited aspect of the present invention, the power equalization device includes at least one acousto-optic tunable filter driven by the RF acoustic control signals from the feedback loop in a bar-state, wherein the acousto-optic tunable filter functions as a multi-channel notch filer.
In accordance with a more limited aspect of the present invention, the power equalization device includes at least one acousto-optic tunable filter driven by the RF acoustic control signals from the feedback loop in a cross-state, wherein the
Cai Jin-Xin
Feng Kai-Ming
Huang Shouhua
Smith David A.
Wilner Alan E.
Case Western Reserve University
Fay Sharpe Fagan Minnich & McKee LLP
Pascal Leslie
Singh Dalzid
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