WO1999000925A1

WO1999000925A1 - Method and apparatus for dynamically equalizing gain in an optical network

Info

Publication number: WO1999000925A1
Application number: PCT/US1998/013474
Authority: WO
Inventors: Gary A. Ball
Original assignee: Uniphase Telecommunications Products, Inc.
Priority date: 1997-06-30
Filing date: 1998-06-25
Publication date: 1999-01-07
Also published as: AU8175098A

Abstract

An apparatus and method for substantially equalizing the gain of a plurality of component signals of a multi-wavelength optical beam, each component signal has an amplitude and a unique wavelength. The method includes determining the amplitude of each component signal, and in turn adjusting the amplitudes of each component signal in dependence on the determined amplitude of each component signal, such that the adjusted amplitudes are at least approximately equal. It is preferred that the amplitudes are adjusted by using a plurality of adjustable-gain filters, such as wavelenght-tunable fiber Bragg gratings. The filters adjust the amplitude of at least one component signal and pass substantially unadjusted the amplitude of at least one other component signal. A controller is provided for adjusting the gain of each filter to set the adjusted amplitudes to be at least approximately equal in magnitude.

Description

METHOD AND APPARATUS FOR DYNAMICALLY EQUALIZING GAIN IN AN OPTICAL NETWORK

FIELD OF THE INVENTION The present invention relates to optical networks, and in particular relates to optical networks in which the signal amplitudes of a plurality of network channels are dynamically adjusted, thereby substantially equalizing all signal amplitudes for all channels.

CROSS REFERENCE TO RELATED APPLICATIONS

Some of the matter contained herein is disclosed and claimed in the commonly owned U.S. Patent Application Serial No. 08/885,428, entitled "Process For Fabrication And Independent Tuning Of Multiple Integrated Optical Directional Couplers On A Single Substrate;" U.S. Patent Application Serial No. 08/885,427, entitled "Loop Status Monitor For Determining The Amplitude Of The Signal Components Of A Multi-Wavelength Optical Beam;" and U.S. Patent Application Serial No. 08/885,747, entitled "Dynamic Optical Amplifier," all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

In optical networks, a technique known as wavelength division multiplexing (WDM) allows a single optical transmission medium, such as a fiber optic cable, to transmit a plurality of component signals ("channels"), in which each component signal has a unique wavelength. The plurality of component optical signals are combined by an optical combiner into a single, multi- wavelength optical beam which is transmitted across the optical transmission medium. Thereafter, the single, multi-wavelength beam is demultiplexed, yielding one or more of the component signals. Signals which are transmitted through portions (stages) of optical networks are attenuated during transmission by well known loss mechanisms. Accordingly, optical networks employ one or more amplifiers to amplify signals after transmission through a stage in an attempt to correct the attenuation. The component signals should ideally be approximately of equal amplitudes and signal-to-noise ratios after transmission. However, a problem with optical networks which employ WDM is that all channels in the single, multi- wavelength signal may not be amplified by the same amount. This is especially true in networks which rely on erbium doped fiber amplifiers since the gain of these amplifiers varies significantly as a function of wavelength.

Although it may conceivably be possible to estimate the variable-gain characteristics of an amplifier, and in turn attempt to correct for such variable- gain characteristics, it is extremely difficult to do so effectively. The variable-gain characteristics typically change dynamically with variations in, for example, the wavelength of each component signal and the number of component signals. This can be problematic, since in optical networks there may be a wide range of operating conditions, such as the dynamic addition and /or subtraction of channels.

Moreover, significant differences in signal strength and signal to noise ratio which occur in prior art networks over multiple stages present a substantial burden to the network, requiring more expensive componentry and system overhead.

Accordingly, it is an object of the present invention to provide a method and apparatus for substantially equalizing the gain of a plurality of component signals in a multi-wavelength optical signal. It is another object of the present invention to provide a method and apparatus of the foregoing type that allows optical networks to avoid large differences in gain and signal to noise ratio characteristic of networks without compensation on a stage by stage basis. SUMMARY OF THE INVENTION

The present invention is embodied in a low-loss, dynamic gain equalization module. In accordance with the present invention, a method is provided for substantially equalizing the gain of a plurality of component signals of a multi-wavelength optical signal, in which each component signal has an amplitude and a unique wavelength. The method includes determining the amplitude of each component signal, and in turn adjusting the amplitudes of each component signal in dependence on the determined amplitude of each component signal, such that the adjusted amplitudes are at least approximately equal.

It is further preferred that the amplitudes are adjusted by using a plurality of adjustable-gain filters, such as wavelength-tunable fiber Bragg gratings. The filters adjust the amplitude of at least one component signal and pass substantially unadjusted the amplitude of at least one other component signal. A means for adjusting the gain of each filter sets the adjusted amplitudes to be at least approximately equal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical network having an apparatus provided in accordance with the present invention for dynamically equalizing gain.

FIG. 2 is a diagrammatical illustration of the transmission characteristics of a fiber Bragg grating used with the apparatus of FIG. 1.

FIG. 3 is a diagrammatical illustration of the transmission characteristics of a fiber Bragg grating after tuning from a first wavelength to a second wavelength.

FIG. 4 is a flow chart illustrating a method by which channels are equalized. FIG. 5 is a schematic illustration of an alternative embodiment of the apparatus of FIG. 1.

FIG. 6 diagrammatically shows strain tuned gain profiles as a function of wavelength for several filters in the apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, an apparatus 10 for dynamically equalizing gain in an optical network 18 that includes an optical transmission medium 12, such as a fiber optic cable, for transmitting a plurality of component signals ("channels") of a multi-wavelength optical beam from a signal source 6 before presentation to a receiver 7. The signal source 6 is typically a multiplexer 8 for combining the component signals, each having a different wavelength, from a corresponding plurality of optical sources 9. There is also one or more amplifiers 15 of a type known in the art each of which amplifies each of the component signals a known amount. The receiver 7 is typically a demultiplexer 11 for separating the optical beam into the corresponding plurality of component signals, each having a different wavelength, onto a plurality of channel lines for presentation to an array 13 of detectors. Although only one apparatus 10 is shown in the optical network 18, those skilled in the art will note that more than one apparatus 10 of the present invention can be included in the network in dependence on networks parameters, such as the network length.

In a typical multi-wavelength signal, there may be up to forty component signals, and each component signal has a unique wavelength in a range between 1530 and 1560 nanometers (nm). Furthermore, successive wavelengths are typically separated by approximately 0.8 nm (100 GHz) or some fraction, or some multiple thereof. As is known in the art, the number of component signals, the component signal wavelengths, and the separation between wavelengths may vary from the exemplary amounts provided herein. Optical isolators 14 and 16 may be included in the network and are connected to the optical transmission medium 12 to prevent signals from being transmitted back through the medium. Thus, the optical isolators 14 and 16 serve to separate stages of an optical network by preventing unwanted noise from successive stages.

A plurality of adjustable-gain filters 20, 22 and 24 are connected to the optical transmission medium 12 for adjusting the amplitudes of each component signal (channel) in the multi-wavelength signal in a manner described in detail below. Naturally, there may be more or less than three such adjustable-gain filters. A gain controller 26 receives a portion of the optical beam as feedback from fiber optic coupler 27, determines the amplitude of each component signal, and adjusts the gain of each filter such that the adjusted amplitudes of the component signals are at least approximately equal. Several methods are known for determining the amplitude of each component signal, such as, for example, using an acousto-optical tunable filter (AOTF) or a tunable fiber Fabry Perot filter as a spectrum analyzer. Methods for adjusting the gain of each filter are described below.

The filters 20, 22 and 24 are preferably wavelength-tunable fiber Bragg gratings. As is known in the art, fiber Bragg gratings are notch-filters which substantially attenuate transmitted signals within a range of wavelengths and which pass substantially unattenuated those signals which are not within the range of wavelengths. The fiber Bragg gratings used by the present invention are of a type known in the are such as disclosed in U.S. Patent No. 5,077,816 incorporated herein by reference. Ideally, the wavelengths of the component signals and the attenuation characteristics of the filters 20, 22 and 24 are such that each filter attenuates at most one component signal, and passes the remaining component signals substantially unattenuated. Accordingly, there should ideally be one filter for each possible channel in the multi-wavelength signal. Turning now to FIG. 2, a chart 30 illustrates the degree to which a fiber Bragg grating attenuates signals having different wavelengths. The fiber Bragg grating represented by the chart 30 attenuates signals having a wavelength l_b more than signals having other wavelengths, and preferably attenuates signals having a wavelength l_b substantially completely. Accordingly, "maximal attenuation" occurs at the wavelength l_b , and the fiber Bragg grating is said to

"have a wavelength l_b". The fiber Bragg grating furthermore attenuates signals having a wavelength in a range 32 by varying amounts depending on, for example, the difference between the signal wavelength and the wavelength l_b. The range 32 is known as the bandwidth of the fiber Bragg grating. Signals having a wavelength outside of the range 32, such as those wavelengths indicated by reference numerals 34 and 36, are substantially unattenuated by the fiber Bragg grating.

Each of the adjustable-gain filters 20, 22 and 24 is preferably for adjusting the amplitude of one component signal. By adjusting the wavelength of each fiber Bragg grating, which is described below, the amount which the grating attenuates a signal can thus be changed as well. Thus, the gains of the filters 20, 22 and 24 may be adjusted such that the component signals, after attenuation by the filters 20, 22 and 24, have approximately equal amplitudes. Turning now to FIG. 3, the chart 40 depicts a curve 42 illustrating the attenuation characteristics when the filter has a wavelength and a curve 44 illustrating the attenuation characteristics when the filter has a wavelength 1₂. A signal having a wavelength indicated by the reference numeral 46 is attenuated to an amplitude 48 when the filter has a wavelength l_α . Similarly, the signal is attenuated to an amplitude 50, more than the amplitude 48 when the filter has a wavelength 1₂ . Accordingly, adjusting the wavelength of the filter adjusts the attenuation of signals having a prescribed wavelength.

In accordance with the present invention, the wavelength of the fiber Bragg gratings may be adjusted in a number of ways. In a first embodiment, the fiber Bragg gratings employ strain-tuning in which the wavelengths vary with the physical stress applied to each grating. Typically, the wavelength of the grating is adjusted ("tuned") by 1.2 nm per me of strain in filters having a wavelength of approximately 1540 nm. Strain is preferably applied by coupling i at least one piezo-electric device to each fiber Bragg grating, and adjusting the current applied to each piezo-electric device from the gain controller. Table 1 below illustrates the effects of different amounts of strain on a fiber Bragg grating having a wavelength of 1540 nm.

Pressure (kpsi) Strain (%) Tuning (nm)

5 0.05 0.6

10 0.10 1.2

25 0.25 3.0

Table 1

In a second embodiment shown schematically in FIG. 5, an apparatus 52 receiving amplified component signals from amplifier 55 and employs fiber Bragg gratings 54 which are thermally tuned such that the wavelengths vary with the temperature of each grating. Typically, the wavelength is tuned by 0.011 nm per degree Centigrade in filters having a wavelength of approximately 1550 nm. The temperature of each grating can be adjusted by applying an electrically-resistive coating 56 to or near each grating, and varying the current applied to each electrically-resistive coating from gain controller 58.

Turning to FIG. 4, a flow chart 60 illustrates the process by which the gain controller 26 (FIG. 1) equalizes the channels of a multi-wavelength signal. The gain controller 26 determines the amplitudes of all channels (step 62), and therefrom determines the lowest amplitude of all channels (step 64). Next, the gain controller 26 determines the degree of attenuation required to attenuate each channel (except the lowest amplitude channel) such that each attenuated amplitude is substantially equal to the lowest amplitude (step 66). The degree of attenuation (scaling factor) is the ratio

A₀ /A_x where:

A₀ is the lowest amplitude; and

A_x is the amplitude of a signal to attenuate.

The gain controller 26 then sets the attenuations of the filters in accordance with the determined degrees of attenuation (step 68). It will be understood by those skilled in the art that other methods of equalizing the channels of a multi- wavelength signal may be utilized without departing from the scope of the present invention, including more complicated algorithms which can be used in dual stage amplifiers with passive gain flattening. The present invention in operation can be seen by way of reference to FIG.

6 which diagramatically shows strain tuned gain profiles as a function of wavelength for several filters in the apparatus of FIG. 1. Curves 70-76 indicate reflectivity as a function of wavelength. Curves 78-84 correspond to the individual WDM signal wavelengths. Note that each pass band has been adjusted to a different value of reflectivity for each wavelength. In the example shown in Fig. 6, the greatest attenuation has been applied to 1₃ and the least attenuation to 1₄ Wavelengths l_x and 1₂ suffer intermediate amounts of reflection.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it would be understood by those skilled in the art that other various changes, omissions and additions thereto may be made without departing from the spirit and scope of the present invention. For example, the present invention may be embodied in a device which dynamically corrects for amplifier gain which is not equal for different wavelength signals, or a device which dynamically corrects for transmission medium attenuation which is not equal for different wavelength signals. Also, other methods of adjusting the wavelength of the fiber Bragg gratings may be employed, and other types of adjustable-gain filters besides fiber Bragg gratings may be employed. Other embodiments of the present invention include open loop systems in which the system parameters are characterized in advanced so that the component signal amplitudes and amount of adjustment there to are pre-established.

Claims

What is claimed is:

1. A method for substantially equalizing the gain of a plurality of component signals of a multi-wavelength optical beam transmitting an optical network in which each component signal has an amplitude and a unique wavelength, said method comprising the steps of: determining the amplitude of each component signal; and adjusting the amplitudes of each component signal in dependence on the determined amplitude of each component signal, such that the adjusted amplitudes are approximately equal.

2. The method of claim 1, wherein the step of adjusting further comprises: transmitting the multi-wavelength optical signal through a plurality of adjustable-gain filters in which each filter adjusts the amplitude of at least one component signal and does not adjust the amplitude of at least one other component signal; measuring said component signal magnitudes; and adjusting the gain of each filter such that the adjusted signal amplitudes are at least approximately equal in magnitude.

3. The method of claim 2, wherein the step of transmitting further comprises transmitting the multi-wavelength optical signal through a plurality of wavelength-tunable fiber Bragg gratings.

4. The method of claim 3, wherein the step of adjusting the gain of each filter further comprises adjusting strain applied to each wavelength-tunable fiber Bragg grating.

5. The method of claim 4, wherein the step of adjusting strain further comprises adjusting current applied to a plurality of piezo-electric devices, each of which is coupled to a respective wavelength-tunable fiber Bragg grating.

6. The method of claim 3, wherein the step of adjusting the gain of each filter further comprises adjusting the temperature of each wavelength- tunable fiber Bragg grating.

7. A method for substantially equalizing the gain of a plurality of component signals of a multi-wavelength optical signal, in which each component signal has an amplitude and a unique wavelength, said method comprising the steps of: transmitting the multi-wavelength optical signal through a plurality of adjustable-gain filters in which each filter adjusts the amplitude of at least one component signal and does not adjust the amplitude of at least one other component signal; measuring said component signal magnitudes; and adjusting the component signal amplitudes except that signal having the least amplitude to be approximately equal to the least signal amplitude, thereby generating approximately equal adjusted signal amplitudes.

8. An apparatus in an optical network for substantially equalizing the gain of a plurality of component signals of a multi-wavelength optical beam, in which each component signal has an amplitude and a unique wavelength, comprising: means for determining the amplitude of each component signal; and means for adjusting the amplitudes of each component signal in dependence on the determined amplitude of each component signal, such that the adjusted amplitudes are at least approximately equal.

9. The apparatus of claim 8, wherein the means for adjusting comprises: a plurality of adjustable-gain filters, each for adjusting the amplitude of at least one component signal and for passing substantially unadjusted the amplitude of at least one other component signal; feedback means for providing feedback signals indicative of the relative magnitudes of the respective component signals; and control means for adjusting the gain of each filter such that the adjusted amplitudes are at least approximately equal in response to said feedback signals.

10. The apparatus of claim 9, wherein the plurality of adjustable-gain filters comprise a plurality of wavelength-tunable fiber Bragg gratings.

11. The apparatus of claim 10, wherein the means for adjusting the gain of each filter comprises means for adjusting strain applied to each wavelength- tunable fiber Bragg grating.

12. The apparatus of claim 11, wherein the means for adjusting strain comprises: a plurality of piezo-electric devices, each of which is coupled to at least one wavelength-tunable fiber Bragg grating; and means for adjusting current applied to each of the plurality of piezo- electric devices in response to strain command signals from said control means.

13. The apparatus of claim 9 further comprising a means for adjusting the temperature of each of said fiber Bragg gratings in response to temperature command signals from said control means.

14. The apparatus of claim 9 wherein the optical network further comprises a signal source having a multiplexer for combining the plurality of component signals, each having a different wavelength, from a corresponding plurality of optical sources, an amplifier for amplifying each of the component signals, said optical network further comprising a receiver with a demultiplexer for separating the optical beam into the corresponding plurality of component signals onto a plurality of channel lines for presentation to an array of detectors.