MXPA01002242A - Method and apparatus for optical system link control - Google Patents

Abstract

A method for tuning and improving the performance of an optical communication system comprising a link that includes optical amplifiers and, optionally, active and/or passive optical components such as, e.g., DWDM's, WADM's, and optical cross-connects, includes preferentially shaping and, in particular, flattening, with respect to the input power spectrum, the output power spectrum from the amplifier, component or of the link. A flattened output power spectrum is obtained by modifying the gain spectrum operating on the respective input power spectrum. Feedback for such gain modifications is typically provided by optimizing the optical signal to noise ratio of each channel of the respective output power spectrum. A system link, an optical amplifier, and optical components having flattened output power spectra are also described.

Description

METHOD AND APPARATUS FOR OPTICAL SYSTEM LINK CONTROL FIELD OF THE INVENTION Fiber-optic communications systems continue to grow dramatically in terms of numbers, capacity and complexity. One of the factors responsible for this trend is the increasing sophistication of optical amplifiers, particularly amplifiers of erbium doped fiber (EDFAs). In addition, the technology driving the EDFAs is expanding the spectral bandwidth where said amplifier can amplify input signals so that the scores of channels with sub-nanometer spacing can be amplified and transmitted in current systems. Other optical network elements such as DWDMs, WADMs, optical cross connections, etc., which are compatible for use in communication systems employing 80+ channel amplifiers are also available today. As data regimes, bandwidths, and system architectures continue to grow with increasing demand, system performance continues to be the baseline criterion, which becomes increasingly challenging. EDFAs, for example, present characteristic gain spectra that dictate the transition of an attenuated input signal into an amplified output signal. In the time of systems ^^ s ^^^ ¡^^^^^^^^^^^^^^^^^^^ X ^^^^^^^^^^^^^^^^^^^^^ ^^^^ ^^^^^^^^^^ individual optics or few channels, the signal transmission could be selected in one or more spectral windows corresponding to flatter portions of the gain spectrum of the optical amplifier, however, for a fiber amplifier doped with 80-channel erbium, the gain spectrum from 1520 nm to 1565 nm is only flat. The unmodified gain spectrum for a typical erbium doped aluminosilicate fiber has a strong peak at about 1532 nm. As such, the input channels in the spectral window that undergo a significantly greater gain can reach power levels where the non-linear optical effects seriously degrade the performance of the system. On the other hand, signal channels that experience a lower gain will typically have reduced signal-to-noise ratios, also contributing to the degradation of system performance. The non-flat gain spectrum of an erbium-doped fiber amplifier is somehow subject to flattening through the use of gain flattening filters (GFF's). A disadvantage of this approach is that even a perfect GFF will only produce a flat gain amplifier spectrum at a specific operating point. As the operating conditions change (ie, the investment values change as a result of a change in, for example, input power, spectral hole burn, wavelength direction of ^^ l? ^^^^^^^^^ w? ^ ^ ^ pumping, etc.) the gain setting will change and therefore the GFF will no longer produce a flat gain spectrum. It is also possible to compensate for the changing operating conditions previously discussed by adjusting the gain setting. This can be achieved, for example, by tuning the pump wavelength, controlling the temperature of the doped fiber with erbium, controlling the pumping power, adjusting the gain of the amplifier through a variable optical attenuator, using a tunable filter, and through obvious means to those skilled in the art. Typically, however, the amplified systems using such control techniques as discussed above are directed to flattening the gain spectrum of the amplifier or of each amplifier in a link having a plurality of amplifiers. In practice, even a finely tuned control scheme in combination with a GFF does not produce a completely flat gain spectrum. Both the control scheme and the GFF introduce certain flattening gain errors. Gain detection or gain flattening applied to an individual amplifier is only able to detect and correct gain flattening errors due to that specific amplifier. Unavoidable detection errors due to the intermediate optical connection and filter fidelityBurning the local spectral hole due to a detection channel, or other causes, results in a gain flattening error of each amplifier. When such random detection errors cause several amplifiers in ^^^^^^^^^^^ X ^ S¡ ^ ^ a link tilt their gain spectrum in a way that correlate, appear degradations of the general link power flattening and the ratio of optical signal to noise. If the gain flattening of each amplifier is optimized, these impairments can only be avoided by using extremely precise gain flattening detectors. Even if the amplifiers are identical and each is individually optimized for flattening, all gain flattening errors will accumulate. Therefore, power flattening errors due to control errors accumulate with each amplifier in the link. In this way, the inventors realized the need to improve the optical amplifier, optical network element, and link performance of the system beyond what was obtained by flattening the gain spectrum to maintain and improve performance. general of the optical communications system. BRIEF DESCRIPTION OF THE INVENTION This and other objects, and the advantages associated therewith can be obtained in accordance with the present invention wherein the output power spectrum 20 of an amplifier, optical network element or an optical network link using these components is configured preferably, and in particular, flattened to provide a power spectrum of ^^^^^^^^^^^^^^ g ^^ ß ^^^^^^ ^^^^^ i ^ S ^^^^, ^ á ^ a ^ -i? l? i? ? t- '' 'i ?? r- rW .t-r • --- IT. flattened input to an amplifier- or immediately consecutive component or to a receiving device. According to a first embodiment of the invention, in a multi-channel, multi-wavelength optical transmission system including a network transmission link having N (N> 1) optical amplifiers, each of which it provides gain to an attenuated input signal spectrum to extract an amplified signal spectrum, a method of tuning and improving the performance of the system includes the steps of detecting a characteristic of an optical signal (e.g., peak power) and a noise value) at the end of the link (for example, at the output of amplifier N) and control the gain spectra of the amplifiers in the link to maximize the optical-to-noise signal (OSNR) ratio in the end of the link. In another embodiment of the invention, a method for tuning and improving the performance of an optical link in an optical transmission system that includes one or more optical amplifiers, involves monitoring the output power spectrum of each amplifier and modifying the gain spectrum. of the conforming amplifier operates on a spectrum of input power to the amplifier to provide a flattened output power spectrum. In one aspect of this mode, the output power spectrum is flattened by optimizing the lowest OSNR for all output channels of the amplifier. ...._ _._, _-, -_ »»! "__. __. . . . " . ^ .- _-B. ».-._. -. * .- .. ", *.» * "Another embodiment of the invention includes an amplifier, or a network optical element that provides at least some equalization between the input and output signals, or a network link that comprises the optical amplifier and / or network optical element wherein the optical amplifier 5 and / or the optical network element and / or the end of the link has a power spectrum profile that is flattened, and more flat than the profile of the respective input power spectrum. In one aspect of this embodiment, an exemplary amplifier is an EDFA, and exemplary optical network elements include DWDM's, WADM's, optical cross connections, others. These exemplary devices are in no way intended to limit the invention since any active or passive device capable of producing a modified output power spectrum is suitable. In an aspect of all the above embodiments, a portion of the output of the respective device or link is diverted through a15 coupler or the like to a device such as an optical spectrum analyzer, which is coupled to means for modifying the gain spectrum or an equalization spectrum of the respective device. Exemplary modalities of techniques for modifying the gain or equalization spectra include gain inclination, input signal power level, spectral profile of input signal, wavelength tuning of pumping, and / or generally modification of the inversion level of an active fiber. In all the above embodiments, the output power per channel of each amplifier is preferably subjected to a maximum ^^^^^ «l. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^ fe ^ g ^^^ Jg ^^^^^^^^^^^^^^^^ g maximum channel power (Pcanai) and total output power (SPcanai), which are determined empirically for avoid disadvantages due to the lack of optical linearity in the transmission fiber.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become apparent from the following description of the preferred embodiments with reference to the accompanying drawings, wherein: Figure 1 shows a generic uni / bidirectional optical transmission system including a network link with N amplifiers Optical couplings and optional optical network elements; Figure 2 shows a representative 3-channel input power spectrum for input to a link or a component in a link; Figure 3 shows a representative 3-channel output power spectrum resulting from a flat gain spectrum applied to the input power spectrum of Figure 2, and a representative noise spectrum resulting from the amplification procedure; Figure 4 shows the output power spectrum of the figure 3, after it has been flattened according to one embodiment of the invention; -__. Figure 5 shows a spectrum of input power and the resulting output spectrum due to a constant or flat gain spectrum; Y Figure 6 schematically shows an embodiment of the invention wherein a coupler diverts a portion of the output power from an amplifier in an optical spectrum analyzer, whose data is used to modify the gain spectrum of the amplifier to obtain a flattened output.

DETAILED DESCRIPTION OF THE INVENTION In a preferred embodiment of the invention, a method for tuning and in particular for improving the performance of an optical transmission system is described. An optical transmission system, generally illustrated in Figure 1, typically comprises at least two terminal stations 10, 12, each of which includes a transmitter and / or receiver depending on whether the system is unidirectional or bidirectional, a network link 14 which typically includes a plurality of optical amplifiers 16 connected by various lengths of transmission waveguides such as optical fibers 18, and optionally comprising various passive and / or active optical network elements 20 including, but not limited to, DWDM's, WADM's, optical cross connections, etc. The invention that is modalized herein refers to tuning, and very particularly, to improving the ^^^^^^^^^^^^ # ^ 2 # ^^^^ performance of the transmission system by modifying or improving the performance of the link. This can be achieved by flattening the output power spectrum of the link (shown not flattened in Figure 3 and flattened in Figure 4). This may include flattening the output power spectrum of the final output of the link at the end of the link compared to the input power spectrum (shown schematically in Figure 2) at the start of the link, or very preferably comprises flattening the output power spectrum of each optical amplifier and optical network element in the link experiencing an input and an output such that the input power spectrum to the next component in the propagation direction of the optical signals has a preferred configuration, for example flattened. Those skilled in the art will appreciate that the invention can be applied to any optical network element that has some kind of equalization capability between signals input to the device and extracted from the device (obviously, for example, an amplifier, but can also be applied, for example, to a WADM). Also, when referring to a component, the referred component will be an optical amplifier and preferably an erbium impurified fiber amplifier. The invention, however, is not limited in this respect since an EDFA best illustrates the method and structure of the invention. In a multi-wavelength, amplified multiple wavelength optical transmission system 1 including a network link 14 that ^ -___ a_iaa -_-__-___ _--. ___ > ._ _____ contains a plurality of optical amplifiers interconnected 16 between the start 8 and the link term 9 (Fig. 1), the output will be different from the input as shown in Figs. 2 and 3; that is, the multiple wavelength output of an amplifier can be characterized by a spectrum that depends on the wavelength 32 which will be a function of the input power spectrum 22 (Fig. 2) and a gain spectrum 52 (Fig. it is shown flattened in FIG. 5) of the amplifier having a configuration that depends at least in part on the particular operating conditions of the amplifier such as, for example, the input power to the amplifier. Because the output power spectrum of an amplifier or a link becomes the input power spectrum for another component or segment of an optical transmission system, and the output power spectrum depends on the gain provided to the spectrum of respective input power, where the gain per se depends on the input power spectrum, a non-flat output power spectrum 22 has an important potential to degrade the performance and quality of the system. As described in the previous section, however, securing a flattened gain spectrum 52 will not by itself ensure a flat or smooth output power spectrum 56 for example, when the input power spectrum 54 is not flat, as it is shown in Figure 5. It is well known to those skilled in the art that the amplification of an input signal inherently produces a certain amount of noise, PASE, and hence the term optical to noise signal ratio. * • ^ "- ^ - - - - - ^^ - • ^^^^. ^ '^ & ^^. ^, .. ^ - ^^^^ ^ ,, __ ^ __.?,., ^ "^ __ _ (OSNR) defines the magnitude of the signal relative to the magnitude of the noise Figure 3 is a representative graph of an amplified output spectrum of multiple channels 32 for channels?.,? 2, and? 3, and PASE broadband noise, 34, due to the amplification procedure, taking the OSNR as the difference between a wavelength channel peak power (dB) and the noise power (dB) in the wavelength, you can see that? 2 has a relatively good OSNR (OSNR2), whereas? .is not as good (OSNR-i) and? 3 (OSNR3) is the worst. (Note that although the noise spectrum , PASE, appears relatively flat on the wavelength spectrum, this is merely illustrative since the spectral density of noise does not need to be flat.The value of? Ase is typically measured on a window of finite wavelength, for example, in units of pM / 0.1 nm, so both for the purpose of describing the invention, the OSNR will be defined as the difference between the peak power at a specified wavelength and the magnitude of noise at a wavelength, in dB, as represented by the formula OSNR (?) = Pout (dB) (?) - PASE (dB) (?).

The worst OSNR of a particular amplifier, or link, limits the overall performance and quality of the system. Therefore, optimizing and / or maximizing the worst or lowest OSNR of each output power spectrum will result in a flattened power output spectrum and - ___.__- ___ _ ___ __ _- _-_. _. ijfiii.fr *? n1_üfít-? f- irthüT? tt-tlH¡r? r-irrttr - * - rt f • • * - '-_'-___! therefore a flattened input power spectrum will be provided to the next amplifier or, a flattened output spectrum will be supplied to a receiver component at the end of the link. According to one embodiment of the invention, the OSNR can be maximized at the output of each successive amplifier in the link, and therefore maximize the OSNR at the end of the link, minimizing differences in output powers. of the channels being amplified in each stage of amplification. One way to achieve this, as shown schematically in figure 6, is to monitor and measure an output signal level and noise level of at least a portion of the amplifier output spectrum for each amplifier in the link. In an exemplary embodiment of the invention, an intermediate power connection or coupler, preferably an achromatic fiber coupler 62, positioned at the output of the amplifier 64, diverts a portion of the output spectrum in a spectrum analyzer 66 that monitors and displays the peaks and depressions of the output, representing the power and noise levels of each output signal. When the worst OSNR channel is detected, the gain spectrum of the amplifier is adjusted so that the channel with the lowest OSNR is maximized. There are numerous techniques for adjusting and / or modifying the gain spectrum of an amplifier, including, for example, modifying the input signal power, modifying the spectral profile of input signal, introducing gain inclination, tuning the wavelength of amplifier pumping light as described in the co-pending application, ». ^« «A. - - _ ^ - > ^ Z ^ _i_ ^ serial number 09 / 016,184 entitled Pump Wavelength Tuning of Optical Amplifiers and Use of Same in Wavelength Division Multiplexed Systems and incorporated herein by reference in its entirety, or by modifying the level of investment of a fiber active in the amplifier as described in the co-pending application entitled Thermal Tuning of Optical Amplifiers and Use of Same in Wavelength Multiplexed Systems Division, filed on June 30, 1998, and incorporated herein by reference in its entirety. Although the invention has been described in terms of flattening the output power spectrum of each successive amplifier in the direction of propagation of the signal, the invention also comprises applying the same general techniques to the output power spectrum at the terminus of the link. All mentioned techniques for modifying the gain spectrum of the amplifiers or other equalization approaches for optical network elements can be achieved in real time, for example, by means of a search algorithm such as a decreasing maximum gradient which by itself is not a part of the invention and therefore does not require additional description to understand the invention. '$ ¿&

Claims (20)

NOVELTY OF THE INVENTION CLAIMS

1. - In an amplified multiplex wavelength optical transmission system comprising a link having a start and a term, said link includes a plurality of optical amplifiers each for extracting a signal having an output power that is greater than a respective input signal power in each respective amplifier, wherein each amplifier is characterized by a gain spectrum having a configuration depending at least in part on the operating condition, said operating condition includes a power of input to the amplifier, a method for tuning the performance of the system, comprising the steps of: a) detecting a characteristic of an optical signal at the end of the link; and b) controlling said plurality of amplifiers so that an OSNR is maximized at the end of the link.

2. The method according to claim 1, further characterized in that the step of controlling said plurality of amplifiers comprises minimizing a deviation of the powers of the output signals from each respective amplifier in the link.

3. The method according to claim 2, further characterized in that it comprises successively minimizing the deviation of the powers of the output signals in the order of the direction of propagation.

4. The method according to claim 1, further characterized in that the step of controlling said plurality of amplifiers comprises measuring and maximizing the OSNR at the output of each successive amplifier in the link.

5. The method according to claim 4, further characterized in that it comprises the steps of: monitoring and / or measuring an output signal level and a noise level of at least a portion of an amplifier output spectrum for each amplifier in the link.

6. The method according to claim 5, further characterized in that it comprises the steps of: a) diverting the portion of an amplifier output spectrum from a main signal transmission line connected to the output of the amplifier; b) enter the deviated portion of the spectrum in a medium to measure a peak and a depression of each output signal of the output spectrum from each amplifier in the link.

7. The method according to claim 1, further characterized in that the step of controlling said plurality of amplifiers so as to maximize a OSNR at the end of the link comprises adjusting the gain spectrum of the amplifier until a channel that has the lowest OSNR to be maximized. , ______ »___ __a ^ ._

8. - The method according to claim 7, further characterized in that the step of adjusting the gain spectrum of the amplifier comprises changing at least one of the parameters influencing the gain inclination, input signal level, spectral profile of input signal , wavelength of pumping light, and inversion level of an active fiber in the amplifier.

9. The method according to claim 1, further characterized in that the step of controlling said plurality of amplifiers in such a way that an OSRN is maximized at the end of the link comprises: a) measuring the OSNR at the end of the link, and b) maximizing the lowest channel OSNR.

10. The method according to claim 9, further characterized in that the step of maximizing the lowest channel OSNT comprises modifying the gain spectra of an amplifier in the link.

11. The method according to claim 10, further characterized in that the step of modifying the gain spectra of an amplifier in the link comprises changing at least one of the parameters including the gain inclination, input signal level, spectral profile of input signal, wavelength of pumping light, and inversion level of an active fiber in the amplifier.

12. A method for tuning the performance of an optical link in an optical transmission system having a plurality of "_ ^« W ___ a- «¿-K ^ ¿. < * =. .. , * __ .. «...,, optical amplifiers each having an input and an output, and a waveguide connecting the output of an amplifier to the input of a different amplifier, where each amplifier has a wavelength which depends on the output power spectrum, which comprises step 5 of: flattening said output power spectrum of each amplifier before inputting said amplifier output into a device downstream of said amplifier.

13. The method according to claim 12, further characterized in that it comprises optimizing a signal ratio 10 optics to noise of an output signal channel with a lower signal-to-noise ratio.

14. A passive active optical component that forms a portion of a link of an optical transmission system, having an input and an output for at least one optical signal, wherein a plurality of

15 signals at the input are characterized by an input power spectrum, said component provides a plurality of signals at the output characterized by an output power spectrum, characterized in that the output power spectrum is flattened relative to the power spectrum of entry. 15. The component according to claim 14, further characterized in that it comprises means for monitoring and modifying the configuration of the output power spectrum. ^^ ¡^^^^^^^^^^ ^ ^ ^ ^ ^^^^^^^^^^^^^^^^

16. - The component according to claim 14, further characterized in that each plurality of output signals is characterized by a OSNR defined as the difference between a peak amplitude of the signal and a noise amplitude at the signal wavelength, in addition wherein a deviation between the OSNRs of all the signals is a minimum value, whereby the output power spectrum is relatively flat.

17. A link in an optical transmission system, comprising a plurality of active and / or passive optical components, wherein a plurality of signals having an input power spectrum are input at a start of the link and a plurality of signals having a spectrum of output power are extracted at the terminus of the link, characterized in that the output power spectrum has a configuration that is flatter than the input power spectrum.

18. The link according to claim 17, further characterized in that the plurality of active and / or passive optical components comprise an optical amplifier and at least one other optical amplifier, a WDM, a WADM, and an optical cross connection, interconnected to a respective input and output of each component by an optical waveguide, further wherein an output power spectrum of each component is a function of an input power spectrum to said component and at least one of a gain spectrum and an equalization spectrum provided by said respective component.

19. - The link according to claim 18, further characterized in that it comprises an optical coupler coupled to the output of at least one of the components to propagate at least a portion of the output to a device capable of monitoring a signal and a value of output noise, and means to modify the gain spectrum and the equalization spectrum to minimize an OSNR of the output.

20. The link according to claim 19, further characterized in that the optical coupler is an achromatic coupler.