WO2001084725A2

WO2001084725A2 - Optical transmission system

Info

Publication number: WO2001084725A2
Application number: PCT/US2001/013944
Authority: WO
Inventors: Glen Koste
Original assignee: Tellabs Operations, Inc.
Priority date: 2000-05-01
Filing date: 2001-05-01
Publication date: 2001-11-08
Also published as: WO2001084725A3; AU2001278845A1

Abstract

An optical transmission system includes a first amplifier that tilts optical signals in a first direction, an optical loss element, over which the optical signals are transmitted, and a second amplifier that tilts the optical signals in a second direction. The first amplifier tilts the optical signals in the first direction in response to an increase in an amount of power associated with the optical signals. The second amplifier tilts the optical signals in the second direction in response to a decrease in an amount of power associated with the optical signals.

Description

OPTICAL TRANSMISSION SYSTEM

TECHNICAL FIELD

This invention relates to an optical transmission system for transmitting wavelength-division-multiplexed optical signals across an optical span.

BACKGROUND

Optical transmission systems are used to transmit optical signals from one location to another location. Amplifiers in those system amplify multiplexed signals prior to transmitting those signals over an optical span, such as fiber optic cable. Such amplifiers are designed to provide a flat gain over an optical span having a predetermined length. If the length of that span changes for some reason (e.g., due to repairs, re-routing, or re-deployment), the loss of the span will likewise change. Accordingly, it may be desirable to increase the gain of an optical amplifier to compensate for such losses. Increasing the gain level of an optical amplifier beyond that for which it was designed can have different effects for signals having different wavelengths. For example, shorter wavelength (higher frequency) signals may experience less gain than longer wavelength (lower frequency) signals when passed through the same amplifiers. The differences in gain result in a wider range of powers in the signals after amplification, which sometimes can cause signal degradation.

SUMMARY

In general, in one aspect, the invention is directed to transmitting optical signals through an optical loss element. This aspect features controlling a first amplifier to tilt the optical signals in a first direction, transmitting the optical signals over the optical loss element from the first amplifier to a second amplifier, and controlling the second amplifier to tilt the optical signals in a second direction. By tilting the optical signals in first and second (e.g., opposite) directions, this aspect of the invention makes it possible to produce a substantially flat gain, and thus a substantially flat power, for signals across the transmission medium. As a result, signal degradations can be reduced.

This aspect may include one or more of the following features/functions. Controlling the first amplifier includes increasing an amount of power associated with the optical signals supplied to the first amplifier. Controlling the second amplifier includes decreasing an amount of power associated with the optical signals supplied to the second amplifier. The second direction is different from the first direction. The first and second optical amplifiers include erbium-doped fibers for tilting the optical signals in the first and second directions, respectively. The optical signals may have a range of wavelengths. Tilting the optical signals in the first direction may include applying a smaller gain to signals at a low end of the range of wavelengths than to signals at a high end of the range of wavelengths. Tilting the optical signals in the second direction may include applying a larger gain to signals at the low end of the range of wavelengths than to signals at the high end of the range of wavelengths. Tilting the optical signals in the first direction and tilting the optical signals in the second direction results in optical signals that have a substantially flat gain over the range of wavelengths.

Other features and advantages will become apparent from the following description, including the claims and drawings.

DESCRIPTION OF DRAWINGS

Fig. 1 shows an optical transmission system according to one embodiment of the invention.

Figs. 2 and 3 are graphs showing tilts in gain versus wavelength curves that are applied to optical signals by an optical amplifier. Fig. 4 is a flowchart showing a process for controlling optical amplifiers to produce a substantially flat gain.

Fig. 5 is a graph showing the substantially flat gain resulting from controlling optical amplifiers according to the process of Fig. 4.

Figs. 6 to 12 shows alternative embodiments of the optical transmission system of Fig. 1. DETAILED DESCRIPTION

Fig. 1 shows an optical transmission system 10 according to one embodiment of the invention. Optical transmission system 10 is a wavelength division multiplexing (WDM) system which transmits optical signals between remote locations, such as two cities. Optical transmission system 10 includes optical transmitters 11 (for example, lasers), multiplexer (MUX) 12, node output amplifier (NOA) 14, optical span 15, node input amplifier (NIA) 16, demultiplexer (DEMUX) 17, and receivers 19.

Multiplexer 12 receives individual optical signals 20 from transmitters 11. Optical signals 20 may include signals that range from relatively short wavelength (high frequency) signals to relatively long wavelength (low frequency) signals. Multiplexer 12 combines optical signals 20 and transmits the combined optical signals 21 to NOA 14.

NOA 14 and NIA 16 are optical amplifiers that apply gains to combined optical signals 21. NOA 14 and NIA 16 are designed so that their gains are flat, meaning substantially the same, for optical signals having a predetermined power level (or gain) over a range of wavelengths. Changing the power level of the optical signals, e.g., by increasing or decreasing the power level of optical signals, changes the gains applied to optical signals 21 by NOA 14 and NIA 16. The result is that, when taken individually, NOA 14 and NIA 16 no longer provide a flat gain over the range of wavelengths. Rather, the gain is "tilted" over the range of wavelengths, meaning that the amount of gain applied to individual optical signals varies with the wavelengths of the optical signals.

For example, as shown in Fig. 2, increasing the power of the optical signals applied to an optical amplifier, such as NOA 14 or NIA 16, tilts the output optical signals 26a in a direction 22a such that a smaller gain is applied to optical signals at the low end 24 of the range of wavelengths than to optical signals at the high end 25 of the range of wavelengths. This can be described as a positive tilt, since the slope of the gain versus (vs.) wavelength curve is positive. In contrast, as shown in Fig. 3, decreasing the power of the optical signals applied to the optical amplifier tilts the output optical signals 26a in a direction 29 such that a larger gain is applied to optical signals at a low end 24 of the range of wavelengths than to optical signals at a high end 25 of the range of wavelengths. This can be described as a negative tilt, since the slope of the gain versus (vs.) wavelength curve is negative.

As shown in Figs. 2 and 3, the amount of the tilt varies in accordance with the change in the power (or gain) of the optical signals. For example, referring to legend 30 in Fig. 2, a 0 dB (decibel) increase results in substantially no tilt in optical signals 26b, a +4 dB increase results in tilt 22c in optical signals 26c, a +8.5 dB increase results in a greater tilt 22d in optical signals 26d, and a +13 dB increase results in a still greater tilt 22a in optical signals 26a. A substantially equal, but opposite, effect is shown in Fig. 3. In Fig. 3, the signal power is decreased by the same amount that the signal power was increased in Fig. 2. This produces similar amounts of tilt in the optical signals, but in the opposite directions from the tilts shown in Fig. 2.

Referring back to Fig. 1, NOA 14 and NIA 16 use erbium (Er) doped fiber to produce gain. Combined optical signals 21 pass through optical span 15 between NOA 14 and NIA 16. A power loss in combined optical signals 21 is associated with optical span 15. The amount of this loss corresponds to the length of optical span 15. Generally speaking, longer optical spans result in more loss than shorter optical spans. The operation of optical transmission system 10 is as follows. Optical signals 20 are transmitted from transmitters 11 to multiplexer 12. Multiplexer 12 combines these signals into combined optical signals 21 and transmits combined optical signals 21 to NOA 14 over a transmission medium 31, such as fiber optic cable.

Alternatively, NOA 14 and multiplexer 12 may be in the same physical location, removing the need for transmission medium 31. NOA 14 amplifies combined optical signals 21 by applying a predetermined gain thereto and transmits combined optical signals 21 to NIA 16 over optical span 15. NIA 16 amplifies combined optical signals 21, e.g., to compensate for the loss associated with optical span 15, and transmits combined optical signals 21 to demultiplexer 17 over a transmission medium 32, such as fiber optic cable. Alternatively, NIA 16 and demultiplexer 17 may be in the same physical location, removing the need for transmission medium 32. Demultiplexer 17 separates the combined optical signals 21 according to wavelength (frequency) and transmits the optical signals to their corresponding intended receivers 19.

Fig. 4 explains how combined optical signals 21 are transmitted between NOA 14 and NIA 16 to obtain a substantially flat gain across optical transmission system 10. To begin, NOA 14 is controlled (401) to tilt combined optical signals 21 in a first direction. For example, NOA 14 may be controlled to tilt combined optical signals 21 in the direction 22a shown in Fig. 2. NOA 14 is controlled in this manner by increasing the power of optical signals 20. This is done, for example, by increasing the power of transmitters 11.

Combined optical signals 21 are transmitted (402) over optical span 15 to NIA 16. NIA 16 is controlled (403) to tilt combined optical signals 21 in a second direction (e.g., the direction 29 of Fig. 3), which is substantially opposite to the first direction above. NIA 16 is controlled by decreasing the amount of power of the combined optical signals applied to NIA 16 by roughly the amount that power was increased in 401. This may be done, for example, by increasing the length of optical span 15 (the increase in span length may, in fact, precede the increase in power to NOA 14 and thus dictate the amount by which that power must be increased to produce a flat gain). However, other means of decreasing the power of combined optical signals 21 may be used, such as attenuators or the like. Because the gain tilt produced by NIA 16 is substantially the opposite of the gain tilt produced by NOA 14, the signals output from NIA 16 have a substantially flat gain and, thus, a substantially flat power. This is shown in Fig. 5, which depicts a substantially flat power regardless of the change in power of original optical signals (e.g., 0 dB, 4dB, 8 dB and 12 dB). By providing a substantially flat gain, the invention reduces signal transmission errors.

The invention is not limited to the embodiment described above. For instance, the invention can be used with optical amplifiers that do not use Er-doped fibers to produce gains and can transmit any number of optical signals having any wavelengths/frequencies. Several amplifiers can be "chained" in a single optical transmission system and controlled in accordance with the process described above to provide a substantially flat gain over the length of the system. For example, as shown in Fig. 6, multiple NOAs 35 and NIAs 36 could be controlled in the manner described above to provide a substantially flat gain. In this case, the amplifiers are controlled to provide portions of the overall power gains. Likewise, as shown in Figs. 7 to 11, optical line amplifiers (OLAs) situated along various portions of an optical span (with or without NOAs and/or NIAs) can be controlled in accordance with the invention. The invention could be used with any optical system having an optical loss between two optical WDM amplifiers. The optical cross-connect of Fig. 12 is an example of such a system. OLA 51 and OLA 50 can be controlled to compensate for changes to the loss of switch fabric 60. The invention can be used with any type of optical loss element, such as the optical span described above and/or the cross- connect of Fig. 12.

Other embodiments not described herein are also within the scope of the following claims. WHAT IS CLAIMED IS:

Claims

1. A method of transmitting optical signals through an optical loss element, comprising: controlling a first amplifier to tilt the optical signals in a first direction; transmitting the optical signals through the optical loss element from the first amplifier to a second amplifier; and controlling the second amplifier to tilt the optical signals in a second direction.

2. The method of claim 1, wherein controlling the first amplifier comprises increasing an amount of power associated with the optical signals supplied to the first amplifier.

3. The method of claim 1, wherein controlling the second amplifier comprises decreasing an amount of power associated with the optical signals supplied to the second amplifier.

4. The method of claim 1, wherein the second direction is different from the first direction.

5. The method of claim 1, wherein: the optical signals have a range of wavelengths; tilting the optical signals in the first direction comprises applying a smaller gain to signals at a low end of the range of wavelengths than to signals at a high end of the range of wavelengths; and tilting the optical signals in the second direction comprises applying a larger gain to signals at the low end of the range of wavelengths than to signals at the high end of the range of wavelengths.

6. The method of claim 5, wherein tilting the optical signals in the first direction and tilting the optical signals in the second direction results in optical signals that have a substantially flat gain over the range of wavelengths.

7. The method of claim 1, wherein the first and second optical amplifiers include erbium-doped fibers for tilting the optical signals in the first and second directions, respectively.

8. The method of claim 1, wherein the optical loss element comprises fiber optic transmission cable.

9. The method of claim 1, wherein the optical loss element comprises an optical assembly.

10. An optical transmission system, comprising: a first amplifier which tilts optical signals in a first direction; an optical loss element, through which the optical signals are transmitted; and a second amplifier which tilts the optical signals in a second direction.

11. The system of claim 10, wherein the first amplifier tilts the optical signals in the first direction in response to an increase in an amount of power associated with the optical signals.

12. The system of claim 10, wherein the second amplifier tilts the optical signals in the second direction in response to a decrease in an amount of power associated with the optical signals.

13. The system of claim 10, wherein the second direction is different from the first direction.

14. The system of claim 10, wherein: the optical signals have a range of wavelengths; tilting the optical signals in the first direction comprises applying a smaller gain to signals at a low end of the range of wavelengths than to signals at a high end of the range of wavelengths; and tilting the optical signals in the second direction comprises applying a larger gain to signals at the low end of the range of wavelengths than to signals at the high end of the range of wavelengths.

15. The system of claim 14, wherein tilting the optical signals in the first direction and tilting the optical signals in the second direction results in optical signals that have a substantially flat gain over the range of wavelengths.

16. The system of claim 10, wherein the optical loss element comprises fiber optic transmission cable.

17. The system of claim 10, wherein the optical loss element comprises an optical assembly.

18. The system of claim 10, wherein the first and second optical amplifiers include erbium-doped fibers for tilting the optical signals in the first and second directions, respectively.