Managing gain tilt in an optically amplified transmission system

1. An optical transmission system comprising:

a plurality of higher gain optical amplifiers, each having a higher nominal gain value across a plurality of optical signal channels;

a plurality of lower gain optical amplifiers, each having a lower nominal gain value across said plurality of optical signal channels lower than said higher nominal gain value;

a plurality of optical fiber spans coupling said optical amplifiers, wherein said higher gain optical amplifiers and said lower gain optical amplifiers are arranged to allow a net gain across said plurality of optical signal channels to vary within a predetermined acceptable net gain excursion at an output of each of said higher gain optical amplifiers and said lower gain optical amplifiers; and

wherein a net gain vector in one direction through said optical fiber spans and said optical amplifiers is the same as a net gain vector in the other direction through said optical fiber spans and said optical amplifiers, said net gain vector being represented by values of net gain at outputs of said optical amplifiers, and wherein said net gain at a respective output of one of said optical amplifiers is the sum of cabled fiber losses at the respective output and nominal gains of said optical amplifiers at the respective output.

2. The optical transmission system of claim 1 wherein a nominal span loss value of each of said optical fiber spans is lower than said higher nominal gain value and higher than said lower nominal gain value.

3. The optical transmission system of claim 2 wherein said nominal span loss value of each of said optical fiber spans is essentially the same.

4. The optical transmission system of claim 1 wherein said net gain at one end of said optical transmission system is approximately zero.

5. The optical transmission system of claim 1 wherein said predetermined acceptable net gain excursion is less than about 10 dB.

6. The optical transmission system of claim 1 wherein at least one of said optical fiber spans includes a plurality of fibers spliced together.

7. The optical transmission system of claim 1 wherein said repeaters have essentially the same spacing.

8. The optical transmission system of claim 1 , further comprising an optical transmitter for transmitting an optical signal through said optical fiber spans.

9. The optical transmission system of claim 8 , further comprising an optical receiver for receiving an optical signal transmitted through said optical fiber spans.

10. The system of claim 1 wherein said optical amplifiers are erbium doped fiber amplifiers.

11. An optical transmission system comprising:

a plurality of transmission spans coupled together, each of said transmission spans having a nominal span loss value across a plurality of optical signal channels and comprising a fiber optic cable span and an optical repeater coupled to said fiber optic cable span, wherein said optical repeater in each of said transmission spans has either a higher nominal gain value across said plurality of optical signal channels higher than said nominal span loss value or a lower nominal gain value across said plurality of optical signal channels lower than said nominal span loss value to allow a net gain across said plurality of optical signal channels to vary within a predetermined acceptable net gain excursion at an output of said optical repeater in each of said transmission spans, wherein a net gain vector in one direction through said transmission spans is the same as a net gain vector in the other direction through said repeaters and said optical transmission spans.

12. The optical transmission system of claim 11 wherein said net gain varies above and below zero.

13. The optical transmission system of claim 11 wherein said predetermined acceptable net gain excursion is less than about 10 dB.

14. The optical transmission system of claim 11 wherein each of said fiber optic cable spans includes at least first and second optical fiber spans for transmitting optical signals in at least first and second directions.

15. The optical transmission system of claim 14 wherein each of said repeaters includes at least first and second optical amplifiers for amplifying optical signals transmitted in said at least first and second directions.

16. A method for managing gain tilt in an optical transmission segment, said method comprising:

determining a nominal span loss value for transmission spans in said optical transmission segment;

providing a plurality of higher gain repeaters each having a higher nominal gain value higher than said nominal span loss value;

providing a plurality of lower gain repeaters each having a lower nominal gain value lower than said nominal span loss value; and

arranging said higher gain repeaters and said lower gain repeaters in said optical transmission segment such that a net gain at different points in said optical transmission segment varies within a predetermined acceptable net gain excursion and such that a net gain vector in one direction through said repeaters and said optical transmission spans is the same as a net gain vector in the other direction through said repeaters and said optical transmission spans.

17. The method of claim 16 wherein said net gain at one end of said optical transmission segment is about zero.

18. The method of claim 16 wherein said predetermined acceptable net gain excursion is less than about 10 dB.

19. A method of constructing an optical transmission segment to manage gain tilt in an optical system, said method comprising:

determining a repeater spacing in said optical transmission segment based on a nominal span loss and nominal repeater gain substantially equal to said nominal span loss;

providing a plurality of fiber optic cable spans corresponding to said repeater spacing;

providing a plurality of higher gain repeaters each having a higher nominal gain value;

providing a plurality of lower gain repeaters each having a lower nominal gain value lower than said higher nominal gain value;

connecting said repeaters to said fiber optic cable spans to form transmission spans, wherein said higher gain repeaters and said lower gain repeaters are arranged such that a net gain at different points in said optical transmission segment varies within a predetermined acceptable net gain excursion, wherein connecting said repeaters comprises connecting a higher gain repeater at a location where negative gain tilt accumulates; and

connecting a line buildout (LBO) at a location where positive gain tilt accumulates.

20. The method of claim 19 wherein said lower nominal gain value is equivalent to a nominal span loss value.

21. The method of claim 19 wherein said higher nominal gain value is higher than said nominal span loss value, and wherein said lower nominal gain value is lower than said nominal span loss value.