WDM optical fiber system using Raman amplification

Details WDM optical fiber system using Raman amplification WDM optical fiber system using Raman amplification

1996-07-15

2001-02-20

Mehta, Bhavesh (Department: 2721)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S334000, C359S345000

Reexamination Certificate

active

06191877

ABSTRACT:

TECHNICAL FIELD
Wavelength Division Multiplexed (WDM) optical fiber communications.
DESCRIPTION OF RELATED ART
The erbium-doped silica-based fiber amplifier (EDFA) has had a large impact on fiber communications. Its operating wavelength range is in the 1550 nm low-loss wavelength region of silica-based fiber. Its substantial bandwidth permits simultaneous amplification of an entire set of WDM channels, and makes WDM operation feasible.
State-of-the-art optical fiber systems operate at 2.5 Gb/s or 5 Gb/s, at a nominal system wavelength of 1550 nm, using EDFAs spaced up to 120 km apart. Planned four-channel WDM systems quadruple that capacity, and more channels are contemplated. Attention is being given to next generation communication systems of still greater capacity. Consideration has been given to incorporation of a second set of WDM channels at a system wavelength of 1310 nm. Praseodymium, while of appropriate emission wavelength, does not offer a 1310 nm optical amplifier analogous to the 1550 nm EDFA, due to a radiationless transition in the silica fiber. Alternative approaches have considered fluoride-based fiber, and single crystal amplifiers.
Needs of soliton systems, which require operation within a critical intensity range to maintain pulse shape, at any system wavelength (at 1550 nm as well as 1310 nm), are not easily satisfied with the EDFA. It has been recognized for some time that Raman amplification, in principle, offers a solution. Postulated soliton systems have used the entire fiber as the Raman amplification medium, with pump-injection points at 20-60 km spacing to maintain signal intensity sufficiently constant. See, Stewart E. Miller and Ivan P. Kaminow,
Optical Fiber Telecommunications II,
Academic Press, Inc. 1988, at p. 97.
A reliable, truly distributed, fiber amplifier would avoid the alternating increasing and decreasing intensities of lumped amplifiers and lessen non-linear effects in conventional systems as well.
The Raman mechanism is operative in the backward pumping, as well as the forward pumping, direction. See, article by W. Jiang and P. Ye,
J. Lightwave Tech.,
vol. 7, no. 9, pp. 1407-1411, September 1989, in which the authors acknowledge the possibility. Accordingly, elements required for substitution for the EDFA seemed to be in place.
While true of single-channel systems, pump depletion modulation (PDM), however, imposes constraints on Raman amplified WDM systems. Crosstalk among WDM channels is caused by a two-step process in which: a) extraction of energy from the initially cw pump wave by a first modulated channel effectively modulates the pump by depletion; and b) the now-modulated pump in turn modulates a second channel being amplified. This pump-mediated crosstalk is of constant amplitude as modulation frequency increases, but beyond some threshold, decreases to a tolerable level (in accordance with a relationship dependent on the reciprocal of frequency). Using proposed systems, pump depletion modulation inhibits WDM operation on signals with significant modulation components below tens of megahertz. This effectively precludes contemplated WDM operation.
Jiang and Ye, J. Lightwave Tech. cited above, postulate that crosstalk in backward Raman amplification is smaller, but dismiss the approach based on other performance characteristics.
By analogy to usual EDFA designs, proposed Raman systems use bidirectional pumping for best results—to maintain constancy of intensity; to minimize number of amplifiers; to avoid excessive signal levels with their non-linear effects.
IEEE J. Quant. Elec.
vol. QE-22, no. 1, pp. 157-173, January 1986; and
Optics Lett.
vol. 13, no. 8, pp. 675-677, August 1988 both describe WDM systems using bidirectionally-pumped distributed Raman amplification.
SUMMARY OF THE INVENTION
Under real system conditions, PDM crosstalk is found to be sharply reduced—by more than three orders of magnitude using backward pumping. The extent of improvement means that there is no meaningful performance penalty due to PDM. Raman amplified optical fiber WDM systems using backward pumping, and operating at per-channel bit rates of gbits, and higher, become practical. Systems are preferably distributed and use the entire transmission fiber for amplification, but may use lumped amplifiers. All systems using intensity-modulation are included—digital systems, prevalent for long distance use; and analog systems, e.g. for cable TV and “fiber to the home”.
Preferred systems completely forgo forward pumping. Using reverse pumping alone, the low frequency limit imposed by PDM, is reduced by a factor of 10
3
, to tens of kilohertz.
Terminology
Raman Amplification—Amplification by which energy is transferred from an electromagnetic pump wave to a lower frequency signal wave via a molecular vibration. The responsible mechanism is stimulated Raman scattering (SRS).
Regenerator—Apparatus in which an optical signal to be amplified, is converted to an electrical signal, amplified, and converted back to an optical signal.
Pump Depletion Modulation (PDM)—Channel-to-channel crosstalk due to depletion of the pump during amplification of a first channel and subsequent transfer of that first channel information to a second channel being amplified.
Lumped Amplifier—An identifiable length of fiber, with substantial responsibility for amplification in series with transmission line. As applied to Raman amplification, the terminology requires pump injection directly to the amplifying fiber.
Distributed Amplifier—In the context of Raman amplification, an amplifier constituted of all or a substantial length of the transmission fiber itself. As usually contemplated, the distributed amplification fiber is unmodified transmission fiber.
Span or Fiber Span—Without further modifier, refers to the length of transmission fiber serviced by a single Raman amplifier. For distributed amplification, the terminology refers to the length of transmission fiber between successive pump injection points.
Regenerator Span—Length of optical fiber between successive regenerators.
Model System—System including at least one regenerator span of 360 km in length and provided with three pump injection points. This is a formalization, intended only to expedite discussion, and is not intended as a prediction of the form Raman amplified systems will take.


REFERENCES:
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patent: 5392377 (1995-02-01), Auracher
Peng, “Step-by-Step BSRS Amplification in Long-Span Optical Fiber Communication,”Electronic Letters, vol. 26, No. 5, 1990, pp. 334-336.
Zhang et al, “Backward Raman Amplification Used to Alleviate SRS Limitations in High-Density WDM Systems”, IEE Colloq., No. 159, 1990, pp. 5/1-5/3.
Stewart E. Miller and Ivan P. Kaminow,Optical Fiber Telecommunications II, Academic Press, Inc. 1988, p. 97.
W. Jiang and P. Ye,J. Lightwave Tech., vol. 7, No. 9, pp. 1407-1411, Sep. 1989.
L.F. Mollenauer, J.P. Gordon and M.N. Islam,IEEE J. Quant. Elec., vol. QE-22, No. 1, pp. 157-173, Jan. 1986.
L.F. Mollenauer and K. Smith,Optics Lett., vol. 13, No. 8, pp. 675-677, Aug. 1988.
S. Bahsoun, D.A. Fishman and J.A. Nagel,SPIE, vol. 1789, pp. 260-266, 1992.
H. Po, J.D Cao, B.M. Laliberte, R.A. Minns, et al.Elec. Lett., vol. 29, No. 17, p. 1500, Aug. 1993.
Li, “The impact of Optical Amplifiers on Long-Distance Lightwave Telecommunications”,Proceedings of IEEE, vol. 81, No. 11, Nov. 1993, pp. 1568-1579.
Kao, “Signal light amplification by Stimulated Raman scattering in an N-channel WDM Optical Fiber Communication System”,Journal of Lightwave Technology, vol. 7, No. 9, Sep. 1989, pp. 1290-1299.

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